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Showing papers by "Fabian Walter published in 2011"


Journal ArticleDOI
TL;DR: In this paper, the authors used the IRAM HERACLES survey to study CO emission from 33 nearby spiral galaxies down to very low intensities, using 21 cm line atomic hydrogen (H I) data, mostly from THINGS, to predict the local mean CO velocity based on the mean H I velocity.
Abstract: We use the IRAM HERACLES survey to study CO emission from 33 nearby spiral galaxies down to very low intensities. Using 21 cm line atomic hydrogen (H I) data, mostly from THINGS, we predict the local mean CO velocity based on the mean H I velocity. By re-normalizing the CO velocity axis so that zero corresponds to the local mean H I velocity we are able to stack spectra coherently over large regions. This enables us to measure CO intensities with high significance as low as I{sub CO} {approx} 0.3 K km s{sup -1} ({Sigma}{sub H{sub 2}}{approx}1 M{sub sun} pc{sup -2}), an improvement of about one order of magnitude over previous studies. We detect CO out to galactocentric radii r{sub gal} {approx} r{sub 25} and find the CO radial profile to follow a remarkably uniform exponential decline with a scale length of {approx}0.2 r{sub 25}. Here we focus on stacking as a function of radius, comparing our sensitive CO profiles to matched profiles of H I, H{alpha}, far-UV (FUV), and Infrared (IR) emission at 24 {mu}m and 70 {mu}m. We observe a tight, roughly linear relationship between CO and IR intensity that does not show any notable break between regionsmore » that are dominated by molecular gas ({Sigma}{sub H{sub 2}}>{Sigma}{sub H{sub i}}) and those dominated by atomic gas ({Sigma}{sub H{sub 2}}<{Sigma}{sub H{sub i}}). We use combinations of FUV+24 {mu}m and H{alpha}+24 {mu}m to estimate the recent star formation rate (SFR) surface density, {Sigma}{sub SFR}, and find approximately linear relations between {Sigma}{sub SFR} and {Sigma}{sub H{sub 2}}. We interpret this as evidence of stars forming in molecular gas with little dependence on the local total gas surface density. While galaxies display small internal variations in the SFR-to-H{sub 2} ratio, we do observe systematic galaxy-to-galaxy variations. These galaxy-to-galaxy variations dominate the scatter in relationships between CO and SFR tracers measured at large scales. The variations have the sense that less massive galaxies exhibit larger ratios of SFR-to-CO than massive galaxies. Unlike the SFR-to-CO ratio, the balance between atomic and molecular gas depends strongly on the total gas surface density and galactocentric radius. It must also depend on additional parameters. Our results reinforce and extend to lower surface densities, a picture in which star formation in galaxies can be separated into two processes: the assembly of star-forming molecular clouds and the formation of stars from H{sub 2}. The interplay between these processes yields a total gas-SFR relation with a changing slope, which has previously been observed and identified as a star formation threshold.« less

584 citations


Journal ArticleDOI
TL;DR: In this article, mass models for the dark matter component of seven dwarf galaxies taken from 'The H I Nearby Galaxy Survey' (THINGS) and compare these with those taken from numerical {Lambda} cold dark matter (LambDA}CDM) simulations are presented.
Abstract: We present mass models for the dark matter component of seven dwarf galaxies taken from 'The H I Nearby Galaxy Survey' (THINGS) and compare these with those taken from numerical {Lambda} cold dark matter ({Lambda}CDM) simulations. The THINGS high-resolution data significantly reduce observational uncertainties and thus allow us to derive accurate dark matter distributions in these systems. We here use the bulk velocity fields when deriving the rotation curves of the galaxies. Compared to other types of velocity fields, the bulk velocity field minimizes the effect of small-scale random motions more effectively and traces the underlying kinematics of a galaxy more properly. The 'Spitzer Infrared Nearby Galaxies Survey' 3.6 {mu}m and ancillary optical data are used for separating the baryons from their total matter content in the galaxies. The sample dwarf galaxies are found to be dark matter dominated over most radii. The relation between total baryonic (stars + gas) mass and maximum rotation velocity of the galaxies is roughly consistent with the baryonic Tully-Fisher relation calibrated from a larger sample of gas-dominated low-mass galaxies. We find discrepancies between the derived dark matter distributions of the galaxies and those of {Lambda}CDM simulations, even after corrections for non-circular motions have beenmore » applied. The observed solid body-like rotation curves of the galaxies rise too slowly to reflect the cusp-like dark matter distribution in cold dark matter halos. Instead, they are better described by core-like models such as pseudo-isothermal halo models dominated by a central constant-density core. The mean value of the logarithmic inner slopes of the mass density profiles is {alpha} = -0.29 {+-} 0.07. They are significantly different from the steep slope of {approx} - 1.0 inferred from previous dark-matter-only simulations, and are more consistent with shallower slopes found in recent {Lambda}CDM simulations of dwarf galaxies in which the effects of baryonic feedback processes are included.« less

434 citations


01 Jan 2011
TL;DR: In this paper, the authors proposed a method to find the minimum number of stars in the UHRA data set, which is not available in the data set available in this paper.
Abstract: Original article can be found at: http://www.iop.org/EJ/journal/1538-3881 Copyright American Astronomical Society. DOI: 10.1088/0004-6256/137/6/4670 [Full text of this article is not available in the UHRA]

371 citations


Journal ArticleDOI
TL;DR: In this paper, a direct comparison of the derived dark matter (DM) distributions between hydrodynamical simulations of dwarf galaxies assuming a ΛCDM cosmology and the observed dwarf galaxies sample from the THINGS survey in terms of rotation curve shape and the logarithmic inner density slope α of mass density profiles is made.
Abstract: We make a direct comparison of the derived dark matter (DM) distributions between hydrodynamical simulations of dwarf galaxies assuming a ΛCDM cosmology and the observed dwarf galaxies sample from the THINGS survey in terms of (1) the rotation curve shape and (2) the logarithmic inner density slope α of mass density profiles. The simulations, which include the effect of baryonic feedback processes, such as gas cooling, star formation, cosmic UV background heating, and most importantly, physically motivated gas outflows driven by supernovae, form bulgeless galaxies with DM cores. We show that the stellar and baryonic mass is similar to that inferred from photometric and kinematic methods for galaxies of similar circular velocity. Analyzing the simulations in exactly the same way as the observational sample allows us to address directly the so-called cusp/core problem in the ΛCDM model. We show that the rotation curves of the simulated dwarf galaxies rise less steeply than cold dark matter rotation curves and are consistent with those of the THINGS dwarf galaxies. The mean value of the logarithmic inner density slopes α of the simulated galaxies' DM density profiles is ~–0.4 ± 0.1, which shows good agreement with α = –0.29 ± 0.07 of the THINGS dwarf galaxies. The effect of non-circular motions is not significant enough to affect the results. This confirms that the baryonic feedback processes included in the simulations are efficiently able to make the initial cusps with α ~–1.0 to –1.5 predicted by DM-only simulations shallower and induce DM halos with a central mass distribution similar to that observed in nearby dwarf galaxies.

294 citations


Journal ArticleDOI
TL;DR: In this article, far-infrared and sub-millimeter photometry from the Herschel Space Observatory is presented for 61 nearby galaxies from the Key Insights on Nearby Galaxies: A Far-Infrared Survey with Herschel (KINGFISH) sample.
Abstract: New far-infrared and sub-millimeter photometry from the Herschel Space Observatory is presented for 61 nearby galaxies from the Key Insights on Nearby Galaxies: A Far-Infrared Survey with Herschel (KINGFISH) sample. The spatially-integrated fluxes are largely consistent with expectations based on Spitzer far-infrared photometry and extrapolations to longer wavelengths using popular dust emission models. Dwarf irregular galaxies are notable exceptions, as already noted by other authors, as their 500um emission shows evidence for a sub-millimeter excess. In addition, the fraction of dust heating attributed to intense radiation fields associated with photo-dissociation regions is found to be (21+/-4)% larger when Herschel data are included in the analysis. Dust masses obtained from the dust emission models of Draine & Li are found to be on average nearly a factor of two higher than those based on single-temperature modified blackbodies, as single blackbody curves do not capture the full range of dust temperatures inherent to any galaxy. The discrepancy is largest for galaxies exhibiting the coolest far-infrared colors.

277 citations


Journal ArticleDOI
TL;DR: In this article, the authors present the most extensive consistent analysis to date of 4 4 sample and the log (L bol/L Edd) = −0.80 (l bol/l Edd ~ 0.16) with a scatter of 0.24
Abstract: Quasars (QSOs) at the highest known redshift (z ~ 6) are unique probes of the early growth of supermassive black holes (BHs). Until now, only the most luminous QSOs have been studied, often one object at a time. Here we present the most extensive consistent analysis to date of 4 4 sample and the log (L bol/L Edd) = –0.80 (L bol/L Edd ~ 0.16) with a scatter of 0.24 dex for the 0.35 4 sources are accreting significantly faster than the lower-redshift ones. We show that the derived Fe II/Mg II ratios depend sensitively on the analysis performed: our self-consistent, homogeneous analysis significantly reduces the Fe II/Mg II scatter found in previous studies. The measured Fe II/Mg II line ratios show no sign of evolution with cosmic time in the redshift range 4 < z < 6.5. If the Fe II/Mg II line ratio is used as a secondary proxy of the Fe/Mg abundance ratio, this implies that the QSOs in our sample have undergone a major episode of Fe enrichment in the few 100 Myr preceding the cosmic age at which they are observed.

247 citations


Journal ArticleDOI
TL;DR: In this article, the CO ground-level transition in high-redshift quasi-stellar objects (QSOs) was detected using the NRAO Green Bank Telescope (GBT) and the MPIfR Effelsberg 100 m telescope.
Abstract: We present ^(12)CO(J = 1 → 0) observations of the high-redshift quasi-stellar objects (QSOs) BR 1202-0725 (z = 4.69), PSS J2322+1944 (z = 4.12), and APM 08279+5255 (z = 3.91) using the NRAO Green Bank Telescope (GBT) and the MPIfR Effelsberg 100 m telescope. We detect, for the first time, the CO ground-level transition in BR 1202-0725. For PSS J2322+1944 and APM 08279+5255, our observations result in line fluxes that are consistent with previous NRAO Very Large Array (VLA) observations, but they reveal the full line profiles. We report a typical lensing-corrected velocity-integrated intrinsic ^(12)CO(J = 1 → 0) line luminosity of L'_(CO) = 5 × 10^(10) K km s^(-1) pc^2 and a typical total H_2 mass of M(H_2) = 4 × 10^(10) M_☉ for the sources in our sample. The CO/FIR luminosity ratios of these high-z sources follow the same trend as seen for low-z galaxies, leading to a combined solution of log L_(FIR) = (1.39 ± 0.05) log L_(CO) - 1.76. It has previously been suggested that the molecular gas reservoirs in some quasar host galaxies may exhibit luminous, extended ^(12)CO(J = 1 → 0) components that are not observed in the higher J CO transitions. Using the line profiles and the total intensities of our observations and large velocity gradient (LVG) models based on previous results for higher J CO transitions, we derive that emission from all CO transitions is described well by a single gas component in which all molecular gas is concentrated in a compact nuclear region. Thus, our observations and models show no indication of a luminous extended, low surface brightness molecular gas component in any of the high-redshift QSOs in our sample. If such extended components exist, their contribution to the overall luminosity is limited to at most 30%.

225 citations


Journal ArticleDOI
TL;DR: In this paper, photometric redshifts from 17-band optical to mid-infrared photometry of 78 robust radio, 24-mu m and Spitzer IRAC counterparts to 72 of the 126 submillimetre galaxies (SMGs) selected at 870 mu m by LABOCA observations in the Extended Chandra Deep Field-South (ECDF-S).
Abstract: We derive photometric redshifts from 17-band optical to mid-infrared photometry of 78 robust radio, 24-mu m and Spitzer IRAC counterparts to 72 of the 126 submillimetre galaxies (SMGs) selected at 870 mu m by LABOCA observations in the Extended Chandra Deep Field-South (ECDF-S). We test the photometric redshifts of the SMGs against the extensive archival spectroscopy in the ECDF-S. The median photometric redshift of identified SMGs is z = 2.2 +/- 0.1, the standard deviation is sigma(z) = 0.9 and we identify 11 (similar to 15 per cent) high-redshift (z >= 3) SMGs. A statistical analysis of sources in the error circles of unidentified SMGs identifies a population of possible counterparts with a redshift distribution peaking at z = 2.5 +/- 0.2, which likely comprises similar to 60 per cent of the unidentified SMGs. This confirms that the bulk of the undetected SMGs are coeval with those detected in the radio/mid-infrared. We conclude that at most similar to 15 per cent of all the SMGs are below the flux limits of our IRAC observations and thus may lie at z greater than or similar to 3 and hence at most similar to 30 per cent of all SMGs have z greater than or similar to 3. We estimate that the full S(870 mu m) > 4mJy SMG population has a median redshift of 2.5 +/- 0.5. In contrast to previous suggestions, we find no significant correlation between submillimetre flux and redshift. The median stellar mass of the SMGs derived from spectral energy distribution fitting is (9.1 +/- 0.5) x 10(10)M(circle dot) although we caution that the uncertainty in the star formation histories results in a factor of similar to 5 uncertainty in these stellarmasses. Using a single temperature modified blackbody fit with beta = 1.5, the median characteristic dust temperature of SMGs is 37.4 +/- 1.4K. The infrared luminosity function shows that SMGs at z = 2-3 typically have higher far-infrared luminosities and luminosity density than those at z = 1-2. This is mirrored in the evolution of the star formation rate density (SFRD) for SMGs which peaks at z similar to 2. The maximum contribution of bright SMGs to the global SFRD (similar to 5 per cent for SMGs with S(870 mu m) greater than or similar to 4mJy or similar to 50 per cent extrapolated to SMGs with S(870 mu m) > 1mJy) also occurs at z similar to 2.

222 citations


Journal ArticleDOI
TL;DR: In this paper, the authors presented the most extensive consistent analysis to date of z>4 QSOs with observed NIR spectra, combining three new z~6 objects from our ongoing VLT-ISAAC program with nineteen 4 =-037 (Lbol/LEdd~043) with a scatter of 020 dex for the z >4 sample and the =-080 (lbol/Ldd~016) with an average of 024 dex for each of the 035 4 sources.
Abstract: [abridged] Quasars (QSOs) at the highest known redshift (z~6) are unique probes of the early growth of supermassive black holes (BHs) Until now, only the most luminous QSOs have been studied, often one object at a time Here we present the most extensive consistent analysis to date of z>4 QSOs with observed NIR spectra, combining three new z~6 objects from our ongoing VLT-ISAAC program with nineteen 4 =-037 (Lbol/LEdd~043) with a scatter of 020 dex for the z>4 sample and the =-080 (Lbol/LEdd~016) with a scatter of 024 dex for the 035 4 sources are accreting significantly faster than the lower-redshift ones We show that the derived FeII/MgII ratios depend sensitively on the performed analysis: our self-consistent, homogeneous analysis significantly reduces the FeII/MgII scatter found in previous studies The measured FeII/MgII line ratios show no sign of evolution with cosmic time in the redshift range 4

212 citations


Journal ArticleDOI
TL;DR: In this paper, the spectral energy distributions (SEDs) were used to estimate the total emission from dust and stars of 62 nearby galaxies in the Kingfish survey in a way that is as empirical and model independent as possible.
Abstract: Using new far-infrared imaging from the Herschel Space Observatory with ancillary data from ultraviolet (UV) to submillimeter wavelengths, we estimate the total emission from dust and stars of 62 nearby galaxies in the KINGFISH survey in a way that is as empirical and model independent as possible. We collect and exploit these data in order to measure from the spectral energy distributions (SEDs) precisely how much stellar radiation is intercepted and re-radiated by dust, and how this quantity varies with galaxy properties. By including SPIRE data, we are more sensitive to emission from cold dust grains than previous analyses at shorter wavelengths, allowing for more accurate estimates of dust temperatures and masses. The dust/stellar flux ratio, which we measure by integrating the SEDs, has a range of nearly three decades (from 10^(−2.2) to 10^(0.5)). The inclusion of SPIRE data shows that estimates based on data not reaching these far-IR wavelengths are biased low by 17% on average. We find that the dust/stellar flux ratio varies with morphology and total infrared (IR) luminosity, with dwarf galaxies having faint luminosities, spirals having relatively high dust/stellar ratios and IR luminosities, and some early types having low dust/stellar ratios. We also find that dust/stellar flux ratios are related to gas-phase metallicity (log(f_(dust)/f_∗) = −0.66 ± 0.08 and −0.22 ± 0.12 for metal-poor and intermediate-metallicity galaxies, respectively), while the dust/stellar mass ratios are less so (differing by ≈0.2 dex); the more metal-rich galaxies span a much wider range of the flux ratios. In addition, the substantial scatter between dust/stellar flux and dust/stellar mass indicates that the former is a poor proxy of the latter. Comparing the dust/stellar flux ratios and dust temperatures, we also show that early types tend to have slightly warmer temperatures (by up to 5 K) than spiral galaxies, which may be due to more intense interstellar radiation fields, or possibly to different dust grain compositions. Finally, we show that early types and early-type spirals have a strong correlation between the dust/stellar flux ratio and specific star formation rate, which suggests that the relatively bright far-IR emission of some of these galaxies is due to ongoing (if limited) star formation as well as to the radiation field from older stars, which is heating the dust grains.

207 citations


Journal ArticleDOI
TL;DR: In this article, the gas-to-dust mass ratio (M_gas/M_d) and the CO luminosity-tom_gas conversion factor (α_(CO)) of two well-studied galaxies in the Great Observatories Origins Deep Survey North field were explored.
Abstract: We explore the gas-to-dust mass ratio (M_gas/M_d) and the CO luminosity-to-M_gas conversion factor (α_(CO)) of two well-studied galaxies in the Great Observatories Origins Deep Survey North field that are expected to have different star-forming modes, the starburst GN20 at z = 4.05 and the normal star-forming galaxy BzK-21000 at z = 1.52. Detailed sampling is available for their Rayleigh-Jeans emission via ground-based millimeter (mm) interferometry (1.1-6.6 mm) along with Herschel PACS and SPIRE data that probe the peak of their infrared emission. Using the physically motivated Draine & Li models, as well as a modified blackbody function, we measure the dust mass (M_(dust)) of the sources and find (2.0^(+0.7)_(–0.6) × 10^9) M_☉for GN20 and (8.6^(+0.6)_(–0.9) × 10^8) M_☉ for BzK-21000. The addition of mm data reduces the uncertainties of the derived M_(dust) by a factor of ~2, allowing the use of the local M_(gas)/M_d versus metallicity relation to place constraints on the αCO values of the two sources. For GN20 we derive a conversion factor of α_(CO) < 1.0 M_☉ pc^(–2) (K km s^(–1))^(–1), consistent with that of local ultra-luminous infrared galaxies, while for BzK-21000 we find a considerably higher value, α_(CO) ~4.0 M_☉ pc^(–2) (K km s^(–1))^(–1), in agreement with an independent kinematic derivation reported previously. The implied star formation efficiency is ~25 L_☉/M_☉ for BzK-21000, a factor of ~5-10 lower than that of GN20. The findings for these two sources support the existence of different disk-like and starburst star formation modes in distant galaxies, although a larger sample is required to draw statistically robust results.

Journal ArticleDOI
TL;DR: In this paper, the gas-to-dust mass ratio (G/D) and the CO luminosity-toMgas conversion factor (a_co) of two well studied galaxies in the GOODS-N field, that are expected to have different star forming modes, were explored.
Abstract: We explore the gas-to-dust mass ratio (G/D) and the CO luminosity-to-Mgas conversion factor (a_co) of two well studied galaxies in the GOODS-N field, that are expected to have different star forming modes, the starburst GN20 at z=4.05 and the normal star-forming galaxy BzK-21000 at z=1.52. Detailed sampling is available for their Rayleigh-Jeans emission via ground based mm interferometry (1.1-6.6mm) along with Herschel, PACS and SPIRE data that probe the peak of their infrared emission. Using the physically motivated Draine & Li (2007) models, as well as a modified black body function, we measure the dust mass (Md) of the sources and find 2.0^{+0.7}_{-0.6} x 10^{9} Msun for GN20 and 8.6^{+0.6}_{-0.9} x 10^{8} Msun for BzK-21000. The addition of mm data reduces the uncertainties of the derived Md by a factor of ~2, allowing the use of the local G\D vs metallicity relation to place constraints on the a_co values of the two sources. For GN20 we derive a conversion factor of a_co < 1.0 Msun pc^{-2}(K km s^{-1})^{-1}, consistent with that of local ULIRGs, while for BzK-21000 we find a considerably higher value, a_co ~4.0 Msun pc^{-2}(K km s^{-1})^{-1}, in agreement with an independent kinematic derivation reported previously. The implied star formation efficiency is ~25 Lsun/Msun for BzK-21000, a factor of ~5-10 lower than that of GN20. The findings for these two sources support the existence of different disk-like and starburst star-formation modes in distant galaxies, although a larger sample is required to draw statistically robust results

Journal ArticleDOI
TL;DR: In this article, the authors reported the detection of spatially resolved CO(J = 1→0) emission in the z ~ 3.4 submillimeter galaxies (SMGs) SMM J09431+4700 and SMMJ13120+4242, using the Expanded Very Large Array (EVLA).
Abstract: We report the detection of spatially resolved CO(J = 1→0) emission in the z ~ 3.4 submillimeter galaxies (SMGs) SMM J09431+4700 and SMM J13120+4242, using the Expanded Very Large Array (EVLA). SMM J09431+4700 is resolved into the two previously reported millimeter sources H6 and H7, separated by ~30 kpc in projection. We derive CO(J = 1→0) line luminosities of L'_(CO(1-0)) = (2.49 ± 0.86) and (5.82 ± 1.22) × 10^(10) K km s^–1 pc^2 for H6 and H7, and L'_(CO(1-0)) = (23.4 ± 4.1) × 10^(10) K km s^(–1) pc^2 for SMM J13120+4242. These are ~1.5-4.5 times higher than what is expected from simple excitation modeling of higher-J CO lines, suggesting the presence of copious amounts of low-excitation gas. This is supported by the finding that the CO(J = 1→0) line in SMM J13120+4242, the system with the lowest CO excitation, appears to have a broader profile and more extended spatial structure than seen in higher-J CO lines (which is less prominently seen in SMM J09431+4700). Based on L'_(CO(1-0)) and excitation modeling, we find M_(gas) = 2.0-4.3 and 4.7-12.7×10^(10) M _☉ for H6 and H7, and M_(gas) = 18.7-69.4×10^(10) M_☉ for SMM J13120+4242. The observed CO(J = 1→0) properties are consistent with the picture that SMM J09431+4700 represents an early-stage, gas-rich major merger and that SMM J13120+4242 represents such a system in an advanced stage. This study thus highlights the importance of spatially and dynamically resolved CO(J = 1→0) observations of SMGs to further understand the gas physics that drive star formation in these distant galaxies, which is possible only now that the EVLA is rising to its full capabilities.

Journal ArticleDOI
TL;DR: In this paper, the authors present an analysis of the properties of H I holes detected in 20 galaxies that are part of 'The H I Nearby Galaxy Survey' and derive the surface and volume porosity.
Abstract: We present an analysis of the properties of H I holes detected in 20 galaxies that are part of 'The H I Nearby Galaxy Survey'. We detected more than 1000 holes in total in the sampled galaxies. Where they can be measured, their sizes range from about 100 pc (our resolution limit) to about 2 kpc, their expansion velocities range from 4 to 36 km s{sup -1}, and their ages are estimated to range between 3 and 150 Myr. The holes are found throughout the disks of the galaxies, out to the edge of the H I disk; 23% of the holes fall outside R{sub 25}. We find that shear limits the age of holes in spirals (shear is less important in dwarf galaxies) which explains why H I holes in dwarfs are rounder, on average than in spirals. Shear, which is particularly strong in the inner part of spiral galaxies, also explains why we find that holes outside R{sub 25} are larger and older. We derive the scale height of the H I disk as a function of galactocentric radius and find that the disk flares up in all galaxies. We proceed to derive the surface and volume porositymore » (Q{sub 2D} and Q{sub 3D}) and find that this correlates with the type of the host galaxy: later Hubble types tend to be more porous. The size distribution of the holes in our sample follows a power law with a slope of a{sub {nu}} {approx} -2.9. Assuming that the holes are the result of massive star formation (SF), we derive values for the supernova rate and star formation rate (SFR) which scales with the SFR derived based on other tracers. If we extrapolate the observed number of holes to include those that fall below our resolution limit, down to holes created by a single supernova, we find that our results are compatible with the hypothesis that H I holes result from SF.« less

Journal ArticleDOI
Fabian Walter1, Axel Weiß1, D. Downes, Roberto Decarli1, Carsten Henkel1 
TL;DR: In this paper, a survey of atomic carbon (C I) emission in high-redshift (z > 2) submillimeter galaxies and quasar host galaxies is presented.
Abstract: We present a survey of atomic carbon (C I) emission in high-redshift (z > 2) submillimeter galaxies and quasar host galaxies. Sensitive observations of the C I ( 3 P 1 → 3 P 0 ) and C I ( 3 P 2 → 3 P 1 ) lines have been obtained at the IRAM Plateau de Bure interferometer and the IRAM 30 m telescope. A total of 16 C I lines have been targeted in 10 sources, leading to a total of 10 detected lines—this doubles the number of C I observations at high redshift to date. We include previously published C I observations (an additional five detected sources) in our analysis. Our main finding is that the C I properties of the high-redshift galaxies studied here do not differ significantly from what is found in low-redshift systems, including the Milky Way. The CI ( 3 P 2 → 3 P 1 )/CI ( 3 P 1 → Po) and the CI ( 3 P 1 → 3 P 0 )/ 12 CO(3―2) line luminosity (L') ratios change little in our sample, with respective ratios of 0.55 ± 0.15 and 0.32 ± 0.13. The C I lines are not an important contributor to cooling of the molecular gas (average L CI /L FIR ∼ (7.7 ± 4.6) x 10 ―6 ). We derive a mean carbon excitation temperature of 29.1 ± 6.3 K, broadly consistent with dust temperatures derived for high-redshift star-forming systems, but lower than gas temperatures typically derived for starbursts in the local universe. The carbon abundance of X[CI]/X[H 2 ] ∼ (8.4±3.5)×10 ―5 is of the same order as found in the Milky Way and nearby galaxies. This implies that the high-z galaxies studied here are significantly enriched in carbon on galactic scales, even though the look-back times are considerable (the average redshift of the sample sources corresponds to an age of the universe of ∼2 Gyr).

Journal ArticleDOI
TL;DR: In this article, a survey of atomic carbon (CI) emission in high-redshift (z>2) submillimeter galaxies and quasar host galaxies (QSOs) is presented.
Abstract: We present a survey of atomic carbon (CI) emission in high-redshift (z>2) submillimeter galaxies (SMGs) and quasar host galaxies (QSOs). Sensitive observations of the CI(3P_1->3P_0) and CI(3P_2->3P_1) lines have been obtained at the IRAM Plateau de Bure interferometer and the IRAM 30m telescope. A total of 16 CI lines have been targeted in 10 sources, leading to a total of 10 detected lines --- this doubles the number of CI observations at high redshift to date. We include previously published CI observations (an additional 5 detected sources) in our analysis. Our main finding is that the CI properties of the studied high-redshift systems do not differ significantly from what is found in low-redshift systems, including the Milky Way. The CI(3P_2->3P_1)/CI(3P_1->3P_0) and the CI(3P_1->3P_0)/12CO(3-2) line luminosity (L') ratios change little in our sample, with respective ratios of 0.55+/-0.15 and 0.32+/-0.13. The CI lines are not an important contributor to cooling of the molecular gas (average L_CI/L_FIR ~ (7.7+/-4.6) x 10^-6). We derive a mean carbon excitation temperature of 29.1+/-6.3 K, broadly consistent with dust temperatures derived for high-redshift starforming systems, but lower than gas temperatures typically derived for starbursts in the local universe. The carbon abundance of X_CI/X_H2~8.4+/-3.5 x 10^-5 is of the same order as found in the Milky Way and nearby galaxies. This implies that the high-z galaxies studied here are significantly enriched in carbon on galactic scales, even though the look-back times are considerable (the average redshift of the sample sources corresponds to an age of the universe of ~2 Gyr).

Journal ArticleDOI
TL;DR: In this article, the authors presented new millimeter and radio observations of nine z ~ 6 quasars discovered in deep optical and near-infrared surveys, and combined with previous farinfrared (FIR) and radio emission and quasar-host galaxy evolution.
Abstract: We present new millimeter and radio observations of nine z ~ 6 quasars discovered in deep optical and near-infrared surveys. We observed the 250 GHz continuum in eight of the nine objects and detected three of them. New 1.4 GHz radio continuum data have been obtained for four sources, and one has been detected. We searched for molecular CO (6-5) line emission in the three 250 GHz detections and detected two of them. Combined with previous millimeter and radio observations, we study the far-infrared (FIR) and radio emission and quasar-host galaxy evolution with a sample of 18 z ~ 6 quasars that are faint at UV and optical wavelengths (rest-frame 1450 A magnitudes of m_(1450) ≥ 20.2). The average FIR-to-active galactic nucleus (AGN) UV luminosity ratio of this faint quasar sample is about two times higher than that of the bright quasars at z ~ 6 (m_ (1450) < 20.2). A fit to the average FIR and AGN bolometric luminosities of both the UV/optically faint and bright z ~ 6 quasars, and the average luminosities of samples of submillimeter/millimeter-observed quasars at z ~ 2-5, yields a relationship of L_(FIR) ~ L_(bol)^(0.62). Five of the 18 faint z ~ 6 quasars have been detected at 250 GHz. These 250 GHz detections, as well as most of the millimeter-detected optically bright z ~ 6 quasars, follow a shallower trend of L_(FIR) ~ L_(bol)^(0.45) defined by the starburst-AGN systems in local and high-z universe. The millimeter continuum detections in the five objects and molecular CO detections in three of them reveal a few × 10^8 M_☉ of FIR-emitting warm dust and 10^(10)M_☉ of molecular gas in the quasar host galaxies. All these results argue for massive star formation in the quasar host galaxies, with estimated star formation rates of a few hundred M_☉ yr^(–1). Additionally, the higher FIR-to-AGN luminosity ratio found in these 250 GHz detected faint quasars also suggests a higher ratio between star formation rate and supermassive black hole accretion rate than the UV/optically most luminous quasars at z ~ 6.

Journal ArticleDOI
TL;DR: In this article, a joint analysis of the overlapping Balloon-borne Large Aperture Submillimetre Telescope (BLAST) 250, 350, 500 μm and LABOCA 870 μm observations is presented.
Abstract: We present a joint analysis of the overlapping Balloon-borne Large Aperture Submillimetre Telescope (BLAST) 250, 350, 500 μm, and LABOCA 870 μm observations [from the LABOCA ECDFS Submm Survey (LESS) survey] of the Extended Chandra Deep Field-South Out to z∼ 3, the BLAST filters sample near the peak wavelength of thermal far-infrared (FIR) emission from galaxies (rest-frame wavelengths ∼60–200 μm), primarily produced by dust heated through absorption in star-forming clouds However, identifying counterparts to individual BLAST peaks is very challenging, given the large beams [full-width at half-maximum (FWHM) 36–60 arcsec] In contrast, the ground-based 870 μm observations have a significantly smaller 19 arcsec FWHM beam, and are sensitive to higher redshifts (z∼ 1–5, and potentially beyond) due to the more favourable negative K-correction We use the LESS data, as well as deep Spitzer and VLA imaging, to identify 118 individual sources that produce significant emission in the BLAST bands We characterize the temperatures and FIR luminosities for a subset of 69 sources which have well-measured submillimetre (submm) spectral energy distributions (SEDs) and redshift measurements out to z∼ 3 For flux-limited sub-samples in each BLAST band, and a dust emissivity index β= 20, we find a median temperature T= 30 K (all bands) as well as median redshifts: z= 11 (interquartile range 02–19) for S250 > 40 mJy; z= 13 (interquartile range 06–21) for S350 > 30 mJy; and z= 16 (interquartile range 13–23) for S500 > 20 mJy Taking into account the selection effects for our survey (a bias towards detecting lower-temperature galaxies), we find no evidence for evolution in the local FIR–temperature correlation out to z∼ 25 Comparing with star-forming galaxy SED templates, about 8 per cent of our sample appears to exhibit significant excesses in the radio and/or mid-IR, consistent with those sources harbouring active galactic nuclei (AGN) Since our statistical approach differs from most previous studies of submm galaxies, we describe the following techniques in two appendices: our ‘matched filter’ for identifying sources in the presence of point-source confusion; and our approach for identifying counterparts using likelihood ratios This study is a direct precursor to future joint FIR/submm surveys, for which we outline a potential identification and SED measurement strategy

Journal ArticleDOI
TL;DR: In this article, the angular cross-correlation (CC) between two pairs of tracers is calculated using all azimuthal positions among pairs of these tracers; the peak of the CC should be offset from zero, in different directions inside and outside the corotation radius.
Abstract: We test whether the spiral patterns apparent in many large disk galaxies should be thought of as dynamical features that are stationary in a corotating frame for t dyn, as implied by the density wave approach for explaining spiral arms. If such spiral arms have enhanced star formation (SF), observational tracers for different stages of the SF sequence should show a spatial ordering, from upstream to downstream in the corotating frame: dense H I, CO, tracing molecular hydrogen gas, 24 ?m emission tracing enshrouded SF, and UV emission tracing unobscured young stars. We argue that such a spatial ordering should be reflected in the angular cross-correlation (CC, in polar coordinates) using all azimuthal positions among pairs of these tracers; the peak of the CC should be offset from zero, in different directions inside and outside the corotation radius. Recent spiral SF simulations by Dobbs & Pringle show explicitly that for the case of a stationary spiral arm potential such angular offsets between gas and young stars of differing ages should be observable as cross-correlation offsets. We calculate the angular cross-correlations for different observational SF sequence tracers in 12 nearby spiral galaxies, drawing on a data set with high-quality maps of the neutral gas (H I, THINGS) and molecular gas (CO, HERACLES), along with 24 ?m emission (Spitzer, SINGS); we include FUV images (GALEX) and 3.6 ?m emission (Spitzer, IRAC) for some galaxies, tracing aging stars and longer timescales. In none of the resulting tracer cross-correlations for this sample do we find systematic angular offsets, which would be expected for a stationary dynamical spiral pattern of well-defined pattern speed. This result indicates that spiral density waves in their simplest form are not an important aspect of explaining spirals in large disk galaxies.

Journal ArticleDOI
TL;DR: In this article, the effects of different assumptions and systematics on the determination of the local, spatially resolved star formation law were explored using four star formation rate (SFR) tracers (H{alpha} with azimuthally averaged extinction correction, mid-infrared 24 {mu}m, combined H{alpha and mid-inrared 24 ¾m, and combined far-ultraviolet and mid infrared 24 Þm), several fitting procedures, and different sampling strategies.
Abstract: This study explores the effects of different assumptions and systematics on the determination of the local, spatially resolved star formation law. Using four star formation rate (SFR) tracers (H{alpha} with azimuthally averaged extinction correction, mid-infrared 24 {mu}m, combined H{alpha} and mid-infrared 24 {mu}m, and combined far-ultraviolet and mid-infrared 24 {mu}m), several fitting procedures, and different sampling strategies, we probe the relation between SFR and molecular gas at various spatial resolutions (500 pc and larger) and surface densities ({Sigma}{sub H{sub 2}})approx. 10-245 M{sub sun} pc{sup -2}) within the central {approx}6.5 kpc in the disk of NGC 4254. We explore the effect of diffuse emission using an unsharp masking technique with varying kernel size. The fraction of diffuse emission, f{sub DE}, thus determined is a strong inverse function of the size of the filtering kernel. We find that in the high surface brightness regions of NGC 4254 the form of the molecular gas star formation law is robustly determined and approximately linear ({approx}0.8-1.1) and independent of the assumed fraction of diffuse emission and the SFR tracer employed. When the low surface brightness regions are included, the slope of the star formation law depends primarily on the assumed fraction of diffuse emission. Inmore » such a case, results range from linear when the fraction of diffuse emission in the SFR tracer is f{sub DE} {approx} 50%. We find that the tightness of the correlation between gas and star formation varies with the choice of star formation tracer. The 24 {mu}m SFR tracer by itself shows the tightest correlation with the molecular gas surface density, whereas the H{alpha} corrected for extinction using an azimuthally averaged correction shows the highest dispersion. We find that for R < 0.5R{sub 25} the local star formation efficiency is constant and similar to that observed in other large spirals, with a molecular gas depletion time {tau}{sub dep} {approx} 2 Gyr.« less

Journal ArticleDOI
TL;DR: In this paper, radio and infrared (3.6-24 μm) counterparts to submillimetre galaxies (SMGs) detected in the Extended Chandra Deep Field-South with the Large APEX Bolometer Camera (LABOCA) 870-μm bolometer camera on the 12m Atacama Pathfinder Experiment.
Abstract: We present radio and infrared (3.6-24 μm) counterparts to submillimetre galaxies (SMGs) detected in the Extended Chandra Deep Field-South with the Large APEX Bolometer Camera (LABOCA) 870-μm bolometer camera on the 12-m Atacama Pathfinder Experiment. Using the Very Large Array at 1.4 GHz and Spitzer, we have identified secure counterparts to 79 of the 126 SMGs [signal-to-noise ratio (S/N) > 3.7, S870 > 4.4 mJy] in the field, 62 via their radio and/or 24-μm emission, the remainder using a colour-flux cut on Infrared Array Camera 3.6- and 5.8-μm sources chosen to maximize the number of secure, coincident radio and 24-μm counterparts. In constructing our radio catalogue, we have corrected for the effects of 'flux boosting', then used the corrected flux densities to estimate the redshifts of the SMGs based on the radio/submm spectral indices. The effect of the boosting correction is to increase the median redshift by 0.2 resulting in a value of ? (1σ errors) for the secure radio counterparts, in agreement with other studies, both spectroscopic and photometric.

Journal ArticleDOI
TL;DR: In this article, the authors reported the detection of CO(1-0) emission in the strongly lensed high-redshift quasars IRAS F10214+4724 (z=2.286), the Cloverleaf (z =2.558), RX J0911+0551 (z.796), SMM J04135+10277 (z 2.846), and MG 0751+2716 (z 3.200), using the Expanded Very Large Array and the Green Bank Telescope.
Abstract: We report the detection of CO(1-0) emission in the strongly lensed high-redshift quasars IRAS F10214+4724 (z=2.286), the Cloverleaf (z=2.558), RX J0911+0551 (z=2.796), SMM J04135+10277 (z=2.846), and MG 0751+2716 (z=3.200), using the Expanded Very Large Array and the Green Bank Telescope. We report lensing-corrected CO(1-0) line luminosities of L'(CO) = 0.34-18.4 x 10^10 K km/s pc^2 and total molecular gas masses of M(H2) = 0.27-14.7 x 10^10 Msun for the sources in our sample. Based on CO line ratios relative to previously reported observations in J>=3 rotational transitions and line excitation modeling, we find that the CO(1-0) line strengths in our targets are consistent with single, highly-excited gas components with constant brightness temperature up to mid-J levels. We thus do not find any evidence for luminous extended, low excitation, low surface brightness molecular gas components. These properties are comparable to those found in z>4 quasars with existing CO(1-0) observations. These findings stand in contrast to recent CO(1-0) observations of z~2-4 submillimeter galaxies (SMGs), which have lower CO excitation and show evidence for multiple excitation components, including some low-excitation gas. These findings are consistent with the picture that gas-rich quasars and SMGs represent different stages in the early evolution of massive galaxies.

Journal ArticleDOI
TL;DR: In this article, a CO line survey of 30 galaxies at moderate redshift (z ∼ 0.2−0.6), with the IRAM 30 m telescope, with the goal to follow galaxy evolution and in particular the star formation efficiency (SFE) as defined by the ratio between far-infrared luminosity and molecular gas mass (LFIR/M(H2)).
Abstract: We present the results of a CO line survey of 30 galaxies at moderate redshift (z ∼ 0.2−0.6), with the IRAM 30 m telescope, with the goal to follow galaxy evolution and in particular the star formation efficiency (SFE) as defined by the ratio between far-infrared luminosity and molecular gas mass (LFIR/M(H2)). The sources are selected to be ultra-luminous infra-red galaxies (ULIRGs), with LFIR larger than 2.8 × 10 12 L� , experiencing starbursts; adopting a low ULIRG CO-to-H2 conversion factor, their gas consumption time-scale is lower than 10 8 yr. To date only very few CO observations exist in this redshift range that spans nearly 25% of the universe’s age. Considerable evolution of the star formation rate is already observed during this period. 18 galaxies out of our sample of 30 are detected (of which 16 are new detections), corresponding to a detection rate of 60%. The average CO luminosity for the 18 galaxies detected is L � = 2 × 10 10 Kk m s −1 pc 2 , corresponding to an average H2 mass of 1.6 × 10 10 M� . The FIR luminosity correlates well with the CO luminosity, in agreement with the correlation found for low and high redshift ULIRGs. Although the conversion factor between CO luminosity and H2 mass is uncertain, we find that the maximum amount of gas available for a single galaxy is quickly increasing as a function of redshift. Using the same conversion factor, the SFEs for z ∼ 0.2−0.6 ULIRGs are found to be significantly higher, by a factor 3, than for local ULIRGs, and are comparable to high redshift ones. We compare this evolution to the expected cosmic H2 abundance and the cosmic star formation history.

Journal ArticleDOI
TL;DR: In this article, the authors reported new observations of CO (2-1) line emission toward five z ~ 6 quasars using the Ka-band receiver system on the Expanded Very Large Array (EVLA).
Abstract: We report new observations of CO (2-1) line emission toward five z ~ 6 quasars using the Ka-band receiver system on the Expanded Very Large Array (EVLA). Strong detections were obtained in two of them, SDSS J092721.82+200123.7 and CFHQS J142952.17+544717.6, and a marginal detection was obtained in another source, SDSS J084035.09+562419.9. Upper limits of the CO (2-1) line emission have been obtained for the other two objects. The CO (2-1) line detection in J0927+2001 together with previous measurements of the CO (6-5) and (5-4) lines reveal important constraints on the CO excitation in the central ~10 kpc region of the quasar host galaxy. The CO (2-1) line emission from J1429+5447 is resolved into two distinct peaks separated by 1."2 (~6.9 kpc), indicating a possible gas-rich, major merging system, and the optical quasar position is consistent with the west peak. This result is in good agreement with the picture in which intense host galaxy star formation is coeval with rapid supermassive black hole (SMBH) accretion in the most distant universe. The two EVLA detections are ideal targets for further high-resolution imaging (e.g., with ALMA or EVLA observations) to study the gas distribution, dynamics, and SMBH-bulge-mass relation in these earliest quasar host galaxy systems.

Journal ArticleDOI
TL;DR: In this paper, the authors reported the detection of spatially extended CO(J = 1→0) and CO (J = 5→4) emission in the z = 2.49 submillimeter galaxy (SMG) J123707+6214, using the Expanded Very Large Array and the Plateau de Bure Interferometer.
Abstract: We report the detection of spatially extended CO(J = 1→0) and CO(J = 5→4) emission in the z = 2.49 submillimeter galaxy (SMG) J123707+6214, using the Expanded Very Large Array and the Plateau de Bure Interferometer. The large molecular gas reservoir is spatially resolved into two CO(J = 1→0) components (northeast and southwest; previously identified in CO J = 3→2 emission) with respective gas masses of 4.3 and 3.5×10^(10) (α_(CO)/0.8) M_☉. We thus find that the optically invisible northeast component slightly dominates the gas mass in this system. The total molecular gas mass derived from the CO(J = 1→0) observations is ≳2.5× larger than estimated from CO(J = 3→2). The two components are at approximately the same redshift, but separated by ~20 kpc in projection. The morphology is consistent with that of an early-stage merger. The total amount of molecular gas is sufficient to maintain the intense 500 M_☉ yr^(–1) starburst in this system for at least ~160 Myr. We derive line brightness temperature ratios of r_(31) = 0.39 ± 0.09 and 0.37 ± 0.10, and r_(51) = 0.26 ± 0.07 and 0.25 ± 0.08 in the two components, respectively, suggesting that the J ≥ 3 lines are substantially subthermally excited. This also suggests comparable conditions for star formation in both components. Given the similar gas masses of both components, this is consistent with the comparable starburst strengths observed in the radio continuum emission. Our findings are consistent with other recent studies that find evidence for lower CO excitation in SMGs than in high-z quasar host galaxies with comparable gas masses. This may provide supporting evidence that both populations correspond to different evolutionary stages in the formation of massive galaxies.

Journal ArticleDOI
TL;DR: In this paper, the authors present the detection of four rotational emission lines of water vapor, from energy levels E u/k = 101-454 K, in the gravitationally lensed z = 3.9 QSO host galaxy APM?08279+5255.
Abstract: We present the detection of four rotational emission lines of water vapor, from energy levels E u/k = 101-454 K, in the gravitationally lensed z = 3.9 QSO host galaxy APM?08279+5255. While the lowest H2 O lines are collisionally excited in clumps of warm, dense gas (density of hydrogen nuclei , gas temperature T g ~ 105 ? 21 K), we find that the excitation of the higher lines is dominated by the intense local infrared radiation field. Since only collisionally excited emission contributes to gas cooling, we conclude that H2 O is not a significant coolant of the warm molecular gas. Our excitation model requires the radiatively excited gas to be located in an extended region of high 100 ? m opacity (?100 = 0.9 ? 0.2). Locally, such extended infrared-opaque regions are found only in the nuclei of ultraluminous infrared galaxies. We propose a model where the infrared-opaque circumnuclear cloud, which is penetrated by the X-ray radiation field of the QSO nucleus, contains clumps of massive star formation where the H2 O emission originates. The radiation pressure from the intense local infrared radiation field exceeds the thermal gas pressure by about an order of magnitude, suggesting close to Eddington-limited star formation in these clumps.

Journal ArticleDOI
TL;DR: In this article, the GN20 proto-cluster of galaxies was detected using the expanded very large array (EVLA) for CO 2-1 emission, with gas masses (H$_2$) between $10^{10}$ and $10''11} \times (\alpha/0.8)$ M$_\odot$
Abstract: We present observations of the molecular gas in the GN20 proto-cluster of galaxies at $z =4.05$ using the Expanded Very Large Array (EVLA). This group of galaxies is the ideal laboratory for studying the formation of massive galaxies via luminous, gas-rich starbursts within 1.6 Gyr of the Big Bang. We detect three galaxies in the proto-cluster in CO 2-1 emission, with gas masses (H$_2$) between $10^{10}$ and $10^{11} \times (\alpha/0.8)$ M$_\odot$. The emission from the brightest source, GN20, is resolved with a size $\sim 2"$, and has a clear north-south velocity gradient, possibly indicating ordered rotation. The gas mass in GN20 is comparable to the stellar mass ($1.3\times 10^{11} \times (\alpha/0.8)$ M$_\odot$ and $2.3\times 10^{11}$ M$_\odot$, respectively), and the sum of gas plus stellar mass is comparable to the dynamical mass of the system ($\sim 3.4\times 10^{11} [sin(i)/sin(45^o)]^{-2}$ M$_\odot$), within a 5kpc radius. There is also evidence for a tidal tail extending another $2"$ north of the galaxy with a narrow velocity dispersion. GN20 may be a massive, gas rich disk that is gravitationally disturbed, but not completely disrupted. There is one Lyman-break galaxy (BD29079) in the GN20 proto-cluster with an optical spectroscopic redshift within our search volume, and we set a 3$\sigma$ limit to the molecular gas mass of this galaxy of $1.1\times 10^{10} \times (\alpha/0.8)$ M$_\odot$.

Journal ArticleDOI
TL;DR: In this paper, the GN20 proto-cluster of galaxies was detected using the expanded very large array (EVLA) for CO 2-1 emission, with gas masses (H$_2$) between $10^{10}$ and $10''11} \times (\alpha/0.8)$ M$_\odot$
Abstract: We present observations of the molecular gas in the GN20 proto-cluster of galaxies at $z =4.05$ using the Expanded Very Large Array (EVLA). This group of galaxies is the ideal laboratory for studying the formation of massive galaxies via luminous, gas-rich starbursts within 1.6 Gyr of the Big Bang. We detect three galaxies in the proto-cluster in CO 2-1 emission, with gas masses (H$_2$) between $10^{10}$ and $10^{11} \times (\alpha/0.8)$ M$_\odot$. The emission from the brightest source, GN20, is resolved with a size $\sim 2"$, and has a clear north-south velocity gradient, possibly indicating ordered rotation. The gas mass in GN20 is comparable to the stellar mass ($1.3\times 10^{11} \times (\alpha/0.8)$ M$_\odot$ and $2.3\times 10^{11}$ M$_\odot$, respectively), and the sum of gas plus stellar mass is comparable to the dynamical mass of the system ($\sim 3.4\times 10^{11} [sin(i)/sin(45^o)]^{-2}$ M$_\odot$), within a 5kpc radius. There is also evidence for a tidal tail extending another $2"$ north of the galaxy with a narrow velocity dispersion. GN20 may be a massive, gas rich disk that is gravitationally disturbed, but not completely disrupted. There is one Lyman-break galaxy (BD29079) in the GN20 proto-cluster with an optical spectroscopic redshift within our search volume, and we set a 3$\sigma$ limit to the molecular gas mass of this galaxy of $1.1\times 10^{10} \times (\alpha/0.8)$ M$_\odot$.

Journal ArticleDOI
TL;DR: In this article, the authors used Very Large Array Hi line data to estimate the energy required to create the centrally dominant hole in each galaxy and compared this energy estimate to the past energy released by the underlying stellar populations computed from SFHs derived from data taken with the Hubble Space Telescope.
Abstract: The origin of kpc-scale holes in the atomic hydrogen (H i) distributions of some nearby dwarf irregular galaxies presents an intriguing problem. Star formation histories (SFHs) derived from resolved stars give us the unique opportunity to study past star-forming events that may have helped shape the currently visible Hi distribution. Our sample of five nearby dwarf irregular galaxies spans over an order of magnitude in both total Hi mass and absolute B-band magnitude and is at the low-mass end of previously studied systems. We use Very Large Array Hi line data to estimate the energy required to create the centrally dominant hole in each galaxy. We compare this energy estimate to the past energy released by the underlying stellar populations computed from SFHs derived from data taken with the Hubble Space Telescope. The inferred integrated stellar energy released within the characteristic ages exceeds our energy estimates for creating the holes in all cases, assuming expected efficiencies. Therefore, it appears that stellar feedback provides sufficient energy to produce the observed holes. However, we find no obvious signature of single star-forming events responsible for the observed structures when comparing the global SFHs of each galaxy in our sample to each other or to those of dwarf irregular galaxies reported in the literature. We also fail to find evidence of a central star cluster in FUV or Hα imaging. We conclude that large Hi holes are likely formed from multiple generations of star formation and only under suitable interstellar medium conditions.

Journal ArticleDOI
TL;DR: In this article, the relationship between molecular gas and current star formation rate surface density at sub-kpc and kpc scales in a sample of 14 nearby star-forming galaxies was analyzed.
Abstract: We present an analysis of the relationship between molecular gas and current star formation rate surface density at sub-kpc and kpc scales in a sample of 14 nearby star-forming galaxies. Measuring the relationship in the bright, high molecular gas surface density ($\Shtwo\gtrsim$20 \msunpc) regions of the disks to minimize the contribution from diffuse extended emission, we find an approximately linear relation between molecular gas and star formation rate surface density, $ mol\sim0.96\pm0.16$, with a molecular gas depletion time $\tdep\sim2.30\pm1.32$ Gyr. We show that, in the molecular regions of our galaxies there are no clear correlations between \tdep\ and the free-fall and effective Jeans dynamical times throughout the sample. We do not find strong trends in the power-law index of the spatially resolved molecular gas star formation law or the molecular gas depletion time across the range of galactic stellar masses sampled (\mstar $\sim$$10^{9.7}-10^{11.5}$ \msun). There is a trend, however, in global measurements that is particularly marked for low mass galaxies. We suggest this trend is probably due to the low surface brightness CO, and it is likely associated with changes in CO-to-H2 conversion factor.