Showing papers by "Fabian Walter published in 2010"

Very High Gas Fractions and Extended Gas Reservoirs in z = 1.5 Disk Galaxies

Abstract: We present evidence for very high gas fractions and extended molecular gas reservoirs in normal, near-infrared selected (BzK) galaxies at z~1.5, based on multi-configuration CO[2-1] observations obtained at the IRAM PdBI. Six of the six galaxies observed were securely detected. High resolution observations resolve the CO emission in four of them, implying sizes of order of 6-11 kpc and suggesting the presence of rotation. The UV morphologies are consistent with clumpy, unstable disks, and the UV sizes are consistent with the CO sizes. The star formation efficiencies are homogeneously low and similar to local spirals - the resulting gas depletion times are ~0.5 Gyr, much higher than what is seen in high-z submm galaxies and quasars. The CO luminosities can be predicted to within 0.15 dex from the star formation rates and stellar masses, implying a tight correlation of the gas mass with these quantities. We use dynamical models of clumpy disk galaxies to derive dynamical masses. These models are able to reproduce the peculiar spectral line shapes of the CO emission. After accounting for the stellar and dark matter masses we derive gas masses of 0.4-1.2x10^11 Msun. The conversion factor is very high: alpha_CO=3.6+-0.8, consistent with the Galaxy but four times higher than that of local ultra-luminous IR galaxies. The gas accounts for an impressive 50-65% of the baryons within the galaxies' half light radii. We are witnessing truly gas-dominated galaxies at z~1.5, a finding that explains the high specific SFRs observed for z>1 galaxies. The BzK galaxies can be viewed as scaled-up versions of local disk galaxies, with low efficiency star formation taking place inside extended, low excitation gas disks. They are markedly different than local ULIRGs and high-z submm galaxies, which have more excited and compact gas.

Different star formation laws for disks versus starbursts at low and high redshifts

Abstract: We present evidence that bona fide disks and starburst systems occupy distinct regions in the gas mass versus star formation rate (SFR) plane, both for the integrated quantities and for the respective surface densities This result is based on carbon monoxide (CO) observations of galaxy populations at low and high redshifts, and on the current consensus for the CO luminosity to gas mass conversion factors The data suggest the existence of two different SF regimes: a long-lasting mode for disks and a more rapid mode for starbursts, the latter probably occurring during major mergers or in dense nuclear SF regions Both modes are observable over a large range of SFRs The detection of CO emission from distant near-IR selected galaxies reveals such bimodal behavior for the first time, as they allow us to probe gas in disk galaxies with much higher SFRs than are seen locally The different regimes can potentially be interpreted as the effect of a top-heavy initial mass function in starbursts However, we favor a different physical origin related to the fraction of molecular gas in dense clouds The IR luminosity to gas mass ratio (ie, the SF efficiency) appears to be inversely proportional to the dynamical (rotation) timescale Only when accounting for the dynamical timescale, a universal SF law is obtained, suggesting a direct link between global galaxy properties and the local SFR

Extremely Inefficient Star Formation in the Outer Disks of Nearby Galaxies

Abstract: We combine data from The Hi Nearby Galaxy Survey and the GALEX Nearby Galaxy Survey to study the relationship between atomic hydrogen (Hi) and far-ultraviolet (FUV) emission outside the optical radius (r25 )i n 17 spiral and 5 dwarf galaxies. In this regime, Hi is likely to represent most of the interstellar medium (ISM) and FUV emission to trace recent star formation with little bias due to extinction, so that the two quantities closely trace the underlying relationship between gas and star formation rate (SFR). The azimuthally averaged Hi and FUV intensities both decline with increasing radius in this regime, with the scale length of the FUV profile typically half that of the Hi profile. Despite the mismatch in profiles, there is a significant spatial correlation (at 15 �� resolution) between local FUV and Hi intensities; near r25 this correlation is quite strong, in fact stronger than anywhere inside r25 (where Hi is not a good tracer for the bulk of the ISM), and shows a decline toward larger radii. The star formation efficiency (SFE)—defined as the ratio of FUV/Hi and thus the inverse of the gas depletion time—decreases with galactocentric radius across the outer disks, though much shallower than across the optical disks. On average, we find the gas depletion times to be well above a Hubble time (∼10 11 yr). We observe a clear relationship between FUV/Hi and Hi column in the outer disks, with the SFE increasing with increasing Hi column. Despite observing systematic variations in FUV/Hi, we find no clear evidence for stepfunction-type star formation thresholds, though we emphasize that it may not be realistic to expect them. When compared with results from insider25, we find outer disk star formation to be distinct in several ways: it is extremely inefficient (depletion times of many Hubble times which are also long compared to either the free fall or orbital timescale) with column densities and SFRs lower than found anywhere inside the optical disks. It appears that the Hi column is one of the key environmental factors—perhaps the key factor—in setting the SFR in outer galaxy disks.

The Calibration of Monochromatic Far-Infrared Star Formation Rate Indicators

Abstract: Spitzer data at 24, 70, and 160 μm and ground-based Hα images are analyzed for a sample of 189 nearby star-forming and starburst galaxies to investigate whether reliable star formation rate (SFR) indicators can be defined using the monochromatic infrared dust emission centered at 70 and 160 μm. We compare recently published recipes for SFR measures using combinations of the 24 μm and observed Hα luminosities with those using 24 μm luminosity alone. From these comparisons, we derive a reference SFR indicator for use in our analysis. Linear correlations between SFR and the 70 μm and 160 μm luminosity are found for L(70) ≳ 1.4 × 10^(42) erg s^(–1) and L(160) ≳ 2 × 10^(42) erg s^(–1), corresponding to SFR ≳ 0.1-0.3 M_☉ yr^(–1), and calibrations of SFRs based on L(70) and L(160) are proposed. Below those two luminosity limits, the relation between SFR and 70 μm (160 μm) luminosity is nonlinear and SFR calibrations become problematic. A more important limitation is the dispersion of the data around the mean trend, which increases for increasing wavelength. The scatter of the 70 μm (160 μm) data around the mean is about 25% (factor ~2) larger than the scatter of the 24 μm data. We interpret this increasing dispersion as an effect of the increasing contribution to the infrared emission of dust heated by stellar populations not associated with the current star formation. Thus, the 70 (160) μm luminosity can be reliably used to trace SFRs in large galaxy samples, but will be of limited utility for individual objects, with the exception of infrared-dominated galaxies. The nonlinear relation between SFR and the 70 and 160 μm emission at faint galaxy luminosities suggests a variety of mechanisms affecting the infrared emission for decreasing luminosity, such as increasing transparency of the interstellar medium, decreasing effective dust temperature, and decreasing filling factor of star-forming regions across the galaxy. In all cases, the calibrations hold for galaxies with oxygen abundance higher than roughly 12 +log(O/H) ~ 8.1. At lower metallicity, the infrared luminosity no longer reliably traces the SFR because galaxies are less dusty and more transparent.

Black hole accretion and star formation as drivers of gas excitation and chemistry in Markarian 231

Abstract: We present a full high resolution SPIRE FTS spectrum of the nearby ultraluminous infrared galaxy Mrk 231. In total 25 lines are detected, including CO J = 5-4 through J = 13-12, 7 rotational lines of H2O, 3 of OH+ and one line each of H2O+, CH+, and HF. We find that the excitation of the CO rotational levels up to J = 8 can be accounted for by UV radiation from star formation. However, the approximately flat luminosity distribution of the CO lines over the rotational ladder above J = 8 requires the presence of a separate source of excitation for the highest CO lines. We explore X-ray heating by the accreting supermassive black hole in Mrk 231 as a source of excitation for these lines, and find that it can reproduce the observed luminosities. We also consider a model with dense gas in a strong UV radiation field to produce the highest CO lines, but find that this model strongly overpredicts the hot dust mass in Mrk 231. Our favoured model consists of a star forming disk of radius 560 pc, containing clumps of dense gas exposed to strong UV radiation, dominating the emission of CO lines up to J = 8. X-rays from the accreting supermassive black hole in Mrk 231 dominate the excitation and chemistry of the inner disk out to a radius of 160 pc, consistent with the X-ray power of the AGN in Mrk 231. The extraordinary luminosity of the OH+ and H2O+ lines reveals the signature of X-ray driven excitation and chemistry in this region.

Black hole accretion and star formation as drivers of gas excitation and chemistry in Mrk231

Abstract: We present a full high resolution SPIRE FTS spectrum of the nearby ultraluminous infrared galaxy Mrk231. In total 25 lines are detected, including CO J=5-4 through J=13-12, 7 rotational lines of H2O, 3 of OH+ and one line each of H2O+, CH+, and HF. We find that the excitation of the CO rotational levels up to J=8 can be accounted for by UV radiation from star formation. However, the approximately flat luminosity distribution of the CO lines over the rotational ladder above J=8 requires the presence of a separate source of excitation for the highest CO lines. We explore X-ray heating by the accreting supermassive black hole in Mrk231 as a source of excitation for these lines, and find that it can reproduce the observed luminosities. We also consider a model with dense gas in a strong UV radiation field to produce the highest CO lines, but find that this model strongly overpredicts the hot dust mass in Mrk231. Our favoured model consists of a star forming disk of radius 560 pc, containing clumps of dense gas exposed to strong UV radiation, dominating the emission of CO lines up to J=8. X-rays from the accreting supermassive black hole in Mrk231 dominate the excitation and chemistry of the inner disk out to a radius of 160 pc, consistent with the X-ray power of the AGN in Mrk231. The extraordinary luminosity of the OH+ and H2O+ lines reveals the signature of X-ray driven excitation and chemistry in this region.

Molecular Gas in z ~ 6 Quasar Host Galaxies

Abstract: We report our new observations of redshifted carbon monoxide emission from six z similar to 6 quasars, using the IRAM Plateau de Bure Interferometer. CO (6-5) or (5-4) line emission was detected in all six sources. Together with two other previous CO detections, these observations provide unique constraints on the molecular gas emission properties in these quasar systems close to the end of the cosmic re-ionization. Complementary results are also presented for low-J CO lines observed at the Green Bank Telescope and the Very Large Array, and dust continuum from five of these sources with the SHARC-II bolometer camera at the Caltech Submillimeter Observatory. We then present a study of the molecular gas properties in our combined sample of eight CO-detected quasars at z similar to 6. The detections of high-order CO line emission in these objects indicates the presence of highly excited molecular gas, with estimated masses on the order of 10(10) M(circle dot) within the quasar host galaxies. No significant difference is found in the gas mass and CO line width distributions between our z similar to 6 quasars and samples of CO-detected 1.4 <= z <= 5 quasars and submillimeter galaxies. Most of the CO-detected quasars at z similar to 6 follow the far-infrared-CO luminosity relationship defined by actively star-forming galaxies at low and high redshifts. This suggests that ongoing star formation in their hosts contributes significantly to the dust heating at FIR wavelengths. The result is consistent with the picture of galaxy formation co-eval with supermassive black hole (SMBH) accretion in the earliest quasar-host systems. We investigate the black hole-bulge relationships of our quasar sample, using the CO dynamics as a tracer for the dynamical mass of the quasar host. The median estimated black hole-bulge mass ratio is about 15 times higher than the present-day value of similar to 0.0014. This places important constraints on the formation and evolution of the most massive SMBH-spheroidal host systems at the highest redshift.

Molecular Gas in Redshift 6 Quasar Host Galaxies

Abstract: We report our new observations of redshifted carbon monoxide emission from six z~6 quasars, using the PdBI. CO (6-5) or (5-4) line emission was detected in all six sources. Together with two other previous CO detections, these observations provide unique constraints on the molecular gas emission properties in these quasar systems close to the end of the cosmic reionization. Complementary results are also presented for low-J CO lines observed at the GBT and the VLA, and dust continuum from five of these sources with the SHARC-II bolometer camera at the CSO. We then present a study of the molecular gas properties in our combined sample of eight CO-detected quasars at z~6. The detections of high-order CO line emission in these objects indicates the presence of highly excited molecular gas, with estimated masses on the order of 10^10 M_sun within the quasar host galaxies. No significant difference is found in the gas mass and CO line width distributions between our z~6 quasars and samples of CO-detected $1.4\leq z\leq5$ quasars and submillimeter galaxies. Most of the CO-detected quasars at z~6 follow the far infrared-CO luminosity relationship defined by actively star-forming galaxies at low and high redshifts. This suggests that ongoing star formation in their hosts contributes significantly to the dust heating at FIR wavelengths. The result is consistent with the picture of galaxy formation co-eval with supermassive black hole (SMBH) accretion in the earliest quasar-host systems. We investigate the black hole--bulge relationships of our quasar sample, using the CO dynamics as a tracer for the dynamical mass of the quasar host. The results place important constraints on the formation and evolution of the most massive SMBH-spheroidal host systems at the highest redshift.

The central slope of dark matter cores in dwarf galaxies: Simulations vs. THINGS

Abstract: We make a direct comparison of the derived dark matter (DM) distributions between hydrodynamical simulations of dwarf galaxies assuming a LCDM 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 alpha 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 (SNe), 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 LCDM model. We show that the rotation curves of the simulated dwarf galaxies rise less steeply than CDM rotation curves and are consistent with those of the THINGS dwarf galaxies. The mean value of the logarithmic inner density slopes alpha of the simulated galaxies' dark matter density profiles is ~ -0.4 +- 0.1, which shows good agreement with \alpha = -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 \alpha ~ -1.0 to -1.5 predicted by dark-matter-only simulations shallower, and induce DM halos with a central mass distribution similar to that observed in nearby dwarf galaxies.

The scale dependence of the molecular gas depletion time in m33

Abstract: We study the Local Group spiral galaxy M33 to investigate how the observed scaling between the (kpc-averaged) surface density of molecular gas (ΣH2) and recent star formation rate (ΣSFR) relates to individual star-forming regions. To do this, we measure the ratio of CO emission to extinction-corrected Hα emission in apertures of varying sizes centered both on peaks of CO and Hα emission. We parameterize this ratio as the molecular gas (H2) depletion time (τdep). On large (kpc) scales, our results are consistent with a molecular star formation law (ΣSFR ~ Σ b H2) with b ~ 1.1-1.5 and a median τdep ~ 1 Gyr, with no dependence on type of region targeted. Below these scales, τdep is a strong function of the adopted angular scale and the type of region that is targeted. Small (300 pc) apertures centered on CO peaks have very long τdep (i.e., high CO-to-Hα flux ratio) and small apertures targeted toward Hα peaks have very short τdep. This implies that the star formation law observed on kpc scales breaks down once one reaches aperture sizes of 300 pc. For our smallest apertures (75 pc), the difference in τdep between the two types of regions is more than one order of magnitude. This scale behavior emerges from averaging over star-forming regions with a wide range of CO-to-Hα ratios with the natural consequence that the breakdown in the star formation law is a function of the surface density of the regions studied. We consider the evolution of individual regions the most likely driver for region-to-region differences in τdep (and thus the CO-to-Hα ratio).

The LABOCA survey of the Extended Chandra Deep Field South: A photometric redshift survey of submillimetre galaxies

Abstract: [abridged] We derive photometric redshifts from 17-band optical to mid-IR photometry of 74 robust counterparts to 68 of the 126 submillimetre galaxies (SMGs) selected at 870um by LABOCA observations in the ECDFS. The median photometric redshift of identified SMGs is z=2.2\pm0.1, the interquartile range is z=1.8-2.7 and we identify 10 (~15%) high-redshift (z>3) SMGs. We derive a simple redshift estimator for SMGs based on the 3.6 and 8um fluxes, which is accurate to Delta_z~0.4 for SMGs at z 3 and hence ~30% of all SMGs have z>3. We estimate that the full S_870um>4mJy SMG population has a median redshift of 2.5\pm0.6. In contrast to previous suggestions we find no significant correlation between S_870um and redshift. The median stellar mass of the SMGs derived from SED fitting is (9.2\pm0.9)x10^10Msun and the interquartile range is (4.7-14)x10^10Msun, although we caution that uncertainty in the star-formation histories results in a factor of ~5 uncertainty in these stellar masses. The median characteristic dust temperature of SMGs is 35.9\pm1.4K and the interquartile range is 28.5-43.3K. The infrared luminosity function shows that SMGs at z=2-3 typically have higher far-IR 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~2. The maximum contribution of bright SMGs to the global SFRD (~5% for SMGs with S_870um>4mJy; ~50% for SMGs with S_870um>1mJy) also occurs at z~2.

The laboca survey of the extended chandra deep field south: two modes of star formation in active galactic nucleus hosts?

Abstract: We study the co-existence of star formation and active galactic nucleus (AGN) activity in Chandra X-ray-selected AGN by analyzing stacked 870 μm submillimeter emission from a deep and wide map of the Extended Chandra Deep Field South (ECDFS), obtained with the LABOCA instrument at the APEX telescope. The total X-ray sample of 895 sources with median redshift z ~ 1 drawn from the combined (E)CDFS X-ray catalogs is detected at >11σ significance at a mean submillimeter flux of 0.49 ± 0.04 mJy, corresponding to a typical star formation rate (SFR) around 30 M sun yr-1 for a T = 35 K, β = 1.5 graybody far-infrared spectral energy distribution. The good signal-to-noise ratio permits stacking analyses for major subgroups, splitting the sample by redshift, intrinsic luminosity, and AGN obscuration properties. We observe a trend of SFR increasing with redshift. An increase of SFR with AGN luminosity is indicated at the highest L 2-10 keV >~ 1044 erg s-1 luminosities only. Increasing trends with X-ray obscuration as expected in some AGN evolutionary scenarios are not observed for the bulk of the X-ray AGN sample but may be present for the highest intrinsic luminosity objects with L 2-10 keV >~ 1044 erg s-1. This behavior suggests a transition between two modes in the co-existence of AGN activity and star formation. For the bulk of the sample, the X-ray luminosity and obscuration of the AGN are not intimately linked to the global SFR of their hosts. The hosts are likely massive and forming stars secularly, at rates similar to the pervasive star formation seen in massive galaxies without an AGN at similar redshifts. In these systems, star formation is not linked to a specific state of the AGN and the period of moderately luminous AGN activity may not highlight a major evolutionary transition of the galaxy. The change indicated toward more intense star formation, and a more pronounced increase in SFRs between unobscured and obscured AGN reported in the literature at highest (L 2-10 keV >~ 1044 erg s-1) luminosities suggests that these luminous AGNs follow an evolutionary path on which obscured AGN activity and intense star formation are linked, possibly via merging. Comparison to local hard X-ray-selected AGN supports this interpretation. SFRs in the hosts of moderate luminosity AGN at z ~ 1 are an order of magnitude higher than at z ~ 0, following the increase in the non-AGN massive galaxy population. At high AGN luminosities, hosts on the evolutionary link/merger path emerge from this secular level of star formation.

Imaging the Molecular Gas in a Submillimeter Galaxy at z = 4.05: Cold Mode Accretion or a Major Merger?

Abstract: We present a high-resolution (down to 0."18), multi-transition imaging study of the molecular gas in the z = 4.05 submillimeter galaxy GN20. GN20 is one of the most luminous starburst galaxy known at z>4, and is a member of a rich proto-cluster of galaxies at z = 4.05 in GOODS-North. We have observed the CO 1-0 and 2-1 emission with the Very Large Array (VLA), the CO 6-5 emission with the Plateau de Bure Interferometer, and the 5-4 emission with Combined Array for Research in Millimeter Astronomy. The H_2 mass derived from the CO 1-0 emission is 1.3 × 10^(11)(α/0.8) M_☉. High-resolution imaging of CO 2-1 shows emission distributed over a large area, appearing as partial ring, or disk, of ~10 kpc diameter. The integrated CO excitation is higher than found in the inner disk of the Milky Way, but lower than that seen in high-redshift quasar host galaxies and low-redshift starburst nuclei. The CO 4-3 integrated line strength is more than a factor of 2 lower than expected for thermal excitation. The excitation can be modeled with two gas components: a diffuse, lower excitation component with a radius ~4.5 kpc and a filling factor ~0.5, and a more compact, higher excitation component (radius ~2.5 kpc, filling factor ~0.13). The lower excitation component contains at least half the molecular gas mass of the system, depending on the relative conversion factor. The VLA CO 2-1 image at 0"2 resolution shows resolved, clumpy structure, with a few brighter clumps with intrinsic sizes ~2 kpc. The velocity field determined from the CO 6-5 emission is consistent with a rotating disk with a rotation velocity of ~570 km s^(–1) (using an inclination angle of 45°), from which we derive a dynamical mass of 3 × 10^(11) M_☉ within about 4 kpc radius. The star formation distribution, as derived from imaging of the radio synchrotron and dust continuum, is on a similar scale as the molecular gas distribution. The molecular gas and star formation are offset by ~1" from the Hubble Space Telescope I-band emission, implying that the regions of most intense star formation are highly dust obscured on a scale of ~10 kpc. The large spatial extent and ordered rotation of this object suggests that this is not a major merger, but rather a clumpy disk accreting gas rapidly in minor mergers or smoothly from the proto-intracluster medium. Qualitatively, the kinematic and structural properties of GN20 compare well to the most rapid star formers fed primarily by cold accretion in cosmological hydrodynamic simulations. Conversely, if GN20 is a major, gas-rich merger, then some process has managed to ensure that the star formation and molecular gas distribution has not been focused into one or two compact regions.

Imaging the molecular gas in a submm galaxy at z = 4.05: cold mode accretion or a major merger?

Abstract: We present a high resolution (down to 0.18"), multi-transition imaging study of the molecular gas in the z = 4.05 submillimeter galaxy GN20. GN20 is one of the most luminous starburst galaxy known at z > 4, and is a member of a rich proto-cluster of galaxies at z = 4.05 in GOODS-North. We have observed the CO 1-0 and 2-1 emission with the VLA, the CO 6-5 emission with the PdBI Interferometer, and the 5-4 emission with CARMA. The H_2 mass derived from the CO 1-0 emission is 1.3 \times 10^{11} (\alpha/0.8) Mo. High resolution imaging of CO 2-1 shows emission distributed over a large area, appearing as partial ring, or disk, of ~ 10kpc diameter. The integrated CO excitation is higher than found in the inner disk of the Milky Way, but lower than that seen in high redshift quasar host galaxies and low redshift starburst nuclei. The VLA CO 2-1 image at 0.2" resolution shows resolved, clumpy structure, with a few brighter clumps with intrinsic sizes ~ 2 kpc. The velocity field determined from the CO 6-5 emission is consistent with a rotating disk with a rotation velocity of ~ 570 km s^{-1} (using an inclination angle of 45^o), from which we derive a dynamical mass of 3 \times 10^{11} \msun within about 4 kpc radius. The star formation distribution, as derived from imaging of the radio synchrotron and dust continuum, is on a similar scale as the molecular gas distribution. The molecular gas and star formation are offset by ~ 1" from the HST I-band emission, implying that the regions of most intense star formation are highly dust-obscured on a scale of ~ 10 kpc. The large spatial extent and ordered rotation of this object suggests that this is not a major merger, but rather a clumpy disk accreting gas rapidly in minor mergers or smoothly from the proto-intracluster medium. ABSTRACT TRUNCATED

The LABOCA survey of the Extended Chandra Deep Field South: Two modes of star formation in AGN hosts?

Abstract: We study the co-existence of star formation and AGN activity in X-ray selected AGN by analyzing stacked 870um submm emission from a deep and wide map of the Extended Chandra Deep Field South, obtained with LABOCA at the APEX telescope. The total X-ray sample of 895 sources with median redshift z~1 is detected at a mean submm flux of 0.49+-0.04mJy, corresponding to a typical star formation rate around 30Msun/yr for a T=35K, beta=1.5 greybody far-infrared SED. The good S/N permits stacking analyses for subgroups. We observe a trend of star formation rate increasing with redshift. An increase of star formation rate with AGN luminosity is indicated at the highest L_2-10>~1E44erg/s luminosities only. Increasing trends with X-ray obscuration as expected in some AGN evolutionary scenarios are not observed for the bulk of the X-ray AGN sample but may be present for the highest intrinsic luminosity objects. This suggests a transition between two modes in the coexistence of AGN activity and star formation. For the bulk of the sample, the X-ray luminosity and obscuration of the AGN are not intimately linked to the global star formation rate of their hosts. The hosts are likely massive and forming stars secularly, at rates similar to the pervasive star formation seen in massive galaxies without an AGN at similar redshifts. The change indicated towards more intense star formation, and a more pronounced increase in star formation rates between unobscured and obscured AGN at highest luminosities suggests that luminous AGN follow an evolutionary path on which obscured AGN activity and intense star formation are linked, possibly via merging. Comparison to local hard X-ray selected AGN supports this interpretation. [Abridged]

BLAST: the far-infrared/radio correlation in distant galaxies

Abstract: We investigate the correlation between far-infrared (FIR) and radio luminosities in distant galaxies, a lynchpin of modern astronomy. We use data from the Balloon-borne Large Aperture Submillimetre Telescope (BLAST), Spitzer, the Large Apex BOlometer CamerA (LABOCA), the Very Large Array and the Giant Metre-wave Radio Telescope (GMRT) in the Extended Chandra Deep Field South (ECDFS). For a catalogue of BLAST 250-μm-selected galaxies, we remeasure the 70–870-μm flux densities at the positions of their most likely 24-μm counterparts, which have a median [interquartile] redshift of 0.74 [0.25, 1.57]. From these, we determine the monochromatic flux density ratio, q_(250)(= log_(10) [ S_(250 μm)/S_(1400 MHz)]), and the bolometric equivalent, q_(IR). At z ≈ 0.6 , where our 250-μm filter probes rest-frame 160-μm emission, we find no evolution relative to q_(160) for local galaxies. We also stack the FIR and submm images at the positions of 24-μm- and radio-selected galaxies. The difference between q_(IR) seen for 250-μm- and radio-selected galaxies suggests that star formation provides most of the IR luminosity in ≲100-μJy radio galaxies, but rather less for those in the mJy regime. For the 24-μm sample, the radio spectral index is constant across 0 < z < 3 , but q_(IR) exhibits tentative evidence of a steady decline such that q_(IR) ∝ (1 +z)^(−0.15±0.03) – significant evolution, spanning the epoch of galaxy formation, with major implications for techniques that rely on the FIR/radio correlation. We compare with model predictions and speculate that we may be seeing the increase in radio activity that gives rise to the radio background.

The Scale Dependence of the Molecular Gas Depletion Time in M33

Abstract: We study the Local Group spiral galaxy M33 to investigate how the observed scaling between the (kpc-averaged) surface density of molecular gas (\Sigma_H2) and recent star formation rate (\Sigma_SFR) relates to individual star-forming regions. To do this, we measure the ratio of CO emission to extinction-corrected Halpha emission in apertures of varying sizes centered both on peaks of CO and Halpha emission. We parameterize this ratio as a molecular gas (H_2) depletion time (\tau_dep). On large (kpc) scales, our results are consistent with a molecular star formation law (Sigma_SFR \sim Sigma_H2^b) with b \sim 1.1 - 1.5 and a median \tau_dep \sim 1 Gyr, with no dependence on type of region targeted. Below these scales, \tau_dep is a strong function of adopted angular scale and the type of region that is targeted. Small (\lesssim 300pc) apertures centered on CO peaks have very long \tau_dep (i.e., high CO-to-Halpha flux ratio) and small apertures targeted toward Halpha peaks have very short \tau_dep. This implies that the star formation law observed on kpc scales breaks down once one reaches aperture sizes of \lesssim 300pc. For our smallest apertures (75pc), the difference in \tau_dep between the two types of regions is more than one order of magnitude. This scale behavior emerges from averaging over star-forming regions with a wide range of CO-to-Halpha ratios with the natural consequence that the breakdown in the star formation law is a function of the surface density of the regions studied. We consider the evolution of individual regions the most likely driver for region-to-region differences in \tau_dep (and thus the CO-to-Halpha ratio).

Dust-free quasars in the early Universe

Abstract: More than 40 quasars have been discovered at redshifts of z ≈ 6, at an epoch when the Universe was less than a billion years old or just 7% of its current age. Surprisingly, the properties of these distant quasars seem almost indistinguishable from those at lower redshifts, suggesting that they are evolved objects. Now with the discovery of a second z ≈ 6 quasar without hot-dust emission, and evidence from others that hot dust accumulates in tandem with the growth of the central black hole, comes confirmation that these extremely distant quasars are indeed less evolved than their lower-redshift equivalents. The two dust-free quasars may be first-generation quasars born in dust-free environments that are too young to have formed a detectable amount of hot dust around them. The most distant quasars known, at redshifts z ≈ 6, generally have the same properties as lower-redshift quasars, implying that although the Universe was young at z ≈ 6, such quasars are still evolved objects. One z ≈ 6 quasar was shown to have no detectable emission from hot dust, but it was not clear whether it was an outlier. Now, a second quasar without hot-dust emission has been discovered in a sample of 21 z ≈ 6 quasars. Moreover, hot-dust abundance in these quasars builds up as the central black hole grows. The most distant quasars known, at redshifts z ≈ 6, generally have properties indistinguishable from those of lower-redshift quasars in the rest-frame ultraviolet/optical and X-ray bands1,2,3. This puzzling result suggests that these distant quasars are evolved objects even though the Universe was only seven per cent of its current age at these redshifts. Recently one z ≈ 6 quasar was shown not to have any detectable emission from hot dust4, but it was unclear whether that indicated different hot-dust properties at high redshift or if it is simply an outlier. Here we report the discovery of a second quasar without hot-dust emission in a sample of 21 z ≈ 6 quasars. Such apparently hot-dust-free quasars have no counterparts at low redshift. Moreover, we demonstrate that the hot-dust abundance in the 21 quasars builds up in tandem with the growth of the central black hole, whereas at low redshift it is almost independent of the black hole mass. Thus z ≈ 6 quasars are indeed at an early evolutionary stage, with rapid mass accretion and dust formation. The two hot-dust-free quasars are likely to be first-generation quasars born in dust-free environments and are too young to have formed a detectable amount of hot dust around them.

IONIZATION NEAR ZONES ASSOCIATED WITH QUASARS AT z ∼ 6 ∗

Abstract: We analyze the size evolution of HII regions around 27 quasars between z = 5.7 and 6.4 ("quasar near zones" or NZs). We include more sources than previous studies, and we use more accurate redshifts for the host galaxies, with eight CO molecular line redshifts and nine Mg II redshifts. We confirm the trend for an increase in NZ size with decreasing redshift, with the luminosity- normalized proper size evolving as R(NZ, corrected) = (7.4 +/- 0.3) - (8.0 +/- 1.1) x (z - 6) Mpc. While derivation of the absolute neutral fraction remains difficult with this technique, the evolution of the NZ sizes suggests a decrease in the neutral fraction of intergalactic hydrogen by a factor similar to 9.4 from z = 6.4 to 5.7, in its simplest interpretation. Alternatively, recent numerical simulations suggest that this rapid increase in NZ size from z = 6.4 to 5.7 is due to the rapid increase in the background photo-ionization rate at the end of the percolation or overlap phase, when the average mean-free path of ionizing photons increases dramatically. In either case, the results are consistent with the idea that z similar to 6-7 corresponds to the tail end of cosmic reionization. The scatter in the normalized NZ sizes is larger than expected simply from measurement errors, and likely reflects intrinsic differences in the quasars or their environments. We find that the NZ sizes increase with quasar UV luminosity, as expected for photo-ionization dominated by quasar radiation.

A joint analysis of BLAST 250--500um and LABOCA 870um observations in the Extended Chandra Deep Field South

Abstract: We present a joint analysis of the overlapping BLAST 250, 350, 500um, and LABOCA 870um observations (from the LESS survey) of the ECDF-S. Out to z~3, the BLAST filters sample near the peak wavelength of far-infrared (FIR) emission from galaxies (rest-frame wavelengths ~60--200um), 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 (FWHM 36--60"). In contrast, the 870um observations have a significantly smaller 19" 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 submm SEDs and redshift measurements out to z~3. For flux-limited sub-samples in each BLAST band, and a dust emissivity index \beta=2.0, we find a median temperature T=30K (all bands) as well as median redshifts: z=1.1 (interquartile range 0.2--1.9) for S250 > 40mJy; z=1.3 (interquartile range 0.6--2.1) for S350 > 30mJy; and z=1.6 (interquartile range 1.3--2.3) for S500 > 20mJy. Taking into account selection effects for our survey (a bias toward detecting lower-temperature galaxies), we find no evidence for evolution in the local FIR-temperature correlation out to z~2.5. Comparing with star-forming galaxy SED templates, about 8% of our sample appears to exhibit significant excesses in the radio and/or mid-IR, consistent with those sources harbouring an AGN. 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.

Herschel observations of water vapour in Markarian 231

Abstract: The Ultra luminous infrared galaxy (ULIRG) Mrk 231 reveals up to seven rotational lines of water (H2O) in emission, including a very high-lying (Eupper = 640 K) line detected at a 4 sigma level, within the Herschel/SPIRE wavelength range (190 < lambda(mu m) < 640), whereas PACS observations show one H2O line at 78 mu m in absorption, as found for other H2O lines previously detected by ISO. The absorption/emission dichotomy is caused by the pumping of the rotational levels by far-infrared radiation emitted by dust, and subsequent relaxation through lines at longer wavelengths, which allows us to estimate both the column density of H2O and the general characteristics of the underlying far-infrared continuum source. Radiative transfer models including excitation through both absorption of far-infrared radiation emitted by dust and collisions are used to calculate the equilibrium level populations of H2O and the corresponding line fluxes. The highest-lying H2O lines detected in emission, with levels at 300-640 K above the ground state, indicate that the source of far-infrared radiation responsible for the pumping is compact (radius = 110-180 pc) and warm (T-dust = 85-95 K), accounting for at least 45% of the bolometric luminosity. The high column density, N(H2O) similar to 5 x 10(17) cm(-2), found in this nuclear component, is most probably the consequence of shocks/cosmic rays, an XDR chemistry, and/or an "undepleted chemistry" where grain mantles are evaporated. A more extended region, presumably the inner region of the 1-kpc disk observed in other molecular species, could contribute to the flux observed in low-lying H2O lines through dense hot cores, and/or shocks. The H2O 78 mu m line observed with PACS shows hints of a blue-shifted wing seen in absorption, possibly indicating the occurrence of H2O in the prominent outflow detected in OH (Fischer et al. 2010, A&A, 518, L41). Additional PACS/HIFI observations of H2O lines are required to constrain the kinematics of the nuclear component, as well as the distribution of H2O relative to the warm dust.

Arm and Interarm Star Formation in Spiral Galaxies

Abstract: We investigate the relationship between spiral arms and star formation in the grand-design spirals NGC 5194 and NGC 628 and in the flocculent spiral NGC 6946. Filtered maps of near-IR (3.6 {mu}m) emission allow us to identify 'arm regions' that should correspond to regions of stellar mass density enhancements. The two grand-design spirals show a clear two-armed structure, while NGC 6946 is more complex. We examine these arm and interarm regions, looking at maps that trace recent star formation-far-ultraviolet (GALEX NGS) and 24 {mu}m emission (Spitzer SINGS)-and cold gas-CO (HERACLES) and H I (THINGS). We find the star formation tracers and CO more concentrated in the spiral arms than the stellar 3.6 {mu}m flux. If we define the spiral arms as the 25% highest pixels in the filtered 3.6 {mu}m images, we find that the majority (60%) of star formation tracers occur in the interarm regions; this result persists qualitatively even when considering the potential impact of finite data resolution and diffuse interarm 24 {mu}m emission. Even with a generous definition of the arms (45% highest pixels), interarm regions still contribute at least 30% to the integrated star formation rate (SFR) tracers. We look for evidence that spiral arms triggermore » star or cloud formation using the ratios of SFR (traced by a combination of FUV and 24 {mu}m emission) to H{sub 2} (traced by CO) and H{sub 2} to H I. Any enhancement of SFR/M(H{sub 2}) in the arm region is very small (less than 10%) and the grand-design spirals show no enhancement compared to the flocculent target. Arm regions do show a weak enhancement in H{sub 2}/H I compared to the interarm regions, but at a fixed gas surface density there is little clear enhancement in the H{sub 2}/H I ratio in the arm regions. Thus, it seems that spiral arms may only act to concentrate the gas to higher densities in the arms.« less

The LABOCA survey of the Extended Chandra Deep Field South - radio and mid-infrared counterparts to submillimetre galaxies

Abstract: We present radio and infrared (3.6-24um) counterparts to submillimetre galaxies (SMGs) detected in the Extended Chandra Deep Field South with the LABOCA 870-um bolometer camera on the 12-m Atacama Pathfinder Experiment. Using the Very Large Array at 1.4GHz and Spitzer we have identified secure counterparts to 79 of the 126 SMGs (SNR>3.7, S_870>4.4mJy) in the field, 62 via their radio and/or 24-um emission, the remainder using a colour-flux cut on IRAC 3.6- and 5.8-um sources chosen to maximise the number of secure, coincident radio and 24-um 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 z=2.2 (+0.7-0.8) (1-sigma errors) for the secure radio counterparts, in agreement with other studies, both spectroscopic and photometric.

COLD MOLECULAR GAS IN MASSIVE, STAR-FORMING DISK GALAXIES AT z = 1.5*

Abstract: We report the detection of the CO J = 1-0 emission line in three near-infrared selected star-forming galaxies at z ~ 1.5 with the Very Large Array and the Green Bank Telescope. These observations directly trace the bulk of molecular gas in these galaxies. We find H2 gas masses of 8.3 ± 1.9 × 1010 M ☉, 5.6 ± 1.4 × 1010 M ☉, and 1.23 ± 0.34 × 1011 M ☉ for BzK-4171, BzK-21000, and BzK-16000, respectively, assuming a conversion αCO = 3.6 M ☉ (K km s–1 pc2)–1. We combined our observations with previous CO 2-1 detections of these galaxies to study the properties of their molecular gas. We find brightness temperature ratios between the CO 2-1 and CO 1-0 emission lines of 0.80+0.35 –0.22, 1.22+0.61 –0.36, and 0.41+0.23 –0.13 for BzK-4171, BzK-21000, and BzK-16000, respectively. At the depth of our observations it is not possible to discern between thermodynamic equilibrium or sub-thermal excitation of the molecular gas at J = 2. However, the low temperature ratio found for BzK-16000 suggests sub-thermal excitation of CO already at J = 2. For BzK-21000, a large velocity gradient model of its CO emission confirms previous results of the low excitation of the molecular gas at J = 3. From a stacked map of the CO 1-0 images, we measure a CO 2-1 to CO 1-0 brightness temperature ratio of 0.92+0.28 –0.19. This suggests that, on average, the gas in these galaxies is thermalized up to J = 2, has star formation efficiencies of ~100 L ☉ (K km s–1 pc2)–1, and gas consumption timescales of ~0.4 Gyr, unlike submillimeter galaxies and quasi-stellar objects at high redshifts.

Cold molecular gas in massive disk galaxies at z=1.5

Abstract: We report the detection of the CO J=1-0 emission line in three near-infrared selected star-forming galaxies at z~1.5 with the Very Large Array (VLA) and the Green Bank telescope (GBT). These observations directly trace the bulk of molecular gas in these galaxies. We find H_2 gas masses of 8.3 \pm 1.9 x 10^{10} M_sun, 5.6 \pm 1.4 x 10^{10} M_sun and 1.23 \pm 0.34 x 10^{11} M_sun for BzK-4171, BzK-21000 and BzK-16000, respectively, assuming a conversion alpha_CO=3.6 M_sun (K km s^{-1} pc^{2})^{-1}. We combined our observations with previous CO 2-1 detections of these galaxies to study the properties of their molecular gas. We find brightness temperature ratios between the CO 2-1 and CO 1-0 emission lines of 0.80_{-0.22}^{+0.35}, 1.22_{-0.36}^{+0.61} and 0.41_{-0.13}^{+0.23} for BzK-4171, BzK-21000 and BzK-16000, respectively. At the depth of our observations it is not possible to discern between thermodynamic equilibrium or sub-thermal excitation of the molecular gas at J=2. However, the low temperature ratio found for BzK-16000 suggests sub-thermal excitation of CO already at J=2. For BzK-21000, a Large Velocity Gradient model of its CO emission confirms previous results of the low-excitation of the molecular gas at J=3. From a stacked map of the CO 1-0 images, we measure a CO 2-1 to CO 1-0 brightness temperature ratio of 0.92_{-0.19}^{+0.28}. This suggests that, on average, the gas in these galaxies is thermalized up to J=2, has star-formation efficiencies of ~100 L_sun (K km s^{-1} pc^2)^{-1} and gas consumption timescales of ~0.4 Gyr, unlike SMGs and QSOs at high redshifts.

Tightly correlated h i and fuv emission in the outskirts of m83

Abstract: We compare sensitive H I data from The H I Nearby Galaxy Survey and deep far-ultraviolet (FUV) data from the Galaxy Evolution Explorer in the outer disk of M83. The FUV and H I maps show a stunning spatial correlation out to almost 4 optical radii (r 25), roughly the extent of our maps. This underscores that H I traces the gas reservoir for outer-disk star formation (SF), and it implies that massive (at least low level) SF proceeds almost everywhere that H I is observed. Whereas the average FUV intensity steadily decreases with increasing radius before leveling off at ~1.7 r 25, the decline in H I surface density is more subtle. Low H I columns (2 M ? pc?2) contribute most of the mass in the outer disk, which is not the case within r 25. The time for SF to consume the available H I, inferred from the ratio of H I to FUV intensity, rises with increasing radius before leveling off at ~100?Gyr, i.e., many Hubble times, near ~1.7 r 25. Assuming that the relatively short H2 depletion times observed in the inner parts of galaxies hold in outer disks, the conversion of H I into bound, molecular clouds seems to limit SF in outer galaxy disks. The long consumption times suggest that most of the extended H I observed in M83 will not be consumed by in situ SF. However, even these low SF rates are enough for moderate chemical enrichment in a closed outer disk to be expected.

Arm & Interarm Star Formation in Spiral Galaxies

Abstract: We investigate the relationship between spiral arms and star formation in the grand-design spirals NGC 5194 and NGC 628 and in the flocculent spiral NGC 6946. Filtered maps of near-IR (3.6 micron) emission allow us to identify "arm regions" that should correspond to regions of stellar mass density enhancements. The two grand-design spirals show a clear two-armed structure, while NGC 6946 is more complex. We examine these arm and interarm regions, looking at maps that trace recent star formation - far-ultraviolet (GALEX NGS) and 24 micron emission (Spitzer, SINGS) - and cold gas - CO (Heracles) and HI (Things). We find the star formation tracers and CO more concentrated in the spiral arms than the stellar 3.6 micron flux. If we define the spiral arms as the 25% highest pixels in the filtered 3.6 micron images, we find that the majority (60%) of star formation tracers occurs in the interarm regions; this result persists qualitatively even when considering the potential impact of finite data resolution and diffuse interarm 24 micron emission. Even with a generous definition of the arms (45% highest pixels), interarm regions still contribute at least 30% to the integrated star formation rate tracers. We look for evidence that spiral arms trigger star or cloud formation using the ratios of star formation rate (SFR, traced by a combination of FUV and 24 micron emission) to H_2 (traced by CO) and H_2 to HI. Any enhancement of SFR / M(H_2) in the arm region is very small (less than 10%) and the grand design spirals show no enhancement compared to the flocculent target. Arm regions do show a weak enhancement in H_2/HI compared to the interarm regions, but at a fixed gas surface density there is little clear enhancement in the H_2/HI ratio in the arm regions. Thus, it seems that spiral arms may only act to concentrate the gas to higher densities in the arms.

[CII] line emission in BRI1335-0417 at z=4.4

Abstract: Using the 12m APEX telescope, we have detected redshifted emission from the 157.74micron [CII] line in the z=4.4074 quasar BRI1335-0417. The linewidth and redshift are in good agreement with previous observations of high-J CO line emission. We measure a [CII] line luminosity, L_[CII] = (16.4 +/- 2.6)x10^9 Lsun, making BRI~1335-0417 the most luminous, unlensed [CII] line emitter known at high-redshift. The [CII]-to-FIR luminosity ratio of (5.3+/-0.8)x10^-4 is ~3x higher than expected for an average object with a FIR luminosity L_FIR = 3.1x10^13 Lsun, if this ratio were to follow the trend observed in other FIR-bright galaxies that have been detected in [CII] line emission. These new data suggest that the scatter in the [CII]-to-FIR luminosity ratio could be larger than previously expected for high luminosity objects. BR1335-0417 has a similar FIR luminosity and [CII]/CO luminosity compared to local ULIRGS and appears to be a gas-rich merger forming stars at a rate of a few thousand solar masses per year.

CARMA Survey Toward Infrared-bright Nearby Galaxies (STING): Molecular Gas Star Formation Law in NGC4254

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 micron, combined H\alpha and mid-infrared 24 micron, and combined far-ultraviolet and mid-infrared 24 micron), several fitting procedures, and different sampling strategies we probe the relation between SFR and molecular gas at various spatial resolutions and surface densities within the central 6.5 kpc in the disk of NGC4254. We find that in the high surface brightness regions of NGC4254 the form of the molecular gas star formation law is robustly determined and approximately linear 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. In such case, results range from linear when the fraction of diffuse emission in the SFR tracer is ~30% or less (or when diffuse emission is removed in both the star formation and the molecular gas tracer), to super-linear when the diffuse fraction is ~50% and above. We find that the tightness of the correlation between gas and star formation varies with the choice of star formation tracer. The 24 micron 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_25 the local star formation efficiency is constant and similar to that observed in other large spirals, with a molecular gas depletion time ~2 Gyr.

Tightly Correlated HI and FUV Emission in the Outskirts of M83

Abstract: We compare sensitive HI data from The HI Nearby Galaxy Survey (THINGS) and deep far UV (FUV) data from GALEX in the outer disk of M83. The FUV and HI maps show a stunning spatial correlation out to almost 4 optical radii (r25), roughly the extent of our maps. This underscores that HI traces the gas reservoir for outer disk star formation and it implies that massive (at least low level) star formation proceeds almost everywhere HI is observed. Whereas the average FUV intensity decreases steadily with increasing radius before leveling off at ~1.7 r25, the decline in HI surface density is more subtle. Low HI columns (<2 M_solar/pc^2) contribute most of the mass in the outer disk, which is not the case within r25. The time for star formation to consume the available HI, inferred from the ratio of HI to FUV intensity, rises with increasing radius before leveling off at ~100 Gyr, i.e., many Hubble times, near ~1.7 r25. Assuming the relatively short H2 depletion times observed in the inner parts of galaxies hold in outer disks, the conversion of HI into bound, molecular clouds seems to limit star formation in outer galaxy disks. The long consumption times suggest that most of the extended HI observed in M83 will not be consumed by in situ star formation. However, even these low star formation rates are enough to expect moderate chemical enrichment in a closed outer disk.