Showing papers by "Fabian Walter published in 2005"

Star formation in NGC 5194 (M51a): The Panchromatic view from GALEX to Spitzer

Abstract: (Abridged) Far ultraviolet to far infrared images of the nearby galaxy NGC5194, from Spitzer, GALEX, Hubble Space Telescope and ground--based data, are used to investigate local and global star formation, and the impact of dust extinction in HII-emitting knots. In the IR/UV-UV color plane, the NGC5194 HII knots show the same trend observed for normal star-forming galaxies, having a much larger dispersion than starburst galaxies. We identify the dispersion as due to the UV emission predominantly tracing the evolved, non-ionizing stellar population, up to ages 50-100 Myr. While in starbursts the UV light traces the current SFR, in NGC5194 it traces a combination of current and recent-past SFR. Unlike the UV emission, the monochromatic 24 micron luminosity is an accurate local SFR tracer for the HII knots in NGC5194; this suggests that the 24 micron emission carriers are mainly heated by the young, ionizing stars. However, preliminary results show that the ratio of the 24 micron emission to the SFR varies by a factor of a few from galaxy to galaxy. While also correlated with star formation, the 8 micron emission is not directly proportional to the number of ionizing photons. This confirms earlier suggestions that the carriers of the 8 micron emission are heated by more than one mechanism.

Star Formation in NGC5194 (M51a): The Panchromatic View from GALEX to Spitzer

Abstract: (Abridged) Far ultraviolet to far infrared images of the nearby galaxy NGC5194, from Spitzer, GALEX, Hubble Space Telescope and ground--based data, are used to investigate local and global star formation, and the impact of dust extinction in HII-emitting knots. In the IR/UV-UV color plane, the NGC5194 HII knots show the same trend observed for normal star-forming galaxies, having a much larger dispersion than starburst galaxies. We identify the dispersion as due to the UV emission predominantly tracing the evolved, non-ionizing stellar population, up to ages 50-100 Myr. While in starbursts the UV light traces the current SFR, in NGC5194 it traces a combination of current and recent-past SFR. Unlike the UV emission, the monochromatic 24 micron luminosity is an accurate local SFR tracer for the HII knots in NGC5194; this suggests that the 24 micron emission carriers are mainly heated by the young, ionizing stars. However, preliminary results show that the ratio of the 24 micron emission to the SFR varies by a factor of a few from galaxy to galaxy. While also correlated with star formation, the 8 micron emission is not directly proportional to the number of ionizing photons. This confirms earlier suggestions that the carriers of the 8 micron emission are heated by more than one mechanism.

First detection of [CII]158 μm at high redshift : vigorous star formation in the early universe

Abstract: We report the detection of the 2 P 3/2 → 2 P 1/2 fine-structure line of C + at 157.74 μm in SDSS J114816.64+525150.3 (hereafter J1148+5251), the most distant known quasar, at z = 6.42, using the IRAM 30-m telescope. This is the first detection of the [CII] line at high redshift, and also the first detection in a Hyperluminous Infrared Galaxy (L FIR > 10 13 L ○. ). The [CII] line is detected at a significance level of 8σ and has a luminosity of 4.4 x 10 9 L ○. . The L [CII] /L FIR ratio is 2 × 10 -4 , about an order of magnitude smaller than observed in local normal galaxies and similar to the ratio observed in local Ultraluminous Infrared Galaxies. The [CII] line luminosity indicates that the host galaxy of this quasar is undergoing an intense burst of star formation with an estimated rate of 3000 M ○. yr -1 . The detection of C + in SDSS J 1148+5251 suggests a significant enrichment of metals at z ∼ 6 (age of the universe ∼870 Myr), although the data are consistent with a reduced carbon to oxygen ratio as expected from chemical evolutionary models of the early phases of galaxy formation.

Infrared Spectral Energy Distributions of Nearby Galaxies

Abstract: The Spitzer Infrared Nearby Galaxies Survey (SINGS) is carrying out a comprehensive multiwavelength survey on a sample of 75 nearby galaxies. The 1-850 μm spectral energy distributions (SEDs) are presented using broadband imaging data from Spitzer, 2MASS, ISO, IRAS, and SCUBA. The infrared colors derived from the globally integrated Spitzer data are generally consistent with the previous generation of models that were developed using global data for normal star-forming galaxies, although significant deviations are observed. Spitzer's excellent sensitivity and resolution also allow a detailed investigation of the infrared SEDs for various locations within the three large, nearby galaxies NGC 3031 (M81), NGC 5194 (M51), and NGC 7331. A wide variety of spectral shapes is found within each galaxy, especially for NGC 3031, the closest of the three targets and thus the galaxy for which the smallest spatial scales can be explored. Strong correlations exist between the local star formation rate and the infrared colors fν(70 μm)/fν(160 μm) and fν(24 μm)/fν(160 μm), suggesting that the 24 and 70 μm emission are useful tracers of the local star formation activity level. Preliminary evidence indicates that variations in the 24 μm emission, and not variations in the emission from polycyclic aromatic hydrocarbons at 8 μm, drive the variations in the fν(8.0 μm)/fν(24 μm) colors within NGC 3031, NGC 5194, and NGC 7331. If the galaxy-to-galaxy variations in SEDs seen in our sample are representative of the range present at high redshift, then extrapolations of total infrared luminosities and star formation rates from the observed 24 μm flux will be uncertain at the factor of 5 level (total range). The corresponding uncertainties using the redshifted 8.0 μm flux (e.g., observed 24 μm flux for a z = 2 source) are factors of 10-20. Considerable caution should be used when interpreting such extrapolated infrared luminosities.

First detection of [CII]158um at high redshift: vigorous star formation in the early universe

Abstract: We report the detection of the 2P_3/2 -> 2P_1/2 fine-structure line of C+ at 157.74 micron in SDSSJ114816.64+525150.3 (hereafter J1148+5251), the most distant known quasar, at z=6.42, using the IRAM 30-meter telescope. This is the first detection of the [CII] line at high redshift, and also the first detection in a Hyperluminous Infrared Galaxy (L_FIR > 10^13 Lsun). The [CII] line is detected at a significance level of 8 sigma and has a luminosity of 4.4 x 10^9 Lsun. The L_[CII]/L_FIR ratio is 2 x 10^-4, about an order of magnitude smaller than observed in local normal galaxies and similar to the ratio observed in local Ultraluminous Infrared Galaxies. The [CII] line luminosity indicates that the host galaxy of this quasar is undergoing an intense burst of star formation with an estimated rate of ~3000 Msun/yr. The detection of C+ in SDSS J1148+5251 suggests a significant enrichment of metals at z ~ 6 (age of the universe ~870 Myr), although the data are consistent with a reduced carbon to oxygen ratio as expected from chemical evolutionary models of the early phases of galaxy formation.

Infrared Spectral Energy Distributions of Nearby Galaxies

Abstract: The Spitzer Infrared Nearby Galaxies Survey (SINGS) is carrying out a comprehensive multi-wavelength survey on a sample of 75 nearby galaxies. The 1-850um spectral energy distributions are presented using broadband imaging data from Spitzer, 2MASS, ISO, IRAS, and SCUBA. The infrared colors derived from the globally-integrated Spitzer data are generally consistent with the previous generation of models that were developed based on global data for normal star-forming galaxies, though significant deviations are observed. Spitzer's excellent sensitivity and resolution also allow a detailed investigation of the infrared spectral energy distributions for various locations within the three large, nearby galaxies NGC3031 (M81), NGC5194 (M51), and NGC7331. Strong correlations exist between the local star formation rate and the infrared colors f_nu(70um)/f_nu(160um) and f_nu(24um)/f_nu(160um), suggesting that the 24 and 70um emission are useful tracers of the local star formation activity level. Preliminary evidence indicates that variations in the 24um emission, and not variations in the emission from polycyclic aromatic hydrocarbons at 8um, drive the variations in the f_nu(8.0um)/f_nu(24um) colors within NGC3031, NGC5194, and NGC7331. If the galaxy-to-galaxy variations in spectral energy distributions seen in our sample are representative of the range present at high redshift then extrapolations of total infrared luminosities and star formation rates from the observed 24um flux will be uncertain at the factor-of-five level (total range). The corresponding uncertainties using the redshifted 8.0um flux (e.g. observed 24um flux for a z=2 source) are factors of 10-20. Considerable caution should be used when interpreting such extrapolated infrared luminosities.

The Spectral Energy Distribution of CO lines in M82

Abstract: We present an analysis of the excitation conditions of the molecular gas in the streamers and the outflow of M82 based on observations obtained at the IRAM 30m telescope. Our analysis of J=1-0 and J=2-1 transitions of CO and 13CO and the CO(3-2) line in 13 regions outside the central starburst disk shows that the gas density within the streamer/outflow system is about an orderof magnitude lower (n(H2) ~ 10^3 cm^-3) than in the central molecular disk. We have used an LVG model and data from the literature to constrain the flux density in each CO transition (the `CO line SED') arising from the streamer/outflow system and the central starburst disk itself. Globally, we find that the CO flux density up to the J=3-2 line is dominated by the diffuse outer regions while lines above the J=5-4 transition are almost exclusively emitted by the central starburst disk. We compare the CO line SED of M82 to CO observations of galaxies at high redshift and suggest that small high-J/low-J CO flux density ratios (observed in some of these sources) are not necessarily caused by a different excitation of the central molecular gas concentration, but may result from an additional, more extended and diffuse gas reservoir around these systems, reminiscent of the situation in M82.

The spectral energy distribution of CO lines in M 82

Abstract: We present an analysis of the excitation conditions of the molecular gas in the streamers and the outflow of M82 based on observations obtained at the IRAM 30 m telescope. Our analysis of J = 1→0 and J = 2→1 transitions of CO and 13 CO and the CO(J = 3→2) line in 13 regions outside the central starburst disk shows that the gas density within the streamer/outflow system is about an order of magnitude lower (log(n H2 ) 3.0 cm -3 ) than in the central molecular disk. We have used an LVG model and data from the literature to constrain the flux density in each CO transition (the CO line SED) arising from the streamer/outflow system and the central starburst disk itself. Globally, we find that the CO flux density up to the J = 3→2 line is dominated by the diffuse outer regions while lines above the./ = 5→4 transition are almost exclusively emitted by the central starburst disk. We compare the CO line SED of M 82 to CO observations of galaxies at high redshift and suggest that small high-J/low-./ CO flux density ratios (observed in some of these sources) are not necessarily caused by a different excitation of the central molecular gas concentration, but may result from an additional, more extended and diffuse gas reservoir around these systems, reminiscent of the situation in M 82.

The Dwarf Galaxy DDO 47 as a Dark Matter Laboratory: Testing Cusps Hiding in Triaxial Halos

Abstract: We present an analysis of high-resolution H I data of the dwarf galaxy DDO 47, aimed at testing the hypothesis that dark halo triaxiality might induce noncircular motions resulting in rotation curves best fitted by cored halos, even if the dark matter halo is intrinsically cuspy. This hypothesis could be invoked in order to reconcile the predictions of the standard ΛCDM theory with the rotation curves of disk galaxies. DDO 47 is an ideal case to test this hypothesis because it has a very regular velocity field, its rotation curve is best fitted by a cored halo, and an NFW halo is inconsistent with the data. We analyze the velocity field through the higher order harmonic terms in order to search for kinematical signatures of alleged noncircular motions needed to "hide" a cusp: the result is that globally noncircular motions are at a level of 2-3 km s-1, and they are more likely to be associated with the presence of some spiral structure than with a global elongated potential (e.g., a triaxial halo). These noncircular motions are far from being sufficient to account for the discrepancy with the ΛCDM predictions. We therefore conclude that the dark matter halo around the dwarf galaxy DDO 47 is truly cored and that a cusp cannot be hidden by noncircular motions.

The Temperature Distribution of Dense Molecular Gas in the Center of NGC 253

Abstract: We present interferometric maps of ammonia (NH3) of the nearby starburst galaxy NGC 253. The observations have been taken with the Australia Telescope Compact Array and include the para-NH3 (1, 1) and (2, 2) and the ortho-NH3 (3, 3) and (6, 6) inversion lines. Six major complexes of dense ammonia are identified, three of them on either side of the starburst center, out to projected galactocentric radii of ~250 pc. Rotational temperatures are derived toward selected individual positions, as well as for the entire southeastern and northwestern molecular complexes. The application of radiative transfer large velocity gradient models reveals that the bulk of the ammonia molecules is embedded in a one-temperature gas phase. Kinetic temperatures of this gas are ~200 and 140 K toward the southwest and northeast, respectively. The temperatures under which ammonia was formed in the past are with 30 K also warmer toward the southwest than toward the northeast (~15-20 K). This is indicated by the ortho-ammonia-to-para-ammonia ratio, which is ~1 and 1.5-2.5 toward the southwest and northeast, respectively. Ammonia column densities in the brightest complexes are in the range of (6-11) × 1014 cm-2, which adds up to a total ammonia mass of ~20 M☉, about evenly distributed toward both sides of the nucleus. Ammonia abundances relative to H2 are ~3 × 10-8. In the southwestern complex, the ammonia abundances increase from the starburst center to larger galactocentric radii. Toward the center of NGC 253, NH3 (1, 1), (2, 2), and (6, 6) are detected in absorption against an unresolved continuum source. At the same location, however, ammonia (3, 3) is found in emission, which indicates maser activity. This would be the first detected extragalactic NH3 maser. Evidence for an expanding shell in the southwestern complex is provided. The shell, with a dynamical age of ~1.3 Myr, is centered on an X-ray point source that must be located within the dense gas of NGC 253. The shell and X-ray properties can be reproduced by the energy input of a highly obscured young stellar cluster with a mass of ~105 M☉, which also heats the dense gas. A current star formation rate of ~2.8 M☉ yr-1 is derived for the nuclear starburst in NGC 253 based on its 1.2 cm continuum emission.

A Chandra X-ray survey of nearby dwarf starburst galaxies: II. Starburst properties and outflows

Abstract: We present a comprehensive comparison of the X-ray properties of a sample of eight dwarf starburst galaxies observed with Chandra (IZw18, VIIZw403, NGC1569, NGC3077, NGC4214, NGC4449, NGC5253, He2–10). In PaperI we presented in detail the data reduction and analysis of the individual galaxies. For the unresolved X-ray sources we find the following: point sources are in general located close to bright H ii regions, rims of superbubbles, or young stellar clusters. The number of X-ray point sources appears to be a function of the current star formation rate and the blue luminosity of the hosts. Ultraluminous X-ray sources are only found in those dwarf galaxies which are currently interacting. The power law index of the combined cumulative X-ray point source luminosity function is α = 0.24 ± 0.06, shallower than that of more massive starburst galaxies (α = 0.4 − 0.8) and of non-starburst galaxies (α ∼ 1.2). For those galaxies showing extended X-ray emission (6 out of the 8 galaxies), we derive the following: Superwinds develop along the steepest gradient of the H i distribution with volume densities of 0.02 −0.06cm 3 , pressures of 1 −3 ×10 5 Kcm 3 , thermal energies of 2 − 30 × 10 54 erg, and hot gas masses of 2 − 20 × 10 6 M⊙ (∼ 1 per cent of the H i masses. On global scales, the distribution of the X-ray emission looks remarkably similar to that seen in Hα (comparing azimuthal averages); locally however their distribution is clearly distinct in many cases – this can be explained by the different emission mechanisms (forward vs. reverse shocks). Mass-loading of order 1 to 5 is required to explain the differences between the amount of hot gas and and the modelled mass-loss from massive stars. The metallicity of the dwarf galaxies correlates with the diffuse X-ray luminosity and anti-correlates with the cooling time of the hot gas. The diffuse X-ray luminosity is also a function of the current star formation rate. The mechanical luminosities of the developing superwinds are energetic enough to overcome the gravitational potentials of their host galaxies. This scenario is supported by the overpressures of the hot gas compared to the ambient ISM. Extended H i envelopes such as tidal tails, however, may delay outflows on timescales exceeding those of the cooling time of the hot gas.

Atomic carbon at redshift ~2.5

Abstract: Using the IRAM 30 m telescope we detected the lower fine structure line of neutral carbon (CI( 3 P 1 → 3 P 0 ), ν rest = 492 GHz) towards three high-redshift sources: IRAS FSC 10214 (z = 2.3), sMMJ14011+0252 (z = 2.5) and H1413+117 (Cloverleaf quasar, z = 2.5). SMM J14011+0252 is the first high-redshift, non-AGN source in which Cl has been detected. The C I( 3 P 1 → 3 P 0 ) line from FSC 10214 is almost an order of magnitude weaker than previously claimed, while our detection in the Cloverleaf is in good agreement with earlier observations. The C 1( 3 P 1 → 3 P 0 ) linewidths are similar to the CO widths, indicating that both lines trace similar regions of molecular gas on galactic scales. Derived C I( 3 P 1 → 3 P 0 ) masses for all three objects are of order few ×10 7 M ○. and the implied CI( 3 P 1 → 3 P 0 ) 12 CO(J = 3 → 2) line luminosity ratio is about 0.2. This number is similar to values found in local galaxies. We derive a C abundance of 5 x 10 -5 which implies significant metal enrichment of the cold molecular gas at redshifts 2.5 (age of the universe 2.7 Gyr). We conclude that the physical properties of systems at large lookback times are similar to today's starburst/AGN environments.

Multiple CO lines in SMM J16359+6612 -- Further evidence for a merger

Abstract: Using the IRAM 30m telescope, we report the detection of the CO(3--2), CO(4--3), CO(5--4) and CO(6--5) lines in the gravitational lensed submm galaxy SMM J16359+6612 at z=2.5. The CO lines have a double peak profile in all transitions. From a Gaussian decomposition of the spectra we show that the CO line ratios, and therefore the underlying physical conditions of the gas, are similar for the blue and the redshifted component. The CO line Spectral Energy Distribution (SED; i.e. flux density vs. rotational quantum number) turns over already at the CO(5--4) transition which shows that the molecular gas is less excited than in nearby starburst galaxies and high--z QSOs. This difference mainly arises from a lower average H2 density, which indicates that the gas is less centrally concentrated than in nuclear starburst regions in local galaxies. We suggest that the bulk of the molecular gas in SMM J16359+6612 may arise from an overlap region of two merging galaxies. The low gas density and clear velocity separation may reflect an evolutionary stage of the merger event that is in between those seen in the Antennae and in the more evolved ultraluminous infrared galaxies (ULIRGs) like e.g. Mrk231.

Intraday Variability of Sagittarius A* at 3 Millimeters

Abstract: We report observations and analysis of flux monitoring of Sagittarius A* at 3 mm wavelength using the Owens Valley Radio Observatory millimeter interferometer over a period of 8 days (2002 May 23-30). Frequent phase and flux referencing (every 5 minutes) with the nearby calibrator source J1744-312 was employed to control for instrumental and atmospheric effects. Time variations are sought by computing and subtracting, from each visibility in the database, an average visibility obtained from all the data acquired in our monitoring program having similar u-v spacings. This removes the confusing effects of baseline-dependent, correlated flux interference caused by the static, thermal emission from the extended source Sgr A West. Few-day variations up to ~20% and intraday variability of ~20% and in some cases up to ~40% on few-hour timescales emerge from the differenced data on Sgr A*. Power spectra of the residuals indicate the presence of hourly variations on all but 2 of the 8 days. Monte Carlo simulation of red-noise light curves indicates that the hourly variations are well described by a red-noise power spectrum with P(f) ∝ f-1. Of particular interest is an ~2.5 hr variation seen prominently on 2 consecutive days. An average power spectrum from all 8 days of data reveals noteworthy power on this timescale. There is some indication that few-hour variations are more pronounced on days when the average daily flux is highest. We briefly discuss the possibility that these few-hour variations are due to the dynamical modulation of accreting gas around the central supermassive black hole as well as the implications for the structure of the Sgr A* photosphere at 3 mm. Finally, these data have enabled us to produce a high-sensitivity 3 mm map of the extended thermal emission surrounding Sgr A*.

Multiple CO lines in SMM J16359+6612 -- Further evidence for a merger

Abstract: Using the IRAM 30 m telescope, we report the detection of the CO(3–2), CO(4–3), CO(5–4) and CO(6–5) lines in the gravitational lensed submm galaxy SMM J16359+6612 at $z=2.5$. The CO lines have a double peak profile in all transitions. From a Gaussian decomposition of the spectra we show that the CO line ratios, and therefore the underlying physical conditions of the gas, are similar for the blue and the redshifted component. The CO line Spectral Energy Distribution (SED; i.e. flux density vs. rotational quantum number) turns over already at the CO 5–4 transition which shows that the molecular gas is less excited than in nearby starburst galaxies and high- z QSOs. This difference mainly arises from a lower average H 2 density, which indicates that the gas is less centrally concentrated than in nuclear starburst regions in local galaxies. We suggest that the bulk of the molecular gas in SMM J16359+6612 may arise from an overlap region of two merging galaxies. The low gas density and clear velocity separation may reflect an evolutionary stage of the merger event that is in between those seen in the Antennae and in the more evolved ultraluminous infrared galaxies (ULIRGs) like e.g. Mrk 231.

Intra-day Variability of Sagittarius A* at 3 Millimeters

Abstract: We report observations and analysis of flux monitoring of Sagittarius A* at 3-mm wavelength using the OVRO millimeter interferometer over a period of eight days (2002 May 23-30). Frequent phase and flux referencing (every 5 minutes) with the nearby calibrator source J1744-312 was employed to control for instrumental and atmospheric effects. Time variations are sought by computing and subtracting, from each visibility in the database, an average visibility obtained from all the data acquired in our monitoring program having similar uv spacings. This removes the confusing effects of baseline-dependent, correlated flux interference caused by the static, thermal emission from the extended source Sgr A West. Few-day variations up to ~20% and intra-day variability of \~20% and in some cases up to ~40% on few-hour time scales emerge from the differenced data on SgrA*. Power spectra of the residuals indicate the presence of hourly variations on all but two of the eight days. Monte Carlo simulation of red-noise light curves indicates that the hourly variations are well described by a red-noise power spectrum with P(f) ~ f^(-1). Of particular interest is a ~2.5 hour variation seen prominently on two consecutive days. An average power spectrum from all eight days of data reveals noteworthy power on this time scale. There is some indication that few-hour variations are more pronounced on days when the average daily flux is highest. We briefly discuss the possibility that these few-hour variations are due to the dynamical modulation of accreting gas around the central supermassive black hole, as well as the implications for the structure of the SgrA* photosphere at 3 mm. Finally, these data have enabled us to produce a high sensitivity 3-mm map of the extended thermal emission surrounding SgrA*.

A Search for Dense Molecular Gas in High-Redshift Infrared-Luminous Galaxies

Abstract: We present a search for HCN emission from four high redshift far infrared (IR) luminous galaxies. Current data and models suggest that these high $z$ IR luminous galaxies represent a major starburst phase in the formation of spheroidal galaxies, although many of the sources also host luminous active galactic nuclei (AGN), such that a contribution to the dust heating by the AGN cannot be precluded. HCN emission is a star formation indicator, tracing dense molecular hydrogen gas within star-forming molecular clouds (n(H$_2$) $\\sim 10^5$ cm$^{-3}$). HCN luminosity is linearly correlated with IR luminosity for low redshift galaxies, unlike CO emission which can also trace gas at much lower density. We report a marginal detection of HCN (1-0) emission from the $z=2.5832$ QSO J1409+5628, with a velocity integrated line luminosity of $L_{\\rm HCN}\'=6.7\\pm2.2 \\times10^{9}$ K km s$^{-1}$ pc$^2$, while we obtain 3$\\sigma$ upper limits to the HCN luminosity of the $z=3.200$ QSO J0751+2716 of $L_{\\rm HCN}\'=1.0\\times10^{9}$ K km s$^{-1}$ pc$^2$, $L_{\\rm HCN}\'=1.6\\times10^{9}$ K km s$^{-1}$ pc$^2$ for the $z= 2.565$ starburst galaxy J1401+0252, and $L_{\\rm HCN}\'=1.0\\times10^{10}$ K km s$^{-1}$ pc$^2$ for the $z = 6.42$ QSO J1148+5251. We compare the HCN data on these sources, plus three other high-$z$ IR luminous galaxies, to observations of lower redshift star-forming galaxies. The values of the HCN/far-IR luminosity ratios (or limits) for all the high $z$ sources are within the scatter of the relationship between HCN and far-IR emission for low $z$ star-forming galaxies (truncated).

A Chandra X-ray survey of nearby dwarf starburst galaxies - I. Data reduction and results

Abstract: We present an analysis of Chandra X-ray observations of a sample of eight dwarf starburst galaxies (IZw18, VIIZw403, NGC1569, NGC3077, NGC4214, NGC4449, NGC5253, and He2–10). Extended, diffuse X-ray emission is detected in all but two of the objects. Unresolved sources were found within all dwarf galaxies (total: 55 sources). These point sources are well fit by power law, thermal plasma, or black body models. Ten of the point sources exceed an X-ray luminosity of 10 39 erg s 1 (ultraluminous X-ray sources). In those galaxies where diffuse X-ray emission is detected, this emission (with X-ray luminosities ranging from 4 × 10 38 erg s 1 to 2 × 10 40 erg s 1 ) contains most (60-80 per cent) of the X-ray photons. This diffuse emission can be well fit by MeKaL one-temperature thermal plasma models once the contribution from the unresolved point sources is subtracted properly. The diffuse X-ray component is significantly extended, reaching as far as 0.5 5kpc into the outskirts of their hosts. Azimuthally averaged X-ray surface brightness profiles are well approximated by exponential functions. Temperatures of various regions within the galaxies range from 1.6 7.6 × 10 6 K. With few exceptions, temperatures of the hot gas are remarkably uniform, hovering around 2 3×10 6 K. Temperatures of the coronal gas in the outer regions are in general � 2 3 times lower than those found in the central regions. Fits to the diffuse emission do not allow strong constraints to be put on the metallicities of the emitting plasmas. However, the derived metallicities are compatible with those determined from their H ii regions. An α/F e ratio of � 2 is indicated for the hot gas within at least three objects (NGC1569, NGC4449, and He2–10). Shadowing of the diffuse X-ray emission by the cooler disc gas is used to constrain the orientation of the galaxies.

The Star Formation Threshold in NGC 6822

Abstract: We investigate the star formation threshold in NGC 6822, a nearby Local Group dwarf galaxy, on sub-kpc scales using high-resolution, wide-field, deep HI, Halpha and optical data. In a study of the HI velocity profiles we identify a cool and warm neutral component in the Interstellar Medium of NGC 6822. We show that the velocity dispersion of the cool component (~4 km/s) when used with a Toomre-Q criterion gives an optimal description of ongoing star formation in NGC 6822, superior to that using the more conventional dispersion value of 6 km/s. However, a simple constant surface density criterion for star formation gives an equally superior description. We also investigate the two-dimensional distribution of Q and the star formation threshold and find that these results also hold locally. The range in gas density in NGC 6822 is much larger than the range in critical density, and we argue that the conditions for star formation in NGC 6822 are fully driven by this density criterion. Star formation is local, and in NGC 6822 global rotational or shear parameters are apparently not important.

The Temperature Distribution of Dense Molecular Gas in the Center of NGC 253

Abstract: [abridged] We present interferometric maps of ammonia (NH3) of the nearby starburst galaxy NGC 253 [star formation rate: ~2.8 Mo yr^(-1)]. The observations have been taken with the Australia Telescope Compact Array and include the para-NH3 (1,1), (2,2), and the ortho-NH3 (3,3) and (6,6) inversion lines. Six major complexes of dense ammonia are identified, three of them on either side of the starburst center, out to projected galactocentric radii of \~250 pc. [...] The application of radiative transfer large velocity gradient models reveals that the bulk of the ammonia molecules is embedded in a one-temperature gas phase. Kinetic temperatures of this gas are ~200 and 140 K toward the south-west and north-east [of the nucleus of NGC 253], respectively. The temperatures under which ammonia was formed in the past are with >~30 K also warmer toward the south-west than toward the north-east (~15-20 K). This is indicated by the ortho-to-para ammonia ratio which is ~1 and 1.5-2.5 toward the south-west and north-east, respectively. Ammonia column densities in the brightest complexes are in the range of 6-11x10^(14) cm^(-2), which adds up to a total ammonia mass of ~20 Mo, about evenly distributed toward both sides of the nucleus. [...] Toward the center of NGC 253, NH3 (1,1), (2,2), and (6,6) is detected in absorption against an unresolved continuum source. At the same location, however, ammonia (3,3) is found in emission which indicates maser activity. This would be the first detected extragalactic NH3 maser. Evidence for an expanding shell in the south-western complex is provided. [...] The shell and X-ray properties can be reproduced by the energy input of a highly obscured young stellar cluster with a mass of ~10^5 Mo which also heats the dense gas.

Spitzer Observations of the Supergiant Shell Region in IC 2574

Abstract: We present spatially resolved Spitzer Space Telescope imaging of the supergiant shell region of the M81 group dwarf galaxy IC 2574 obtained as part of the Spitzer Infrared Nearby Galaxies Survey. This region harbors one of the best nearby examples of a kinematically distinct H I shell, with an associated remnant stellar cluster; the shell is initiating sequential star formation as it interacts with the surrounding interstellar medium. This region dominates the infrared luminosity of IC 2574 and is spatially resolved in all Spitzer imaging bands. We study the differences in dust temperature as a function of local environment and compare local star formation rates as inferred from Hα and total infrared luminosities. We find that the strong Hα sources are associated with regions of warm dust; however, the most luminous infrared and Hα sources are not necessarily cospatial. The coolest dust is found in the regions farthest from the rim of the shell; these regions show the best agreement between star formation rates derived from Hα and from total infrared luminosities (although discrepancies at the factor of 3-4 level still exist). There is considerable variation in the radio-far-infrared correlation in different regions surrounding the shell. The low dust content of the region may influence the scatter seen in these relations; these data demonstrate that the expanding shell is dramatically affecting its surroundings by triggering star formation and altering the dust temperature.

The Nature of Radio Continuum Emission at Very Low Metallicity: Very Large Array Observations of I Zw 18

Abstract: We present the first resolved study of the radio continuum properties of I Zw 18, the dwarf galaxy with the lowest known nebular metal abundance in the local universe. New Very Large Array radio continuum images at 20 and 3.6 cm are compared to various Hubble Space Telescope images, and we find a striking morphological similarity between high-resolution Hα and short-wavelength radio continuum emission, especially in the Hα shell in the northwest region. We separate thermal and nonthermal components of the emission and find a large synchrotron halo surrounding the galaxy. Comparison between Hα and X-band fluxes suggests that the emission at 3.6 cm is dominated by thermal processes; an additional synchrotron component dominates the flux at 20 cm and produces a modest fraction of the detected flux at 3.6 cm. The fluxes of three of the four major emission peaks show a mix of thermal and nonthermal processes, while one shows a nearly flat spectral index. The strong synchrotron component argues for active star formation throughout the disk for at least the last ~30 Myr. These sensitive observations provide a new, detailed view of the nature of radio continuum emission in the very low metallicity interstellar medium. Comparing with the literature, the role of metallicity in the evolution of radio continuum emission seems to be secondary to other factors such as the recent star formation history and the presence or absence of outflows from star formation regions.

VLA Imaging of the Intriguing HI Cloud HIJASS J1021+6842 in the M81 Group

Abstract: We present VLA HI 21cm observations of HIJASS J1021+6842 which has been discovered in the direction of the M81 group. Our synthesis imaging reveals that the HI is distributed over a larger angular extent and velocity range than the single dish discovery observations. Assuming that HIJASS J1021+6842 is at the distance of the M81 group, we detect 1.5 x 10^8 M_sun of HI distributed over as much as 30 kpc, i.e., substantially larger than the biggest dwarf galaxies in the same group. At the depth of our imaging, the HI appears to be confined to at least 7 clouds. Peak HI column densities are ~1.8 x 10^20 atoms cm^-2 which is well below the canonical star formation threshold of ~10^21 atoms cm^-2 and therefore consistent with the fact that no optical counterpart has as yet been identified. A gradient in velocity is observed across the extent of the detected HI; assuming that the object is gravitationally bound we derive a dynamical mass of 7 x 10^9 M_sun and a dark-to-luminous mass ratio of >10. Alternatively, a tidal origin may also result in the observed velocity gradient which would lead to a considerably lower dynamical mass. Given the above properties and the absence of evidence of a stellar population, HIJASS J1021+6842 is unique amongst the other systems in the M81 group.

VLA Imaging of the Intriguing H I Cloud HIJASS J1021+6842 in the M81 Group

Abstract: We present VLA H i 21 cm observations of HIJASS J10216842, which has been discovered in the direction of the M81 Group. Our synthesis imaging reveals that the H i is distributed over a larger angular extent and velocity range than the single-dish discovery observations. Assuming that HIJASS J10216842 is at the distance of the M81 Group, we detect M, of H i distributed over as much as 30 kpc, i.e., substantially larger 8 1.5 # 10 than the biggest dwarf galaxies in the same group. At the depth of our imaging, the H i appears to be confined to at least seven clouds. Peak H i column densities are ∼ atoms cm 2 , which is well below the canonical 20 1.8 # 10 star formation threshold of ∼10 21 atoms cm 2 and therefore consistent with the fact that no optical counterpart has as yet been identified. A gradient in velocity is observed across the extent of the detected H i; assuming that the object is gravitationally bound we derive a dynamical mass of M, and a dark-to-luminous mass 9 7 # 10 ratio of 110. Alternatively, a tidal origin may also result in the observed velocity gradient, which would lead to a considerably lower dynamical mass. Given the above properties and the absence of evidence of a stellar population, HIJASS J10216842 is unique amongst the other systems in the M81 Group. Subject headings: galaxies: clusters: general — galaxies: dwarf — galaxies: formation — galaxies: individual (HIJASS J10216842) — radio lines: galaxies

Intra-day Variability of Sagittarius Aat 3 Millimeters

Abstract: We report observations and analysis of flux monitoring of Sagittarius A ∗ at 3mm wavelength using the OVRO millimeter interferometer over a period of eight days (2002 May 23–30). Frequent phase and flux referencing (every 5 minutes) with the nearby calibrator source J1744 312 was employed to control for instrumental and atmospheric effects. Time variations are sought by computing and subtracting, from each visibility in the database, an average visibility obtained from all the data acquired in our monitoring program having similar uv spacings. This removes the confusing effects of baseline-dependent, correlated flux interference caused by the static, thermal emission from the extended source Sgr A West. Few-day variations up to � 20% and intra-day variability of � 20% and in some cases up to � 40% on few-hour time scales emerge from the differenced data on SgrA ∗ . Power spectra of the residuals indicate the presence of hourly variations on all but two of the eight days. Monte Carlo simulation of red-noise light curves indicates that the hourly variations are well described by a red-noise power spectrum with P(f) / f −1 . Of particular interest is a � 2.5 hour variation seen prominently on two consecutive days. An average power spectrum from all eight days of data reveals noteworthy power on this time scale. There is some indication that few-hour variations are more pronounced on days when the average daily flux is highest. We briefly discuss the possibility that these few-hour variations are due to the dynamical modulation of accreting gas around the central supermassive black hole, as well as the implications for the structure of the SgrA ∗ photosphere at 3 mm. Finally, these data have enabled us to produce a high sensitivity 3-mm map of the extended thermal emission surrounding SgrA ∗ .

First Results from THINGS: The HI Nearby Galaxy Survey

Abstract: Original paper can be found at: http://www.astrosociety.org/pubs/cs/328.html--Copyright Astronomical Society of the Pacific

Infrared Spectral Energy Distributions of Nearby Galaxies

Abstract: The Spitzer Infrared Nearby Galaxies Survey (SINGS) is carrying out a comprehensive multiwavelength survey on a sample of 75 nearby galaxies. The 1‐850 ! m spectral energy distributions (SEDs) are presented using broadband imaging data from Spitzer, 2MASS, ISO, IRAS, and SCUBA. The infrared colors derived from the globally integrated Spitzer data are generally consistent with the previous generation of models that were developed using global data for normal star-forming galaxies, although significant deviations are observed. Spitzer’s excellent sensitivity and resolution also allow a detailed investigation of the infrared SEDs for various locations within the three large, nearby galaxies NGC 3031 (M81), NGC 5194 (M51), and NGC 7331. Awide variety of spectral shapes is foundwithineachgalaxy,especiallyforNGC3031,theclosestofthethreetargetsandthusthegalaxyforwhichthe smallest spatial scales can be explored. Strong correlations exist between the local star formation rate and the infrared colors f" (70 ! m)/f" (160 ! m) and f" (24 ! m)/f" (160 ! m), suggesting that the 24 and 70 ! m emission are useful tracers of the local star formation activity level. Preliminary evidence indicates that variations in the 24 ! m emission,and not variations in the emission from polycyclic aromatic hydrocarbons at 8 ! m, drive the variations in the f" (8:0 ! m)/f" (24 ! m) colors within NGC 3031, NGC 5194, and NGC 7331. If the galaxy-to-galaxy variations in SEDs seen in our sample are representative of the range present at high redshift, then extrapolations of total infrared luminosities and star formation rates from the observed 24 ! m flux will be uncertain at the factor of 5 level (total range). The corresponding uncertainties using the redshifted 8.0 ! m flux (e.g., observed 24 ! m flux for a z ! 2 source) are factors of 10‐20. Considerable caution should be used when interpreting such extrapolated infrared luminosities. Subject headingg: infrared: galaxies — infrared: ISM Online material: color figures

The Nature Of Radio Continuum Emission At Very Low Metallicity: Very Large Array Observations of I Zw 18

Abstract: We present the first resolved study of the radio continuum properties of I Zw 18, the dwarf galaxy with the lowest known nebular metal abundance in the local universe. New Very Large Array radio continuum images at 20 and 3.6 cm are compared to various Hubble Space Telescope images, and we find a striking morphological similarity between high-resolution H alpha and short wavelength radio continuum emission, especially in the H alpha shell in the northwest region. We separate thermal and nonthermal components of the emission, and find a large synchrotron halo surrounding the galaxy. Comparison between H alpha and X-band fluxes suggests that the emission at 3.6 cm is dominated by thermal processes; an additional synchrotron component dominates the flux at 20 cm and produces a modest fraction of the detected flux at 3.6 cm. The fluxes of three of the four major emission peaks show a mix of thermal and nonthermal processes, while one shows a nearly flat spectral index. The strong synchrotron component argues for active star formation throughout the disk for at least the last ~ 30 Myr. These sensitive observations provide a new, detailed view of the nature of radio continuum emission in the very low metallicity interstellar medium. Comparing with the literature, the role of metallicity in the evolution of radio continuum emission seems to be secondary to other factors such as the recent star formation history and the presence or absence of outflows from star formation regions.

Resolved Molecular Gas Emission in a QSO Host Galaxy at Z = 6.4

Abstract: We present high-resolution VLA observations of the molecular gas in the host galaxy of the highest-redshift quasar currently known, SDSS J1148+5251 (z = 642) Our VLA data of the CO(3−2) emission have a maximum resolution of 017 × 013 arcsec2 (≤ 1 kpc2), and enable us to resolve the molecular gas emission both spatially and in velocity The molecular gas in J1148+5251 is extended to a radius of 25 kpc, and the central region shows 2 peaks, separated by 03 arcsec (17 kpc) Assuming that the molecular gas is gravitationally bound, we estimate a dynamical mass of ∼ 45 × 1010M☉ within a radius of 25 kpc This dynamical mass estimate leaves little room for matter other than the detected molecular gas, and — in particular — the data are inconsistent with a ∼ 1012 M☉ stellar bulge, which would be predicted based on the M BH —σ bulge relation

The dwarf galaxy DDO 47 as a dark matter laboratory: testing cusps hiding in triaxial halos

Abstract: We present an analysis of high resolution HI data of the dwarf galaxy DDO 47, aimed at testing the hypothesis that dark halo triaxiality might induce non-circular motions resulting in rotation curves best fitted by cored halos, even if the dark matter halo is intrinsically cuspy. This hypothesis could be invoked in order to reconcile the predictions of the standard LambdaCDM theory with the rotation curves of disk galaxies. DDO 47 is an ideal case to test this hypothesis because it has a very regular velocity field, its rotation curve is best fitted by a cored halo and a NFW halo is inconsistent with the data. We analysed the velocity field through the higher-order harmonic terms in order to search for kinematical signatures of alleged non-circular motions needed to "hide" a cusp: the result is that globally non-circular motions are at a level of 2-3 km/s, and they are more likely to be associated to the presence of some spiral structure than to a global elongated potential (e.g. a triaxial halo). These non-circular motions are far from being sufficient to account for the discrepancy with the LambdaCDM predictions. We therefore conclude that the dark matter halo around the dwarf galaxy DDO 47 is truly cored and that a cusp cannot be hidden by non-circular motions.