Adam K. Leroy

University of Virginia, Charlottesville, Virginia, United States

Are you Adam K. Leroy?

Claim your profile

Publications (155)565.93 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We use a sample of 36 galaxies from the KINGFISH (Herschel IR), HERACLES (IRAM CO), and THINGS (VLA HI) surveys to study empirical relations between Herschel infrared (IR) luminosities and the total mass of the interstellar gas (H2+HI). Such a comparison provides a simple empirical relationship without introducing the uncertainty of dust model fitting. We find tight correlations, and provide fits to these relations, between Herschel luminosities and the total gas mass integrated over entire galaxies, with the tightest, almost linear, correlation found for the longest wavelength data (SPIRE500). However, we find that accounting for the gas-phase metallicity (affecting the dust-to-gas ratio) is crucial when applying these relations to low-mass, and presumably high-redshift, galaxies. The molecular (H2) gas mass is found to be better correlated with the peak of the IR emission (e.g. PACS160), driven mostly by the correlation of stellar mass and mean dust temperature. When examining these relations as a function of galactocentric radius we find the same correlations, albeit with a larger scatter, up to a radius of 0.7 r_25 (within which most of the galaxy's baryonic mass resides). However, beyond this radius the same correlations no longer hold, with the gas mass (predominantly HI) increasing relative to the infrared emission. The tight relations found for the bulk of the galaxy's baryonic content suggest that the total gas masses of disk-like (non-merging) galaxies can be inferred from far-infrared continuum measurements in situations where only the latter are available, e.g. in ALMA continuum observations of high-redshift galaxies.
    11/2014;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We use ALMA to derive the mass, length, and time scales associated with the nuclear starburst in NGC 253. This region forms ~2 M_sun/yr of stars and resembles other starbursts in scaling relations, with star formation consuming the gas reservoir 10 times faster than in galaxy disks. We present observations of CO, the high effective density transitions HCN(1-0), HCO+(1-0), CS(2-1), and their isotopologues. We identify ten clouds that appear as peaks in line emission and enhancements in the HCN-to-CO ratio. These clouds are massive (~10^7 M_sun) structures with sizes (~30 pc) similar to GMCs in other systems. Compared to disk galaxy GMCs, they show high line widths (~20-40 km/s) given their size, with implied Mach numbers ~90. The clouds also show high surface (~6,000 M_sun/pc^2) and volume densities (n_H2~2,000 cm^-3). Given these, self-gravity can explain the line widths. This short free fall time (~0.7 Myr) helps explain the more efficient star formation in NGC 253. We also consider the starburst region as a whole. The geometry is confused by the high inclination, but simple models support a non-axisymmetric, bar-like geometry with a compact, clumpy region of high gas density embedded in an extended CO distribution. Even for the whole region, the surface density still exceeds that of a disk galaxy GMC. The orbital time (~10 Myr), disk free fall time (<~ 3 Myr), and disk crossing time (<~ 3 Myr) are each much shorter than in a normal spiral galaxy disk. Some but not all aspects of the structure correspond to predictions from assuming vertical dynamical equilibrium or a marginally stable rotating disk. Finally, the CO-to-H2 conversion factor implied by our cloud calculations is approximately Galactic, contrasting with results showing a low value for the whole starburst region. The contrast provides resolved support for the idea of mixed molecular ISM phases in starburst galaxies.
    11/2014;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present new Karl G. Jansky Very Large Array radio continuum images of the nuclei of Arp 220, the nearest ultra-luminous infrared galaxy. These images have both the angular resolution to study detailed morphologies of the two nuclei that power the system and sensitivity to a wide range of spatial scales. At 33 GHz, and with a resolution of 0".081 x 0".063 (29.9 x 23.3 pc), we resolve the emission surrounding both nuclei and conclude that is mostly synchrotron in nature. The spatial distributions of radio emission in both nuclei are well described by exponential profiles. These have deconvolved half-light radii of 51 and 35 pc for the eastern and western nuclei, and they match the number density profile of radio supernovae observed with very long baseline interferometry. This similarity might be due to the fast cooling of cosmic rays electrons caused by the presence of a strong (~ mG) magnetic field in this system. We estimate high luminosity surface densities of $\mathrm{\Sigma_{IR} \sim 4.2^{+1.6}_{-0.7} \times 10^{13}}$ (east) and $\mathrm{\sim 9.7^{+3.7}_{-2.4} \times 10^{13}~(west)~L_{\odot}~kpc^{-2}}$, and star formation rate surface densities of $\mathrm{\Sigma_{SFR} \sim 10^{3.7\pm0.1}}$ (east) and $\mathrm{\sim 10^{4.1\pm0.1}~(west)~M_{\odot}~yr^{-1}~kpc^{-2}}$. These values, especially for the western nucleus are, to our knowledge, the highest luminosity and star formation rate surface densities measured for any star-forming system. Despite these high values, the nuclei lie below the dusty Eddington limit in which radiation pressure is balanced only by self-gravity. The small measured sizes also imply that the nuclei of Arp 220 are only transparent in the frequency range ~ 5 to 350 GHz. Our results offer no clear evidence that an active galactic nucleus dominates the emission from either nucleus at 33 GHz.
    11/2014;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The [CII] 158 micron line is one of the strongest emission lines observed in star-forming galaxies, and has been empirically measured to correlate with the star formation rate (SFR) globally and on ~kpc scales. However, due to the multi-phase origins of [CII], one might expect this relation to break down at small scales. We investigate the origins of [CII] emission by examining high spatial resolution observations of [CII] in M31, with the Survey of Lines in M31 (SLIM). We present five ~700x700 pc (3"x3") Fields mapping the [CII] emission, Halpha emission, combined with ancillary infrared (IR) data. We spatially separate star-forming regions from diffuse gas and dust emission on ~50 pc scales. We find that the [CII] - SFR correlation holds even at these scales, although the relation typically has a flatter slope than found at larger (~kpc) scales. While the Halpha emission in M31 is concentrated in the SFR regions, we find that a significant amount (~20-90%) of the [CII] emission comes from outside star-forming regions, and that the total IR (TIR) emission has the highest diffuse fraction of all SFR tracers. We find a weak correlation of the [CII]/TIR to dust color in each Field, and find a large scale trend of increasing [CII]/TIR with galactocentric radius. The differences in the relative diffuse fractions of [CII], Halpha and IR tracers are likely caused by a combination of energetic photon leakage from HII regions and heating by the diffuse radiation field arising from older (B-star) stellar populations. However, we find that by averaging our measurements over ~kpc scales, these effects are minimized, and the relation between [CII] and SFR found in other nearby galaxy studies is retrieved.
    10/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present the highest spatial resolution (~0.5") CO (3-2) observations to date of the "overlap" region in the merging Antennae galaxies (NGC 4038/39), taken with the ALMA. We report on the discovery of a long (3 kpc), thin (aspect ratio 30/1), filament of CO gas which breaks up into roughly ten individual knots. Each individual knot has a low internal velocity dispersion (~10 km/s), and the dispersion of the ensemble of knots in the filament is also low (~10 km/s). At the other extreme, we find that the individual clouds in the Super Giant Molecular Cloud 2 region discussed by Wilson and collaborators have a large range of internal velocity dispersions (10 to 80 km/s), and a large dispersion amongst the ensemble (~80 km/s). We use a combination of optical and near-IR data from HST, radio continuum observations taken with the VLA, and CO data from ALMA to develop an evolutionary classification system which provides a framework for studying the sequence of star cluster formation and evolution, from diffuse SGMCs, to proto, embedded, emerging, young, and intermediate/old clusters. The relative timescales have been assessed by determining the fractional population of sources at each evolutionary stage. Using the evolutionary framework, we estimate the maximum age range of clusters in a single SGMC is ~10 Myr, which suggests that the molecular gas is removed over this timescale resulting in the cessation of star formation and the destruction of the GMC within a radius of about 200 pc. (abridged)
    The Astrophysical Journal 10/2014; 795(2). · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The [CII] 157.74 $\mu$m transition is the dominant coolant of the neutral interstellar gas, and has great potential as a star formation rate (SFR) tracer. Using the Herschel KINGFISH sample of 46 nearby galaxies, we investigate the relation of [CII] surface brightness and luminosity with SFR. We conclude that [CII] can be used for measurements of SFR on both global and kiloparsec scales in normal star-forming galaxies in the absence of strong active galactic nuclei (AGN). The uncertainty of the $\Sigma_{\rm [CII]}-\Sigma_{\rm SFR}$ calibration is $\pm$0.21 dex. The main source of scatter in the correlation is associated with regions that exhibit warm IR colors, and we provide an adjustment based on IR color that reduces the scatter. We show that the color-adjusted $\Sigma_{\rm[CII]}-\Sigma_{\rm SFR}$ correlation is valid over almost 5 orders of magnitude in $\Sigma_{\rm SFR}$, holding for both normal star-forming galaxies and non-AGN luminous infrared galaxies. Using [CII] luminosity instead of surface brightness to estimate SFR suffers from worse systematics, frequently underpredicting SFR in luminous infrared galaxies even after IR color adjustment (although this depends on the SFR measure employed). We suspect that surface brightness relations are better behaved than the luminosity relations because the former are more closely related to the local far-UV field strength, most likely the main parameter controlling the efficiency of the conversion of far-UV radiation into gas heating. A simple model based on Starburst99 population-synthesis code to connect SFR to [CII] finds that heating efficiencies are $1\%-3\%$ in normal galaxies.
    09/2014;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Observed HI accretion around nearby galaxies can only account for a fraction of the gas supply needed to sustain the currently observed star formation rates. It is possible that additional accretion happens in the form of low column density cold flows, as predicted by numerical simulations of galaxy formation. To contrain the presence and properties of such flows, we present deep HI observations obtained with the NRAO Green Bank Telescope of an area measuring 4 by 4 degrees around NGC 2403. These observations, with a 5 sigma detection limit of 2.4 x 10^18 cm^-2 over a 20 km/s linewidth, reveal the presence of a low-column density, extended cloud outside the main HI disk, about 17' (~16 kpc or ~2R25) to the NW of the center of the galaxy. The total HI mass of the cloud is 6.3 x 10^6 Msun, or 0.15 percent of the total HI mass of NGC 2403. The cloud is associated with an 8-kpc anomalous-velocity HI filament in the inner disk, previously observed in deep VLA observations by Fraternali et al. (2001, 2002). We discuss several scenarios for the origin of the cloud, and conclude that it is either accreting from the intergalactic medium, or is the result of a minor interaction with a neigbouring dwarf galaxy.
    Astronomy and Astrophysics 07/2014; 569. · 5.08 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We use observed radial profiles of mass surface densities of total, $\Sigma_g$, & molecular, $\Sigma_{\rm H2}$, gas, rotation velocity & star formation rate (SFR) surface density, $\Sigma_{\rm sfr}$, of the molecular-rich ($\Sigma_{\rm H2}\ge\Sigma_{\rm HI}/2$) regions of 16 nearby disk galaxies to test several star formation laws: a Kennicutt-Schmidt law, $\Sigma_{\rm sfr}=A_g\Sigma_{g,2}^{1.5}$; a Constant Molecular law, $\Sigma_{\rm sfr}=A_{\rm H2}\Sigma_{\rm H2,2}$; the turbulence-regulated laws of Krumholz & McKee (KM05) and Krumholz et al. (KMT09), a Gas-$\Omega$ law, $\Sigma_{\rm sfr}=B_\Omega\Sigma_g\Omega$; and a shear-driven GMC Collision law, $\Sigma_{\rm sfr}=B_{\rm CC}\Sigma_g\Omega(1-0.7\beta)$, where $\beta\equiv d {\rm ln} v_{\rm circ}/d {\rm ln} r$. If allowed one free normalization parameter for each galaxy, these laws predict the SFR with rms errors of factors of 1.4 - 1.8. If a single normalization parameter is used by each law for the entire galaxy sample, then rms errors range from factors of 1.5 - 2.1. Although the Constant Molecular law gives the smallest errors, the improvement over KMT, Kennicutt-Schmidt & GMC Collision laws is not especially significant, particularly given the different observational inputs that the laws utilize and the scope of included physics, which ranges from empirical relations to detailed treatment of interstellar medium processes. We next search for variation of star formation law parameters with local & global galactic dynamical properties of disk shear rate (related to $\beta$), rotation speed & presence of a bar. We demonstrate with high significance that higher shear rates enhance star formation efficiency per local orbital time. Such a trend is expected if GMC collisions play an important role in star formation, while an opposite trend would be expected if development of disk gravitational instabilities is the controlling physics.
    The Astrophysical Journal 04/2014; 787(1). · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The kinematic complexity and the favorable position of M51 on the sky make this galaxy an ideal target to test different theories of spiral arm dynamics. Taking advantage of the new high-resolution PdBI Arcsecond Whirlpool Survey data, we undertake a detailed kinematic study of M51 to characterize and quantify the origin and nature of the non-circular motions. Using a tilted-ring analysis supported by several other archival data sets, we update the estimation of M51's position angle (P.A. = (173 ± 3)°) and inclination (i = (22 ± 5)°). Harmonic decomposition of the high-resolution (~40 pc) CO velocity field shows the first kinematic evidence of an m = 3 wave in the inner disk of M51 with a corotation at R CR, m = 3 = 1.1 ± 0.1 kpc and a pattern speed of Ω p, m = 3 ≈ 140 km s-1 kpc-1. This mode seems to be excited by the nuclear bar, while the beat frequencies generated by the coupling between the m = 3 mode and the main spiral structure confirm its density-wave nature. We observe also a signature of an m = 1 mode that is likely responsible for the lopsidedness of M51 at small and large radii. We provide a simple method to estimate the radial variation of the amplitude of the spiral perturbation (V sp) attributed to the different modes. The main spiral arm structure has langV sprang = 50-70 km s-1, while the streaming velocity associated with the m = 1 and m = 3 modes is, in general, two times lower. Our joint analysis of H I and CO velocity fields at low and high spatial resolution reveals that the atomic and molecular gas phases respond differently to the spiral perturbation due to their different vertical distribution and emission morphology. Based on observations carried out with the IRAM Plateau de Bure Interferometer and 30 m telescope. IRAM is operated by INSY/CNRS (France), MPG (Germany), and IGN (Spain).
    The Astrophysical Journal 03/2014; 784(1):4. · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We study the spatially resolved Radio Continuum-Star Formation Rate (RC-SFR) relation using state-of-the-art star-formation (SF) tracers in a sample of 17 THINGS galaxies. We use hybrid Sigma_SFR maps (GALEX FUV plus Spitzer 24 mu), RC maps at 22/18 cm from the WSRT SINGS survey, and H-alpha maps to correct for thermal RC emission. We compare azimuthally averaged radial profiles of the RC and FUV/MIR-based Sigma_SFR maps and study pixel-by-pixel correlations at fixed linear scales of 1.2 and 0.7 kpc. The ratio of the integrated SFRs from the RC emission to that of the FUV/MIR-based SF tracers is R_int = 0.78 +/- 0.38, consistent with Condon's relation. We find a tight correlation between the radial profiles of the radio and FUV/MIR-based Sigma_SFR for the entire extent of the disk. The ratio R of the azimuthally averaged radio to FUV/MIR-based Sigma_SFR agrees with the integrated ratio with only small quasi-random fluctuations as function of radius. Pixel-by-pixel plots show a tight correlation in log-log diagrams of radio to FUV/MIR-based Sigma_SFR, with a typical standard deviation of a factor of two. Averaged over our sample we find (Sigma_SFR)_RC ~ (Sigma_SFR)_hyb^{0.63+/-0.25} implying that data points with high Sigma_SFR are relatively radio bright, whereas the reverse is true for low Sigma_SFR. We interpret this as a result of spectral ageing of CRe, which is supported by the radio spectral index: data points dominated by young CRe are relatively radio dim, those dominated by old CRe are relatively radio dim. The ratio of radio to FUV/MIR-based integrated SFR is independent of global galaxy parameters, suggesting that we can use RC emission as a universal SF tracer for galaxies, if we restrict ourselves to global or azimuthally averaged measurements. A magnetic field-SFR relation, B ~ SFR_hyb^{0.30+/-0.02}, holding both globally and locally, can explain our results. (abridged)
    02/2014; 147(5).
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Using data from the PdBI Arcsecond Whirlpool Survey (PAWS), we have generated the largest extragalactic giant molecular cloud (GMC) catalog to date, containing 1507 individual objects. GMCs in the inner M51 disk account for only 54% of the total 12CO(1-0) luminosity of the survey, but on average they exhibit physical properties similar to Galactic GMCs. We do not find a strong correlation between the GMC size and velocity dispersion, and a simple virial analysis suggests that ~30% of GMCs in M51 are unbound. We have analyzed the GMC properties within seven dynamically motivated galactic environments, finding that GMCs in the spiral arms and in the central region are brighter and have higher velocity dispersions than inter-arm clouds. Globally, the GMC mass distribution does not follow a simple power-law shape. Instead, we find that the shape of the mass distribution varies with galactic environment: the distribution is steeper in inter-arm region than in the spiral arms, and exhibits a sharp truncation at high masses for the nuclear bar region. We propose that the observed environmental variations in the GMC properties and mass distributions are a consequence of the combined action of large-scale dynamical processes and feedback from high-mass star formation. We describe some challenges of using existing GMC identification techniques for decomposing the 12CO(1-0) emission in molecule-rich environments, such as M51's inner disk. Based on observations carried out with the IRAM Plateau de Bure Interferometer and 30 m telescope. IRAM is operated by INSY/CNRS (France), MPG (Germany) and IGN (Spain).
    The Astrophysical Journal 01/2014; 784(1):3. · 6.73 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The [CII] 158 micron line is typically the brightest far-IR emission line from star-forming galaxies. To use this line as a tracer of star-formation and a diagnostic of ISM conditions, we must understand which phases of the ISM and what gas heating sources are contributing to it. As a massive, nearby galaxy, Andromeda is ideal for studying [CII] because we can resolve individual star-forming regions in the galaxy, but it is representative of more distant galaxies. To address the origins of [CII], we have assembled a unique set of observations including: [CII] 158 micron and [OI] 63 micron lines from Herschel PACS; fully sampled optical integral field spectroscopy from PPAK on the Calar Alto 3.5m, and Herschel dust continuum mapping from 70-500 microns. These observations span a range of conditions across Andromeda. We present first results on how [CII] correlates with the far-IR continuum on ~50 pc scales. In particular, we find that star-forming regions in M31 do not exhibit a "[CII] line deficit" even in regions where the dust is very warm. Using the optical line emission, we determine the fraction of [CII] emission spatially associated with star-forming regions. Our method implies a high fraction ~40-75% of [CII] emission is coming from diffuse regions. These diffuse regions appear to dominated by the UV interstellar radiation field, which we infer from the Pan-Chromatic Hubble Andromeda Treasury data to be dominated by B stars. Our results suggest that studies using [CII] to trace the massive star-formation rate must take into account the the contribution of older stellar populations in heating the ISM gas.
    01/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: One of the key questions in astronomy today is how galaxies accrete their gas and then convert that gas into stars. The HI Nearby Galaxies Survey (THINGS), conducted with the VLA, has made great strides towards answering these questions, but it lacked sensitivity to the extended, diffuse HI in and around galaxies. To rectify this problem, my collaborators and I have recently completed a Green Bank Telescope (GBT) HI survey of the THINGS galaxies. The GBT's clean beam, low system temperature and good angular resolution make it the ideal single-dish telescope for observing low column density HI. Our survey was capable of detecting HI emission from analogs to Lyman limit systems, provided it fills the GBT beam. I will present the first results from our search for low column density tidal features and infall from the ``cosmic web", and I will detail my future plans.
    01/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: In both the Milky Way and nearby galaxies, the presence of dense molecular gas is correlated with recent star formation, suggesting that the formation of this gas may represent a key regulating step in the star formation process. Testing this idea requires wide-area, high-resolution maps of dense molecular gas in galaxies to explore how local physical conditions drive dense gas formation. Until now, these observations have been limited by the faintness of dense gas tracers like HCN and HCO+, but new instruments like the 4mm receiver on Robert C. Byrd Green Bank Telescope (GBT) -- the largest single-dish millimeter telescope -- are poised to change this picture. We present GBT maps of the dense gas tracers HCN and HCO+ in the prototypical nearby starburst galaxy M82. The HCN and HCO+ in the disk of M82 correlates both with recent star formation and the diffuse molecular gas and shows kinematics consistent with a rotating torus. HCO+ emission is also associated with the outflow of molecular gas previously identified in CO. These observations mark the first time that dense molecular gas like HCO+ has been associated with an outflow in a nearby galaxy and suggests that the outflow of dense molecular gas from the center of galaxies like M82 may regulate the star formation globally. Finally, the CO-to-HCN and CO-to-HCO+ line ratios reveal that there is more dense gas at the center of M82, pointing to the starburst as a key driver of this relationship. These results establish that the GBT can efficiently map the dense molecular gas at 90 GHz in nearby galaxies; this capability will increase further with the 16-element feed array currently being built for the GBT.
    01/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Galaxies observed at redshift z > 6, when the Universe was less than a billion years old, thus far very rarely show evidence of the cold dust that accompanies star formation in the local Universe, where the dust-to-gas mass ratio is around one per cent. A prototypical example is the galaxy Himiko (z = 6.6), which-a mere 840 million years after the Big Bang-is forming stars at a rate of 30-100 solar masses per year, yielding a mass assembly time of about 150 × 10(6) years. Himiko is thought to have a low fraction (2-3 per cent of the Sun's) of elements heavier than helium (low metallicity), and although its gas mass cannot yet be determined its dust-to-stellar mass ratio is constrained to be less than 0.05 per cent. The local dwarf galaxy I Zwicky 18, which has a metallicity about 4 per cent that of the Sun's and is forming stars less rapidly (assembly time about 1.6 × 10(9) years) than Himiko but still vigorously for its mass, is also very dust deficient and is perhaps one of the best analogues of primitive galaxies accessible to detailed study. Here we report observations of dust emission from I Zw 18, from which we determine its dust mass to be 450-1,800 solar masses, yielding a dust-to-stellar mass ratio of about 10(-6) to 10(-5) and a dust-to-gas mass ratio of 3.2-13 × 10(-6). If I Zw 18 is a reasonable analogue of Himiko, then Himiko's dust mass must be around 50,000 solar masses, a factor of 100 below the current upper limit. These numbers are quite uncertain, but if most high-z galaxies are more like Himiko than like the very-high-dust-mass galaxy SDSS J114816.64 + 525150.3 at z ≈ 6, which hosts a quasar, then our prospects for detecting the gas and dust inside such galaxies are much poorer than hitherto anticipated.
    Nature 12/2013; · 38.60 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Observations of the Milky Way and nearby galaxies show that dense molecular gas correlates with recent star formation, suggesting that the formation of this gas phase may help regulate star formation. A key test of this idea requires wide-area, high-resolution maps of dense molecular gas in galaxies to explore how local physical conditions drive dense gas formation, but these observations have been limited because of the faintness of dense gas tracers like HCN and HCO+. Here we demonstrate the power of the Robert C. Byrd Green Bank Telescope -- the largest single-dish millimeter radio telescope -- for mapping dense gas in galaxies by presenting the most sensitive maps yet of HCN and HCO+ in the starburst galaxy M82. The HCN and HCO+ in the disk of this galaxy correlates with both recent star formation and more diffuse molecular gas and shows kinematics consistent with a rotating torus. The HCO+ emission extending to the north and south of the disk is coincident with the outflow previously identified in CO and traces the eastern edge of the hot outflowing gas. The central starburst region has a higher ratio of star formation to dense gas than the outer regions, pointing to the starburst as a key driver of this relationship. These results establish that the GBT can efficiently map the dense molecular gas at 90 GHz in nearby galaxies, a capability that will increase further with the 16 element feed array under construction.
    The Astrophysical Journal 12/2013; 780(1). · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We compare the properties of giant molecular clouds (GMCs) in M51 identified by the Plateau de Bure Interferometer Whirlpool Arcsecond Survey (PAWS) with GMCs identified in wide-field, high resolution surveys of CO emission in M33 and the Large Magellanic Cloud (LMC). We find that GMCs in M51 are larger, brighter and have higher velocity dispersions relative to their size than equivalent structures in M33 and the LMC. These differences imply that there are genuine variations in the average mass surface density of the different GMC populations. To explain this, we propose that the pressure in the interstellar medium surrounding the GMCs plays a role in regulating their density and velocity dispersion. We find no evidence for a correlation between size and linewidth in any of M51, M33 or the LMC when the CO emission is decomposed into GMCs, although moderately robust correlations are apparent when regions of contiguous CO emission (with no size limitation) are used. Our work demonstrates that observational bias remains an important obstacle to the identification and study of extragalactic GMC populations using CO emission, especially in molecule-rich galactic environments.
    The Astrophysical Journal 12/2013; 779(1):46. · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The Plateau de Bure Interferometer Arcsecond Whirlpool Survey has mapped the molecular gas in the central ~9 kpc of M51 in its 12CO(1-0) line emission at a cloud-scale resolution of ~40 pc using both IRAM telescopes. We utilize this data set to quantitatively characterize the relation of molecular gas (or CO emission) to other tracers of the interstellar medium, star formation, and stellar populations of varying ages. Using two-dimensional maps, a polar cross-correlation technique and pixel-by-pixel diagrams, we find: (1) that (as expected) the distribution of the molecular gas can be linked to different components of the gravitational potential; (2) evidence for a physical link between CO line emission and radio continuum that seems not to be caused by massive stars, but rather depends on the gas density; (3) a close spatial relation between polycyclic aromatic hydrocarbon (PAH) and molecular gas emission, but no predictive power of PAH emission for the molecular gas mass; (4) that the I - H color map is an excellent predictor of the distribution (and to a lesser degree, the brightness) of CO emission; and (5) that the impact of massive (UV-intense) young star-forming regions on the bulk of the molecular gas in central ~9 kpc cannot be significant due to a complex spatial relation between molecular gas and star-forming regions that ranges from cospatial to spatially offset to absent. The last point, in particular, highlights the importance of galactic environment—and thus the underlying gravitational potential—for the distribution of molecular gas and star formation.
    The Astrophysical Journal 12/2013; 779(1):42. · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present the data of the Plateau de Bure Arcsecond Whirlpool Survey, a high spatial and spectral resolution 12CO (1-0) line survey of the inner ~10 × 6 kpc of the M51 system, and the first wide-field imaging of molecular gas in a star-forming spiral galaxy with resolution matched to the typical size of giant molecular clouds (40 pc). We describe the observation, reduction, and combination of the Plateau de Bure Interferometer (PdBI) and IRAM-30 m "short spacing" data. The final data cube attains 1.''1 resolution over the ~270'' × 170'' field of view, with sensitivity to all spatial scales from the combination of PdBI and IRAM-30 m data, and a brightness sensitivity of 0.4 K (1σ) in each 5 km s-1-wide channel map. We find a CO luminosity of 9 × 108 K km s-1 pc2, corresponding to a molecular gas mass of 4 × 109 M ⊙ for a standard CO-to-H2 conversion factor. Unexpectedly, we find that a large fraction of this emission, (50 ± 10)%, arises mostly from spatial scales larger than 36'' ~= 1.3 kpc. Through a series of tests, we demonstrate that this extended emission does not result from a processing artifact. We discuss its origin in light of the stellar component, the 12CO/13CO ratio, and the difference between the kinematics and structure of the PdBI-only and hybrid synthesis (PdBI + IRAM-30 m) images. The extended emission is consistent with a thick, diffuse disk of molecular gas with a typical scale height of ~200 pc, substructured in unresolved filaments that fill ~0.1% of the volume.
    The Astrophysical Journal 12/2013; 779(1):43. · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We use the high spatial and spectral resolution of the PAWS CO(1-0) survey of the inner 9 kpc of the iconic spiral galaxy M51 to examine the effect of gas streaming motions on the star-forming properties of individual GMCs. We compare our view of gas flows in M51 -- which arise due to departures from axi-symmetry in the gravitational potential (i.e. the nuclear bar and spiral arms) -- with the global pattern of star formation as traced by Halpha and 24\mu m emission. We find that the dynamical environment of GMCs strongly affects their ability to form stars, in the sense that GMCs situated in regions with large streaming motions can be stabilized, while similarly massive GMCs in regions without streaming go on to efficiently form stars. We argue that this is the result of reduced surface pressure felt by clouds embedded in an ambient medium undergoing large streaming motions, which prevents collapse. Indeed, the variation in gas depletion time expected based on the observed streaming motions throughout the disk of M51 quantitatively agrees with the variation in observed gas depletion time scale. The example of M51 shows that streaming motions, triggered by gravitational instabilities in the form of bars and spiral arms, can alter the star formation law; this can explain the variation in gas depletion time among galaxies with different masses and morphologies. In particular, we can explain the long gas depletion times in spiral galaxies compared to dwarf galaxies and starbursts. We suggest that adding a dynamical pressure term to the canonical free-fall time produces a single star formation law that can be applied to all star-forming regions and galaxies, across cosmic time.
    The Astrophysical Journal 12/2013; 779:45. · 6.73 Impact Factor

Publication Stats

3k Citations
565.93 Total Impact Points

Institutions

  • 2014
    • University of Virginia
      • Department of Astronomy
      Charlottesville, Virginia, United States
    • University of Florida
      • Department of Physics
      Gainesville, Florida, United States
  • 2006–2014
    • National Radio Astronomy Observatory
      Charlottesville, Virginia, United States
    • University of Victoria
      Victoria, British Columbia, Canada
  • 2013
    • Max Planck Institute for Radio Astronomy
      Bonn, North Rhine-Westphalia, Germany
  • 2006–2012
    • Max Planck Institute for Astronomy
      Heidelburg, Baden-Württemberg, Germany
  • 2011
    • Nagoya University
      Nagoya, Aichi, Japan
  • 2010
    • Ecole Normale Supérieure de Paris
      Lutetia Parisorum, Île-de-France, France
  • 2001–2008
    • University of California, Berkeley
      • • Department of Astronomy
      • • Radio Astronomy Laboratory
      Berkeley, CA, United States
  • 2007
    • University of Maryland, College Park
      • Department of Astronomy
      College Park, MD, United States