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Title: MOLECULAR GAS AND STAR FORMATION IN NEARBY DISK GALAXIES

Abstract

We compare molecular gas traced by {sup 12}CO (2-1) maps from the HERACLES survey, with tracers of the recent star formation rate (SFR) across 30 nearby disk galaxies. We demonstrate a first-order linear correspondence between {Sigma}{sub mol} and {Sigma}{sub SFR} but also find important second-order systematic variations in the apparent molecular gas depletion time, {tau}{sub dep}{sup mol}={Sigma}{sub mol}/{Sigma}{sub SFR}. At the 1 kpc common resolution of HERACLES, CO emission correlates closely with many tracers of the recent SFR. Weighting each line of sight equally, using a fixed {alpha}{sub CO} equivalent to the Milky Way value, our data yield a molecular gas depletion time, {tau}{sub dep}{sup mol}={Sigma}{sub mol}/{Sigma}{sub SFR}{approx}2.2 Gyr with 0.3 dex 1{sigma} scatter, in very good agreement with recent literature data. We apply a forward-modeling approach to constrain the power-law index, N, that relates the SFR surface density and the molecular gas surface density, {Sigma}{sub SFR}{proportional_to}{Sigma}{sub mol}{sup N}. We find N = 1 {+-} 0.15 for our full data set with some scatter from galaxy to galaxy. This also agrees with recent work, but we caution that a power-law treatment oversimplifies the topic given that we observe correlations between {tau}{sub dep}{sup mol} and other local and global quantities. Themore » strongest of these are a decreased {tau}{sub dep}{sup mol} in low-mass, low-metallicity galaxies and a correlation of the kpc-scale {tau}{sub dep}{sup mol} with dust-to-gas ratio, D/G. These correlations can be explained by a CO-to-H{sub 2} conversion factor ({alpha}{sub CO}) that depends on dust shielding, and thus D/G, in the theoretically expected way. This is not a unique interpretation, but external evidence of conversion factor variations makes this the most conservative explanation of the strongest observed {tau}{sub dep}{sup mol} trends. After applying a D/G-dependent {alpha}{sub CO}, some weak correlations between {tau}{sub dep}{sup mol} and local conditions persist. In particular, we observe lower {tau}{sub dep}{sup mol} and enhanced CO excitation associated with nuclear gas concentrations in a subset of our targets. These appear to reflect real enhancements in the rate of star formation per unit gas, and although the distribution of {tau}{sub dep} does not appear bimodal in galaxy centers, {tau}{sub dep} does appear multivalued at fixed {Sigma}{sub H2}, supporting the idea of ''disk'' and ''starburst'' modes driven by other environmental parameters.« less

Authors:
;  [1]; ; ; ; ;  [2];  [3];  [4];  [5];  [6];  [7];  [8];  [9];  [10]
  1. National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903 (United States)
  2. Max Planck Institute fuer Astronomie, Koenigstuhl 17, D-69117 Heidelberg (Germany)
  3. California Institute for Technology, 1200 East California Boulevard, Pasadena, CA 91125 (United States)
  4. Theoretische Astrophysik, Albert-Ueberle-Str. 2, D-69120 Heidelberg (Germany)
  5. Department of Astronomy, University of Maryland, College Park, MD (United States)
  6. Centre for Astrophysics Research, University of Hertfordshire, Hatfield AL10 9AB (United Kingdom)
  7. Astrophysics, Cosmology and Gravity Centre, Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701 (South Africa)
  8. University of British Columbia, Okanagan Campus, Kelowna, BC (Canada)
  9. IRAM, 300 rue de la Piscine, F-38406 St. Martin d'Heres (France)
  10. Observatorio Astronomico Nacional, C/ Alfonso XII, 3, E-28014 Madrid (Spain)
Publication Date:
OSTI Identifier:
22118719
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astronomical Journal (New York, N.Y. Online); Journal Volume: 146; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ASTRONOMY; ASTROPHYSICS; CARBON MONOXIDE; COMPARATIVE EVALUATIONS; CORRELATIONS; COSMIC DUST; DENSITY; ELEMENT ABUNDANCE; EXCITATION; HYDROGEN; INDEXES; MILKY WAY; MOLECULES; PHOTON EMISSION; STAR EVOLUTION; STARS; VARIATIONS

Citation Formats

Leroy, Adam K., Munoz-Mateos, Juan-Carlos, Walter, Fabian, Sandstrom, Karin, Meidt, Sharon, Rix, Hans-Walter, Schinnerer, Eva, Schruba, Andreas, Bigiel, Frank, Bolatto, Alberto, Brinks, Elias, De Blok, W. J. G., Rosolowsky, Erik, Schuster, Karl-Friedrich, and Usero, Antonio. MOLECULAR GAS AND STAR FORMATION IN NEARBY DISK GALAXIES. United States: N. p., 2013. Web. doi:10.1088/0004-6256/146/2/19.
Leroy, Adam K., Munoz-Mateos, Juan-Carlos, Walter, Fabian, Sandstrom, Karin, Meidt, Sharon, Rix, Hans-Walter, Schinnerer, Eva, Schruba, Andreas, Bigiel, Frank, Bolatto, Alberto, Brinks, Elias, De Blok, W. J. G., Rosolowsky, Erik, Schuster, Karl-Friedrich, & Usero, Antonio. MOLECULAR GAS AND STAR FORMATION IN NEARBY DISK GALAXIES. United States. doi:10.1088/0004-6256/146/2/19.
Leroy, Adam K., Munoz-Mateos, Juan-Carlos, Walter, Fabian, Sandstrom, Karin, Meidt, Sharon, Rix, Hans-Walter, Schinnerer, Eva, Schruba, Andreas, Bigiel, Frank, Bolatto, Alberto, Brinks, Elias, De Blok, W. J. G., Rosolowsky, Erik, Schuster, Karl-Friedrich, and Usero, Antonio. 2013. "MOLECULAR GAS AND STAR FORMATION IN NEARBY DISK GALAXIES". United States. doi:10.1088/0004-6256/146/2/19.
@article{osti_22118719,
title = {MOLECULAR GAS AND STAR FORMATION IN NEARBY DISK GALAXIES},
author = {Leroy, Adam K. and Munoz-Mateos, Juan-Carlos and Walter, Fabian and Sandstrom, Karin and Meidt, Sharon and Rix, Hans-Walter and Schinnerer, Eva and Schruba, Andreas and Bigiel, Frank and Bolatto, Alberto and Brinks, Elias and De Blok, W. J. G. and Rosolowsky, Erik and Schuster, Karl-Friedrich and Usero, Antonio},
abstractNote = {We compare molecular gas traced by {sup 12}CO (2-1) maps from the HERACLES survey, with tracers of the recent star formation rate (SFR) across 30 nearby disk galaxies. We demonstrate a first-order linear correspondence between {Sigma}{sub mol} and {Sigma}{sub SFR} but also find important second-order systematic variations in the apparent molecular gas depletion time, {tau}{sub dep}{sup mol}={Sigma}{sub mol}/{Sigma}{sub SFR}. At the 1 kpc common resolution of HERACLES, CO emission correlates closely with many tracers of the recent SFR. Weighting each line of sight equally, using a fixed {alpha}{sub CO} equivalent to the Milky Way value, our data yield a molecular gas depletion time, {tau}{sub dep}{sup mol}={Sigma}{sub mol}/{Sigma}{sub SFR}{approx}2.2 Gyr with 0.3 dex 1{sigma} scatter, in very good agreement with recent literature data. We apply a forward-modeling approach to constrain the power-law index, N, that relates the SFR surface density and the molecular gas surface density, {Sigma}{sub SFR}{proportional_to}{Sigma}{sub mol}{sup N}. We find N = 1 {+-} 0.15 for our full data set with some scatter from galaxy to galaxy. This also agrees with recent work, but we caution that a power-law treatment oversimplifies the topic given that we observe correlations between {tau}{sub dep}{sup mol} and other local and global quantities. The strongest of these are a decreased {tau}{sub dep}{sup mol} in low-mass, low-metallicity galaxies and a correlation of the kpc-scale {tau}{sub dep}{sup mol} with dust-to-gas ratio, D/G. These correlations can be explained by a CO-to-H{sub 2} conversion factor ({alpha}{sub CO}) that depends on dust shielding, and thus D/G, in the theoretically expected way. This is not a unique interpretation, but external evidence of conversion factor variations makes this the most conservative explanation of the strongest observed {tau}{sub dep}{sup mol} trends. After applying a D/G-dependent {alpha}{sub CO}, some weak correlations between {tau}{sub dep}{sup mol} and local conditions persist. In particular, we observe lower {tau}{sub dep}{sup mol} and enhanced CO excitation associated with nuclear gas concentrations in a subset of our targets. These appear to reflect real enhancements in the rate of star formation per unit gas, and although the distribution of {tau}{sub dep} does not appear bimodal in galaxy centers, {tau}{sub dep} does appear multivalued at fixed {Sigma}{sub H2}, supporting the idea of ''disk'' and ''starburst'' modes driven by other environmental parameters.},
doi = {10.1088/0004-6256/146/2/19},
journal = {Astronomical Journal (New York, N.Y. Online)},
number = 2,
volume = 146,
place = {United States},
year = 2013,
month = 8
}
  • We present the results of large-area {sup 12}CO J = 3-2 emission mapping of three nearby field galaxies, NGC 628, NGC 3521, and NGC 3627, completed at the James Clerk Maxwell Telescope as part of the Nearby Galaxies Legacy Survey. These galaxies all have moderate to strong {sup 12}CO J = 3-2 detections over large areas of the fields observed by the survey, showing resolved structure and dynamics in their warm/dense molecular gas disks. All three galaxies were part of the Spitzer Infrared Nearby Galaxies Survey sample, and as such have excellent published multiwavelength ancillary data. These data sets allowmore » us to examine the star formation properties, gas content, and dynamics of these galaxies on sub-kiloparsec scales. We find that the global gas depletion time for dense/warm molecular gas in these galaxies is consistent with other results for nearby spiral galaxies, indicating this may be independent of galaxy properties such as structures, gas compositions, and environments. Similar to the results from The H I Nearby Galaxy Survey, we do not see a correlation of the star formation efficiency with the gas surface density consistent with the Schmidt-Kennicutt law. Finally, we find that the star formation efficiency of the dense molecular gas traced by {sup 12}CO J = 3-2 is potentially flat or slightly declining as a function of molecular gas density, the {sup 12}CO J = 3-2/J = 1-0 ratio (in contrast to the correlation found in a previous study into the starburst galaxy M83), and the fraction of total gas in molecular form.« less
  • This study explores the effects of different assumptions and systematics on the determination of the local, spatially resolved star formation law. Using four star formation rate (SFR) tracers (H{alpha} with azimuthally averaged extinction correction, mid-infrared 24 {mu}m, combined H{alpha} and mid-infrared 24 {mu}m, and combined far-ultraviolet and mid-infrared 24 {mu}m), several fitting procedures, and different sampling strategies, we probe the relation between SFR and molecular gas at various spatial resolutions (500 pc and larger) and surface densities ({Sigma}{sub H{sub 2}})approx. 10-245 M{sub sun} pc{sup -2}) within the central {approx}6.5 kpc in the disk of NGC 4254. We explore the effectmore » of diffuse emission using an unsharp masking technique with varying kernel size. The fraction of diffuse emission, f{sub DE}, thus determined is a strong inverse function of the size of the filtering kernel. We find that in the high surface brightness regions of NGC 4254 the form of the molecular gas star formation law is robustly determined and approximately linear ({approx}0.8-1.1) and independent of the assumed fraction of diffuse emission and the SFR tracer employed. When the low surface brightness regions are included, the slope of the star formation law depends primarily on the assumed fraction of diffuse emission. In such a case, results range from linear when the fraction of diffuse emission in the SFR tracer is f{sub DE} {approx}< 30% (or when diffuse emission is removed in both the star formation and the molecular gas tracer) to super-linear ({approx}1.4) when f{sub DE} {approx}> 50%. We find that the tightness of the correlation between gas and star formation varies with the choice of star formation tracer. The 24 {mu}m SFR tracer by itself shows the tightest correlation with the molecular gas surface density, whereas the H{alpha} corrected for extinction using an azimuthally averaged correction shows the highest dispersion. We find that for R < 0.5R{sub 25} the local star formation efficiency is constant and similar to that observed in other large spirals, with a molecular gas depletion time {tau}{sub dep} {approx} 2 Gyr.« less
  • We use the IRAM HERACLES survey to study CO emission from 33 nearby spiral galaxies down to very low intensities. Using 21 cm line atomic hydrogen (H I) data, mostly from THINGS, we predict the local mean CO velocity based on the mean H I velocity. By re-normalizing the CO velocity axis so that zero corresponds to the local mean H I velocity we are able to stack spectra coherently over large regions. This enables us to measure CO intensities with high significance as low as I{sub CO} {approx} 0.3 K km s{sup -1} ({Sigma}{sub H{sub 2}}{approx}1 M{sub sun} pc{supmore » -2}), an improvement of about one order of magnitude over previous studies. We detect CO out to galactocentric radii r{sub gal} {approx} r{sub 25} and find the CO radial profile to follow a remarkably uniform exponential decline with a scale length of {approx}0.2 r{sub 25}. Here we focus on stacking as a function of radius, comparing our sensitive CO profiles to matched profiles of H I, H{alpha}, far-UV (FUV), and Infrared (IR) emission at 24 {mu}m and 70 {mu}m. We observe a tight, roughly linear relationship between CO and IR intensity that does not show any notable break between regions that are dominated by molecular gas ({Sigma}{sub H{sub 2}}>{Sigma}{sub H{sub i}}) and those dominated by atomic gas ({Sigma}{sub H{sub 2}}<{Sigma}{sub H{sub i}}). We use combinations of FUV+24 {mu}m and H{alpha}+24 {mu}m to estimate the recent star formation rate (SFR) surface density, {Sigma}{sub SFR}, and find approximately linear relations between {Sigma}{sub SFR} and {Sigma}{sub H{sub 2}}. We interpret this as evidence of stars forming in molecular gas with little dependence on the local total gas surface density. While galaxies display small internal variations in the SFR-to-H{sub 2} ratio, we do observe systematic galaxy-to-galaxy variations. These galaxy-to-galaxy variations dominate the scatter in relationships between CO and SFR tracers measured at large scales. The variations have the sense that less massive galaxies exhibit larger ratios of SFR-to-CO than massive galaxies. Unlike the SFR-to-CO ratio, the balance between atomic and molecular gas depends strongly on the total gas surface density and galactocentric radius. It must also depend on additional parameters. Our results reinforce and extend to lower surface densities, a picture in which star formation in galaxies can be separated into two processes: the assembly of star-forming molecular clouds and the formation of stars from H{sub 2}. The interplay between these processes yields a total gas-SFR relation with a changing slope, which has previously been observed and identified as a star formation threshold.« less
  • We present an analysis of the relationship between molecular gas and current star formation rate surface density at sub-kiloparsec and kiloparsec scales in a sample of 14 nearby star-forming galaxies. Measuring the relationship in the bright, high molecular gas surface density ({Sigma}{sub H{sub 2}}{approx}>20 M{sub Sun} pc{sup -2}) regions of the disks to minimize the contribution from diffuse extended emission, we find an approximately linear relation between molecular gas and star formation rate surface density, N{sub mol} {approx} 0.96 {+-} 0.16, with a molecular gas depletion time, {tau}{sup mol}{sub dep} {approx} 2.30 {+-} 1.32 Gyr. We show that in themore » molecular regions of our galaxies there are no clear correlations between {tau}{sup mol}{sub dep} and the free-fall and effective Jeans dynamical times throughout the sample. We do not find strong trends in the power-law index of the spatially resolved molecular gas star formation law or the molecular gas depletion time across the range of galactic stellar masses sampled (M{sub *} {approx} 10{sup 9.7}-10{sup 11.5} M{sub Sun }). There is a trend, however, in global measurements that is particularly marked for low-mass galaxies. We suggest that this trend is probably due to the low surface brightness CO J = 1-0, and it is likely associated with changes in CO-to-H{sub 2} conversion factor.« less
  • We investigate the formation and evolution of giant molecular clouds (GMCs) in a Milky-Way-like disk galaxy with a flat rotation curve. We perform a series of three-dimensional adaptive mesh refinement numerical simulations that follow both the global evolution on scales of {approx}20 kpc and resolve down to scales {approx}<10 pc with a multiphase atomic interstellar medium. In this first study, we omit star formation and feedback, and focus on the processes of gravitational instability and cloud collisions and interactions. We define clouds as regions with n {sub H} {>=} 100 cm{sup -3} and track the evolution of individual clouds asmore » they orbit through the galaxy from their birth to their eventual destruction via merger or via destructive collision with another cloud. After {approx}140 Myr a large fraction of the gas in the disk has fragmented into clouds with masses {approx}10{sup 6} M {sub sun} and a mass spectrum similar to that of Galactic GMCs. The disk settles into a quasi-steady-state in which gravitational scattering of clouds keeps the disk near the threshold of global gravitational instability. The cloud collision time is found to be a small fraction, {approx}1/5, of the orbital time, and this is an efficient mechanism to inject turbulence into the clouds. This helps to keep clouds only moderately gravitationally bound, with virial parameters of order unity. Many other observed GMC properties, such as mass surface density, angular momentum, velocity dispersion, and vertical distribution, can be accounted for in this simple model with no stellar feedback.« less