A MOLECULAR STAR FORMATION LAW IN THE ATOMIC-GAS-DOMINATED REGIME IN NEARBY GALAXIES

By A. Schruba, A. Leroy, F. Walter, F. Bigiel, E. Brinks, W. D. de Blok, G. Dumas, C. Kramer, Erik Rosolowsky, K. Sandstrom and 4 more. 2011

423 citations recorded by the source corpus.

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 ICO ≈ 0.3 K km s−1 ( M☉ pc−2), an improvement of about one order of magnitude over previous studies. We detect CO out to galactocentric radii rgal ∼ r25 and find the CO radial profile to follow a remarkably uniform exponential decline with a scale length of ∼0.2 r25. Here we focus on stacking as a function of radius, comparing our sensitive CO profiles to matched profiles of H i, Hα, far-UV (FUV), and Infrared (IR) emission at 24 μm and 70 μm. 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 _{\rm H\,\mathsc{i}}$?>) and those dominated by atomic gas (). We use combinations of FUV+24 μm and Hα+24 μm to estimate the recent star formation rate (SFR) surface density, ΣSFR, and find approximately linear relations between ΣSFR and . 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-H2 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 H2. 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.

Subfields: Astronomy and Astrophysics, Computational Mechanics

View Paper: https://www.semanticscholar.org/paper/132f518ff2de0f457cc000eadd059f5fab148337

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Citations in the corpus

Implementing molecular hydrogen in hydrodynamic simulations of galaxy formation (2012, background): “Extending this study to greater radii in the galaxies by stacking spectra, Schruba et al. (2011) verified that this relationship held out to r25, whereH2 had been previously undetected.”
Cold gas properties of the Herschel Reference Survey - II. Molecular and total gas scaling relations (2014, background): “The observational evidence collected so far indicates that, although local y the process of star formation is mainly related to the molecular gas (Schruba et al. 2011), the total gas content is the gaseous phase that mainly drives galaxy evolution.”
The SILCC (SImulating the LifeCycle of molecular Clouds) project - II. Dynamical evolution of the supernova-driven ISM and the launching of outflows (2015, methodology): “We account for the variations and uncertainties in the KS relation (Bigiel et al. 2008; Schruba et al. 2011; Leroy et al. 2013; Shetty, Kelly & Bigiel 2013; Shetty et al. 2014) by varying the SN rate by a factor of three above and below the standard value.”
An uncertainty principle for star formation - I. Why galactic star formation relations break down below a certain spatial scale (2014, background): “…SF relations are often characterised by defining the gas depletion time-scaletdepl ≡ Mgas/SFR, which allows the scatter of the SF relation to be quantified as the root-mean-square (RMS) scatter of the depletion time-scaleσlog t (e.g. Bigiel et al. 2008; Schruba et al. 2011; Leroy et al. 2013).”
BARYONS MATTER: WHY LUMINOUS SATELLITE GALAXIES HAVE REDUCED CENTRAL MASSES (2012, result): “…formation in the new model is tied directly to the local H2 abundance, which is regulated by the gas metallicity and the ability of the gas to self-shield, in accordance with observational results (Leroy et al. 2008; Bigiel et al. 2008; Blanc et al. 2009; Bigiel et al. 2010; Schruba et al. 2011).”
Dusty Star Forming Galaxies at High Redshift (2014, neutral): “This result was extended to the atomic gas-dominated outskirts of nearby galaxies by Schruba et al. (2011).”
The RMS survey : Galactic distribution of massive star formation (2013, background): “Several factors are thought to contribute to the comparative lack of molecular clouds, including lower metallicity, gas density and ambient pressure (Snell et al. 2002; Leroy et al. 2008; Schruba et al. 2011).”
THE CO-TO-H2 CONVERSION FACTOR AND DUST-TO-GAS RATIO ON KILOPARSEC SCALES IN NEARBY GALAXIES (2012, background): “Similar results are found when comparing with the radial profiles determined from the HERACLES CO maps we utilize in this study (see radial profiles in Schruba et al. 2011).”
COMBINED CO AND DUST SCALING RELATIONS OF DEPLETION TIME AND MOLECULAR GAS FRACTIONS WITH COSMIC TIME, SPECIFIC STAR-FORMATION RATE, AND STELLAR MASS (2014, background): “In the IRAM HERACLES survey Bigiel et al. (2008, 2011), Leroy et al. (2008, 2013) and Schruba et al. (2011) studied the spatial distribution of CO 2-1 emission on subgalactic scales (resolution ~1 kpc) in 30 local disk and dwarf star forming galaxies near the main sequence.”
The lifecycle of molecular clouds in nearby star-forming disc galaxies (2019, background): “…Mgas/SFR, which is observed to be tdep ≈ 2 Gyr in nearby star-forming galaxies (Bigiel et al. 2008; Bigiel et al. 2011; Leroy et al. 2008, 2013; Blanc et al. 2009; Schruba et al. 2011) and represents the time necessary to convert the entire reservoir of molecular gas into stars at the current SFR.”
Physical Processes in the Interstellar Medium (2014, background): “Work by a number of groups has shown that on large scales, there is a tight correlation between the surface density of molecular gas and the surface density of star formation in spiral galaxies (Wong & Blitz, 2002; Leroy et al., 2008; Bigiel et al., 2008; Bigiel et al., 2011; Schruba et al., 2011).”
LOW CO LUMINOSITIES IN DWARF GALAXIES (2012, methodology): “We choose this constant line ratio to achieve consistency with Bigiel et al. (2011) and Schruba et al. (2011).”
Feedback-regulated star formation in molecular clouds and galactic discs (2013, background): “In general, star formation correlates strongly with molecular gas but not atomic gas (e.g., Wong & Blitz 2002; Schruba et al. 2011) and the mass spectrum of molecular clouds in nearby galaxies is sufficiently shallow that the mass is concentrated in the most massive objects (e.g., Williams & McKee…”
THE ARECIBO LEGACY FAST ALFA SURVEY: THE GALAXY POPULATION DETECTED BY ALFALFA (2012, background): “Furthermore, Schruba et al. (2011) found a tight and roughly linear relationship between IR (inferringΣSFR) and CO (inferredΣH2 ) intensity, withΣH2/ΣSFR ∼ 1.8 Gyr.”
THE IMPACT OF INTERACTIONS, BARS, BULGES, AND ACTIVE GALACTIC NUCLEI ON STAR FORMATION EFFICIENCY IN LOCAL MASSIVE GALAXIES (2012, background): “This choice is based on the observation that CO and SFR distributions trace each other well in nearby star-forming galaxies (Leroy et al. 2009), even in the HIdominated outer disc regions (Schruba et al. 2011).”
A unified model for galactic discs: star formation, turbulence driving, and mass transport (2017, background): “Consequently, the scale length of the star formation is comparable to the stellar scale length, ≈2–4 kpc, and a factor of 2–3 smaller than the neutral gas scale length (Regan et al. 2001; Leroy et al. 2008; Schruba et al. 2011; Bigiel & Blitz 2012).”
MOLECULAR GAS AND STAR FORMATION IN NEARBY DISK GALAXIES (2013, methodology): “The resulting linear combination resembles that of Kennicutt et al. (2007) but places slightly more weight on the 24µm term.”
STAR FORMATION RELATIONS IN NEARBY MOLECULAR CLOUDS (2014, background): “The strong, linear correlation with molecular gas has been extended to the outer regions of galaxies by a line stacking analysis of CO observations (Schruba et al. 2011).”