THE SCALE DEPENDENCE OF THE MOLECULAR GAS DEPLETION TIME IN M33

By A. Schruba, A. Leroy, F. Walter, K. Sandstrom, Erik Rosolowsky. 2010

154 citations recorded by the source corpus.

We study the Local Group spiral galaxy M33 to investigate how the observed scaling between the (kpc-averaged) surface density of molecular gas (ΣH2) and recent star formation rate (ΣSFR) relates to individual star-forming regions. To do this, we measure the ratio of CO emission to extinction-corrected Hα emission in apertures of varying sizes centered both on peaks of CO and Hα emission. We parameterize this ratio as the molecular gas (H2) depletion time (τdep). On large (kpc) scales, our results are consistent with a molecular star formation law (ΣSFR ∼ ΣbH2) with b ∼ 1.1–1.5 and a median τdep ∼ 1 Gyr, with no dependence on type of region targeted. Below these scales, τdep is a strong function of the adopted angular scale and the type of region that is targeted. Small (≲300 pc) apertures centered on CO peaks have very long τdep (i.e., high CO-to-Hα flux ratio) and small apertures targeted toward Hα peaks have very short τdep. This implies that the star formation law observed on kpc scales breaks down once one reaches aperture sizes of ≲300 pc. For our smallest apertures (75 pc), the difference in τdep between the two types of regions is more than one order of magnitude. This scale behavior emerges from averaging over star-forming regions with a wide range of CO-to-Hα ratios with the natural consequence that the breakdown in the star formation law is a function of the surface density of the regions studied. We consider the evolution of individual regions the most likely driver for region-to-region differences in τdep (and thus the CO-to-Hα ratio).

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/b3824b8fa05fbaf06ae33ce6167338646ef8d859

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In Astronomy and Astrophysics

1. P. Hopkins
2. L. Hernquist
3. Mark R. Krumholz
4. A. Dekel
5. R. Somerville
6. R. Dav'e
7. E. Quataert
8. T. Naab

In Instrumentation

1. P. Hopkins
2. A. Dekel
3. L. Hernquist
4. R. Somerville
5. R. Dav'e
6. T. Naab
7. Volker Springel
8. E. Quataert

Citations in the corpus

• An uncertainty principle for star formation - I. Why galactic star formation relations break down below a certain spatial scale (2014, background): “Galactic star formation (SF) relations (e.g. Silk 1997; Kennicutt 1998; Elmegreen 2002; Bigiel et al. 2011) break down below a certain spatial scale (Schruba et al. 2010; Onodera et al. 2010; Liu et al. 2011).”
• THE SINS SURVEY OF z ∼ 2 GALAXY KINEMATICS: PROPERTIES OF THE GIANT STAR-FORMING CLUMPS (2010, background): “As a result, the Kennicutt–Schmidt scaling relation in Equation (2) may break down or be significantly altered on small scales (e.g., Schruba et al. 2010 in M33 on 80 pc scales).”
• Cool Gas in High-Redshift Galaxies (2013, methodology): “This has been quantified by Schruba et al. (2010) and Onodera et al. (2010) who have shown that in the case of M 33 the star formation law on large/galactic scales breaks down on scales below ∼300 pc.”
• What controls star formation in the central 500 pc of the Galaxy (2013, background): “…than∼ 500 pc. Smoothing is justified on galactic (>∼ 500 pc) scales, because the lifetime of substructure on these scales is typically of theorder of (or shorter than) an orbital timescale, and accounting for substructure therefore only introduces spurious stochasticity (e.g. Schruba et al. 2010).”
• A MOLECULAR STAR FORMATION LAW IN THE ATOMIC-GAS-DOMINATED REGIME IN NEARBY GALAXIES (2011, background): “On small scales, this scatter arises from the evolution of individual star– forming regions, which vary dramatically in their ratios of CO–to–SFR tracers — leading to a breakdown of scaling relations when a resolution element corresponds to an individual region (Schruba et al. 2010).”
• The lifecycle of molecular clouds in nearby star-forming disc galaxies (2019, background): “Inspired by the interpretation first proposed by Schruba et al. (2010), recent work has now demonstrated that the observed smallscale scatter around the global star formation relation (e.g. Bigiel et al. 2008; Blanc et al. 2009; Onodera et al. 2010; Schruba et al. 2010; Leroy et al. 2013; Kreckel…”
• On the impact of empirical and theoretical star formation laws on galaxy formation (2010, background): “…above attempt to describe the SF activity in galaxies on scales larger than ∼500 pc, averaging over representative volumes of the interstellar medium (ISM) including both star-forming clouds and intercloud gas (e.g. Fumagalli, Krumholz & Hunt 2010; Onodera et al. 2010; Schruba et al. 2010).”
• Global hierarchical collapse in molecular clouds. Towards a comprehensive scenario (2019, background): “…or more, of the type of regions observed in nearby external galaxy surveys (e.g., Bigiel et al. 2008; Leroy et al. 2008; Onodera et al. 2010; Schruba et al. 2010; Liu et al. 2011), which contain ensembles of GMCs as well as more diffuse gas, the GHC interpretation of the low observed ǫff is…”
• The dynamical evolution of molecular clouds near the Galactic Centre - I. Orbital structure and evolutionary timeline (2014, background): “…star formation relations to the cloud-scale physics of star formation (e.g. Bigiel et al. 2008; Heiderman et al. 2010; Lada, Lombardi & Alves 2010; Schruba et al. 2010; Gutermuth et al. 2011; Hopkins, Quataert & Murray 2011; Krumholz, Dekel & McKee 2012; Burkert & Hartmann 2013; Kruijssen &…”
• THE STATE OF THE GAS AND THE RELATION BETWEEN GAS AND STAR FORMATION AT LOW METALLICITY: THE SMALL MAGELLANIC CLOUD (2011, background): “Although the precise values differ, a very similar trend for τmoldep as a function of spatial scale is observed in M 33 (Schruba et al. 2010).”
• Cosmic evolution of the atomic and molecular gas contents of galaxies (2011, background): “Moreover, the surface density of the star formation rate (SFR) correlates with the surface density of molecular hydrogen,H2, in an approximately linear fashion,ΣSFR ∝ ΣH2 (e.g. Bigiel et al. 2008; Schruba et al. 2010).”
• COLD GASS, an IRAM legacy survey of molecular gas in massive galaxies - II. The non-universality of the molecular gas depletion time-scale (2011, background): “It is therefore not surprising that the rate at which gas is being converted to stars in a single galaxy exhibits large scatter from one region to another when studied on small scales (e.g. Schruba et al. 2010).”
• The big problems in star formation: The star formation rate, stellar clustering, and the initial mass function (2014, background): “Between ∼ 100 pc and 1 kpc, the observed correlation between SFR and molecular gas progressively worsens as one moves to smaller scales, reaching multiple order of magnitudelevel scatter at ∼ 100 pc scales (Onodera et al. 2010; Schruba et al. 2010).”
• Self-regulated star formation in galaxies via momentum input from massive stars (2011, background): “Similar gas consumption times are found even in dense regions in galaxies (e.g. Krumholz & Tan 2007; but see also Schruba et al. 2010; Feldmann & Gnedin 2011; Murray 2011).”
• LOW CO LUMINOSITIES IN DWARF GALAXIES (2012, background): “On small scales this shows as offsets between Hα, a tracer of recent star formation, and CO, and induces large scatter in the respective ratios (e.g., Schruba et al. 2010; Onodera et al. 2010) which can be linked to the evolution of starforming regions (Kawamura et al. 2009).”
• ESTIMATING THE STAR FORMATION RATE AT 1 kpc SCALES IN NEARBY GALAXIES (2012, background): “Over a square kpc, we expect to average several populations so that the scatter in intensity should be lower than in Table 3 by ∼ √ N , where N is the number of independent populations that we average (e.g., see Schruba et al. 2010).”
• MOLECULAR GAS AND STAR FORMATION IN NEARBY DISK GALAXIES (2013, background): “As a result, scaling relations between SFR and H2, which capture the timeaveraged relation between gas and star formation, emerge only after averaging together many distinct regions (see Schruba et al. 2010; Feldmann et al. 2011).”
• THE PdBI ARCSECOND WHIRLPOOL SURVEY (PAWS). I. A CLOUD-SCALE/MULTI-WAVELENGTH VIEW OF THE INTERSTELLAR MEDIUM IN A GRAND-DESIGN SPIRAL GALAXY (2013, background): “Both Schruba et al. (2010) and Onodera et al. (2010) report that the relation between (extinction-corrected) Hα emission and CO emission in M 33 breaks on scales below ≈ 300 pc - similar to our observed angular separation between molecular gas and HII regions.”