THE STAR FORMATION LAW IN NEARBY GALAXIES ON SUB-KPC SCALES

By F. Bigiel, A. Leroy, F. Walter, Elias Brinks, W. D. de Blok, B. Madore, M. Thornley. 2008

1,354 citations recorded by the source corpus.

We present a comprehensive analysis of the relationship between star formation rate surface density, ΣSFR, and gas surface density, Σgas, at sub-kpc resolution in a sample of 18 nearby galaxies. We use high-resolution H i data from The H i Nearby Galaxy Survey, CO data from HERACLES and the BIMA Survey of Nearby Galaxies, 24 μm data from the Spitzer Space Telescope, and UV data from the Galaxy Evolution Explorer. We target seven spiral galaxies and 11 late-type/dwarf galaxies and investigate how the star formation law differs between the H2 dominated centers of spiral galaxies, their H i dominated outskirts and the H i rich late-type/dwarf galaxies. We find that a Schmidt-type power law with index N = 1.0 ± 0.2 relates ΣSFR and ΣH2 across our sample of spiral galaxies, i.e., that H2 forms stars at a constant efficiency in spirals. The average molecular gas depletion time is ∼2 × 109 years. The range of ΣH2 over which we measure this relation is ∼3–50 M☉ pc−2, significantly lower than in starburst environments. We find the same results when performing a pixel-by-pixel analysis, averaging in radial bins, or when varying the star formation tracer used. We interpret the linear relation and constant depletion time as evidence that stars are forming in giant molecular clouds with approximately uniform properties and that ΣH2 may be more a measure of the filling fraction of giant molecular clouds than changing conditions in the molecular gas. The relationship between total gas surface density (Σgas) and ΣSFR varies dramatically among and within spiral galaxies. Most galaxies show little or no correlation between ΣHI and ΣSFR. As a result, the star formation efficiency (SFE), ΣSFR/Σgas, varies strongly across our sample and within individual galaxies. We show that this variation is systematic and consistent with the SFE being set by local environmental factors: in spirals the SFE is a clear function of radius, while the dwarf galaxies in our sample display SFEs similar to those found in the outer optical disks of the spirals. We attribute the similarity to common environments (low density, low metallicity, H i dominated) and argue that shear (which is typically absent in dwarfs) cannot drive the SFE. In addition to a molecular Schmidt law, the other general feature of our sample is a sharp saturation of H i surface densities at ΣHI ≈ 9 M☉ pc−2 in both the spiral and dwarf galaxies. In the case of the spirals, we observe gas in excess of this limit to be molecular.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/e85a06911eb99576a78218c50685c1548bc89788

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Citations in the corpus

• PHIBSS: Unified Scaling Relations of Gas Depletion Time and Molecular Gas Fractions (2017, background): “…qualitative agreement in the literature that the depletion time scale is about one Gyr, and dropping by a factor of 2 to 4 between z=0 and 2.5 (Bigiel et al. 2008, Leroy et al. 2013, Saintonge et al. 2011b, 2013, 2016, Tacconi et al. 2010, 2013, Daddi et al. 2010b, Santini et al. 2014,…”
• Is molecular gas necessary for star formation (2011, background): “…observations have demonstrated that on large scales within local galaxies, there is a surprisingly close correlation between the surface density of star formation, ΣSFR, and the surface density of molecular hydrogen, ΣH2 (see e.g. Wong & Blitz 2002; Leroy et al. 2008; Bigiel et al. 2008, 2011).”
• Global hierarchical collapse in molecular clouds. Towards a comprehensive scenario (2019, background): “At the scale of hundreds of parsecs 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…”
• Star Formation in the Milky Way and Nearby Galaxies (2012, methodology): “Recent analyses have become available (Kennicutt et al. 2007; Bigiel et al. 2008, 2010, 2011; Leroy et al. 2008; Blanc et al. 2009; Eales et al. 2010; Verley et al. 2010; Liu et al. 2011; Rahman et al. 2011, 2012; Schruba et al. 2011), and several other major studies are ongoing.”
• An analytic model of angular momentum transport by gravitational torques: from galaxies to massive black holes (2010, result): “This spans the entire observed parameter space of star formation efficiencies on large galactic scales (see e.g. Bigiel et al. 2008), in dense molecular clouds (Krumholz & Tan 2007), and the central regions of AGN (Hicks et al. 2009); our “typical” cases are consistent with the latter observations.”
• VALIDATION OF THE EQUILIBRIUM MODEL FOR GALAXY EVOLUTION TO z ∼ 3 THROUGH MOLECULAR GAS AND DUST OBSERVATIONS OF LENSED STAR-FORMING GALAXIES (2013, background): “…gas is located in the outer regions of galaxies, outside of the actively star-forming disks, but even within the central regions a significant fraction of the cold gas is in atomic form as the HI-to-H2 transition typically occurs at a radius of ∼ 0.4r25 (Bigiel et al. 2008; Leroy et al. 2008).”
• 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).”
• COLD GASS, an IRAM legacy survey of molecular gas in massive galaxies – I. Relations between H2, H i, stellar content and structural properties (2011, background): “These samples are excellent for studying star formation laws within galaxies (e.g. Bigiel et al. 2008), but the number of objects is too small to adequately define global scaling relations.”
• Cloud-scale Molecular Gas Properties in 15 Nearby Galaxies (2018, background): “…(see references in Morris & Rickard 1982) and the correlation between star formation rate (SFR) tracers and molecular gas content (e.g., Rownd & Young 1999; Wong & Blitz 2002; Bigiel et al. 2008; Schruba et al. 2011), suggests that cold molecular gas is the direct gas reservoir for star formation.”
• What controls star formation in the central 500 pc of the Galaxy (2013, background): “Bigiel et al. (2008, 2011) show that thegas depletion time of the galaxies in their sample istdepl = 1–2 Gyr, which is indeed consistent with time-scales listed for the CMZ in Table 1.”
• THE ATOMIC-TO-MOLECULAR TRANSITION IN GALAXIES. II: H i AND H2 COLUMN DENSITIES (2008, background): “…and a similar dependence of molecular fraction on pressure are also seen in newer surveys such as the HERA CO-Line Extragalactic Survey (HERACLES) that cover a broader range of galaxy properties at higher spatial resolution (Walter et al. 2008; Bigiel et al. 2008; Leroy et al. 2008, hereafter L08).”
• The evolution of the dust and gas content in galaxies (2013, background): “Bigiel et al. (2008) have measured steeper slopes for the star formation laws in local galaxies when both the molecular and atomic gas components are considered.”
• PHIBSS: MOLECULAR GAS CONTENT AND SCALING RELATIONS IN z ∼ 1–3 MASSIVE, MAIN-SEQUENCE STAR-FORMING GALAXIES (2012, background): “In summary, the galaxy integrated data of z~1-3 near-main sequence SFGs favor a near-linear molecular gas–star formation relation, similar to the findings at z~0 (Bigiel et al. 2008, 2011.”
• THE DEPENDENCE OF QUENCHING UPON THE INNER STRUCTURE OF GALAXIES AT 0.5 ⩽ z < 0.8 IN THE DEEP2/AEGIS SURVEY (2012, background): “…have stressed that the surface density of gas in these rings is well below the critical threshold for star formation (Kennicutt 1989; Schaye 2004; Bigiel et al. 2008), which represents the threshold for molecular H2 formation, a tracer of gravitationally unstable gas (e.g. Krumholz et al. 2011).”
• The dust content of galaxies from z = 0 to z = 9 (2016, methodology): “The SF in the SAM is modelled based on an empirical relationship between the surface density of molecular hydrogen and the surface density of star-formation (Bigiel et al. 2008; Genzel et al. 2010; Bigiel & Blitz 2012a).”
• THE SCALE DEPENDENCE OF THE MOLECULAR GAS DEPLETION TIME IN M33 (2010, background): “…Germany 2 National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA 3 University of British Columbia Okanagan, 3333 University Way, Kelowna, BC V1V 1V7, Canada 4 Hubble Fellow Kennicutt et al. 2007; Bigiel et al. 2008; Leroy et al. 2008; Wilson et al. 2008).”
• An Overview of the Dwarf Galaxy Survey (2013, background): “Evidence is growing for molecular gas surface density, in particular, as the regulator for star formation in galaxies of the local Universe as well as at high-redshift, and not necessarily the total gas density (e.g., Kennicutt et al. 2007; Bigiel et al. 2008; Genzel et al. 2012).”
• ENVIRONMENTAL DEPENDENCE OF THE KENNICUTT–SCHMIDT RELATION IN GALAXIES (2010, methodology): “…the molecular gas (e.g., Wong & Blitz 2002a; Bigiel et al. 2008), especially with the densest gas traced by HCN emission (Gao & Solomon 2004; Wu et al. 2005), while it only correlates weakly, if at all, with the density of atomic gas (Wong & Blitz 2002a; Kennicutt et al. 2007; Bigiel et al. 2008).”
• On the fraction of star formation occurring in bound stellar clusters (2012, background): “Thirdly, at low densities galaxies deviate from the simple Schmidt power law that we assume to convert our model densities to SFR densities, having a larger spread at a given gas density (e.g. Bigiel et al. 2008).”