The Relationship between Molecular Gas Tracers and Kennicutt-Schmidt Laws

By Mark R. Krumholz, T. Thompson. 2007

147 citations recorded by the source corpus.

We provide a model for how Kennicutt-Schmidt (KS) laws, which describe the correlation between star formation rate and gas surface or volume density, depend on the molecular line chosen to trace the gas. We show that, for lines that can be excited at low temperatures, the KS law depends on how the line critical density compares to the median density in a galaxy's star-forming molecular clouds. High critical density lines trace regions with similar physical properties across galaxy types, and this produces a linear correlation between line luminosity and star formation rate. Low critical density lines probe regions whose properties vary across galaxies, leading to a star formation rate that varies superlinearly with line luminosity. We show that a simple model in which molecular clouds are treated as isothermal and homogenous can quantitatively reproduce the observed correlations between galactic luminosities in far-infrared and in the CO(1→0) and HCN(1→0) lines, and naturally explains why these correlations have different slopes. We predict that IR-line luminosity correlations should change slope for galaxies in which the median density is close to the line critical density. This prediction may be tested by observations of lines such as HCO+(1→0) with intermediate critical densities, or by HCN(1 → 0) observations of intensely star-forming high-redshift galaxies with very high densities. Recent observations by Gao et al. hint at just such a change in slope. We argue that deviations from linearity in the HCN(1→0)-IR correlation at high luminosity are consistent with the assumption of a constant star formation efficiency.

Subfields: Astronomy and Astrophysics, Spectroscopy

View Paper: https://www.semanticscholar.org/paper/b966f4b636cb34598ed4659b3fb3c6abe9b8c089

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Citations in the corpus

A UNIVERSAL, LOCAL STAR FORMATION LAW IN GALACTIC CLOUDS, NEARBY GALAXIES, HIGH-REDSHIFT DISKS, AND STARBURSTS (2011, unknown): “To date the only published theoretical models for the molecular line-star formation rate correlation are those 14 of Krumholz & Thompson (2007) and Narayanan et al. (2008b), who adopt a model for star formation equivalent to the local, volumetric star formation law (equation 1).”
The CO-H2 Conversion Factor in Disc Galaxies and Mergers (2011, methodology): “The cooling rate is calculated via an escape probability formalism utilising the public code of Krumholz & Thompson (2007).”
HYDRODYNAMICS OF HIGH-REDSHIFT GALAXY COLLISIONS: FROM GAS-RICH DISKS TO DISPERSION-DOMINATED MERGERS AND COMPACT SPHEROIDS (2010, background): “If gas-rich mergers at high-redshift are characterized by strong turbulence, the spread of the gas density distribution could be very large, as it scales with the turbulent mach number (Krumholz & Thompson 2007).”
THE IMPACT OF INTERACTIONS, BARS, BULGES, AND ACTIVE GALACTIC NUCLEI ON STAR FORMATION EFFICIENCY IN LOCAL MASSIVE GALAXIES (2012, background): “Krumholz & Thompson (2007) find that a line with a high critical density like HCN(1-0) traces well the ef- ficiently star forming gas between galaxies with different global properties.”
A general model for the CO–H2 conversion factor in galaxies with applications to the star formation law (2011, methodology): “The escape of the photons from the GMCs is calculated via the public escape probability code of Krumholz & Thompson (2007), which we describe in the subsequent section along with the other molecular line radiative transfer equations.”
Dusty Star Forming Galaxies at High Redshift (2014, methodology): “Models that relate the index of the SFR-line luminosity index for various dense gas tracers to the gas density PDF in galaxies were developed by Krumholz and Thompson (2007) (utilizing analytic models for GMC structure), Narayanan et al. (2008c) (utilizing hydrodynamic models for galaxies in…”
THE STAR FORMATION LAW IN NEARBY GALAXIES ON SUB-KPC SCALES (2008, unknown): “We refer the reader to Krumholz & Thompson (2007) for a full treatment of theoretical expectations for changing Schmidt laws.”
EXTREMELY INEFFICIENT STAR FORMATION IN THE OUTER DISKS OF NEARBY GALAXIES (2010, background): “Krumholz & McKee (2005) and Krumholz & Thompson (2007) argued that star-forming structures on many scales convert ∼ 1% of their gas to stars per free-fall time.”
STAR FORMATION RATES IN MOLECULAR CLOUDS AND THE NATURE OF THE EXTRAGALACTIC SCALING RELATIONS (2011, background): “We thank Leo Blitz, Tom Dame, Daniela Calzetti, Bruce Elmegreen, Debbie Elmegreen, Neal Evans, Reinhard Genzel, and Mark Krumholz for informative discussions and comments and Tom Dame for providing us with CO data.”
Star Formation in the Milky Way and Nearby Galaxies (2012, background): “Various authors (Krumholz & Thompson 2007; Narayanan et al. 2008; Juneau et al. 2009) have pointed out that lines like those of HCN J = 1 → 0 are not thermalized at lower densities, so can result in a linear relation even if the underlying star formation relation is a local version of a non-linear…”
Submillimetre galaxies in a hierarchical universe: number counts, redshift distribution, and implications for the IMF (2012, methodology): “We use N = 1.5, which reproduces the global KS law and is consistent with observations of high-redshift disc galaxies (Krumholz & Thompson 2007; Narayanan et al. 2008a, 2011; but see Narayanan et al. 2012b).”
Molecular Hydrogen and Global Star Formation Relations in Galaxies (2007, background): “…that tie star formation to the fraction of gas in the dense ISM are successful in reproducing many observational trends (e.g., Elmegreen 2002; Kravtsov 2003; Krumholz & McKee 2005; Li et al. 2005b, 2006; Tasker & Bryan 2006, 2007; Krumholz & Thompson 2007; Wada & Norman 2007; Tassis 2007).”
Star-formation laws in luminous infrared galaxies. New observational constraints on models (2011, methodology): “As argued by Krumholz & Thompson (2007), the models for normal galaxies and starbursts are seen to converge in the high density regime (¯n≥105cm−3).”
WHAT DOES A SUBMILLIMETER GALAXY SELECTION ACTUALLY SELECT? THE DEPENDENCE OF SUBMILLIMETER FLUX DENSITY ON STAR FORMATION RATE AND DUST MASS (2010, result): “5gas, with a minimum density threshold; this index is consistent with observations of z ∼ 2 disks (Krumholz & Thompson 2007; Narayanan et al. 2008a, 2011).”
THE IMPACT OF COLD GAS ACCRETION ABOVE A MASS FLOOR ON GALAXY SCALING RELATIONS (2009, background): “For a marginally unstable disk with Toomre Q parameter Q ∼ 1, it can be shown (Martin & Kennicutt 2001; Krumholz & Thompson 2007) that Equation 7 is equivalent to the traditional Kennicutt-Schmidt (KS) relation (Schmidt 1959; Kennicutt 1998), which relates the surface densities of gas and SFR via…”
THE PROPERTIES OF MASSIVE, DENSE CLUMPS: MAPPING SURVEYS OF HCN AND CS (2010, background): “For example, Krumholz & Thompson (2007) assumed that a Schmidt law in the form involving volume density describes star formation: ρ̇⋆ ∝ ρ 1.5 g over the full range of densities.”
THE STAR FORMATION RATE AND GAS SURFACE DENSITY RELATION IN THE MILKY WAY: IMPLICATIONS FOR EXTRAGALACTIC STUDIES (2010, background): “These conditions may not be met during the time span of an individual massive clump (Krumholz & Thompson 2007; Urban et al. 2010).”
STAR FORMATION RELATIONS IN NEARBY MOLECULAR CLOUDS (2014, background): “…∝ ρgasx (3) The idea is that the amount of gas divided by the free fall time should be reflected in the star formation rate, and tff ∝ ρ−0.5 leading to x = 1.5, equal within uncertainties to the exponent in the KS relation (Krumholz & McKee 2005; Krumholz & Thompson 2007; Narayanan et al. 2008).”