The global star formation law of galaxies revisited in the radio continuum

Abstract

We study the global star formation law, the relation between the gas and star formation rate (SFR) in a sample of 130 local galaxies with infrared (IR) luminosities spanning over three orders of magnitude (10 9 -10 12 L  ), which includes 91 normal spiral galaxies and 39 (ultra)luminous IR galaxies [(U)LIRGs]. We derive their total (atomic and molecular) gas and dense molecular gas masses using newly available HI, CO and HCN data from the literature. The SFR of galaxies is determined from total IR (8-1000 m) and 1.4 GHz radio continuum (RC) luminosities. The galaxy disk sizes are defined by the de-convolved elliptical Gaussian FWHM of the RC maps. We derive the galaxy disk-averaged SFRs and various gas surface densities, and investigate their relationships. We find that the galaxy disk-averaged surface density of dense molecular gas mass has the tightest correlation with that of SFR (scatter ~0.26 dex), and is linear in log-log space (power-law slope of N=1.03±0.02) across the full galaxy sample. The correlation between the total gas and SFR surface densities for the full sample has a somewhat larger scatter (~0.48 dex), and is best fit by a power-law with slope 1.45±0.02. However, the slope changes from ~1 when only normal spirals are considered, to ~1.5 when more and more (U)LIRGs are included in the fitting. When different CO-to-H 2 conversion factors are used to infer molecular gas masses for normal galaxies and (U)LIRGs, the bi-modal relations claimed recently in CO observations of high-redshift galaxies appear to also exist in local populations of star-forming galaxies. galaxies: evolation, galaxies: ISM, radio lines: galaxies, stars: fromation PACS number(s): 98.62.Ai, 98.58.D6, 98.54.Ep, 98.52.Nr Citation:Liu L J, Gao Y. The global star formation law of galaxies revisited in the radio continuum.Understanding how stars form in galaxies is fundamentally one of the central questions in galactic and extragalactic astronomy, since star formation (SF) plays an integral part of the formation and evolution of galaxies. Studies of the so-called SF law, which relates the SF rate (SFR) to the density of the gas from which the stars form, have proven to be a fruitful way of approaching this problem (e.g., Kennicutt 1998, hereafter K98 [1] ). Schmidt (1959) [2] first introduced the notion of a general SF law by suggesting that the SFR volume density ( SFR ) varies with the interstellar gas volume density ( gas ) to the power n, i.e. SFR gas , n    where n~2. Under the assumption of a constant scale-height for the star-forming gas, this relation translates directly into an equivalent expression for the SFR and gas surface densities: SFR gas . N   K98 studied the global SF law in log surface densities space and found that a power-law index of N~1.4±0.2 provided a surprisingly tight parameterization of the data. These studies of the Kennicutt-Schmidt SF law (KS law) were among the first to highlight the importance of the gas density in regulating the SFR in star-forming galaxies, including luminous and ultralu...

The Global Star Formation Laws of Galaxies From a Radio Continuum Perspective

“…In the work presented here, we scale the various gas components (atomic, molecular, total gas and dense molecular gas) and uniformly calibrated SFRs (either IR or RC) with the consistently derived radio sizes to derive the surface densities and their relations. A brief report of some preliminary results based on a smaller sam-ple can be found in an earlier paper (Liu & Gao 2012). Here, we further increase our sample size by nearly 50% and present a thorough analysis and discussion.…”

DENSE GAS TRACERS AND STAR FORMATION LAWS IN ACTIVE GALAXIES: APEX SURVEY OF HCN J = 4 → 3, HCO + J = 4 → 3, AND CS J = 7 → 6

“…The Kennicutt-Schmidt (K-S) law (Kennicutt 1998) globally correlates the surface densities of star formation rate (Σ SFR traced by Hα) with total gas mass (Σgas traced by CO and H i), with a slope α ∼ 1.4 (Σ SFR ∝ Σ α gas ). However, quite a range of deviations from the K-S law have been observed, and no unique slope has been found in the L CO -L IR correlations as well (e.g., Liu & Gao 2012;Gao & Solomon 2004a).…”

Dense gas in M 33 (HerM33es)

“…Because stars condense out of dense cores of giant molecular clouds (GMCs), both molecules are promising tracers of star formation (SF) and the star formation rate (SFR). A series of papers (Gao & Solomon 2004a,b;Wu et al 2005;Gao et al 2007;Baan et al 2008;Graciá-Carpio et al 2008;Wu et al 2010;García-Burillo et al 2012;Liu & Gao 2012) have recently investigated the correlation of HCN (and partly HCO + ) with far-infrared (FIR) luminosities (L FIR ) in galactic GMCs, centers of nearby galaxies, and (ultra-)luminous galaxies (LIRGs/ULIRGs), showing that HCN is indeed a good tracer of SF and tightly correlated with L FIR . There are, however, findings that complicate this picture (see e.g.…”