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

Abstract

We study the global star formation law -the relation between gas and star formation (SF) rates in a sample of 181 local galaxies with infrared (IR) luminosities spanning almost five orders of magnitude (10 7.8 − 10 12.3 L ⊙ ), which includes 115 normal spiral galaxies and 66 (ultra)luminous IR galaxies [(U)LIRGs, L IR ≥ 10 11 L ⊙ ]. We derive their atomic, molecular gas and dense molecular gas masses using newly available HI, CO and HCN data from the literature, and SF rates are determined both from total IR (8 − 1000 µm) and 1.4 GHz radio continuum (RC) luminosities. In order to derive the disk-averaged surface densities of gas and SF rates, we have taken a novel approach and used high-resolution RC observations to measure the radio sizes for all 181 galaxies. In our sample, we find that the surface density of dense molecular gas (as traced by HCN) has the tightest correlation with that of SF rates (Σ SFR ), and is linear in log − log space (power-law slope of N = 1.01 ± 0.02) across the full galaxy sample. The correlation between surface densities of molecular gas (Σ H2 ,traced by CO) and Σ SFR is sensitive to the adopted value of the CO-to-H 2 conversion factor (α CO ) used to infer molecular gas masses from CO luminosities. For a fixed Galactic value of α CO , a power law index of 1.14 ± 0.02 is found. If instead we adopt values for α CO of 4.6 and 0.8 for disk galaxies and (U)LIRGs, respectively, we find the two galaxy populations separate into two distinct Σ SFR versus Σ H2 relations. Finally, applying a continuously varying α CO to our sample, we recover a single Σ SFR -Σ H2 relation with slope of 1.60 ± 0.03. The Σ SFR is a steeper function of total gas Σ gas (molecular gas with atomic gas) than that of molecular gas Σ H2 , and are tighter among low-luminosity galaxies. We find no correlation between global surface densities of SFRs and atomic gas (H I).

The MALATANG Survey: The LGAS–LIR Correlation on Sub-kiloparsec Scale in Six Nearby Star-forming Galaxies as Traced by HCN J = 4 → 3 and HCO+ J = 4 → 3

“…Our results are consistent with the correlation between the IR and the HCN (1-0) luminosities shown in Gao & Solomon (2004a). Moreover, a tight linear correlation between the surface densities of the dense molecular gas and the SF rates, as well as between the HCN luminosity and the radio continuum luminosity, has been established for a large sample of galaxies (e.g., Liu & Gao 2010;Chen et al 2015Chen et al , 2017Liu et al 2015b). All of these results indicate that the star formation is very likely physically related to the dense molecular gas.…”

“…A least-squares fit and a Spearman test yield log(L IR /L′ HCN (4-3) )=2.1(±0.5) log( f 70 /f 100 ) + 3.8 (r s =0.50, p-value=3.5×10 ), respectively, indicating that there is a statistically significant correlation between L IR /L′ dense and T d . These correlations are slightly stronger than the correlation between L IR /L′ HCN (1-0) and f 60 μm /f 100 μm , but not as strong as the L IR /L′ CO (1-0) versus f 60 μm /f 100 μm correlation (correlation coefficient of 0.85; see Figure 9 in Gao & Solomon 2004a;Liu et al 2015b).…”

“…In contrast, a slope of unity relates Σ SFR and H 2 S for normal and dwarf galaxies (Schruba 2013). However, the slope steepens for the SFR H 2 S S -relation if we expand the sample to include galaxies with extreme starbursts, such as luminous infrared galaxies (LIRGs, L L L 10 10 11 IR 12  <   ) and ultraluminous infrared galaxies (ULIRGs, L L 10 IR 12   ), which is evident from both observations (e.g., Gao & Solomon 2004b;Daddi et al 2010;Liu et al 2015b) and theoretical predictions (e.g., Krumholz & McKee 2005;Elmegreen 2015Elmegreen , 2018. In addition, a breakdown of the KS law is found at giant molecular cloud (GMC) scales of a few tens of parsecs (e.g., Onodera et al 2010;Nguyen-Luong et al 2016), which is attributed to the dynamical evolution of GMCs and the drift of young clusters from their GMCs.…”

Star Formation Relations in the Milky Way

“…The two green points (triangles) are from Chen et al (2015), who observed M51 with 1 kpc resolution; the higher point is for the outer galaxy and the lower point is for inner (r < 1.66 kpc) part of M51. The blue point (pentagon) represents the whole, normal galaxies while the cyan point (hexagon) represents the whole (U)LIRGs, both from Liu et al (2015).…”

“…The (U)LIRGs in Liu et al (2015) are again plotted in cyan, but only one value of conversion from HCN emission to dense gas mass was used by Liu et al (2015). Observations of HCN in M51 with a resolution of 1 kpc bridge the gap between the Milky Way clouds and whole galaxies.…”