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Register a new userThe Gas-Star Formation Cycle in Nearby Star-Forming Galaxies I. Assessment of Multi-scale Variations
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The processes regulating star formation in galaxies are thought to act across a hierarchy of spatial scales. To connect extragalactic star formation relations from global and kpc-scale measurements to recent cloud-scale resolution studies, we have developed a simple, robust method that quantifies the scale dependence of the relative spatial distributions of molecular gas and recent star formation. In this paper, we apply this method to eight galaxies with roughly 1 arcsec resolution molecular gas imaging from the PHANGS-ALMA and PAWS surveys that have matched resolution, high quality narrowband Halpha imaging. At a common scale of 140pc, our massive (log(Mstar/Msun)=9.3-10.7), normally star-forming (SFR/Msun/yr=0.3-5.9) galaxies exhibit a significant reservoir of quiescent molecular gas not associated with star formation as traced by Halpha emission. Galactic structures act as backbones for both molecular and HII region distributions. As we degrade the spatial resolution, the quiescent molecular gas disappears, with the most rapid changes occurring for resolutions up to about 0.5kpc. As the resolution becomes poorer, the morphological features become indistinct for spatial scales larger than about 1kpc. The method is a promising tool to search for relationships between the quiescent or star-forming molecular reservoir and galaxy properties, but requires a larger sample size to identify robust correlations between the star-forming molecular gas fraction and global galaxy parameters.
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We use new ALMA observations to investigate the connection between dense gas fraction, star formation rate, and local environment across the inner region of four local galaxies showing a wide range of molecular gas depletion times. We map HCN (1-0), HCO$^+$ (1-0), CS (2-1), $^{13}$CO (1-0), and C$^{18}$O (1-0) across the inner few kpc of each target. We combine these data with short spacing information from the IRAM large program EMPIRE, archival CO maps, tracers of stellar structure and recent star formation, and recent HCN surveys by Bigiel et al. and Usero et al. We test the degree to which changes in the dense gas fraction drive changes in the SFR. $I_{HCN}/I_{CO}$ (tracing the dense gas fraction) correlates strongly with $I_{CO}$ (tracing molecular gas surface density), stellar surface density, and dynamical equilibrium pressure, $P_{DE}$. Therefore, $I_{HCN}/I_{CO}$ becomes very low and HCN becomes very faint at large galactocentric radii, where ratios as low as $I_{HCN}/I_{CO} sim 0.01$ become common. The apparent ability of dense gas to form stars, $Sigma_{SFR}/Sigma_{dense}$ (where $Sigma_{dense}$ is traced by the HCN intensity and the star formation rate is traced by a combination of H$alpha$ and 24$mu$m emission), also depends on environment. $Sigma_{SFR}/Sigma_{dense}$ decreases in regions of high gas surface density, high stellar surface density, and high $P_{DE}$. Statistically, these correlations between environment and both $Sigma_{SFR}/Sigma_{dense}$ and $I_{HCN}/I_{CO}$ are stronger than that between apparent dense gas fraction ($I_{HCN}/I_{CO}$) and the apparent molecular gas star formation efficiency $Sigma_{SFR}/Sigma_{mol}$. We show that these results are not specific to HCN.
In order to quantify the relationship between gas accretion and star formation, we analyse a sample of 29 nearby galaxies from the WHISP survey which contains galaxies with and without evidence for recent gas accretion. We compare combined radial profiles of FUV (GALEX) and IR 24 {mu}m (Spitzer) characterizing distributions of recent star formation with radial profiles of CO (IRAM, BIMA, or CARMA) and HI (WSRT) tracing molecular and atomic gas contents to examine star formation efficiencies in symmetric (quiescent), asymmetric (accreting), and interacting (tidally disturbed) galaxies. In addition, we investigate the relationship between star formation rate and HI in the outer discs for the three groups of galaxies. We confirm the general relationship between gas surface density and star formation surface density, but do not find a significant difference between the three groups of galaxies.
We examined radial variations in molecular-gas based star formation efficiency (SFE), which is defined as star formation rate per unit molecular gas mass, for 80 galaxies selected from the CO Multi-line Imaging of Nearby Galaxies project (Sorai et al. 2019). The radial variations in SFE for individual galaxies are typically a factor of 2 -- 3, which suggests that SFE is nearly constant along galactocentric radius. We found the averaged SFE in 80 galaxies of $(1.69 pm 1.1) times 10^{-9}$ yr$^{-1}$, which is consistent with Leroy et al. 2008 if we consider the contribution of helium to the molecular gas mass evaluation and the difference in the assumed initial mass function between two studies. We compared SFE among different morphological (i.e., SA, SAB, and SB) types, and found that SFE within the inner radii ($r/r_{25} < 0.3$, where $r_{25}$ is $B$-band isophotal radius at 25 mag arcsec$^{-2}$) of SB galaxies is slightly higher than that of SA and SAB galaxies. This trend can be partly explained by the dependence of SFE on global stellar mass, which probably relates to the CO-to-H$_2$ conversion factor through the metallicity. For two representative SB galaxies in our sample, NGC 3367 and NGC 7479, the ellipse of $r/r_{25}$ = 0.3 seems to cover not only the central region but also the inner part of the disk, mainly the bar. These two galaxies show higher SFE in the bar than in spiral arms. However, we found an opposite trend in NGC 4303; SFE is lower in the bar than in spiral arms, which is consistent with earlier studies (e.g., Momose et al. 2010). These results suggest diversity of star formation activities in the bar.
We present multi-wavelength global star formation rate (SFR) estimates for 326 galaxies from the Star Formation Reference Survey (SFRS) in order to determine the mutual scatter and range of validity of different indicators. The widely used empirical SFR recipes based on 1.4 GHz continuum, 8.0 $mu$m polycyclic aromatic hydrocarbons (PAH), and a combination of far-infrared (FIR) plus ultraviolet (UV) emission are mutually consistent with scatter of $raise{-0.8ex}stackrel{textstyle <}{sim }$0.3 dex. The scatter is even smaller, $raise{-0.8ex}stackrel{textstyle <}{sim }$0.24 dex, in the intermediate luminosity range 9.3<log(L(60 $mu$m/L$_odot$)<10.7. The data prefer a non-linear relation between 1.4 GHz luminosity and other SFR measures. PAH luminosity underestimates SFR for galaxies with strong UV emission. A bolometric extinction correction to far-ultraviolet luminosity yields SFR within 0.2 dex of the total SFR estimate, but extinction corrections based on UV spectral slope or nuclear Balmer decrement give SFRs that may differ from the total SFR by up to 2 dex. However, for the minority of galaxies with UV luminosity ${>}5times10^9$ L$_{odot}$ or with implied far-UV extinction <1 mag, the UV spectral slope gives extinction corrections with 0.22~dex uncertainty.
We study the relationship between dense gas and star formation in the Antennae galaxies by comparing ALMA observations of dense gas tracers (HCN, HCO$^+$, and HNC $mathrm{J}=1-0$) to the total infrared luminosity ($mathrm{L_{TIR}}$) calculated using data from the textit{Herschel} Space Observatory and the textit{Spitzer} Space Telescope. We compare the luminosities of our SFR and gas tracers using aperture photometry and employing two methods for defining apertures. We taper the ALMA dataset to match the resolution of our $mathrm{L_{TIR}}$ maps and present new detections of dense gas emission from complexes in the overlap and western arm regions. Using OVRO CO $mathrm{J}=1-0$ data, we compare with the total molecular gas content, $mathrm{M(H_2)_{tot}}$, and calculate star formation efficiencies and dense gas mass fractions for these different regions. We derive HCN, HCO$^+$ and HNC upper limits for apertures where emission was not significantly detected, as we expect emission from dense gas should be present in most star-forming regions. The Antennae extends the linear $mathrm{L_{TIR}-L_{HCN}}$ relationship found in previous studies. The $mathrm{L_{TIR}-L_{HCN}}$ ratio varies by up to a factor of $sim$10 across different regions of the Antennae implying variations in the star formation efficiency of dense gas, with the nuclei, NGC 4038 and NGC 4039, showing the lowest SFE$_mathrm{dense}$ (0.44 and 0.70 $times10^{-8}$ yr$^{-1}$). The nuclei also exhibit the highest dense gas fractions ($sim 9.1%$ and $sim7.9%$).
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