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Register a new userOn The Origin of HI in Galaxies: Photodissociation and the ``Schmidt Law for Global Star Formation
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Authors
Ronald J. Allen
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Young stars in the disks of galaxies produce HI from their parent H2 clouds by photodissociation. This process is widespread in late-type galaxies, and follows the distribution of Far-UV photons produced primarily by B-type stars. An estimate of the amount of dissociated gas can be made using observed Far-UV fluxes and simple approximations for the physics of photodissociation. This leads to the startling conclusion that much, and perhaps even all, of the HI in galaxy disks can be produced in this way. This result offers a simple, but inverse, cause-effect explanation for the ``Schmidt Law of Global Star Formation in galaxies.
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Measurements of H-alpha, HI, and CO distributions in 61 normal spiral galaxies are combined with published far-infrared and CO observations of 36 infrared-selected starburst galaxies, in order to study the form of the global star formation law, over the full range of gas densities and star formation rates (SFRs) observed in galaxies. The disk-averaged SFRs and gas densities for the combined sample are well represented by a Schmidt law with index N = 1.4+-0.15. The Schmidt law provides a surprisingly tight parametrization of the global star formation law, extending over several orders of magnitude in SFR and gas density. An alternative formulation of the star formation law, in which the SFR is presumed to scale with the ratio of the gas density to the average orbital timescale, also fits the data very well. Both descriptions provide potentially useful recipes for modelling the SFR in numerical simulations of galaxy formation and evolution.
We compile observations of molecular gas contents and infrared-based star formation rates (SFRs) for 112 circumnuclear star forming regions, in order to re-investigate the form of the disk-averaged Schmidt surface density star formation law in starbursts. We then combine these results with total gas and SFR surface densities for 153 nearby non-starbursting disk galaxies from de los Reyes & Kennicutt (2019), to investigate the properties of the combined star formation law, following Kennicutt (1998; K98). We confirm that the combined Schmidt law can be fitted with a single power law with slope $n = 1.5pm0.05$ (including fitting method uncertainties), somewhat steeper than the value $n = 1.4pm0.15$ found by K98. Fitting separate power laws to the non-starbursting and starburst galaxies, however, produces very different slopes ($n = 1.34pm0.07$ and $0.98pm0.07$, respectively), with a pronounced offset in the zeropoint ($sim$0.6,dex) of the starburst relation to higher SFR surface densities. This offset is seen even when a common conversion factor between CO intensity and molecular hydrogen surface density is applied, and is confirmed when disk surface densities of interstellar dust are used as proxies for gas measurements. Tests for possible systematic biases in the starburst data fail to uncover any spurious sources for such a large offset. We tentatively conclude that the global Schmidt law in galaxies, at least as it is conventionally measured, is bimodal or possibly multi-modal. Possible causes may include changes in the small-scale structure of the molecular ISM or the stellar initial mass function. A single $n sim 1.5$ power law still remains as a credible approximation or recipe for analytical or numerical models of galaxy formation and evolution.
Young stars in the disks of galaxies produce HI from their parent H2 clouds by photodissociation. This paper describes the observational evidence for and the morphology of such HI. Simple estimates of the amount of dissociated gas lead to the startling conclusion that much, and perhaps even all, of the HI in galaxy disks can be produced in this way.
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 Lsun), 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 um) 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 relationship. We find that the galaxy disk-averaged surface densities 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-H2 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.
We use a new method to trace backwards the star formation history of the Milky Way disk, using a sample of M dwarfs in the solar neighbourhood which is representative for the entire solar circle. M stars are used because they show H_alpha emission until a particular age which is a well calibrated function of their absolute magnitudes. This allows us to reconstruct the rate at which disk stars have been born over about half the disks lifetime. Our star formation rate agrees well with those obtained by using other, independent, methods and seems to rule out a constant star formation rate. The principal result of this study is to show that a relation of the Schmidt-Kennicut type (which relates the star formation rate to the interstellar gas content of galaxy disks) has pertained in the Milky Way disk during the last 5 Gyr. The star formation rate we derive from the M dwarfs and the interstellar gas content of the disk can be inferred as a function of time from a model of the chemical enrichment of the disk, which is well constrained by the observations indicating that the metallicity of the Galactic disk has remained nearly constant over the timescales involved. We demonstrate that the star formation rate and gas surface densities over the last 5 Gyrs can be accurately described by a Schmidt-Kennicutt law with an index of Gamma = 1.45 (+0.22,-0.09). This is, within statistical uncertainties, the same value found for other galaxies.
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