Measurements of Hα, H I, 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 modeling the SFR in numerical simulations of galaxy formation and evolution.

https://iopscience.iop.org/article/10.1086/305588/pdf

The Global Schmidt law in star forming galaxies

We present far-infrared (FIR) photometry at 150 and 205 micron(s) of eight low-redshift starburst galaxies obtained with the Infrared Space Observatory (ISO) ISOPHOT. Five of the eight galaxies are detected in both wave bands, and these data are used, in conjunction with IRAS archival photometry, to model the dust emission at lambda approximately greater than 40 microns. The FIR spectral energy distributions (SEDs) are best fitted by a combination of two modified Planck functions, with T approx. 40 - 55 K (warm dust) and T approx. 20-23 K (cool dust) and with a dust emissivity index epsilon = 2. The cool dust can be a major contributor to the FIR emission of starburst galaxies, representing up to 60% of the total flux. This component is heated not only by the general interstellar radiation field, but also by the starburst itself. The cool dust mass is up to approx. 150 times larger than the warm dust mass, bringing the gas-to-dust ratios of the starbursts in our sample close to Milky Way values, once resealed for the appropriate metallicity. The ratio between the total dust FIR emission in the range 1-1000 microns and the IRAS FIR emission in the range 40 - 120 microns is approx. 1.75, with small variations from galaxy to galaxy. This ratio is about 40% larger than previously inferred from data at millimeter wavelengths. Although the galaxies in our sample are generally classified as "UV bright," for four of them the UV energy emerging shortward of 0.2 microns is less than 15% of the FIR energy. On average, about 30% of the bolometric flux is coming out in the UV-to-near-IR wavelength range; the rest is emitted in the FIR. Energy balance calculations show that the FIR emission predicted by the dust reddening of the UV-to-near-IR stellar emission is within a factor of approx. 2 of the observed value in individual galaxies and within 20% when averaged over a large sample. If our sample of local starbursts is representative of high-redshift (z approx. greater than 1), UV - bright star-forming galaxies, these galaxies' FIR emission will be generally undetected in submillimeter surveys, unless: (1) their bolometric luminosity is comparable to or larger than that of ultraluminous FIR galaxies and (2) their FIR SED contains a cool dust component.

/pdf/the-dust-content-and-opacity-of-actively-star-forming-3rvrlvzkyq.pdf

The Dust Content and Opacity of Actively Star-Forming Galaxies

We present a comprehensive study of the physical properties of ∼ 10 5 galaxies with measurable star formation in the Sloan Digital Sky Survey (SDSS). By comparing physical information extracted from the emission lines with continuum properties, we build up a picture of the nature of star-forming galaxies at z < 0.2. We develop a method for aperture correction using resolved imaging and show that our method takes out essentially all aperture bias in the star formation rate (SFR) estimates, allowing an accurate estimate of the total SFRs in galaxies. We determine the SFR density to be 1.915 +0.02 −0.01 (random) +0.14

The physical properties of star-forming galaxies in the low-redshift universe

▪ Abstract At luminosities above 1011 , infrared galaxies become the dominant population of extragalactic objects in the local Universe (z ≲ 0.3), being more numerous than optically selected starburst and Seyfert galaxies and quasi-stellar objects at comparable bolometric luminosity. The trigger for the intense infrared emission appears to be the strong interaction/merger of molecular gas-rich spirals, and the bulk of the infrared luminosity for all but the most luminous objects is due to dust heating from an intense starburst within giant molecular clouds. At the highest luminosities (Lir > 1012 ), nearly all objects appear to be advanced mergers powered by a mixture of circumnuclear starburst and active galactic nucleus energy sources, both of which are fueled by an enormous concentration of molecular gas that has been funneled into the merger nucleus. These ultraluminous infrared galaxies may represent an important stage in the formation of quasi-stellar objects and powerful radio galaxies. They may al...

/pdf/luminous-infrared-galaxies-zoi3y3hutv.pdf

Luminous infrared galaxies

▪ Abstract The Local Group dwarf galaxies offer a unique window to the detailed properties of the most common type of galaxy in the Universe. In this review, I update the census of Local Group dwarfs based on the most recent distance and radial velocity determinations. I then discuss the detailed properties of this sample, including (a) the integrated photometric parameters and optical structures of these galaxies, (b) the content, nature, and distribution of their interstellar medium (ISM), (c) their heavy-element abundances derived from both stars and nebulae, (d) the complex and varied star-formation histories of these dwarfs, (e) their internal kinematics, stressing the relevance of these galaxies to the “dark matter problem” and to alternative interpretations, and (f) evidence for past, ongoing, and future interactions of these dwarfs with other galaxies in the Local Group and beyond. To complement the discussion and to serve as a foundation for future work, I present an extensive set of basic observ...

/pdf/dwarf-galaxies-of-the-local-group-4ham6n81f0.pdf

Dwarf galaxies of the local group

Emission from the CO molecule at lambda = 2.6 mm has been observed at 1412 positions in 300 galaxies using the 14 m telescope of the Five College Radio Astronomy Observatory (HPBW = 45"); these data comprise the FCRAO Extragalactic CO Survey. In this paper we describe the galaxy sample, present the data, and determine global CO fluxes and radial distributions for the galaxies in the Survey. Future papers will deal with the data analysis, both with regard to the global properties of galaxies and the radial distributions within them. CO emission was detected in 236 of the 300 Survey galaxies for an overall detection rate of 79%; among the 52 Sc galaxies in the Survey, the detection rate was as high as 96%. most of the 193 galaxies observed in multiple positions exhibit CO distributions which peak at the center. However, a small number (10-primarily Sb galaxies) exhibit CO rings at 45" resolution, and a similar number (18-primarily Sc galaxies) have CO distributions which peak on one side of the center. We derive CO isophotal diameters for 151 galaxies and find the mean ratio of CO to optical isophotal diameters to be 0.5. We also find a trend along the Hubble sequence such that the mean ratio of CO to optical isophotal diameters is smallest among the early-type spirals (SO/a, Sa, and Sab) and the mean ratio increases for Sb, Sbc, and Sc galaxies, finally decreasing among the later types. Comparison of the global fluxes we derive for the Survey galaxies with independent measurements from the literature indicates that the global fluxes we derive are accurate to ~40%.

The FCRAO Extragalactic CO Survey. I. The Data

A systematic search was carried out for high-velocity CO emission associated with bright 100-micron sources from the IRAS Point Source Catalog, in an effort to increase the understanding of the nature and evolutionary status of the objects producing molecular outflows. Eighteen sources with 100-micron flux densities greater than 500 Jy were selected and maps were made in the J = 1-0 (C-12) line around each source. Almost every source observed was found to lie toward, or in the immediate vicinity of, strong CO emission and in most cases close to the maximum of this emission. Thus, all of the far-infrared sources can be associated with molecular clouds. Five of the sources show clear evidence of high-velocity molecular emission and at least three have bipolar morphologies. The newly detected outflow sources are all intrinsically luminous objects, and their outflows energetic. The infrared characteristics of all the sources in the present survey suggest that they are young stellar objects still embedded in their parent molecular clouds. Statistics on the occurrence of outflows from bright far-infrared sources are used to set an upper limit of 400,000 yr for the dispersal time scale of material around young, luminous stellar objects. 31 references.

Molecular outflows associated with bright far-infrared sources

Results of a systematic search for high-velocity CO emission from a sample of bright FIR sources from the IRAS Point Source Catalog are presented. This extends the original survey to a complete flux-limited sample. Results of CO mapping toward 22 FIR sources are given, showing that all of the sources are associated with strong CO emission that is peaked close to the FIR sources. Based on their IR colors and positional coincidence with strong CO emission, it is suggested that the sample is composed entirely of luminous, young stellar objects. New detections of molecular outflows were made toward six of the FIR sources. These outflows are relatively energetic, with typical mechanical energies of 10 to the 45th ergs. 30 refs.

Molecular outflows associated with a flux-limited sample of bright far-infrared sources

The study of extragalactic supernova (SNs) suggests that Type II SNs, not Type I, tend to occur near extreme optical Population I objects, but the detection of these objects in the Galaxy is limited by heavy local obscuration. A CO survey has been conducted toward every confirmed outer Galaxy SNR from l = 70 to 210 deg, for a total of 26, and it is found that roughly half of them, within uncertainties of distance estimates, revealed spatial coincidences with large molecular cloud complexes. Most of the cloud complexes in these coincidences probably are the birthplaces of the progenitors of the corresponding Type II SNRs, because it is statistically improbable that the coincidences result from change superposition.

Molecular clouds and supernova remnants in the outer galaxy

We present the results of systematic, high-resolution J=1→0 CO and HCO + mapping observations of IC 443 carried out with the FCRAO 14 m radio telescope. Five new clumps of perturbed molecular gas have been identified and their relationship to one another clarified. Together with three previously known perturbed regions, the clumps outline a roughly elliptical ring whose major axis is ∼9 pc across

Y. L. Huang

Papers

The FCRAO Extragalactic CO Survey. I. The Data

Molecular outflows associated with bright far-infrared sources

Molecular outflows associated with a flux-limited sample of bright far-infrared sources

Molecular clouds and supernova remnants in the outer galaxy

Structure and kinematics of dense gas associated with the supernova remnant ic-443