Star Formation in the Milky Way and Nearby Galaxies

Abstract

We review progress over the past decade in observations of large-scale star formation, with a focus on the interface between extragalactic and Galactic studies. Methods of measuring gas contents and star formation rates are discussed, and updated prescriptions for calculating star formation rates are provided. We review relations between star formation and gas on scales ranging from entire galaxies to individual molecular clouds. Subject headings: star formation, galaxies, Milky Way 1. OVERVIEW 1.1. Introduction Star formation encompasses the origins of stars and planetary systems, but it is also a principal agent of galaxy formation and evolution, and hence a subject at the roots of astrophysics on its largest scales.The past decade has witnessed an unprecedented stream of new observational information on star formation on all scales, thanks in no small part to new facilities such as the Galaxy Evolution Explorer (GALEX), the Spitzer Space Telescope, the Herschel Space Observatory, the introduction of powerful new instruments on the Hubble Space Telescope (HST), and a host of groundbased optical, infrared, submillimeter, and radio telescopes. These new observations are providing a detailed reconstruction of the key evolutionary phases and physical processes that lead to the formation of individual stars in interstellar clouds, while at the same time extending the reach of integrated measurements of star formation rates (SFRs) to the most distant galaxies known. The new data have also stimulated a parallel renaissance in theoretical investigation and numerical modelling of the star formation process, on scales ranging from individual protostellar and protoplanetary systems to the scales of molecular clouds and star clusters, entire galaxies and ensembles of galaxies, even to the first objects, which are thought to have reionized the Universe and seeded today's stellar populations and Hubble sequence of galaxies.This immense expansion of the subject, both in terms of the volume of results and the range of physical scales explored, may help to explain one of its idiosyncracies, namely the relative isolation between the community studying individual star-forming regions and stars in the Milky Way (usually abbreviated hereafter as MW, but sometimes referred to as "the Galaxy"), and the largely extragalactic community that attempts to characterise the star formation process on galactic and cosmologiElectronic address: robk@ast.cam.as.uk Electronic address: nje@astro.as.utexas.edu cal scales. Some aspects of this separation have been understandable. The key physical processes that determine how molecular clouds contract and fragment into clumps and cores and finally clusters and individual stars can be probed up close only in the Galaxy, and much of the progress in this subject has come from in-depth case studies of individual star-forming regions. Such detailed observations have been impossible to obtain for even relatively nearby galaxies. Instead the extragalactic branch of the subject has focused on the co...

A dusty, normal galaxy in the epoch of reionization

“…For the TIR luminosity we obtain L TIR = 6.2 ± 0.8×10 10 L ⊙ , which is the flux uncertainty only, using our fiducial model, but where the true uncertainty is constrained primarily by the desire to remain consistent within the scatter with the IRX-β UV relation. These values are consistent with those derived from modelling the SED (see below) and result in a SFR TIR = 9±2 M ⊙ yr -1 , using recent calibrations based on the 3-1100 µm IR luminosity 14 . A SFR TIR much below 7 M ⊙ yr -1 is excluded in this model.…”

“…And while the lack of Lyα emission in this galaxy could be explained by IGM absorption, the absence of C III] emission cannot, and is consistent with a more evolved galaxy. The surface area we are observing in UV emission is only approximately 1.5 kpc 2 (lensing-corrected size), thus the star-formation rate per unit area is high, approximately 8 M ⊙ yr -1 kpc -2 , comparable to vigorous starburst galaxies with similar SFRs 14 , but an order of magnitude or more lower per unit surface area than the extreme starbursts found in high redshift submm surveys or QSOs 20 .…”

“…Gas mass and metallicity. We invert the Schmidt-Kennicutt law relating the surface densities of SFR and gas mass 14 to obtain constraints on the gas mass. The dominant uncertainty on this conversion comes from the uncertainty in the slope of the S-K law, which is a ±0.4 dex scatter-the dependence on the surface area of the galaxy is very small (0.04 dex) since it appears in both the SFR and gas mass terms.…”

“…The dominant uncertainty on this conversion comes from the uncertainty in the slope of the S-K law, which is a ±0.4 dex scatter-the dependence on the surface area of the galaxy is very small (0.04 dex) since it appears in both the SFR and gas mass terms. We here use the relation as originally derived, which reproduces even starburst galaxies quite well, assuming a constant CO to gas conversion, X(CO) 14 . Low metallicity galaxies may have lower gas mass for a given SFR 14 , which would only increase the dust-to-gas ratio derived here.…”

“…We can derive an approximate gas mass for this galaxy by inverting the Schmidt-Kennicutt law based on its size and star-formation rate 14 and we deduce a gas mass of 2 +2 -1 ×10 9 M ⊙ . This gives a dustto-gas mass ratio of about 17×10 -3 .…”