Star Formation Rate Determinations

I review determinations of star formation rates (SFR) from the ultraviolet to the infrared, in the context of their use for galaxies and galaxy surveys. The mid-infrared SFR indicators have garnered interest in recent years thanks to the Spitzer capabilities and the opportunities offered by the upcoming Herschel Space Telescope. I discuss what we have learned in the mid-infrared from studies of local galaxies combining Spitzer with HST and other data.Comment: 12 pages, 4 figures; to appear in the Proceedings of the conference `Pathways through an Eclectic Universe', J.H. Knapen, T.J. Mahoney, and A. Vazdekis eds., ASP Conference Serries, 200

Dust in External Galaxies

Existing (Spitzer Space Telescope) and upcoming (Herschel Space Telescope) facilities are deepening our understanding of the role of dust in tracing the energy budget and chemical evolution of galaxies. The tools we are developing while exploring the local Universe will in turn become pivotal in the interpretation of the high redshift Universe when near--future facilities (the Atacama Large Millimeter Array [ALMA], the Sub--Millimeter Array [SMA], the Large Millimeter Telescope [LMT], the James Webb Space Telescope [JWST]), and, possibly, farther--future ones, will begin operations.Comment: 14 pages, 5 figures, invited review at the conference `Cosmic Dust - Near & Far', Heidelberg, Germany, September 2008. ASP Conference Series, Eds. T. Henning et a

UV Opacity in Nearby Galaxies and Application to Distant Galaxies

The effects of dust opacity on the radiation of nearby and distant galaxies are reviewed. The geometrical distribution of the dust inside the galaxy plays a fundamental role in determining the wavelength dependence of the obscuration and the opacity of the galaxy. In the local Universe, late Hubble type galaxies appear to contain enough dust that corrections for the effect of obscuration become important. This is true expecially at blue and UV wavelengths, i.e. in the wavelength range of interest for studies of massive stars and star formation processes. Multiwavelength observations provide a powerful tool for characterizing the reddening caused by dust. A `recipe' is given for removing the dust reddening and recovering the UV and optical light in star-forming galaxies.Comment: 10 pages, Latex, 2 encapsulated Postscript figures, aipproc.sty macro, to appear in the Proceedings of the Conference `The Ultraviolet Universe at Low and High Redshift' (College Park, MD, May 2-4, 1997

Star Formation Rate Indicators

What else can be said about star formation rate indicators that has not been said already many times over? The `coming of age' of large ground-based surveys and the unprecedented sensitivity, angular resolution and/or field-of-view of infrared and ultraviolet space missions have provided extensive, homogeneous data on both nearby and distant galaxies, which have been used to further our understanding of the strengths and pitfalls of many common star formation rate indicators. The synergy between these surveys has also enabled the calibration of indicators for use on scales that are comparable to those of star-forming regions, thus much smaller than an entire galaxy. These are being used to investigate star formation processes at the sub-galactic scale. I review progress in the field over the past decade or so.Comment: 41 pages, 5 figures. Proceedings of the XXIII Canary Islands Winter School of Astrophysics: `Secular Evolution of Galaxies', edited by J. Falcon-Barroso and J.H. Knape

The New Frontier: Galactic-Scale Star Formation

The arena of investigation of star formation and its scaling laws is slowly, but consistently, shifting from the realm of luminous galaxies to that of faint ones and to sub--galactic regions, as existing and new facilities enable investigators to probe regions of the combined parameter space of surface brightness, wavelength, and angular resolution that were inaccessible until a few years ago. We summarize what has been accomplished, and what remain as challenges in the field of galactic--scale star formation.Comment: accepted for publication on PASP, short review for the IYA2009, 12 pages, no figure

Spitzer 70~μ\mum Emission as a SFR Indicator for Sub--Galactic Regions

We use Spitzer 24 $\mu$m, 70 $\mu$m and ground based H$\alpha$ data for a sample of 40 SINGS galaxies to establish a star formation rate (SFR) indicator using 70 $\mu$m emission for sub--galactic ($\sim0.05-2\ \rm{kpc}$) line-emitting regions and to investigate limits in application. A linear correlation between 70 $\mu$m and SFR is found and a star formation indicator SFR(70) is proposed for line-emitting sub-galactic regions as $\rm \Sigma(SFR)\ ({M_{\odot}\cdot yr^{-1}\cdot kpc^{-2}})=9.4\times10^{-44}\ \Sigma(70)\ \rm{(ergs\cdot s^{-1}\cdot kpc^{-2})}$, for regions with$12+\rm{log(O/H)}\gtrsim8.4$and$\rm \Sigma(SFR)\gtrsim10^{-3}\ (M_{\odot}\cdot yr^{-1}\cdot kpc^{-2})$, with a 1-$\sigma$dispersion around the calibration of$\sim0.16$dex. We also discuss the influence of metallicity on the scatter of the data. Comparing with the SFR indicator at 70$\mu$m for integrated light from galaxies, we find that there is$\sim40%$excess 70$\mu$m emission in galaxies, which can be attributed to stellar populations not involved in the current star formation activity.Comment: 36 pages, 1 table, 18 figures, accepted by Ap

The present-day star formation rate of the Milky-Way determined from Spitzer detected young stellar objects

We present initial results from a population synthesis model aimed at determining the star formation rate of the Milky-Way. We find that a total star formation rate of 0.68 to 1.45 Msun/yr is able to reproduce the observed number of young stellar objects in the Spitzer/IRAC GLIMPSE survey of the Galactic plane, assuming simple prescriptions for the 3D Galactic distributions of YSOs and interstellar dust, and using model SEDs to predict the brightness and color of the synthetic YSOs at different wavelengths. This is the first Galaxy-wide measurement derived from pre-main-sequence objects themselves, rather than global observables such as the total radio continuum, Halpha, or FIR flux. The value obtained is slightly lower than, but generally consistent with previously determined values. We will extend this method in the future to fit the brightness, color, and angular distribution of YSOs, and simultaneously make use of multiple surveys, to place constraints on the input assumptions, and reduce uncertainties in the star formation rate estimate. Ultimately, this will be one of the most accurate methods for determining the Galactic star formation rate, as it makes use of stars of all masses (limited only by sensitivity) rather than solely massive stars or indirect tracers of massive stars.Comment: 12 pages, 3 figures, Published in the Astrophysical Journal Letter

Star Formation Laws: the Effects of Gas Cloud Sampling

Recent observational results indicate that the functional shape of the spatially-resolved star formation-molecular gas density relation depends on the spatial scale considered. These results may indicate a fundamental role of sampling effects on scales that are typically only a few times larger than those of the largest molecular clouds. To investigate the impact of this effect, we construct simple models for the distribution of molecular clouds in a typical star-forming spiral galaxy, and, assuming a power-law relation between SFR and cloud mass, explore a range of input parameters. We confirm that the slope and the scatter of the simulated SFR-molecular gas surface density relation depend on the size of the sub-galactic region considered, due to stochastic sampling of the molecular cloud mass function, and the effect is larger for steeper relations between SFR and molecular gas. There is a general trend for all slope values to tend to ~unity for region sizes larger than 1-2 kpc, irrespective of the input SFR-cloud relation. The region size of 1-2 kpc corresponds to the area where the cloud mass function becomes fully sampled. We quantify the effects of selection biases in data tracing the SFR, either as thresholds (i.e., clouds smaller than a given mass value do not form stars) or backgrounds (e.g., diffuse emission unrelated to current star formation is counted towards the SFR). Apparently discordant observational results are brought into agreement via this simple model, and the comparison of our simulations with data for a few galaxies supports a steep (>1) power law index between SFR and molecular gas.Comment: 54 pages, 16 figures; accepted for publication on the Astrophysical Journa