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

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

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

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

The Mid-Infrared Luminosities of Normal Galaxies over Cosmic Time

Modern population synthesis models estimate that 50% of the restframe K-band light is produced by TP-AGB stars during the first Gyr of a stellar population, with a substantial fraction continuing to be produced by the TP-AGB over a Hubble time. Between 0.2 and 1.5 Gyr, intermediate mass stars evolve into TP-AGB C stars which, due to significant amounts of circumstellar dust, emit half their energy in the mid-IR. We combine these results using published mid-IR colors of Galactic TP-AGB M and C stars to construct simple models for exploring the contribution of the TP-AGB to 24micron data as a function of stellar population age. We compare these empirical models with an ensemble of galaxies in the CDFS from z=0 to z=2, and with high quality imaging in M81. Within the uncertainties, the TP-AGB appears responsible for a substantial fraction of the mid-IR luminosities of galaxies from z=0 to z=2, the maximum redshift to which we can test our hypothesis, while, at the same time, our models reproduce much of the detailed structure observed in mid-IR imaging of M81. The mid-IR is a good diagnostic of star formation over timescales of ~1.5 Gyr, but this implies that on-going star formation rates at z=1 may be overestimated by factors of ~1.5-6, depending on the nature of star formation events. Our results, if confirmed through subsequent work, have strong implications for the star formation rate density of the universe and the growth of stellar mass over time.Comment: 6 pages, 4 figures, Accepted for publication in Astrophysical Journal Letter