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GALEX and Star Formation Luciana Bianchi Abstract Wide-field far-UV (FUV, 1344-1786A)˚ and 1 Introduction near-UV (NUV, 1771-2831A)˚ imaging from GALEX provides a deep, comprehensive view of the young stel- The GalaxyEvolutionExplorer(GALEX) has imaged lar populations in hundreds of nearby galaxies, shed- in far-UV and near-UV a wide portion of the sky. Cat- ding new light on the process of star formation (SF) in alogs of hundreds of millions UV sources enable ad- different environments, and on the interplay between vances in a variety of fields, from hot stellar objects in dust and SF. GALEX’s FUV-NUV color is extremely the Milky Way to QSOs (Bianchi et al. 2009) and star- sensitive to stellar populations of ages up to a few hun- forming galaxies, and provide a roadmap for future UV dred Myrs, unambiguously probing their presence and missions (see http://dolomiti.pha.jhu.edu/uvsky). enabling age-dating and stellar mass estimate, together A deep, comprehensive view of the young stellar pop- with the characterization of interstellar dust extinction. ulations in hundreds of nearby galaxies, afforded by The deep sensitivity, combined with the wide field-of- GALEX’s wide-field UV imaging, allows us to char- view, made possile in particular the discovery and char- acterize their spatially-resolved and time-resolved re- acterization of star formation in extremely low-density, cent star formation. In addition, star formation was diffuse gas environments such as outer galaxy disks, revealed in extreme low-density environments, where it tidal tails, low-surface-brightness galaxies (LSB) and is elusive at other wavelengths. UV measurements of dwarf Irregular galaxies, and of rejuvenation episodes in millions of more distant galaxies probe their evolution early-type galaxies. Such results provide several miss- and the Universe’s star-formation history (SFH) since ing links for interpreting galaxy classes in an evolution- redshift ∼2. ary context, extend our knowledge of the star-formation In this review, we first recall the basic characteristics process to previously unexplored conditions, constrain of GALEX data (Section 2.1) and of the UV-emitting models of galaxy disk formation, and clarify the mu- young stellar populations which trace SF sites (Section tual role of dust and star formation. We review a vari- 2.2), and we discuss dust extinction correction (Sec- ety of star-forming environments studied by GALEX, tion 2.3). We review a number of environments where and provide some model analysis tools useful for inter- pretation of GALEX measurements, and potentially SF was discovered from UV imaging and was previ- as basic science planning tools for next-generation UV ously elusive, or thought to not possibly happen (low instruments. gas density conditions) in Section 3. We mention a few starburst places (Section 4) without attempting to be Keywords Astronomical surveys — stars: star forma- complete, which would be impossible. We do not ad- tion — galaxies: evolution — galaxies: stellar popula- dress the abundant statistical studies of distant galaxy tions — ultraviolet: galaxies — interstellar dust: ex- samples, aiming at reconstructing the star-formation tinction history of the universe, because these are described else- where in this book. We provide, instead, a few sample Luciana Bianchi diagnostic plots that may be of general use for a first Johns Hopkins University, Dept. of Physics & Astronomy, Balti- interpretation of GALEX data, in particular for SF more, MD, USA studies in nearby galaxies (Section 2.4), as well as for planning new UV and multi-wavelength observations. 2 2 UV imaging of young stellar populations now included in the GALEX imaging surveys, i.e. over 80% of the total number listed in Hyperleda 2.1 GALEX data database within such velocity limit and no culling cri- teria (Thilker, priv. comm.). GALEX performs wide-field (1.2◦ diameter) imaging in two Ultraviolet (UV) bands simoultaneously: FUV 2.2 The UV-emitting young populations (λeff =1539A,˚ ∆λ = 1344 - 1786A)˚ and NUV (λeff = 2316A,˚ ∆λ = 1771 - 2831A),˚ with a spatial resolu- Why is UV data sensitive to star formation? Young ′′ tion of 4.2/5.3 (FUV/NUV) (Morrissey et al. 2007), massive stars, hot, luminous, and short-lived, are the ′′ ′′ sampled with 1.5 ×1.5 pixels. Its sensitivity reaches unambiguous tracers of star formation. They are lu- ′′ ∼27.5mag/ with ≈1,500sec exposures. GALEX has minous enough that they can be seen in distant galax- also a spectroscopic observing mode, which will not be ies. They evolve on fast timescales (.10 Myrs for O- addressed here. type stars), therefore they also trace the original spatial The photometric system used in the GALEX archive structure of the star-formation episode, before the large 1 is based on the AB magnitude scale (see Morrissey et complexes and stellar associations dissolve. They are al. 2007). For most science analyses it is useful to com- very hot, therefore the UV wavelength range is ideal bine GALEX data with corollary data at other wave- to detect and study them, because (i) UV colors are lengths, especially the optical range where the Vega more sensitive to the temperatures of the hottest stars, magnitude system is often used, therefore we give in enabling e.g. to discern O-types from late-O/early-B, Table 1 the transformation between the two systems in while optical colors are saturated in this regime (e.g. commonly used passbands, as computed by us from the Bianchi 2007), and (ii) UV colors provide precise age- filters’ transmission curves, and the Vega spectrum. dating of integrated stellar populations for ages less than 1Gyr. UV images give therefore an instant snap- Table 1 Vega - AB magnitude transformation shot of young star-forming sites, uncomplicated by pre- vious star-formation history, unlike longer wavelengths Filter magV ega − magAB [mag] GALEX FUV -2.223 where multiple stellar generations contribute signifi- GALEX NUV -1.699 cantly to the light (Fig.s 1 and 2). Finally, UV fluxes SDSS u -0.944 are more sensitive to dust, which plays a major role in SDSS g 0.116 star formation, as will be discussed in Section 2.3. SDSS r -0.131 Massive stars drive the chemical evolution of the SDSS i -0.354 Universe, enriching the interstellar medium (ISM) with SDSS z -0.524 nucleosynthesis products via supenova explosions and Landolt U -0.694 intense mass loss during AGB and planetary nebula Landolt B2/B3 0.125/ 0.123 phases, and driving the dynamical evolution of the ISM Landolt V 0.004 through highly supersonic stellar winds and mass loss ∼ −5 −1 Landolt R 0.180 up to 10 M⊙ yr in their main sequence lifetime. Landolt I -0.423 Therefore, understanding their formation and charac- 2MASSJ -0.901 terizing them in a wide variety of environmental phys- 2MASSH -1.384 ical conditions (galaxy type, metallicity, gas content, 2MASSKs -1.852 dynamics including interaction events) helps our un- derstanding of the overall evolution of the universe. GALEX performs nested surveys with differing sky 2.3 Characterizing dust extinction coverage and depth, see Bianchi (2009) for a summary, Bianchi et al (2010, 2011a), Conti et al. (2011), and 2.3.1 The mutual influence of dust and star formation (Hutchings & Bianchi 2010a) for a description of the source content of currently released catalogs and sample Dust is a minor component of the ISM, yet it has a ma- science applications. jor role in the formation of stars. In turn, hard UV ra- In total, with any level of exposure, over 46,000 1 diation from massive stars can modify the dust grains’ galaxies within 100Mpc (velocity ≤ 7000 km s− ) are size distribution or coating. Extinction properties in the far-UV give information about the distribution of 1 mUV (AB)=-2.5×log(CTRUV )+ZP, and zero-points small dust grains in particular. It is important to un- ZPF UV =18.82 and ZPNUV =20.08; the countrate CTR is derstand the role of dust in the star-formation and ISM the dead-time-corrected, flat-fielded count rate in counts s−1 3 Fig. 1 GALEX (left, FUV: blue and NUV: yellow) and optical (right, on the same scale) color-composite images of selected objects examplifying the power of UV imaging for revealing young stellar populations. In particular, examples of dwarfs and tidal dwarfs inconsipcuous in the optical are also shown, near bright galaxies. From the top: NGC 4656, M81, and M101. 4 Fig. 2 GALEX (left) and optical (right) color-composite images giving typical examples of: extended UV-disks (top, NGC5474), UV-emitting rings around early-type galaxies (middle, the SB0/a galaxy NGC5701) and halo UV emission (bottom: M 82, see Hoopes et al. 2005) 5 enrichment history of galaxies, and the relation of ex- with age and metallicity (Fig. 5), and (ii) that far-IR tinction to local and global galaxy properties such as fluxes and UV fluxes trace the same populations, which metallicity and starburst intensity. It is also neccessary is mostly not the case (Fig. 3, also e.g. Efremova et al. to characterize dust extinction in various conditions, in 2011, Bianchi 2007). As a more detailed discussion is order to interpret integrated properties of distant galax- not possible here, we only mentioned the main caveats ies such as those measured by the GALEX surveys, to of such methods. properly unredden observed fluxes and correctly mea- sure their energy budget and physical properties. Extinction is a combination of scattering and absorp- tion. Geometry, density, grain composition and UV ra- diation from hot stars are relevant parameters. Spectro- scopic studies in the UV range show that in our Galaxy, an average extinction law can reproduce observed ex- tinction curves (range 1250A˚ –3.5µ) in a variety of dust environments (e.g.
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