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A FAR-ULTRAVIOLET VIEW OF STARBURST GALAXIES

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arXiv:astro-ph/0410545 v1 22 Oct 2004A FAR-ULTRAVIOLET VIEW OF STARBURST GALAXIESClaus LeithererSpace Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, [email protected] Recent observational and theoretical results on starburst galaxies related to thewavelength regime below 1200 Aare discussed. The review covers stars, dust, aswell as hot and cold gas. This wavelength region follows trends similar to thoseseen at longer wavelengths, with several notable exceptions. Even the youngeststellar populations show a turn-over in their spectral energy distributions, andline-blanketing is much more pronounced. Furthermore, the O VI line allowsone to probe gas at higher temperatures than possible with lines at longer wave-lengths. Molecular hydrogen lines (if detected) provide a glimpse of the coldphase. I cover the crucial wavelength regime below 912 A and the implicationsof recent attempts to detect the escaping ionizing radiation.1. BackgroundThe astrophysically important wavelength region below ∼1200 A is still rel-atively unexplored, at least at low redshift where restframe observations mustbe obtained from space. Prior to the launch of FUSE (Moos et al. 2000), far-ultraviolet (far-UV) studies were limited to bright objects. The earliest spectraldata for bright stars were obtained by Copernicus (Rogerson et al. 1973) andORFEUS (Grewing et al. 1991), and with the UV spectrometers on the Voy-ager 1 and 2 spacecraft (Longo et al. 1989). Voyager 2 also succeeded inrecording a far-UV spectrum of M33 (Keel 1998). HUT (Davidsen 1993) wasthe first instrument sensitive enough to collect spectra of faint galaxies belowLy-α. The mission was flown on two missions and generated a rich database offar-UV spectra of actively star-forming and starburst galaxies. Subsequently,FUSE with its superior resolution and sensitivity fully opened the far-UV win-dow to starburst galaxies. Most of this review deals with results obtained withFUSE and, to a smaller degree, with HUT.2. Stellar PopulationsThe far-UV spectrum of the archetypal starburst galaxy M83 is reproducedin Fig. 1 (Leitherer et al. 2002). The wavelength region shown covers 900 –2Figure 1. Spectral region between 900 and 1200 A for M83 (= NGC 5236). Major lines arelabeled. Thick line: HUT; thin: FUSE (Leitherer et al. 2002).1200 A, where blanketing is most severe. M83 has supersolar oxygen abun-dance. Therefore, line-blanketing effects due to stellar-wind, stellar photo-spheric, and interstellar lines are particularly strong in this galaxy. The stel-lar features generally originate from higher ionization stages than the featuresfound above 1200 A. The most prominent transition is the O VI resonance dou-blet at 1032,38 A, which displays a spectacular P Cygni profile over a broadrange of spectral types. At the resolution afforded by FUSE, the blueshiftedabsorption component of the P Cygni profile is resolved from nearby Ly-β andcan be distinguished from the narrow interstellar C II at 1036 A. The (red-shifted) emission component of its P Cygni profile is relatively unaffected byinterstellar lines and provides additional diagnostic power. The C III 1176line is at the long-wavelength end of the covered spectral range and can alsobe observed with spectrographs optimized for wavelengths longward of Ly-α.Surprisingly, the line has received relatively little attention in the earlier liter-ature although it is a very good diagnostic of the properties of hot stars. C IIIis not a resonance transition, and consequently does not suffer from contam-ination by an interstellar component. C III, like most other stellar lines, hasa pronounced metallicity dependence, either directly via opacity variations, orindirectly via metallicity-dependent stellar-wind properties.Quantitative modeling of the stellar far-UV lines by means of evolutionarysynthesis was done by Robert et al. (2003). In Fig. 2 I show an evolutionarysequence based on an empirical FUSE library of hot stars (Pellerin et al. 2002).The computed spectra are a good match to the M83 spectrum in Fig. 1. TheFUV View of Starburst Galaxies 31000 1050 1100 1150 1200 0 1 2 3 4 5 6 7 80 Myr1 Myr2 Myr3 Myr5 Myr7 Myr10 MyrFigure 2. Evolution of a synthetic far-UV spectrum following a Salpeter IMF between 0 and10 Myr. Stellar features (bottom), H2lines (top), and other interstellar absorptions (top; verticaldotted lines) are labeled. The emission line at 1026 A is geocoronal (Robert et al. 2003).C III 1176 line is an excellent age diagnostic mirroring the behavior of thewell-studied Si IV 1400 line: when luminous supergiants appear around 3 Myr,wind recombination raises the emission flux (Leitherer et al. 2001). The O VIline, in contrast, is largely decoupled from stellar parameters over a wide rangeof ages. This line is powered by shock heating and remains constant for stellartemperatures above ∼30,000 K. Combining lines with different optical depths,excitation, and ionization parameters allows age and metallicity estimates fromfar-UV spectra analogous to methods calibrated at longer wavelengths (e.g.,Keel et al. 2004).Apart from their sensitivity to metallicity, the far-UV lines are affected thewell-known age vs. initial mass function (IMF) degeneracy. In the absenceof additional constraints, age and IMF can always be traded. This applies tothe far-UV continuum as well, which in addition suffers from an age-reddeningdegeneracy. In contrast to wavelengths above 1200 A, the intrinsic stellar spec-tra below 1200 A are outside the Rayleigh-Jeans regime, and age effects are4no longer negligible for the continuum slope generated by an instantaneouspopulation. Alternatively, for a population of continuously forming stars, theregion between 912 and 1200 A becomes even less age sensitive to populationvariations than the near-UV, because an equilibrium between star formationand stellar death is reached earlier in time (see Leitherer et al. 1999). Agesand star-formation rates in starburst galaxies derived from far-UV spectra areconsistent with the results from longer wavelengths.3. Dust ObscurationIf the age and IMF can be constrained independently, the observed far-UVspectral energy distribution is mostly a measure of the dust attenuation. Thecontinua of star-forming galaxies are known to obey a well-defined averageobscuration curve above 1200 A (Calzetti 2001). The curve accounts for thetotal absorption and encompasses the net effects of dust/star geometry, absorp-tion,


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