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Date and Time of the Query: 2019-03-18 T08:44:09 PDT
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For refcode 1988ApJ...328..440B:
Retrieve 31 NED objects in this reference.
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Copyright by American Astronomical Society. Reproduced by permission
1988ApJ...328..440B THE FAR-ULTRAVIOLET SPECTRA OF EARLY-TYPE GALAXIES DAVID BURSTEIN Department of Physics, Arizona State University F. BERTOLA Department of Astronomy, University of Padova, Italy L. M. BUSON Astronomical Observatory, Padova, Italy S. M. FABER Lick Observatory, University of California, Santa Cruz AND TOD R. LAUER Princeton University Observatory Received 1987 July 20; accepted 1987 November 5 ABSTRACT New and revised ultraviolet erergy distributions are presented based on 97 IUE spectra of 31 early-type galaxies and the bulge of M31. Measurements of the line strength index Mg_2_ and the V magnitude flux with the IUE aperture are included. The average flux between 1250 and 1850 A is combined with the V magnitude to form an ultraviolet/optical (1550 - V) color. Based on optical spectra and morphology, the sample galaxies are divided into three categories: four objects with abnormally high numbers of young stars (star-forming galaxies); four objects with strong, broad nuclear emission lines (active galaxies); and 24 normal objects showing no clear spectral or morphological peculiarities (quiescent galaxies). The major finding of this study is a well-defined, nonlinear relationship between (1550 - V) color and Mg_2_ for the 24 quiescent galaxies. This trend is in the sense suggested by Faber in 1983: (1550 - V) color is bluer at higher line strength, opposite to the behavior of every other known color. Correlations of (1550 - V) color with velocity dispersion and total luminosity have also been tested, but both are weaker. In comparison to the quiescent galaxies the four star-forming galaxies clearly show excess UV flux due to the impact of young stars. The four active galaxies with nuclear emission also show a UV excess, but of much smaller magnitude. The ultraviolet spectra of quiescent galaxies can be modeled as a sum of two components: a normal, stellar population of main-sequence and giant branch stars that is redder at higher Mg_2_ (in the normal sense plus a very blue population having a steeply rising UV flux below 2000 A that increases in strength with metallicity. This blue component affects the whole spectrum shortward of 3200 A, and its much higher amplitude in high-Mg_2_ galaxies is what accounts for their very blue (1500 - V) colors and relatively blue fluxes from 2600 to 3200 A. The ultraviolet spectra of the star-forming galaxies are significantly flatter than the very blue component of quiescent galaxies and are consistent with aging bursts of star formation. The two bluest active galaxies N4486 and N6166, may have excess light at wavelengths between 2000 and 2500 A compared to quiescent galaxies with similar (1500 - V) colors. Two sample population models are explored for the blue stellar component of quiescent galaxies: post-asymptotic giant branch (PAGB) stars and young stars from continuing star formation. Although both models exhibit similar UV energy distributions which approximate the UV spectra of quiescent and active galaxies, there currently exists no satisfactory theoretical explanation of the increase of UV flux with increasing metallicity. Resolution of the origin of the far-UV flux in ellipticals can be tested both with detailed UV imaging and with very accurate spectral energy distributions of galaxies. This is an important problem for the Hubble Space Telescope. Subject headings: galaxies: evolution - galaxies: stellar content - ultraviolet: spectra
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