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Abstract. We present a systematic imaging survey of 37 nearby galaxies observed with the Hubble Space Telescope (HST) Wide Field and Planetary Camera 2 (WFPC2) in the mid-UV F300W filter, centered at 2930Å, as well as in the I-band (F814W) filter at 8230Å. Eleven of these galaxies were also imaged in the F255W filter, centered at 2550Å. Our sample is carefully selected to include galaxies of sufficiently small radius and high predicted mid-UV surface brightness to be detectable with WFPC2 in one orbit, and covers a wide range of Hubble types and inclinations. The mid-UV (2000-3200Å) spans the gap between ground-based UBVR(IJHK) images, which are available or were acquired for the current study, and far-UV images available from the Astro / UIT missions for 15 galaxies in our sample. The first qualitative results from our study are:
(1) Early-type galaxies show a significant decrease in surface brightness going from the red to the mid-UV, reflecting the absence of a dominant young stellar population, and in some cases the presence of significant (central) dust lanes. Galaxies that are early-types in the optical show a variety of morphologies in the mid-UV that can lead to a different morphological classification, although not necessarily as later-type. Some early-type galaxies become dominated by a blue nuclear feature or a point source in the mid-UV, e.g., due to the presence of a Seyfert nucleus or a LINER. This is in part due to our mid-UV surface brightness selection, but it also suggests that part of the strong apparent evolution of weak AGN in early-type galaxies may be due to surface brightness dimming of their UV-faint stellar population, which renders the early-type host galaxies invisible at intermediate to higher redshifts.
(2) About half of the mid-type spiral and star-forming galaxies appear as a later morphological type in the mid-UV, as Astro / UIT also found primarily in the far-UV. Sometimes these differences are dramatic (e.g., NGC 6782 shows a spectacular ring of hot stars in the mid-UV). However, not all mid-type spiral galaxies look significantly different in the mid-UV. Their mid-UV images show a considerable range in the scale and surface brightness of individual star-forming regions. Almost without exception, the mid-type spirals in our sample have their small bulges bisected by a dust-lane, which often appears to be connected to the inner spiral arm structure.
(3) The majority of the heterogeneous subset of late-type, irregular, peculiar and merging galaxies display F300W morphologies that are similar to those seen in F814W, but with important differences due to recognizable dust-features absorbing the bluer light, and due to hot stars, star-clusters, and star-formation "ridges" that are bright in the mid-UV. Less than one third of the galaxies classified as late-type in the optical appears sufficiently different in the mid-UV to result in a different classification.
Our HST mid-UV survey of nearby galaxies shows that -when observed in the rest-frame mid-UV- early- to mid-type galaxies are more likely to be misclassified as later types than late-type galaxies are to be misclassified as earlier types. This is because the later type galaxies are dominated by the same young and hot stars in all filters from the mid-UV to the red, and so have a smaller "morphological K-correction" than true earlier type galaxies. The morphological K-correction can thus explain part, but certainly not all of the excess faint blue late-type galaxies seen in deep HST fields.
Keywords: galaxies: elliptical and lenticular - galaxies: spiral - galaxies: irregular - galaxies: interactions - galaxies: peculiar - ultraviolet: galaxies
Table of Contents
1 Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS 5-26555.
2 Based in part on observations made with the Vatican Advanced Technology Telescope: the Alice P. Lennon Telescope and the Thomas J. Bannan Astrophysics Facility.
3 Department of Physics and Astronomy, Arizona State University, Box 871504, Tempe, AZ 85287-1504
4 Department of Astronomy, Caltech, MS 105-24 Pasadena, CA 91125
5 Institute of Astronomy, Madingley Road, University of Cambridge, Cambridge, CB3 0HA, UK
6 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218
7 Department of Physics and Astronomy, Minnesota State University, Mankato, MN 56003
8 Code S, NASA Headquarters, 300 D Street SW, Washington, DC 20546
9 Department of Astronomy, University of Wisconsin, 475 N. Charter St., Madison, WI 53706-1582
10 National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475
11 Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138
12 Department of Astronomy, University of Virginia, P.O. Box 3818, Charlottesville, VA 22903-0818
13 Visiting Astronomer, Cerro Tololo International Observatory, National Optical Astronomy Observatory, which is operated by AURA, Inc., under cooperative agreement with the National Science Foundation (NSF).
14 Visiting Astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatory, which is operated by AURA, Inc., under cooperative agreement with the NSF.
15 Hubble Fellow.