DISCOVERY OF AN EXTENDED ULTRAVIOLET DISK IN THE NEARBY GALAXY NGC 4625

Recent far-UV (FUV) and near-UV (NUV) observations of the nearby galaxy NGC 4625 made by the Galaxy Evolution Explorer (GALEX) show the presence of an extended UV disk reaching to 4 times the optical radius of the galaxy. The UV-to-optical colors suggest that the bulk of the stars in the disk of NGC 4625 are currently being formed, providing a unique opportunity to study today the physics of star formation under conditions similar to those when the normal disks of spiral galaxies like the Milky Way first formed. In the case of NGC 4625, the star formation in the extended disk is likely to be triggered by interaction with NGC 4618 and possibly also with the newly discovered galaxy NGC 4625A. The positions of the FUV complexes in the extended disk coincide with peaks in the H I distribution. The masses of these complexes are in the range 10^3-10^4 M_☉, with their Hα emission (when present) being dominated by ionization from single stars.

[1]  S. Odewahn.  Properties of the Magellanic-Type Spirals. I. Surface Photometry of NGC 4618 and NGC 4625 , 1991 .

[2]  A. Sandage,et al.  The Carnegie atlas of galaxies , 1994 .

[3]  A. Sandage,et al.  The Carnegie Atlas of Galaxies, Volumes 1 and 2 , 1995 .

[4]  Lars Hernquist,et al.  Using tidal tails to probe dark matter halos , 1996 .

[5]  E. Bertin,et al.  SExtractor: Software for source extraction , 1996 .

[6]  William D. Vacca,et al.  The Lyman-Continuum Fluxes and Stellar Parameters of O and Early B-Type Stars , 1996 .

[7]  The stellar populations of spiral galaxies , 1999, astro-ph/9909402.

[8]  E. Bell,et al.  The Effects of Dust in Simple Environments: Large Magellanic Cloud H II Regions , 2001, astro-ph/0108367.

[9]  U. Milano,et al.  Chemo-spectrophotometric evolution of spiral galaxies—IV. Star formation efficiency and effective ages of spirals , 2000, astro-ph/0009232.

[10]  Jr.,et al.  SINGS: The SIRTF Nearby Galaxies Survey , 2001, astro-ph/0305437.

[11]  Robert C. Kennicutt,et al.  Star Formation Thresholds in Galactic Disks , 2001 .

[12]  M. Balcells,et al.  The Westerbork HI survey of spiral and irregular galaxies - II. R-band surface photometry of late-type dwarf galaxies , 2002, astro-ph/0204526.

[13]  R. Sancisi,et al.  Astronomy & Astrophysics manuscript no. (will be inserted by hand later) The Westerbork HI Survey of Spiral and Irregular Galaxies I. HI Imaging of Late-type Dwarf Galaxies , 2002 .

[14]  A. Sternberg,et al.  Ionizing Photon Emission Rates from O- and Early B-Type Stars and Clusters , 2003, astro-ph/0312232.

[15]  A. Szalay,et al.  Preparing Red-Green-Blue (RGB) Images from CCD Data , 2003 .

[16]  G. Bruzual,et al.  Stellar population synthesis at the resolution of 2003 , 2003, astro-ph/0309134.

[17]  A Comparative Study of the Spatial Distribution of Ultraviolet and Far-Infrared Fluxes from M101 , 2004, astro-ph/0411336.

[18]  E. Wilcots,et al.  Neutral Hydrogen in the Interacting Magellanic Spirals NGC 4618 and NGC 4625 , 2004, astro-ph/0409209.

[19]  Heidelberg,et al.  Modelling the spectral energy distribution of galaxies. III. Attenuation of stellar light in spiral galaxies , 2004, astro-ph/0401630.

[20]  David W. Hogg,et al.  Preparing Red‐Green‐Blue Images from CCD Data , 2003, astro-ph/0312483.

[21]  Luciana Bianchi,et al.  Recent Star Formation in Nearby Galaxies from Galaxy Evolution Explorer Imaging: M101 and M51 , 2005 .

[22]  B. Milliard,et al.  Recent star formation in the extreme outer disk of M83 , 2005 .

[23]  Dust Attenuation in the Nearby Universe: A Comparison between Galaxies Selected in the Ultraviolet and in the Far-Infrared , 2004, astro-ph/0411343.

[24]  A. Szalay,et al.  The Galaxy Evolution Explorer: A Space Ultraviolet Survey Mission , 2004, astro-ph/0411302.