THE OUTER DISKS OF DWARF IRREGULAR GALAXIES

Journal Article · Sat Oct 15 04:00:00 EDT 2011 · Astronomical Journal (New York, N.Y. Online)

DOI:https://doi.org/10.1088/0004-6256/142/4/121· OSTI ID:21582829

Hunter, Deidre A; Massey, Philip; Wilsey, Nick; Riabokin, Malanka ^[1]; Elmegreen, Bruce G ^[2]; Oh, Se-Heon ^[3]; Anderson, Ed ^[4]

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1. Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001 (United States)
2. IBM T. J. Watson Research Center, P.O. Box 218, Yorktown Heights, NY 10598 (United States)
3. Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, Republic of South Africa (South Africa)
4. Department of Physics and Astronomy, Northern Arizona University, P.O. Box 6010, Flagstaff, AZ 86011-6010 (United States)

In order to explore the properties of extreme outer stellar disks, we obtained ultra-deep V and GALEX ultraviolet (UV) images of four dwarf irregular galaxies and one blue compact dwarf galaxy, and ultra-deep B images of three of these. Our V-band surface photometry extends to 29.5 mag arcsec{sup -2}. We convert the FUV and V-band photometry, along with H{alpha} photometry obtained in a larger survey, into radial star formation rate profiles that are sensitive to timescales from 10 Myr to the lifetime of the galaxy. We also obtained H I-line emission data and compare the stellar distributions, surface brightness profiles, and star formation rate profiles to H I-line emission maps, gas surface density profiles, and gas kinematics. Our data lead us to two general observations. First, the exponential disks in these irregular galaxies are extraordinarily regular. We observe that the stellar disks continue to decline exponentially as far as our measurements extend. In spite of lumpiness in the distribution of young stars and H I distributions and kinematics that have significant unordered motions, sporadic processes that have built the disks-star formation, radial movement of stars, and perhaps even perturbations from the outside-have, nevertheless, conspired to produce standard disk profiles. Second, there is a remarkable continuity of star formation throughout these disks over time. In four out of five of our galaxies the star formation rate in the outer disk measured from the FUV tracks that determined from the V-band, to within factors of five, requiring star formation at a fairly steady rate over the galaxy's lifetime. Yet, the H I surface density profiles generally decline with radius more shallowly than the stellar light, and the gas is marginally gravitationally stable against collapse into clouds. Outer stellar disks are challenging our concepts of star formation and disk growth and provide a critical environment in which to understand processes that mold galaxy disks.

• OSTI ID: 21582829
• Journal Information: Astronomical Journal (New York, N.Y. Online), Journal Name: Astronomical Journal (New York, N.Y. Online) Journal Issue: 4 Vol. 142; ISSN 1538-3881
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
BRIGHTNESS
ELECTROMAGNETIC RADIATION
GALAXIES
OPTICAL PROPERTIES
PERTURBATION THEORY
PHOTOMETRY
PHYSICAL PROPERTIES
RADIATIONS
STARS
ULTRAVIOLET RADIATION
VISIBLE RADIATION