Star Formation Triggering Mechanisms in Dwarf Galaxies: The Far-Ultraviolet, Hα, and H I Morphology of Holmberg II

Far-ultraviolet (FUV), Hα, and H I observations of dwarf galaxy Holmberg II are used to investigate the means by which star formation propagates in galaxies lacking global internal triggering mechanisms such as spiral density waves. The observations trace the interaction between sites of massive star formation and the neutral and ionized components of the surrounding ISM in this intrinsically simple system. Both local and large-scale triggering mechanisms related to massive star formation are seen, suggesting that feedback from massive stars is a microscopic process operating in all galaxies to a certain degree. The data emphasize the importance of local conditions in regulating star formation from evidence such as massive stars inside ionized shells, compact H II regions surrounding aging clusters, and stars formed in chains of progressing age. Surface brightness profiles show that current activity correlates with the time-averaged level of past star formation at a given radius demonstrating a reliance on local conditions. Large-scale triggering by H I shells is supported by observations of progenitor populations as well as secondary sites of star formation associated with their dense rims. Analysis of the energy available from massive stars inside H I shells indicates that energy deposited into the ISM from supernovae and stellar winds is sufficient to account for the H I morphology. Ages of individual star-forming regions are derived using B, Hα, and FUV photometry and show both older, diffuse FUV regions and younger, compact H II regions. The distribution of ages is reconciled with the H I morphology, showing a clear preference of young regions for areas of dense H I and old regions for H I voids. Global kinematical properties may also play a role in the star formation process since differences in the rotation characteristics of the neutral gas disk correlate with differences in triggering mechanisms. Large-scale feedback from massive stars is shown to operate in regions that lack differential shear in the gas disk.

[1]  J. Salzer,et al.  A Test of the Standard Hypothesis for the Origin of the H I Holes in Holmberg II , 1999, astro-ph/9904065.

[2]  M. Dopita,et al.  An H I Aperture Synthesis Mosaic of the Large Magellanic Cloud , 1998 .

[3]  R. Hill,et al.  Ultraviolet Imaging of the Irregular Galaxy NGC 4449 with the Ultraviolet Imaging Telescope: Photometry and Recent Star Formation History , 1998, astro-ph/9808039.

[4]  T. Wong,et al.  The Recent Star Formation in Sextans A , 1998, astro-ph/9808001.

[5]  N. Brosch,et al.  MORPHOLOGY OF STAR FORMATION REGIONS IN IRREGULAR GALAXIES , 1998, astro-ph/9807203.

[6]  Jr.,et al.  STAR FORMATION IN GALAXIES ALONG THE HUBBLE SEQUENCE , 1998, astro-ph/9807187.

[7]  M. S. Roberts,et al.  Ultraviolet Imaging Telescope Observations of the Magellanic Clouds , 1998 .

[8]  B. Elmegreen,et al.  Triggered star formation in the LMC4/Constellation III region of the Large Magellanic Cloud , 1998, astro-ph/9805092.

[9]  A. Saha,et al.  Variable Stars in the Holmberg II Dwarf Galaxy , 1998 .

[10]  L. Rickard,et al.  Infrared Properties of Molecular Cirrus. I. Photometry of Extended Sources on IRAS Image Products , 1998 .

[11]  D. Hunter,et al.  The Relationship between Gas, Stars, and Star Formation in Irregular Galaxies: A Test of Simple Models , 1997, astro-ph/9712353.

[12]  L. Pozzetti,et al.  The Star Formation History of Field Galaxies , 1997, astro-ph/9708220.

[13]  D. Hunter.  STAR FORMATION IN IRREGULAR GALAXIES: A REVIEW OF SEVERAL KEY QUESTIONS , 1997 .

[14]  Morton S. Roberts,et al.  THE ULTRAVIOLET IMAGING TELESCOPE: INSTRUMENT AND DATA CHARACTERISTICS , 1997, astro-ph/9704297.

[15]  R. Allen,et al.  Evidence for the Large-Scale Dissociation of Molecular Gas in the Inner Spiral Arms of M81 , 1997, astro-ph/9704218.

[16]  Eric P. Smith,et al.  Ultraviolet Colors and Extinctions of H II Regions in the Whirlpool Galaxy (M51) , 1996, astro-ph/9606168.

[17]  P. Massey,et al.  The UV-brightest Stars of M33 and Its Nucleus: Discovery, Photometry, and Optical Spectroscopy , 1996 .

[18]  P. Prugniel,et al.  CCD and HI observations of LSB dwarf galaxies in the general field , 1996 .

[19]  D. Hunter,et al.  The Massive Star Populations in Ionized Gas Shells in M33 and M31 , 1995 .

[20]  T. Tongue,et al.  Radio Continuum and HI Emission in Holmberg II: Tracing the Energy Balance in the ISM , 1995 .

[21]  C. Leitherer,et al.  The stellar content of 30 doradus derived from spatially integrated ultraviolet spectra: A test of spectral synthesis models , 1995 .

[22]  M. S. Roberts,et al.  Ultraviolet imaging telescope and optical emission-line observations of H II regions in M81 , 1995 .

[23]  Timothy M. Heckman,et al.  Synthetic properties of starburst galaxies , 1995 .

[24]  The RSA survey of dwarf galaxies, 1: Optical photometry , 1994 .

[25]  M. Dopita,et al.  The Relationship between Past and Present Star Formation in Galactic Disks from CCD Surface Photometry , 1994 .

[26]  Garth D. Illingworth,et al.  The Hubble Space Telescope Extragalactic Distance Scale Key Project. 1: The discovery of Cepheids and a new distance to M81 , 1994 .

[27]  B. Miller,et al.  H II Regions in M81 Group Dwarf Galaxies , 1994 .

[28]  M. S. Roberts,et al.  Initial mass functions from ultraviolet stellar photometry: A comparison of Lucke and Hodge OB associations near 30 Doradus with the nearby field , 1994 .

[29]  R. Kennicutt,et al.  THE H II REGIONS OF HOLMBERG II , 1994 .

[30]  M. S. Roberts,et al.  Ultraviolet Imaging Telescope images - Limits on recent star formation in Holmberg IX , 1993 .

[31]  M. Colless.  Book Review: The Stellar Populations of Galaxies, ed. by B. Barbuy & A. Renzini (Kluwer, Dordrecht), 1992. , 1993 .

[32]  D. Burstein,et al.  Spectral Synthesis in the Ultraviolet. IV. A Library of Mean Stellar Groups , 1992 .

[33]  J. D. Stolarik,et al.  The Ultraviolet Imaging Telescope - Design and performance , 1992 .

[34]  J. Roy,et al.  Holmberg II : a laboratory for studying the violent interstellar medium , 1992 .

[35]  G. Byrd,et al.  Tidal Arms are Ubiquitous in Spiral Galaxies , 1992 .

[36]  G. Tenorio-Tagle,et al.  Star formation in stellar systems , 1992 .

[37]  Robert L. Kurucz,et al.  Model Atmospheres for Population Synthesis , 1992 .

[38]  A. Blaauw,et al.  OB Associations and the Fossil Record of Star Formation , 1991 .

[39]  C. Lada,et al.  Book-Review - the Physics of Star Formation and Early Stellar Evolution , 1991 .

[40]  W. Waller.  EMISSION-LINE AND CONTINUUM FLUXES FROM NARROW-AND BROAD-BAND IMAGERY. , 1990 .

[41]  F. Gillett,et al.  IRAS observations of a large sample of normal irregular galaxies , 1989 .

[42]  R. Sancisi,et al.  High-Velocity Gas in M101 , 1988 .

[43]  P. Bodenheimer,et al.  Large-Scale Expanding Superstructures in Galaxies , 1988 .

[44]  L. Tacconi,et al.  THE CO CONTENTS OF DWARF IRREGULAR GALAXIES , 1987 .

[45]  M. Dopita,et al.  Shapley constellation III - a region of self-propagating star formation , 1985 .

[46]  D. Hunter,et al.  Star-forming properties and histories of dwarf irregular galaxies: down but not out , 1985 .

[47]  G. R. Carruthers,et al.  Distribution of hot stars and hydrogen in the Large Magellanic Cloud , 1981 .

[48]  L. Schulman,et al.  Theory of dwarf galaxies , 1980 .

[49]  Blair D. Savage,et al.  Observed Properties of Interstellar Dust , 1979 .

[50]  R. Weaver,et al.  Interstellar bubbles. II - Structure and evolution , 1977 .

[51]  H. Gerola,et al.  Stochastic star formation and spiral structure of galaxies. , 1977 .

[52]  C. Lada,et al.  Sequential formation of subgroups in OB associations , 1977 .

[53]  J. J. Condon,et al.  Accurate optical positions of bright galaxies. , 1976 .

[54]  Peter Nilson,et al.  Uppsala general catalogue of galaxies , 1973 .

[55]  K. Freeman.  On the disks of spiral and SO Galaxies , 1970 .

[56]  E. Salpeter.  The Luminosity function and stellar evolution , 1955 .