Star Formation Properties of a Large Sample of Irregular Galaxies

By D. Hunter, Bruce G. Elmegreen. 2004

227 citations recorded by the source corpus.

We present the results of Hα imaging of a large sample of irregular galaxies. Our sample includes 94 galaxies with morphological classifications of Im, 26 blue compact dwarfs (BCDs), and 20 Sm systems. The sample spans a large range in galactic parameters, including integrated absolute magnitude (MV of -9 to -19), average surface brightness (20–27 mag arcsec-2), current star formation activity (0–1.3 M⊙ yr-1 kpc-2), and relative gas content (0.02–5 M⊙/LB). The Hα images were used to measure the integrated star formation rates, determine the extents of star formation in the disks, and compare azimuthally averaged radial profiles of current star formation to older starlight. The integrated star formation rates of Im galaxies normalized to the physical size of the galaxy span a range of a factor of 104 with 10% Im galaxies and one Sm system having no measurable star formation at the present time. The BCDs fall, on average, at the high star formation rate end of the range. We find no correlation between star formation activity and proximity to other cataloged galaxies. Two galaxies located in voids are similar in properties to the Sm group in our sample. The H II regions in these galaxies are most often found within the Holmberg radius RH, although in a few systems H II regions are traced as far as 1.7RH. Similarly, most of the star formation is found within three disk scale lengths RD, but in some galaxies H II regions are traced as far as 6RD. A comparison of Hα surface photometry with V-band surface photometry shows that the two approximately follow each other with radius in Sm galaxies, but in most BCDs there is an excess of Hα emission in the centers that drops with radius. In approximately half of the Im galaxies Hα and V correspond well, and in the rest there are small to large differences in the relative rate of falloff with radius. The cases with strong gradients in the LHα/LV ratios and with high central star formation rate densities, which include most of the BCDs, require a significant fraction of their gas to migrate to the center in the last gigayear. We discuss possible torques that could have caused this without leaving an obvious signature, including dark matter bars and past interactions or mergers with small galaxies or H I clouds. There is now a substantial amount of evidence for these processes among many surveys of BCDs. We note that such gas migration will also increase the local pressure and possibly enhance the formation of massive dense clusters but conclude that the star formation process itself does not appear to differ much among BCD, Im, and Sm types. In particular, there is evidence in the distribution function for Hα surface brightness that the turbulent Mach numbers are all about the same in these systems. This follows from the Hα distribution functions corrected for exponential disk gradients, which are log-normal with a nearly constant dispersion. Thus, the influence of shock-triggered star formation is apparently no greater in BCDs than in Im and Sm types.

Subfields: Astronomy and Astrophysics, Electrical and Electronic Engineering, Instrumentation

View Paper: https://www.semanticscholar.org/paper/423ff41eeb19d836fe4ee0c93c2be1b35b9cf479

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Citations in the corpus

Structural Parameters of Thin and Thick Disks in Edge-on Disk Galaxies (2005, background): “The disks of lower mass galaxies have systematically lower baryon surface densities (e.g., Swaters et al. 2002; Hunter & Elmegreen 2004) and thus are inefficient at converting gas into stars.”
The Baryon Content of Extremely Low Mass Dwarf Galaxies (2006, background): “Using criteria such as surface brightness and irregularity, several authors have assembled large catalogs of nearby dwarf galaxies (Impey et al. 1996; Schombert et al. 2001; Hunter & Elmegreen 2004; Karachentsev et al. 2004).”
GHASP: an Hα kinematic survey of spiral and irregular galaxies - VI. New Hα data cubes for 108 galaxies (2008, methodology): “This prevented us from computing an accurate velocity field in that region, even though there is some real emission there, as confirmed by Hunter & Elmegreen (2004) line map.”
A GALEX ULTRAVIOLET IMAGING SURVEY OF GALAXIES IN THE LOCAL VOLUME (2010, background): “– 14 – a result of the recent star formation activity in such systems (e.g., van Zee 2001; Hunter & Elmegreen 2004; Lee et al. 2007), while low metallicities (e.g., Skillman et al. 1989; van Zee et al. 1997; Lee et al. 2004) and low dust attenuation (e.g., Lee et al. 2009b) also play a role.”
COMPARISON OF Hα AND UV STAR FORMATION RATES IN THE LOCAL VOLUME: SYSTEMATIC DISCREPANCIES FOR DWARF GALAXIES (2009, background): “…IMF with standard mass limits of 0.1-100M⊙ yr −1, and considering that only stars with masses >18 M⊙ significantly contribute to the ionizing flux (e.g., Hunter & Elmegreen 2004, appendix B), simple integrals show that there are 10 ionizing stars for a total stellar mass of 4.3×103 M⊙ formed.”
Broadband Imaging of a Large Sample of Irregular Galaxies (2005, methodology): “A more complete description of the sample and of its selection is given by Hunter & Elmegreen (2004), where we present the H imaging data.”
DWARF GALAXY STARBURST STATISTICS IN THE LOCAL VOLUME (2008, background): “…generally cycle between similar quiescent and global starburst phases has been the subject of a long-running debate (e.g., Gallagher et al. 1984; Marlowe et al. 1999; Simpson & Gottesman 2000; van Zee 2001; Hunter & Elmegreen 2004; Pelupessy et al. 2004; Gil de Paz & Madore 2005; Bekki 2008).”
Molecular Hydrogen and Global Star Formation Relations in Galaxies (2007, methodology): “While the H2D-SF+ISRF simulation of the dwarf galaxy produces a steady SFR∼ 0.025M⊙yr−1, we note that the galaxy DDO 154 that served as a model for thevcirc = 50km s−1 system has a star formation rate of only SFR∼ 0.004M⊙yr−1 (Hunter & Elmegreen 2004).”
The Metallicity of Galaxy Disks: Infall versus Outflow (2006, background): “Such a systematic rise can only occur if the gas surface density is rising with time, as would be expected for gas infall onto a system with inefficient star formation (as dwarf galaxies are observed to have; e.g. van Zee et al. 1997b; van Zee 2001; Hunter & Elmegreen 2004).”