Star formation in extremely faint dwarf galaxies

• Published in: Monthly notices of the Royal Astronomical Society 2009-08, Vol.397 (3), p.1435-1453
• Main Authors: Roychowdhury, Sambit, Chengalur, Jayaram N., Begum, Ayesha, Karachentsev, Igor D.
• Format: Article
• Language: English
• Subjects: galaxies: dwarf; galaxies: irregular; galaxies: stellar content; Radio astronomy; radio lines: galaxies; Star & galaxy formation; Stars & galaxies; stars: formation; Ultraviolet astronomy; ultraviolet: galaxies
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2009.14931.x

We study the relationship between the gas column density (Σgas) and the star formation rate (SFR) surface density (ΣSFR) for a sample of 23 extremely faint dwarf irregular galaxies drawn from the Faint Irregular Galaxy GMRT (Giant Metrewave Radio Telescope) Survey (FIGGS). Our sample galaxies have a median H i mass of 2.8 × 107 M⊙ and a median blue magnitude MB∼−13.2. ΣSFR is derived from Galaxy Evolution Explorer (GALEX) data, while Σgas is derived from the GMRT-based FIGGS H i 21-cm survey data. We find that Σgas averaged over the star-forming region of the disc lies below most estimates of the ‘threshold density’ for star formation, and that the average ΣSFR is also lower than would be expected from the ‘Kennicutt–Schmidt’ law. This deviation is indicative of an environmental dependence of star formation efficiencies, since the Kennicutt relation was derived from data on the central regions of large spiral and starburst galaxies. The star formation efficiency in small galaxies may be more relevant to modelling of star formation in gas-rich, low-metallicity systems in the early universe. We also use our data to look for small-scale (400 and 200 pc) correlations between ΣSFR and Σgas. For 18 of our 23 galaxies, we find that ΣSFR can be parametrized as having a power-law dependence on Σgas. The power-law relation holds until one reaches the sensitivity limit of the GALEX data, i.e. we find no evidence for a ‘threshold density’ below which star formation is completely cut off. The power-law slopes and coefficients however vary substantially from galaxy to galaxy, and are in general steeper than the value of ∼1.4 derived for large galaxies by Kennicutt. Further, as for the globally averaged quantities, the ΣSFR at 400 pc resolution is in general lower than that predicted by the Kennicutt relation, with the deviation decreasing with increasing Σgas. Our computation of ΣSFR uses a calibration that assumes solar metallicity and a standard Salpeter initial mass function (IMF), similarly, the Σgas we use is not corrected for the molecular gas density. Incorporating corrections for molecular gas and low metallicity will increase the deviation from the Kennicutt relation. Conversely, truncating the IMF at the high-mass end would decrease the deviation from the Kennicutt relation. For the five galaxies for which a power law does not provide a good parametrization of the (ΣSFR, Σgas) relation, there are substantial offsets between the ultraviolet (UV) bright regions