The Recent Star Formation History of GR 8 from [ITAL]Hubble[/ITAL] [ITAL]Space[/ITAL] [ITAL]Telescope[/ITAL] Photometry of the Resolved Stars

Citations Over TimeTop 10% of 1998 papers

Abstract

We have used the Hubble Space T elescope to observe the resolved stars in the dwarf irregular galaxy GR 8 (DDO 155, UGC 8091). The data consisted of dithered Wide Field Planetary Camera 2 images in three bands : F439W (1 hr), F555W (30 minutes), and F814W (30 minutes). The stellar photometry was extracted with a modied version of DoPHOT. Articial star tests showed the data to be 50% complete to V \ 26.3, B \ 25.4, and I \ 25.2. The color-magnitude diagrams contain well-dened populations, including a very young main sequence (MS) (\10 Myr), and a red giant branch as old as several Gyr. These features align well with stellar evolution models of the appropriate metallicity. The distance based on the tip of the red giant branch is in excellent agreement with the Cepheid determination of k \ 26.75 ^0.35 (2.2 Mpc), which we adopted. An extended stellar "" halo was discovered well beyond the H I. Based on the MS and blue HeB luminosity function, we calculated the star formation rate (SFR) over the past 500 Myr. The SFR has been fairly constant, at 400 Myr~1 kpc~2 with up to 60% M _ variations. The blue HeB stars were used as a tracer for the location of star formation over this time period. The star formation occurred in superassociation size regions (100200 pc), which lasted D100 Myr. These regions come and go with no obvious pattern, except that they seem to concentrate in the current locations of H I clumps. This suggested that the H I clumps are long-lived features that support several star-forming events over time. The most likely explanation is that the star-forming regions are gravitationally bound. We estimated the gas-to-star conversion efficiency to be 6%. We compared our results with those of three other dI galaxies : Sextans A, Pegasus DIG, and Leo A. There is a trend of higher SFR per area with larger Also, the star formation pattern is similar in all four galaxies. M H I /L B . Finally, none of the four dI galaxies contained a large starburst, comparable to those in BCD galaxies. Combining the histories of all four galaxies, this implies that less than 5% of dI galaxies are hosting a strong burst of star formation at any given time. Observations of more galaxies are needed to improve this statistic.

Related Papers

• → Deep ACS Imaging of the Halo of NGC 5128: Reaching the Horizontal Branch(2005)122 cited
• → Comparison of Distances from RR Lyrae Stars, the Tip of the Red Giant Branch, and Classical Cepheids(2008)77 cited
• → Deep [ITAL]Hubble Space Telescope[/ITAL] Imaging of Sextans A. II. Cepheids and Distance(2003)63 cited
• → Novel Aspects of Cepheid Metallicity(2009)1 cited