Stellar Populations at the Center of IC 1613

Abstract

We have observed the center of the Local Group dwarf irregular galaxy IC 1613 with WFPC2 aboard the Hubble Space Telescope in the F439W, F555W, and F814W filters. We find a dominant old stellar population (aged ~7 Gyr), identifiable by the strong red giant branch (RGB) and red clump populations. From the (V-I) color of the RGB, we estimate a mean metallicity of the intermediate-age stellar population [Fe/H] = -1.38 +/- 0.31. We confirm a distance of 715 +/- 40 kpc using the I-magnitude of the RGB tip. The main-sequence luminosity function down to I ~25 provides evidence for a roughly constant SFR of approximately 0.00035 solar masses per year across the WFPC2 field of view (0.22 square kpc) during the past 250-350 Myr. Structure in the blue loop luminosity function implies that the SFR was ~50% higher 400-900 Myr ago than today. The mean heavy element abundance of these young stars is 1/10th solar. The best explanation for a red spur on the main-sequence at I = 24.7 is the blue horizontal branch component of a very old stellar population at the center of IC 1613. We have also imaged a broader area of IC 1613 using the 3.5-meter WIYN telescope under excellent seeing conditions. The AGB-star luminosity function is consistent with a period of continuous star formation over at least the age range 2-10 Gyr. We present an approximate age-metallicity relation for IC 1613, which appears similar to that of the Small Magellanic Cloud. We compare the Hess diagram of IC 1613 to similar data for three other Local Group dwarf galaxies, and find that it most closely resembles the nearby, transition-type dwarf galaxy Pegasus (DDO 216).Comment: To appear in the September 1999 Astronomical Journal. LaTeX, uses AASTeX v4.0, emulateapj style file, 19 pages, 12 postscript figures, 2 tables. 5 of the figures available separately via the WW

The star formation history of the Sculptor dwarf irregular galaxy

“…sites of young star formation characterise the majority of the transition-type dwarfs in the Sculptor group (Jerjen & Rejkuba 2001;Lianou et al 2012), while the same is also observed for the young stellar populations of other LG dwarf galaxies (e.g, Dohm-Palmer et al 1997;Gallagher et al 1998;Mateo 1998;Cole et al 1999;McConnachie et al 2006). …”

Near-infrared photometric properties of asymptotic giant branch stars in the dwarf irregular galaxy IC 1613

“…The global metallicity of [Fe/H] = −1.23 ± 0.06 for IC 1613 was estimated from the relation between the C/M ratio and metallicity developed by Cioni (2009). This metallicity value is not much different from the previous results for the metallicity of IC 1613 (Freedman 1988;Cole et al 1999;Zucker & Wyder 2004). More recently, Sibbons et al (2015) also derived the metallicity of [Fe/H] = −1.26±0.07 for IC 1613 from their C/M ratio.…”

“…Several studies with optical data have provided a low-extinction value of E(B − V) = 0.02 ∼ 0.04 (Sandage 1971;Freedman 1988;Cole et al 1999) for IC 1613. The map of Schlegel et al (1998), based on the dust infrared emission feature of the COBE and IRAS data, gives the reddening value of E(B − V) = 0.03 for IC 1613.…”

“…Several previous researchers have studied the star formation history and star formation rate of IC 1613. Cole et al (1999) suggested that there has been a constant and continuous star formation in IC 1613 from the analysis of the AGB stars. Lake & Skillman (1989) reported that they found no significant recent star formation in the center of IC 1613.…”

“…IC 1613 is a gas-rich dwarf irregular galaxy located 730 kpc from our Sun (Dolphin et al 2001), is isolated, and is known to have no interaction with other galaxies. Cole et al (1999) found a significant number of old red giant stars and discussed constant star formation. More recently, Skillman et al (2003Skillman et al ( , 2014 also confirmed that the star formation history of IC 1613 agrees with being constant over the entire lifetime of the galaxy, ruling out a dominant episode of star formation.…”