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Register a new userDeep HST Imaging of IC 1613. I. Variable Stars and Distance
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We present WFPC2 VI photometry of a field in the halo of IC 1613, finding 13 RR Lyraes and 11 Cepheids. Our photometry of the red giant branch tip and red clump is used to derive distances to IC 1613, which are consistent with each other and with distances based on the variable stars. We compare these values with similarly-measured distances for the Magellanic Clouds, and are able to measure metallicity dependencies of the RR Lyrae and Cepheid distances by requiring consistent relative distance measurements from the four techniques. For metallicities of [Fe/H] = -1.3 (RR Lyraes) and -1.0 (Cepheids), we find a relatively steep slope of 0.34 +/- 0.20 magnitudes per dex for the RR Lyraes and a shallow slope of -0.07 +/- 0.16 mag/dex for the Cepheids, both values within the range of theoretical and empirical results in the literature. We find that a dependence of the red clump absolute magnitude on age, in addition to metallicity, is required to produce self-consistent relative distances between IC 1613 and the Magellanic Clouds. Adopting such a red clump calibration and self-consistent calibrations for the other three distance indicators, we find that the distances to all three objects are in excellent agreement. Our best distance modulus to IC 1613 is mu_0 = 24.31 +/- 0.06, corresponding to a distance of 730 +/- 20 kpc. This distance produces an RR Lyrae absolute magnitude of MV = 0.61 +/- 0.08.
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We present deep HST WFPC2 imaging of the Local Group dwarf irregular galaxy IC 1613. The photometry is the deepest to date for an isolated dwarf irregular galaxy. The resulting color-magnitude diagram (CMD) is analyzed using three different methods to derive a star formation history (SFH). All three find an enhanced star formation rate (SFR), from 3 to 6 Gyr ago, and similar age-metallicity relationships (AMR). A comparison of the newly observed outer field with an earlier studied central field of IC 1613 shows that the SFR in the outer field has been significantly depressed during the last Gyr. This implies that the optical scale length of the galaxy has been decreasing with time and that comparison of galaxies at intermediate redshift with present day galaxies should take this effect into account. We find strong similarities between IC 1613 and the more distant Milky Way dSph companions in that all are dominated by star formation at intermediate ages. In particular, the SFH and AMR for IC 1613 and Leo I are indistinguishable. This implies that dIrr galaxies cannot be distinguished from dSphs by their intermediate age stellar populations. This type of a SFH may also be evidence for slower or suppressed early star formation in dwarf galaxies due to photoionization after the reionization of the universe by background radiation. Assuming that IC 1613 is typical of a dIrr evolving in isolation, since most of the star formation occurs at intermediate ages, these dwarf systems cannot be responsible for the fast chemical enrichment of the IGM which is seen at high redshift. There is no evidence for any large amplitude bursts of star formation in IC 1613, and we find it highly unlikely that analogs of IC 1613 have contributed to the excess of faint blue galaxies in existing galaxy redshift surveys.
We are going to apply AGB stars to find star formation history for IC,1613 galaxy, this a new and simple method that works well for nearby galaxies. IC,1613 is a Local Group dwarf irregular galaxy that is located at distance of 750 kpc, a gas rich and isolated dwarf galaxy that has a low foreground extinction. We use the long period variable stars (LPVs) that represent the very final stage of evolution of stars with low and intermediate mass at the AGB phase and are very luminous and cool so that they emit maximum brightness in near--infrared bands. Thus near--infrared photometry with using stellar evolutionary models help us to convert brightness to birth mass and age and from this drive star formation history of the galaxy. We will use the luminosity distribution of the LPVs to reconstruct the star formation history--a method we have successfully applied in other Local Group galaxies. Our analysis shows that the IC 1613 has had a nearly constant star formation rate, without any dominant star formation episode.
IC 1613 is a Local Group dwarf irregular galaxy at a distance of 750 kpc. In this work, we present an analysis of the star formation history (SFH) of a field of $sim200$ square arcmin in the central part of the galaxy. To this aim, we use a novel method based on the resolved population of more highly evolved stars. We identify 53 such stars, 8 of which are supergiants and the remainder are long period variables (LPV), large amplitude variables (LAV) or extreme Asymptotic Giant Branch (x-AGB) stars. Using stellar evolution models, we find the age and birth mass of these stars and thus reconstruct the SFH. The average rate of star formation during the last Gyr is $sim3times10^{-4}$ M$_odot$ yr$^{-1}$ kpc$^{-2}$. The absence of a dominant epoch of star formation over the past 5 Gyr, suggests that IC 1613 has evolved in isolation for that long, spared harrassment by other Local Group galaxies (in particular M 31 and the Milky Way). We confirm the radial age gradient, with star formation currently concentrated in the central regions of IC 1613, and the failure of recent star formation to have created the main HI supershell. Based on the current rate of star formation at $(5.5pm2)times10^{-3}$ M$_odot$ yr$^{-1}$, the interstellar gas mass of the galaxy of $9times10^7$ M$_odot$ and the gas production rate from AGB stars at $sim6times10^{-4}$ M$_odot$ yr$^{-1}$, we conclude that the star formation activity of IC 1613 can continue for $sim18$ Gyr in a closed-box model, but is likely to cease much earlier than that unless gas can be accreted from outside.
King, Modjaz, & Li (1999) discovered Nova 1999 in IC1613 at Lick Observatory. Both Fugazza et al. (2000) and Borissova et al. (2000) questioned this classification, because they were able to detect the star on images obtained in previous years. In infrared frames taken on Oct. 15, 1998, the Nova 1999 has (J-K) = 1.14 and K = 14.69 mag. Our light curve study, based primarily on 92 unfiltered Lick images, suggested that the object could be a Mira-type variable with a period of 640.7 days. This period is very close to that obtained by Fugazza et al. (2000) - 631 days. The star is overluminous with respect to the period-luminosity (PL) relation derived by Feast et al. (1989) for Mira variables in the LMC. At longer periods (P > 400 ~ days), many LMC Miras show such behavior and the PL relation appears to break down. It is possible that the situation in IC1613 is similar. An optical spectrum obtained with the Keck-II telescope shows features typical of M3Ie or M3IIIe stars. We conclude that the star is a normal long-period M-type Mira variable, the first such star confirmed in IC1613.
Determining the star formation history (SFH) is key to understand the formation and evolution of dwarf galaxies. Recovering the SFH in resolved galaxies is mostly based on deep colour--magnitude diagrams (CMDs), which trace the signatures of multiple evolutionary stages of their stellar populations. In distant and unresolved galaxies, the integrated light of the galaxy can be decomposed, albeit made difficult by an age--metallicity degeneracy. Another solution to determine the SFH of resolved galaxies is based on evolved stars; these luminous stars are the most accessible tracers of the underlying stellar populations and can trace the entire SFH. Here we present a novel method based on long period variable (LPV) evolved asymptotic giant branch (AGB) stars and red supergiants (RSGs). We applied this method to reconstruct the SFH for IC 1613, an irregular dwarf galaxy at a distance of 750 kpc. Our results provide an independent confirmation that no major episode of star formation occurred in IC 1613 over the past 5 Gyr.
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