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تسجيل مستخدم جديدRecovering the Star Formation History of IC 1613 Dwarf Galaxy Using Evolved Stars
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Seyed Azim Hashemi
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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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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 met
hod 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.
We present an analysis of the star formation history (SFH) of a field near the half light radius in the Local Group dwarf irregular galaxy IC 1613 based on deep Hubble Space Telescope Advanced Camera for Surveys imaging. Our observations reach the ol
dest main sequence turn-off, allowing a time resolution at the oldest ages of ~1 Gyr. Our analysis shows that the SFH of the observed field in IC 1613 is consistent with being constant over the entire lifetime of the galaxy. These observations rule out an early dominant episode of star formation in IC 1613. We compare the SFH of IC 1613 with expectations from cosmological models. Since most of the mass is in place at early times for low mass halos, a naive expectation is that most of the star formation should have taken place at early times. Models in which star formation follows mass accretion result in too many stars formed early and gas mass fractions which are too low today (the over-cooling problem). The depth of the present photometry of IC 1613 shows that, at a resolution of ~1 Gyr, the star formation rate is consistent with being constant, at even the earliest times, which is difficult to achieve in models where star formation follows mass assembly.
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 an
d 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.
We present sub-arcsecond optical images and low- to moderate-resolution spectra of the compact, X-ray and optically bright supernova remnant known as S8 in the nearby dwarf galaxy IC 1613. Deep H$alpha$ images of the remnant show a sharply defined cr
escent shaped nebula, while narrow passband images reveal a coincident and unexpectedly bright continuum nebulosity exhibiting a size and morphology like that seen for the remnants line emissions. Low-dispersion spectra covering 3600 - 9000 A show numerous low-ionization line emissions such as [O I] and [Fe II], along with higher-ionization emission lines including He II and optical coronal lines [Fe VII], [Fe X], [Fe XI], and [Fe XIV]. This suggests the presence of a wide range of shock velocities from $sim$ 50 to over 350 km s$^{-1}$, corresponding to preshock densities of $sim1 - 30$ cm$^{-3}$. Higher resolution spectra indicate an expansion velocity around 180 km s$^{-1}$ with a $sim45$ km s$^{-1}$ wide central cavity. H$alpha$ emission spans rest frame velocities of $+120$ to $-240$ km s$^{-1}$ and we estimate a total nebula mass of $119 pm 34$ M$_{odot}$. We conclude S8 is a relatively young supernova remnant ($simeq2700 - 4400$ yr) exhibiting properties remarkably like those seen in the young LMC remnant N49, including age, physical size, shock velocities, filament densities, optical line strengths, X-ray and optical luminosities, and coronal line and continuum emissions.
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 t
o 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.
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