arXiv subjects

Ata Sarajedini

Publications and source records attributed to Ata Sarajedini.

At least 55 records · Page 3Linked to original sources

arXiv2011

Globular clusters in the outer Galactic halo: new HST/ACS imaging of 6 globular clusters and the Galactic globular cluster age-metallicity relation

Color-magnitude diagrams (CMDs) derived from Hubble Space Telescope (HST) Advanced Camera for Surveys F606W,F814W photometry of 6 globular clusters (GCs) are presented. The six GCs form two loose groupings in Galactocentric distance (Rgc): IC 4499, NGC 6426, and Ruprecht 106 at ~15-20 kpc and NGC 7006, Palomar 15, and Pyxis at ~40 kpc. The CMDs allow the ages to be estimated from the main sequence turnoff in every case. In addition, the age of Palomar 5 (Rgc ~ 18 kpc) is estimated using archival HST Wide Field Planetary Camera 2 V,I photometry. The age analysis reveals the following: IC 4499, Ruprecht 106, and Pyxis are 1-2 Gyr younger than inner halo GCs with similar metallicities; NGC 7006 and Palomar 5 are marginally younger than their inner halo counterparts; NGC 6426 and Palomar 15, the two most metal-poor GCs in the sample, are coeval with all the other metal-poor GCs within the uncertainties. Combined with our previous efforts, the current sample provides strong evidence that the Galactic GC age-metallicity relation consists of two distinct branches. One suggests a rapid chemical enrichment in the inner Galaxy while the other suggests prolonged GC formation in the outer halo. The latter is consistent with the outer halo GCs forming in dwarf galaxies and later being accreted by the Milky Way.

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arXiv2011

RR Lyrae Variables in M31 and M33

The properties of RR Lyrae variables make them excellent probes of the formation and evolution of a stellar population. The mere presence of such stars necessitates an age greater than ~10 Gyr while their periods and amplitudes can be used to estimate the metal abundance of the cluster or galaxy in which they reside. These and other features of RR Lyraes have been used to study the properties of M31 and M33. Though these studies are generally in their infancy, we have established that M31 and M33 do indeed harbor RR Lyraes in their halos and probably also in their disks suggesting that these two components formed early in the history of M31 and M33. The mean metallicities of the halo RR Lyraes in these galaxies are consistent with those of other halo stellar population tracers such as the dwarf spheroidal satellites of M31 and the halo globular clusters in M33. Little is known about the spatial distribution of the RR Lyraes, especially in M33. This will require wide-field time-series studies with sufficient photometric depth to allow both the identification of RR Lyraes and robust period determination.

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arXiv2010

The ACS Survey of Galactic Globular Clusters. X. New Determinations of Centers for 65 Clusters

We present new measurements of the centers for 65 Milky Way globular clusters. Centers were determined by fitting ellipses to the density distribution within the inner $2\arcmin$ of the cluster center, and averaging the centers of these ellipses. The symmetry of clusters was also analyzed by comparing cumulative radial distributions on opposite sides of the cluster across a grid of trial centers. All of the determinations were done with stellar positions derived from a combination of two single-orbit ACS images of the core of the cluster in $F606W$ and $F814W$. We find that the ellipse-fitting method provides remarkable accuracy over a wide range of core sizes and density distributions, while the symmetry method is difficult to use on clusters with very large cores, or low density. The symmetry method requires a larger field, or a very sharply peaked density distribution.

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arXiv2010

Near-infrared counterparts of Chandra X-ray sources toward the Galactic Center

The Chandra X-ray Observatory has now discovered nearly 10,000 X-ray point sources in the 2 x 0.8 degree region around the Galactic Center (Muno 2009). The sources are likely to be a population of accreting binaries in the Galactic Center, but little else is known of their nature. We obtained JHKs imaging of the 17'x 17' region around Sgr A*, an area containing 4339 of these X-ray sources, with the ISPI camera on the CTIO 4-m telescope. We cross-correlate the Chandra and ISPI catalogs to find potential IR counterparts to the X-ray sources. The extreme IR source crowding in the field means that it is not possible to establish the authenticity of the matches with astrometry and photometry alone. We find 2137 IR/X-ray astrometrically matched sources: statistically we estimate that our catalog contains 289 +/- 13 true matches to soft X-ray sources and 154 +/- 39 matches to hard X-ray sources. However, the fraction of true counterparts to candidate counterparts for hard sources is just 11 %, compared to 60 % for soft sources, making hard source NIR matches particularly challenging for spectroscopic follow-up. We calculate a color-magnitude diagram (CMD) for the matches to hard X-ray sources, and find regions where significant numbers of the IR matches are real. We use their CMD positions to place limits on the absolute Ks band magnitudes of the potential NIR counterparts to hard X-ray sources. We find regions of the counterpart CMD with 9 +/- 3 likely Wolf-Rayet/supergiant binaries (with 4 spectroscopically confirmed in the literature) as well as 44 +/- 13 candidates that could consist of either main sequence high mass X-ray binaries or red giants with an accreting compact companion. (abridged)

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arXiv2010

Photometric Properties of the M33 Star Cluster System

We present a catalog of 2,990 extended sources in a 1deg x1deg area centered on M33 using the MegaCam camera on the 3.6m Canada-France-Hawaii telescope (CFHT). The catalog includes 599 new candidate stellar clusters, 204 previously confirmed clusters, 1,969 likely background galaxies and 218 unknown extended objects. We present ugriz integrated magnitudes of the candidates and confirmed star clusters as well as full width at half maximum, ellipticity and stellarity. Based on the properties of the confirmed star clusters, we select a sub-sample of highly probable clusters composed of 246 objects. The integrated photometry of the complete cluster catalog reveals a wide range of colors from -0.4 < (g-r) < 1.5 and -1.0 < (r-i) < 1.0 with no obvious cluster subpopulations. Comparisons with models of simple stellar populations suggest a large range of ages some as old as ~ 10 Gyrs. In addition, we find a sequence in the color-color diagrams that deviates from the expected direction of evolution. This feature could be associated with very young clusters (< 10^7 yrs) possessing significant nebular emission. Analysis of the radial density distribution suggests that the cluster system of M33 has suffered from significant depletion possibly due to interactions with M31. We also detect a gap in the cluster distribution in the color-color diagram at (g-r) ~ 0.3 and (u-g) ~ 0.8. This gap could be interpreted as an evolutionary effect. This complete catalog provides promising targets for deep photometry and high resolution spectroscopy to study the structure and star formation history of M33.

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arXiv2010

Deep HST/ACS Photometry of the M81 Halo

We present a deep color-magnitude diagram for individual stars in the halo of the nearby spiral galaxy M81, at a projected distance of 19 kpc, based on data taken with the Advanced Camera for Surveys on the Hubble Space Telescope (HST). The color magnitude diagram reveals a red giant branch that is narrow and fairly blue, and a horizontal branch that has stars that lie mostly redward of the RR Lyrae instability strip. We derive a mean metallicity of [M/H] = -1.15 +\- 0.11 and age of 9 +\- 2 Gyr for the dominant population in our field, from the shape of the red giant branch, the magnitude of the red clump, and the location of the red giant branch bump. We compare our metallicity and age results with those found previously for stars in different locations within M81, and in the spheroids of other nearby galaxies.

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arXiv2009

RR Lyrae Variables in the Halo of M33

The properties of RR Lyrae variables make them excellent probes of the formation and evolution of a stellar population. The mere presence of such stars necessitates an age greater than ~10 Gyr while their periods and amplitudes can be used to estimate the metal abundance of the cluster or galaxy in which they reside. These and other features of RR Lyraes have been used to study the Local Group late-type spiral galaxy M33. Though these studies are generally in their infancy, we have established that M33 does indeed harbor RR Lyraes in its halo and probably also in its disk suggesting that these two components formed early in the history of M33. The mean metallicity of the halo RR Lyraes is consistent with that of the halo globular clusters in M33 at [Fe/H]~-1.3. Little is known about the spatial distribution of the RR Lyraes; this will require wide-field time-series studies with sufficient photometric depth to allow both the identification of RR Lyraes and robust period determination.

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arXiv2009

The ACS Survey of Galactic Globular Clusters. IX. Horizontal Branch Morphology and the Second Parameter Phenomenon

The horizontal branch (HB) morphology of globular clusters (GCs) is most strongly influenced by metallicity. The second parameter phenomenon acknowledges that metallicity alone is not enough to describe the HB morphology of all GCs. In particular, the outer Galactic halo contains GCs with redder HBs at a given metallicity than are found inside the Solar circle. Thus, at least a second parameter is required to characterize HB morphology. Here we analyze the median color difference between the HB and the red giant branch (RGB), d(V-I), measured from HST ACS photometry of 60 GCs within ~20 kpc of the Galactic Center. Analysis of this homogeneous data set reveals that, after the influence of metallicity has been removed, the correlation between d(V-I) and age is stronger than that of any other parameter considered. Expanding the sample to include HST photometry of the 6 most distant Galactic GCs lends additional support to the correlation between d(V-I) and age. This result is robust with respect to the adopted metallicity scale and the method of age determination, but must bear the caveat that high quality, detailed abundance information is not available for a significant fraction of the sample. When a subset of GCs with similar metallicities and ages are considered, a correlation between d(V-I) and central luminosity density is exposed. With respect to the existence of GCs with anomalously red HBs at a given metallicity, we conclude that age is the second parameter and central density is most likely the third. Important problems related to HB morphology in GCs, notably multi-modal distributions and faint blue tails, remain to be explained. (Abridged)

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arXiv2009

RR Lyrae Variables in the Local Group Dwarf Galaxy NGC 147

We investigate the RR Lyrae population in NGC 147, a dwarf satellite galaxy of M31 (Andromeda). We used both Thuan-Gunn g-band ground-based photometry from the literature and Hubble Space Telescope Wide Field Planetary Camera 2 archival data in the F555W and F814W passbands to investigate the pulsation properties of RR Lyrae variable candidates in NGC 147. These datasets represent the two extreme cases often found in RR Lyrae studies with respect to the phase coverage of the observations and the quality of the photometric measurements. Extensive artificial variable star tests for both cases were performed. We conclude that neither dataset is sufficient to confidently determine the pulsation properties of the NGC 147 RR Lyraes. Thus, while we can assert that NGC 147 contains RR Lyrae variables, and therefore a population older than ~10 Gyr, it is not possible at this time to use the pulsation properties of these RR Lyraes to study other aspects of this old population. Our results provide a good reference for gauging the completeness of RR Lyrae variable detection in future studies.

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arXiv2009

RR Lyrae Variables in Two Fields in the Spheroid of M31

We present Hubble Space Telescope observations taken with the Advanced Camera for Surveys Wide Field Channel of two fields near M32 - between four and six kpc from the center of M31. The data cover a time baseline sufficient for the identification and characterization of 681 RR Lyrae variables of which 555 are ab-type and 126 are c-type. The mean magnitude of these stars is =25.29 +/- 0.05 where the uncertainty combines both the random and systematic errors. The location of the stars in the Bailey Diagram and the ratio of c-type RR Lyraes to all types are both closer to RR Lyraes in Oosterhoff type I globular clusters in the Milky Way as compared with Oosterhoff II clusters. The mean periods of the ab-type and c-type RR Lyraes are =0.557 +/- 0.003 and =0.327 +/- 0.003, respectively, where the uncertainties in each case represent the standard error of the mean. When the periods and amplitudes of the ab-type RR Lyraes in our sample are interpreted in terms of metallicity, we find the metallicity distribution function to be indistinguishable from a Gaussian with a peak at <[Fe/H]>=-1.50 +/- 0.02, where the quoted uncertainty is the standard error of the mean. Using a relation between RR Lyrae luminosity and metallicity along with a reddening of E(B-V) = 0.08 +/- 0.03, we find a distance modulus of (m-M)o=24.46 +/- 0.11 for M31. We examine the radial metallicity gradient in the environs of M31 using published values for the bulge and halo of M31 as well as the abundances of its dwarf spheroidal companions and globular clusters. In this context, we conclude that the RR Lyraes in our two fields are more likely to be halo objects rather than associated with the bulge or disk of M31, in spite of the fact that they are located at 4-6 kpc in projected distance from the center.

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arXiv2009

Newly Identified Star Clusters in M33. II. Radial HST/ACS Fields

We present integrated photometry and color-magnitude diagrams for 161 star clusters in M33, of which 115 were previously uncataloged, using the Advanced Camera For Surveys Wide Field Channel onboard the Hubble Space Telescope. The integrated V-band magnitudes of these clusters range from Mv~-9 to as faint as Mv~-4, extending the depth of the existing M33 cluster catalogs by ~1 mag. Comparisons of theoretical isochrones to the color-magnitude diagrams using the Padova models yield ages for 148 of these star clusters. The ages range from Log (t)~7.0 to Log (t)~9.0. Our color-magnitude diagrams are not sensitive to clusters older than ~1 Gyr. We find that the variation of the clusters' integrated colors and absolute magnitudes with age is consistent with the predictions of simple stellar population models. These same models suggest that the masses of the clusters in our sample range from 5x10^3 to 5x10^4 *Msun.

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arXiv2009

Deep 2MASS Photometry of M67 and Calibration of the Main Sequence J-Ks Color Difference as an Age Indicator

We present an analysis of Two Micron All Sky Survey (2MASS) calibration photometry of the old open cluster M67 (NGC 2682). The proper motion-cleaned color-magnitude diagram (CMD) resulting from these data extends ~3 magnitudes deeper than one based on data from the point source catalog. The CMD extends from above the helium-burning red clump to a faint limit that is more than 7 magnitudes below the main sequence turnoff in the Ks band. After adopting a reddening of E(B-V) = 0.041 +/- 0.004 and a metal abundance of [Fe/H] = -0.009 +/- 0.009 based on a survey of published values, we fit the unevolved main sequence of M67 to field main sequence stars with 2MASS photometry and Hipparcos parallaxes. This analysis yields distance moduli of (m-M)Ks = 9.72 +/- 0.05 and (m-M)o = 9.70 +/- 0.05, which are consistent with published values. We compare the theoretical isochrones of Girardi et al. and Dotter et al. to the CMD of M67 and comment on the relative merits of each set of models. These comparisons suggest an age between 3.5 and 4.0 Gyr for M67. The depth of the M67 data make them ideal for the calibration of a new age indicator that has recently been devised by Calamida et al.- the difference in (J-Ks) color between the main sequence turnoff (TO) and the point on the lower main sequence where it turns down (TD) and becomes nearly vertical [D(J-Ks)]. Coupled with deep 2MASS photometry for three other open clusters, NGC 2516, M44, and NGC 6791, we calibrate D(J-Ks) in terms of age and find D(J-Ks) = (3.017 +/- 0.347) - (0.259 +/- 0.037)*Log Age (yrs).

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arXiv2009

The Detection of Inside-out Disk Growth in M33

We present resolved stellar photometry of 4 fields along the major axis of the M33 disk from images taken with the Advanced Camera for Surveys aboard the Hubble Space Telescope. The photometry provides a detailed census of the red clump in all fields and reaches the ancient main sequence in the outermost field. Through detailed modeling of the color-magnitude diagrams, we find that the percentage of the stellar mass formed prior to z=1 changes from 71 +/- 9% in the innermost field to 16 +/- 6% in the outermost field. The disk shows a clear trend of increasing scale-length with time, evolving from 1.0 +/- 0.1 kpc 10 Gyr ago to 1.8 +/- 0.1 kpc at times more recent than 5 Gyr ago, in agreement with analytical predictions for disk growth. Beyond the disk truncation radius, however, the stellar density profile steepens with time and the age gradient reverses, in agreement with recent simulations. The late and slow growth of the stellar disk may be due in part to the low mass of M33.

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arXiv2009

"An Era of Precision Astrophysics: Connecting Stars, Galaxies and the Universe," an Astro2010 Science White Paper

Abridged: The golden age of astrophysics is upon us with both grand discoveries (extra-solar planets, dark matter, dark energy) and precision cosmology. Fundamental understanding of the working of stars and galaxies is within reach, thanks to newly available precision measurements. We highlight the importance of distances and model independent distances and masses. Distances are fundamental in astrophysics and their knowledge can change our perception of phenomena dramatically: e.g., in antiquity, the Heliocentric model was rejected because the predicted stellar parallaxes were not observed. Distance measurements are directly related to the history & fate of the uni- verse as they provide 2 of 3 methods available to date the universe. The 1st method is based on the ages of stars, which can be ascertained if their lumi- nosities (distances) are accurately known. The 2nd method relies on cosmolo- gical methods. To 1st order, the age of the universe is the inverse H_0. As stressed by the previous decadal report, "the fundamental goal of ... astrophysics is to understand how the universe ... galaxies [and] stars ... formed, how they evolved, and what their destiny will be." These questions can be answered partly by micro-arcsecond astrometry: 1) Galactic archeology: a detailed reconstruction of the formation history of the Milky Way and other Local Group galaxies, 2) the very oldest stars in the Milky Way and the age of the Universe, and 3) H_0 and concordance cosmology. These goals are achievable by combining muas-arcsecond astrometry from the proposed SIM-Lite mission supplemented with ground-based spectroscopy. The results of our proposed project will force the biggest reassessment of stellar astrophysics in 50 years, which will affect most branches of astrophysics.

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arXiv2008

The ACS Survey of Galactic Globular Clusters. VII. Relative Ages

The ACS Survey of Galactic Globular Clusters is a Hubble Space Telescope (HST) Treasury program designed to provide a new large, deep and homogeneous photometric database. Based on observations from this program, we have measured precise relative ages for a sample of 64 Galactic globular clusters by comparing the relative position of the clusters' main sequence turn offs, using main-sequence fitting to cross-compare clusters within the sample. This method provides relative ages to a formal precision of 2-7%. We demonstrate that the calculated relative ages are independent of the choice of theoretical model. We find that the Galactic globular cluster sample can be divided into two groups -- a population of old clusters with an age dispersion of ~5% and no age-metallicity relation, and a group of younger clusters with an age-metallicity relation similar to that of the globular clusters associated with the Sagittarius dwarf galaxy. These results are consistent with the Milky Way halo having formed in two phases or processes. The first one would be compatible with a rapid (<0.8 Gyr) assembling process of the halo, in which the clusters in the old group were formed. Also these clusters could have been formed before reionization in dwarf galaxies that would later merge to build the Milky Way halo as predicted by Lambda-CDM cosmology. However, the galactocentric metallicity gradient shown by these clusters seems difficult to reconcile with the latter. As for the younger clusters, it is very tempting to argue that their origin is related to their formation within Milky Way satellite galaxies that were later accreted, but the origin of the age-metallicity relation remains unclear.

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arXiv2008

Stellar Populations in the Andromeda V Dwarf Spheroidal Galaxy

Using archival imaging from the Wide Field Planetary Camera 2 aboard the Hubble Space Telescope, we investigate the stellar populations of the Local Group dwarf spheroidal Andromeda V - a companion satellite galaxy of M31. The color-magnitude diagram (CMD) extends from above the first ascent red giant branch (RGB) tip to approximately one magnitude below the horizontal branch (HB). The steep well-defined RGB is indicative of a metal-poor system while the HB is populated predominantly redward of the RR Lyrae instability strip. Utilizing Galactic globular cluster fiducial sequences as a reference, we calculate a mean metallicity of [Fe/H] = -2.20 +/- 0.15 and a distance of (m-M)0 = 24.57 +/- 0.04 after adopting a reddening of E(B-V) = 0.16. This metal abundance places And V squarely in the absolute magnitude - metallicity diagram for dwarf spheroidal galaxies. In addition, if we attribute the entire error-corrected color spread of the RGB stars to an abundance spread, we estimate a range of ~0.5 dex in the metallicities of And V stars. Our analysis of the variable star population of And V reveals the presence of 28 potential variables. Of these, at least 10 are almost certainly RR Lyrae stars based on their time sequence photometry.

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arXiv2008

The Stellar Populations of M33's Outer Regions IV: Inflow History and Chemical Evolution

We have modelled the observed color-magnitude diagram (CMD) at one location in M33's outskirts under the framework of a simple chemical evolution scenario which adopts instantaneous and delayed recycling for the nucleosynthetic products of Type II and Ia supernovae. In this scenario, interstellar gas forms stars at a rate modulated by the Kennicutt-Schmidt relation and gas outflow occurs at a rate proportional to the star formation rate (SFR). With this approach, we put broad constraints on the role of gas flows during this region's evolution and compare its [alpha/Fe] vs. [Fe/H] relation with that of other Local Group systems. We find that models with gas inflow are significantly better than the closed box model at reproducing the observed distribution of stars in the CMD. The best models have a majority of gas inflow taking place in the last 7 Gyr, and relatively little in the last 3 Gyr. These models predict most stars in this region to have [alpha/Fe] ratios lower than the bulk of the Milky Way's halo. The predictions for the present-day SFR, gas mass, and oxygen abundance compare favorably to independent empirical estimates. Our results paint a picture in which M33's outer disc formed from the protracted inflow of gas over several Gyr with at least half of the total inflow occurring since z ~ 1.

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arXiv2008

Globular Clusters in the Outer Galactic Halo: AM-1 and Palomar 14

AM-1, at ~120 kpc, and Pal 14, at ~70 kpc, are two of the most distant Galactic globular clusters known. We present Hubble Space Telescope WFPC2 photometry of AM-1 and Pal 14 that reveals unprecedented depth and detail in the color-magnitude diagrams of these two clusters. Absolute and relative age measurements confirm that both are younger than the inner halo globular cluster M 3 by 1.5-2 Gyr, assuming all three clusters have similar compositions. Thus AM-1 and Pal 14 join Pal 3, Pal 4, and Eridanus (studied by Stetson et al.) as distant Galactic globular clusters with red horizontal branches and young ages relative to the inner halo. Within the context of the entire body of research on the ages of second parameter globular clusters, the observed correlation between age and horizontal branch morphology suggests that age is the best candidate for the second parameter. However, this conclusion is tempered by the lack of precise chemical abundance determinations for a significant fraction of second parameter globular clusters.

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