arXiv subjects

James A. Rose

Publications and source records attributed to James A. Rose.

At least 19 recordsLinked to original sources

arXiv2011

Star Clusters in M31: II. Old Cluster Metallicities and Ages from Hectospec Data

We present new high signal-to-noise spectroscopic data on the M31 globular cluster system, obtained with the Hectospec multifiber spectrograph on the 6.5m MMT. More than 300 clusters have been observed at a resolution of 5A. The primary focus of this paper is the determination of mean cluster metallicities, ages and reddenings. Metallicities were estimated using a calibration of Lick indices with [Fe/H] provided by Galactic GCs. The metallicity distribution is not generally bimodal, in strong distinction with the bimodal Galactic globular distribution. Rather, the M31 distribution shows a broad peak, centered at [Fe/H]=-1, suggesting that the cluster systems of M31 and the Milky Way had different formation histories. Ages for clusters with [Fe/H] > -1 were determined using the automatic stellar population analysis program EZ_Ages. We find no evidence for massive clusters in M31 with intermediate ages. Moreover, we find that the mean ages of the old GCs are remarkably constant over about a decade in metallicity (-0.95 < [Fe/H] < 0.0). (abridged)

astro-ph.GA↗

arXiv2010

Galaxies M32 and NGC 5102 Confirm a Near-infrared Spectroscopic Chronometer

We present near infrared (NIR) IRTF/SpeX spectra of the intermediate-age galaxy M32 and the post-starburst galaxy NGC 5102. We show that features from thermally-pulsing asymptotic giant branch (TP-AGB) and main sequence turn-off (MSTO) stars yield similar ages to those derived from optical spectra. The TP-AGB can dominate the NIR flux of a coeval stellar population between ~0.1 and ~2 Gyr, and the strong features of (especially C-rich) TP-AGB stars are useful chronometers in integrated light studies. Likewise, the Paschen series in MSTO stars is stongly dependent on age and is an indicator of a young stellar component in integrated spectra. We define four NIR spectroscopic indices to measure the strength of absorption features from both C-rich TP-AGB stars and hydrogen features in main sequence stars, in a preliminary effort to construct a robust chronometer that probes the contributions from stars in different evolutionary phases. By comparing the values of the indices measured in M32 and NGC 5102 to those in the Maraston (2005) stellar population synthesis models for various ages and metallicities, we show that model predictions for the ages of the nuclei of M32 and NGC 5102 agree with previous results obtained from integrated optical spectroscopy and CMD analysis of the giant branches. The indices discriminate between an intermediate age population of ~3-4 Gyr, a younger population of <1 Gyr, and can also detect the signatures of very young (<100 Myr) populations.

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arXiv2010

Stellar Population Trends in S0 Galaxies

We present stellar population age and metallicity trends for a sample of 59 S0 galaxies based on optical SDSS and NIR J & H photometry. When combined with optical g and r passband imaging data from the SDSS archive and stellar population models, we obtain radial age and metallicity trends out to at least 5 effective radii for most of the galaxies in our sample. The sample covers a range in stellar mass and light concentration. We find an average central light-weighted age of ~ 4 Gyr and central metallicity [Z/H] ~ 0.2 dex. Almost all galaxies show a negative metallicity gradient from the center out, with an average value of Delta[Z/H]/Delta(log(r/Re)) = -0.6. An age increase, decrease, and minimal change with radius is observed for 58%, 19%, and 23%, respectively, for a mean age gradient of Delta(age)/Delta(log(r/Re)) = 2.3 Gyr dex^{-1}. For 14 out of 59 galaxies, the light-weighted age of the outer region is greater than 10 Gyr. We find that galaxies with both lower mass and lower concentration have younger light-weighted ages and lower light-weighted metallicities. This mass-metallicity relation extends into the outer regions of our S0 galaxies. Our results are consistent with the formation of S0 galaxies through the transformation of spiral galaxy disks. Determining the structural component that makes up the outer region of galaxies with old outksirts is a necessary step to understand the formation history of S0 galaxies.

astro-ph.CO↗

arXiv2009

Star Formation in Partially Gas-Depleted Spiral Galaxies

Broadband B and R and H-alpha images have been obtained with the 4.1-m SOAR telescope atop Cerro Pachon, Chile for 29 spiral galaxies in the Pegasus I galaxy cluster and for 18 spirals in non-cluster environments. Pegasus I is a spiral-rich cluster with a low density intracluster medium and a low galaxy velocity dispersion. When combined with neutral hydrogen (HI) data obtained with the Arecibo 305-m radiotelescope, acquired by Levy et al. (2007) and by Springob et al. (2005a), we study the star formation rates in disk galaxies as a function of their HI deficiency. To quantify HI deficiency, we use the usual logarithmic deficiency parameter, DEF. The specific star formation rate (SSFR) is quantified by the logarithmic flux ratio of H-alpha flux to R band flux, and thus roughly characterizes the logarithmic SFR per unit stellar mass. We find a clear correlation between the global SFR per unit stellar mass and DEF, such that the SFR is lower in more HI-deficient galaxies. This correlation appears to extend from the most gas-rich to the most gas-poor galaxies. We also find a correlation between the central SFR per unit mass relative to the global values, in the sense that the more HI-deficient galaxies have a higher central SFR per unit mass relative to their global SFR values than do gas-rich galaxies. In fact, approximately half of the HI-depleted galaxies have highly elevated SSFRs in their central regions, indicative of a transient evolutionary state. In addition, we find a correlation between gas-depletion and the size of the H-alpha disk (relative to the R band disk); HI-poor galaxies have truncated disks. Moreover, aside from the elevated central SSFR in many gas-poor spirals, the SSFR is otherwise lower in the H-alpha disks of gas-poor galaxies than in gas-rich spirals.

astro-ph.CO↗

arXiv2008

Star Clusters in M31: I. A Catalog and a Study of the Young Clusters

We present an updated catalog of 1300 objects in the field of M31, including 670 likely star clusters of various types. Archival images were inspected to confirm cluster classifications where possible, but most of the classifications were based on spectra taken of ~1000 objects with the Hectospec fiber positioner and spectrograph on the 6.5m MMT. The spectra and images of young clusters are analyzed in detail in this paper. Ages, reddenings and masses of 140 young clusters are derived by comparing the observed spectra and photometry with model spectra. We find these to have masses as great as 10^5 with a median of 10^4 M_sun, and a median age of 0.25 Gyr. Thus these clusters are similar in mass to the LMC young massive clusters, and are in between Milky Way open clusters and the globulars of M31 and the Milky Way. Most of the M31 young clusters have the low concentration typical of Milky Way open clusters, and we expect that most of these will be disrupted in the next Gyr, but a few have high concentrations and will likely survive longer. The spatial distribution of the young clusters is well correlated with the star-forming regions as mapped out by mid-IR emission. A kinematic analysis likewise confirms the spatial association of the young clusters with the young disk in M31.

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arXiv2007

Young star clusters in M31

In our study of M31's globular cluster system with MMT/Hectospec, we have obtained high-quality spectra of 85 clusters with ages less than 1 Gyr. With the exception of Hubble V, the young cluster in NGC 205, we find that these young clusters have kinematics and spatial distribution consistent with membership in M31's young disk. Preliminary estimates of the cluster masses and structural parameters, using spectroscopically derived ages and HST imaging, confirms earlier suggestions that M31 has clusters similar to the LMC's young populous clusters.

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arXiv2006

The Effect of Cluster Environment on Galaxy Evolution in the Pegasus I Cluster

We present HI observations of 54 galaxies in the Pegasus cluster. The observations include single dish HI measurements, obtained with the Arecibo telescope for all 54 galaxies in the sample, as well as HI images, obtained with the VLA for 10 of these. The Arecibo profiles reveal an overall HI deficiency in the cluster, with ~40% of the galaxies in the core of the cluster showing modest deficiencies of typically a factor of 2-3. The HI morphology of some galaxies shows that the HI disk is smaller than the optical disk and slightly offset from the stars. We find a correlation between HI deficiency and the ratio of the HI disk size to optical disk size. More HI deficient galaxies have relatively smaller HI disks, a configuration that is usually attributed to an interaction between the ISM of the galaxy and the hot ICM. Such a result is surprising since the Pegasus cluster has a low level of X-ray emission, and a low velocity dispersion. The low velocity dispersion, coupled with the lack of a dense hot ICM indicate that ram pressure stripping should not play a significant role in this environment. In addition, two of the galaxies, NGC7604 and NGC7648, are morphologically peculiar. Their peculiarities indicate contradictory scenarios of what is triggering their unusual star formation. Halpha imaging, along with long-slit spectroscopy of NGC7648 reveal morphological features which point to a recent tidal interaction. On the other hand, Halpha imaging,along with VLA HI mapping, of NGC7604 reveal morphological features suggestive of a ram pressure event. Our data indicate that ISM-ICM interactions may play a role in a wider variety of environments than suggested by simple ram pressure arguments.

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arXiv2005

A New Definition for the Ca4227 Feature: Is Calcium Really Underabundant in Early-type Galaxies?

We have investigated the abundance of calcium in early-type galaxies by measuring the strength of the Ca I 4227 absorption line in their integrated spectra. The database used is the large sample of early-type galaxy integrated spectra in Caldwell, Rose, & Concannon (2003). We have measured Ca abundances from the Ca I 4227 feature both by using the Lick Ca4227 index and also by defining a new index, Ca4227_r, that avoids the CN4216 molecular band in the continuum on the blueward side of the line. With the new index definition we measure Ca abundances that are systematically ~0.3 dex higher than with the Lick Ca4227 index. The result is that with the new index definition we obtain higher [Ca/Fe] abundances in early-type galaxies which are more consistent with their well known [Mg/Fe] over-abundances. Hence, we suggest that Ca might be slightly enhanced, relative to Fe, in early-type galaxies.

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arXiv2005

A Library of Integrated Spectra of Galactic Globular Clusters

We present a new library of integrated spectra of 40 Galactic globular clusters, obtained with the Blanco 4-m telescope and the R-C spectrograph at the Cerro Tololo Interamerican Observatory. The spectra cover the range ~ 3350 -- 6430 A with ~ 3.1 A (FWHM) resolution. The spectroscopic observations and data reduction were designed to integrate the full projected area within the cluster core radii in order to properly sample the light from stars in all relevant evolutionary stages. The S/N values of the flux-calibrated spectra range from 50 to 240/A at 4000 A and from 125 to 500/A at 5000 A. The selected targets span a wide range of cluster parameters, including metallicity, horizontal-branch morphology, Galactic coordinates, Galactocentric distance, and concentration. The total sample is thus fairly representative of the entire Galactic globular cluster population and should be valuable for comparison with similar integrated spectra of unresolved stellar populations in remote systems. For most of the library clusters, our spectra can be coupled with deep color-magnitude diagrams and reliable metal abundances from the literature to enable the calibration of stellar population synthesis models. In this paper we present a detailed account of the observations and data reduction. The spectral library is publicly available in electronic format from the National Optical Astronomical Observatory website.

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arXiv2004

Radial Age and Metal Abundance Gradients in the Stellar Content of M32

We present long-slit spectroscopy of the elliptical galaxy M32, obtained with the 8-m Subaru telescope at Mauna Kea, the 1.5-m Tillinghast telescope at the F. L. Whipple Observatory, and the 4-m Mayall telescope at the Kitt Peak National Observatory. The spectra cover the Lick index red spectral region as well as higher order Balmer lines in the blue. Spectra have been taken with the slit off-set from the nucleus to avoid scattered light contamination from the bright nucleus of M32. An analysis of numerous absorption features, particularly involving the H$γ$ and H$β$ Balmer lines, reveals that systematic radial trends are evident in the integrated spectrum of M32. Population synthesis models indicate a radial change in both the age and chemical composition of the light-weighted mean stellar population in M32, from the nucleus out to 33", i.e., approximately 1.0 effective radius, R_e. Specifically, the light-weighted mean stellar population at 1 R_e is older, by \~3 Gyr, and more metal-poor, by ~-0.25 dex in [Fe/H], t han the central value of ~4 Gyr and [Fe/H]~0.0. We show that this apparent population trend cannot be attributed to a varying contribution from either hot stars or emission line contamination. The increase in age and decrease in metal-abundance with radius are sufficiently well-matched to explain the flat radial color profiles previously observed in M32. In addition, the ratio of Mg to Fe abundance, [Mg/Fe], increases from ~-0.25 in the nucleus to ~-0.08 at 1 R_e. Finally, we find spuriously pronounced line strength gradients in the Mayall data that are an artifact of scattered light from the bright nucleus. Scattered light issues may explain the lack of consistency among previously published studies of radial line strength gradients in M32.

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arXiv2004

The Identification of Blue Horizontal Branch Stars in the Integrated Spectra of Globular Clusters

A major uncertainty in the spectroscopic dating of extragalactic globular clusters concerns the degenerate effect that age and horizontal branch morphology have on the strength of Balmer lines. In this Letter we show that the ratio between the equivalent widths of Hdelta and Hbeta is far more sensitive to horizontal branch morphology than to age, thus making it possible to break the degeneracy. We show that it is possible to distinguish intermediate-age globular clusters from those whose Balmer lines are strengthened by the presence of blue horizontal branch stars, purely on the basis of the clusters' integrated spectra. The degeneracy between age and horizontal branch morphology can be lifted with Hbeta and Hdelta line strengths from spectra with S/N >= 30 per Angstrom, which is typical of current studies of integrated spectroscopy of extragalactic globular clusters.

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arXiv2004

The Indo-U.S. Library of Coude Feed Stellar Spectra

We have obtained spectra for 1273 stars using the 0.9m Coudé Feed telescope at Kitt Peak National Observatory. This telescope feeds the coudé spectrograph of the 2.1m telescope. The spectra have been obtained with the #5 camera of the coudé spectrograph and a Loral 3K X 1K CCD. Two gratings have been used to provide spectral coverage from 3460 Åto 9464 Å, at a resolution of $\sim$1ÅFWHM and at an original dispersion of 0.44 Å/pixel. For 885 stars we have complete spectra over the entire 3460 Åto 9464 Åwavelength region (neglecting small gaps of $<$ 50 Å), and partial spectral coverage for the remaining stars. The 1273 stars have been selected to provide broad coverage of the atmospheric parameters T$_{eff}$, log g, and [Fe/H], as well as spectral type. The goal of the project is to provide a comprehensive library of stellar spectra for use in the automated classification of stellar and galaxy spectra and in galaxy population synthesis. In this paper we discuss the characteristics of the spectral library, viz., details of the observations, data reduction procedures, and selection of stars. We also present a few illustrations of the quality and information available in the spectra. The first version of the complete spectral library is now publicly available from the National Optical Astronomy Observatory (NOAO) via FTP and HTTP.

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arXiv2003

Population Synthesis in the Blue III. The Integrated Spectrum of M67 and the Spectroscopic Age of M32

We construct an integrated spectrum of the intermediate age, solar metallicity Galactic cluster M67, from individual spectra of cluster members. The integrated spectrum is used as a template to test our stellar population synthesis (SPS) models, in an age and [Fe/H] regime where such models remain largely untested. We show that our models predict a spectroscopic age of 3.5 +/- 0.5 Gyr for M67, in excellent agreement with the age we derive from the color-magnitude diagram of the cluster. The same age is obtained when using Hbeta, Hgamma or Hdelta as the age indicator. Our models predict the abundances of Fe, Mg, C and N in agreement with detailed abundance analyses of cluster stars, to within 0.1 dex. Encouraged by the high degree of consistency of our models, we apply them to the study of the integrated spectrum of the central 3 arcsec of the compact elliptical galaxy M32. The resulting luminosity-weighted age of the galaxy ranges between 2 and 3.5 Gyr, depending on the Balmer line used. According to our models, the center of M32 seems to have a super-solar light-weighted Fe abundance, ranging between [Fe/H] ~ +0.1 and +0.3, depending on the Fe line adopted. Light-weighted abundance ratios found for the center of M32 are as follows: [Mg/Fe] ~ -0.1 -- -0.2, [C/Fe] and [N/Fe] ~ 0. We find that single SPS models with a solar-scaled abundance pattern cannot fit all the Balmer and metal lines in the integrated spectrum of M32. There is a systematic trend in the sense that bluer absorption lines indicate a younger age and a higher [Fe/H]. This slight inconsistency can be due either to (unaccounted for) abundance ratio effects on blue iron and Balmer line indices, or to a spread of the ages of the stellar populations in M32. Current stellar population models cannot break this degeneracy at the level of accuracy required to address this problem.

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arXiv2003

Star formation histories of early-type galaxies. I: Higher order Balmer lines as age indicators

(shortened) We have obtained blue integrated spectra of 175 nearby early-type galaxies, covering a wide range in galaxy velocity dispersion, and emphasizing those with sigma < 100 km/s. Galaxies have been observed both in the Virgo cluster and in lower-density environments. The main goals are the evaluation of higher order Balmer lines as age indicators, and differences in stellar populations as a function of mass, environment and morphology. In this first paper our emphasis is on presenting the evolutionary population synthesis models. Lower-sigma galaxies exhibit a substantially greater intrinsic scatter, in a variety of line strength indicators, than do higher-sigma galaxies, with the large intrinsic scatter setting in below a sigma of 100 km/s. Modeling of the observed spectral indices indicates that the strong Balmer lines found primarily among the low-sigma galaxies are caused by young age, rather than by low metallicity. Thus we find a trend between the population age and the velocity dispersion, such that low sigma galaxies have younger luminosity-weighted mean ages. We have repeated this analysis using several different Balmer lines, and find consistent results from one spectral indicator to another.

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arXiv2002

Population Synthesis in the Blue I. Synthesis of the Integrated Spectrum of 47 Tucanae from its Color-Magnitude Diagram

We perform an empirical synthesis of the blue integrated spectrum of the metal-rich globular cluster 47 Tucanae, based directly on the color-magnitude diagram of the cluster coupled to a moderately high-resolution spectral library. Freed from any significant dependence on theory, we are able to perform a fundamental test of the adequacy of the spectral library and its associated stellar parameters. Excellent fits are achieved for almost all absorption-line indices studied, provided the computations are corrected for two limitations of the spectral library, namely, the lack of a representative set of metal-poor giants and the absence of CN-strong stars. The latter effect is corrected by means of spectrum synthesis from model photospheres, considering the abundance pattern of CN-strong and CN-normal stars. We also need to perform a slight correction of the metallicity of the cluster (by --0.05 dex in relation to the standard value [Fe/H]=-0.7) in order to match the metal-line index measurements in the cluster spectrum. After these relatively small adjustments, the overall spectral agreement is good. Good fits are achieved for Hbeta, Hgamma, Mgb, , Ca4227 and Fe4383, and only Hdelta_F is overpredicted. Thus, ages inferred from Hdelta_F are slightly older than the ages based on the other Balmer lines, by ~ 3 Gyrs. The success of this exercise suggests that previous failures to synthesize the spectrum of 47 Tuc must have arisen from inadequacies in the theoretical evolutionary isochrones and/or luminosity functions. Such a possibility is considered in a companion paper.

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arXiv2002

Population Synthesis in the Blue II. The Spectroscopic Age of 47 Tucanae

We develop a new set of models for intermediate-metallicity single stellar populations in the blue/optical region and use those models to determine the spectroscopic age of 47 Tuc. The models are based on a moderately high-resolution (1.8 A FWHM) empirical spectral library, state-of-the-art theoretical isochrones from M. Salaris and the most recent set from the Padova group, and new semi-empirical calibrations between fundamental stellar parameters and observables. We highlight the importance of correctly modeling the luminosity function (LF) of the cluster at the level of the giant branch, in order to achieve a good reproduction of the integrated spectrum; agreement between the spectroscopic age and the age based on the cluster's color-magnitude diagram (CMD) is achieved only if the observed LF is used rather than the theoretical ones, which either do not include AGB stars (Salaris) or underpredict the total number counts of bright giants in the cluster by a factor of two(Padova). After all corrections are made, the CMD and the spectroscopic ages (from Hgamma and Hbeta) are in close agreement: 11--12 Gyrs for Salaris isochrones and about 13 Gyrs for Padova. The difference between the model ages is due to the inclusion of atomic diffusion in the Salaris models. Previously older spectroscopic ages were due to the underestimate of the number of red giants and/or the use of isochrones that neglected the effects of He-diffusion and alpha-enhancement. A similar underestimate in the bright giant LF, if it exists in current super-solar metallicity models, would cause spectroscopic ages of elliptical galaxies inferred from such models to be too high by approximately 30%.

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arXiv2001

Radial Color Gradients in K+A Galaxies in Distant Clusters of Galaxies

Galaxies in rich clusters with z $\gtrsim$ 0.3 are observed to have a higher fraction of photometrically blue galaxies than their nearby counterparts. This raises the important question of what environmental effects can cause the termination of star formation between z $\approx$ 0.3 and the present. The star formation may be truncated due to ram-pressure stripping, or the gas in the disk may be depleted by an episode of star formation caused by some external perturbation. To help resolve this issue, surface photometry was carried out for a total of 70 early-type galaxies in the cluster Cl1358+62, at z $\sim$ 0.33, using two-color images from the Hubble Archive. The galaxies were divided into two categories based on spectroscopic criteria: 24 are type K+A (e.g., strong Balmer lines, with no visible emission lines), while the remaining 46 are in the control sample with normal spectra. Radial color profiles were produced to see if the K+A galaxies show bluer nuclei in relation to their surrounding disks. Specifically, a linear gradient was fit to the radial color profile of each galaxy. We find that the K+A galaxies on average tend to have slightly bluer gradients towards the center than the normals. A Kolmogorov-Smirnov two-sample test has been applied to the two sets of color gradients. The result of the test indicates that there is only a $\sim$2% probability that the K+A and normal samples are drawn from the same parent distribution. There is a possible complication from a trend in the apparent magnitude vs. color gradient relation, but overall our results favor the centralized star formation scenario as an important process in the evolution of galaxies in dense clusters.

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arXiv2000

47 Tuc: The Spectroscopic Versus CMD Age Discrepancy

We investigate current problems in obtaining reliable ages for old stellar systems based on stellar population synthesis modelling of their integrated spectra. In particular, we address the large ages derived for the globular cluster 47 Tuc, which is at odds with its Color-Magnitude-Diagram (CMD) age. Using a new age indicator, Hgamma_sigma<130, which is particularly effective at breaking the degeneracy between age and metallicity, we confirm the discrepancy between the spectroscopic age and the CMD age of 47 Tuc, in that the spectroscopic age is much older. Nebular emission appears unlikely to be a source for weakening the observed Balmer lines. We then explore a number of key parameters affecting the temperature of Turn-Off stars, which are the main contributors to the Balmer lines for old metal-rich stellar populations. We find that alpha-enhanced isochrones with atomic diffusion included not only provides a good fit to the CMD of 47 Tuc, but also leads to a spectroscopic age in better agreement with the CMD age.

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