arXiv subjects

R. A. Bernstein

Publications and source records attributed to R. A. Bernstein.

52 records · Page 3Linked to original sources

arXiv2013

Chemical Abundance Evidence of Enduring High Star Formation Rates in an Early Type Galaxy: High [Ca/Fe] in NGC 5128 Globular Clusters

We present [Fe/H], ages, and Ca abundances for an initial sample of 10 globular clusters in NGC 5128 obtained from high resolution, high signal-to-noise ratio echelle spectra of their integrated light. All abundances and ages are obtained using our original technique for high resolution integrated light abundance analysis of globular clusters. The clusters have a range in [Fe/H] between -1.6 to -0.2. In this sample, the average [Ca/Fe] for clusters with [Fe/H]<-0.4 is +0.37$\pm$0.07, while the average [Ca/Fe] in our MW and M31 GC samples is +0.29 $\pm$0.09 and +0.24 $\pm$0.10, respectively. This may imply a more rapid chemical enrichment history for NGC 5128 than for either the Milky Way or M31.This sample provides the first quantitative picture of the chemical history of NGC 5128 that is directly comparable to what is available for the Milky Way. Data presented here were obtained with the MIKE echelle spectrograph on the Magellan Clay Telescope.

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arXiv2013

Extragalactic Globular Cluster Populations from High Resolution Integrated Light Spectra

We present a comparison of high-resolution, integrated-light, detailed chemical abundances for Galactic and extragalactic globular clusters in both massive galaxies and dwarf galaxies. We include measurements of Fe, Ca, Si, Na, and Al for globular cluster samples in the Milky Way, M31, Large Magellanic Cloud, and NGC 5128. These and other recent results from our group on M31 and NGC 5128 are the first chemical abundances derived from discrete absorption features in old stars beyond the Milky Way and its nearest neighbors. These abundances can provide both galaxy enrichment histories and constraints on globular cluster formation and evolution.

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arXiv2012

The Detailed Chemical Abundance Patterns of M31 Globular Clusters

We present detailed chemical abundances for $>$20 elements in $\sim$30 globular clusters in M31. These results have been obtained using high resolution ($λ/Δλ\sim$24,000) spectra of their integrated light and analyzed using our original method. The globular clusters have galactocentric radii between 2.5 kpc and 117 kpc, and therefore provide abundance patterns for different phases of galaxy formation recorded in the inner and outer halo of M31. We find that the clusters in our survey have a range in metallicity of $-2.2<$[Fe/H]$<-0.11$. The inner halo clusters cover this full range, while the outer halo globular clusters at R$>$20 kpc have a small range in abundance of [Fe/H]$=-1.6 \pm 0.10$. We also measure abundances of alpha, r- and s-process elements. These results constitute the first abundance pattern constraints for old populations in M31 that are comparable to those known for the Milky Way halo.

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arXiv2012

Comparison of Convective Overshooting Models and Their Impact on Abundances from Integrated Light Spectroscopy of Young ($<$ 3 Gyr) Star Clusters

As part of an ongoing program to measure detailed chemical abundances in nearby galaxies, we use a sample of young to intermediate age clusters in the Large Magellanic Cloud with ages of 10 Myr to 2 Gyr to evaluate the effect of isochrone parameters, specifically core convective overshooting, on Fe abundance results from high resolution, integrated light spectroscopy. In this work we also obtain fiducial Fe abundances from high resolution spectroscopy of the cluster individual member stars. We compare the Fe abundance results for the individual stars to the results from isochrones and integrated light spectroscopy to determine whether isochrones with convective overshooting should be used in our integrated light analysis of young to intermediate age (10 Myr -3 Gyr) star clusters. We find that when using the isochrones from the Teramo group, we obtain more accurate results for young and intermediate age clusters over the entire age range when using isochrones without convective overshooting. While convective overshooting is not the only uncertain aspect of stellar evolution, it is one of the most readily parametrized ingredients in stellar evolution models, and thus important to evaluate for the specific models used in our integrated light analysis. This work demonstrates that our method for integrated light spectroscopy of star clusters can provide unique tests for future constraints on stellar evolution models of young and intermediate age clusters.

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arXiv2011

Globular Cluster Abundances from High-Resolution, Integrated-Light Spectroscopy. IV. The Large Magellanic Cloud: $α$, Fe-peak, Light, and Heavy Elements

We present detailed chemical abundances in 8 clusters in the Large Magellanic Cloud (LMC). We measure abundances of 22 elements for clusters spanning a range in age of 0.05 to 12 Gyr, providing a comprehensive picture of the chemical enrichment and star formation history of the LMC. The abundances were obtained from individual absorption lines using a new method for analysis of high resolution ($R\sim$25,000) integrated light spectra of star clusters. This method was developed and presented in Papers I, II, and III of this series. In this paper, we develop an additional integrated light $χ^2$-minimization spectral synthesis technique to facilitate measurement of weak ($\sim$15 mÅ) spectral lines and abundances in low signal-to-noise ratio data (S/N$\sim$30). Additionally, we supplement the integrated light abundance measurements with detailed abundances that we measure for individual stars in the youngest clusters (Age$<$2 Gyr) in our sample. In both the integrated light and stellar abundances we find evolution of [$α$/Fe] with [Fe/H] and age. Fe-peak abundance ratios are similar to those in the Milky Way, with the exception of [Cu/Fe] and [Mn/Fe], which are sub-solar at high metallicities. The heavy elements Ba, La, Nd, Sm, and Eu are significantly enhanced in the youngest clusters. Also, the heavy to light s-process ratio is elevated relative to the Milky Way ([Ba/Y]$>+0.5$) and increases with decreasing age, indicating a strong contribution of low-metallicity AGB star ejecta to the interstellar medium throughout the later history of the LMC. We also find a correlation of integrated light Na and Al abundances with cluster mass, in the sense that more massive, older clusters are enriched in the light elements Na and Al with respect to Fe, which implies that these clusters harbor star-to-star abundance variations as is common in the Milky Way.

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arXiv2011

Globular Cluster Abundances from High-Resolution, Integrated-Light Spectroscopy. III. The Large Magellanic Cloud: Fe and Ages

In this paper we refine our method for the abundance analysis of high resolution spectroscopy of the integrated light of unresolved globular clusters (GCs). This method was previously demonstrated for the analysis of old ($>$10 Gyr) Milky Way GCs. Here we extend the technique to young clusters using a training set of 9 GCs in the Large Magellanic Cloud (LMC). Depending on the signal-to-noise ratio of the data, we use 20-100 Fe lines per cluster to successfully constrain the ages of old clusters to within a $\sim$5 Gyr range, the ages of $\sim$2 Gyr clusters to a 1-2 Gyr range, and the ages of the youngest clusters (0.05-1 Gyr) to a $\sim$200 Myr range. We also demonstrate that we can measure [Fe/H] in clusters with any age less than 12 Gyrs with similar or only slightly larger uncertainties (0.1-0.25 dex) than those obtained for old Milky Way GCs (0.1 dex); the slightly larger uncertainties are due to the rapid evolution in stellar populations at these ages. In this paper, we present only Fe abundances and ages. In the next paper in this series, we present our complete analysis of the $\sim 20$ elements for which we are able to measure abundances. For several of the clusters in this sample, there are no high resolution abundances in the literature from individual member stars; our results are the first detailed chemical abundances available. The spectra used in this paper were obtained at Las Campanas with the echelle on the du Pont Telescope and with the MIKE spectrograph on the Magellan Clay Telescope.

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arXiv2010

Detailed Chemical Abundances of Globular Clusters in Local Group Dwarf Galaxies

We present detailed chemical abundances of Fe, Ca and Ba for 17 globular clusters (GCs) in 5 Local Group dwarf galaxies: NGC 205, NGC 6822, WLM, the SMC and LMC. These abundances are part of a larger sample of over 20 individual elements measured in GCs in these galaxies using a new analysis method for high resolution, integrated light spectra. Our analysis also provides age and stellar population constraints. The existence of GCs in dwarf galaxies with a range of ages implies that there were episodes of rapid star formation throughout the history of these galaxies; the abundance ratios of these clusters suggest that the duration of these burst varied considerably from galaxy to galaxy. We find evolution of Fe, Ca, and Ba with age in the LMC, SMC, and NGC 6822 that is consistent with extended, lower-efficiency SF between bursts, with an increasing contribution of low-metallicity AGB ejecta at late times. Our sample of GCs in NGC 205 and WLM are predominantly old and metal-poor with high [Ca/Fe] ratios, implying that the early history of these galaxies was marked by consistently high SF rates.

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arXiv2009

M31 Globular Cluster Abundances from High-Resolution, Integrated-Light Spectroscopy

We report the first detailed chemical abundances for 5 globular clusters (GCs) in M31 from high-resolution (R ~ 25,000) spectroscopy of their integrated light. These GCs are the first in a larger set of clusters observed as part of an ongoing project to study the formation history of M31 and its globular cluster population. The data presented here were obtained with the HIRES echelle spectrograph on the Keck I telescope, and are analyzed using a new integrated light spectra analysis method that we have developed. In these clusters, we measure abundances for Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Y, and Ba, ages >10 Gyrs, and a range in [Fe/H] of -0.9 to -2.2. As is typical of Milky Way GCs, we find these M31 GCs to be enhanced in the alpha-elements Ca, Si, and Ti relative to Fe. We also find [Mg/Fe] to be low relative to other [alpha/Fe], and [Al/Fe] to be enhanced in the integrated light abundances. These results imply that abundances of Mg, Al (and likely O, Na) recovered from integrated light do display the inter- and intra-cluster abundance variations seen in individual Milky Way GC stars, and that special care should be taken in the future in interpreting low or high resolution integrated light abundances of globular clusters that are based on Mg-dominated absorption features. Fe-peak and the neutron-capture elements Ba and Y also follow Milky Way abundance trends. We also present high-precision velocity dispersion measurements for all 5 M31 GCs, as well as independent constraints on the reddening toward the clusters from our analysis.

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arXiv2007

Diffuse Optical Light in Galaxy Clusters II: Correlations with Cluster Properties

We have measured the flux, profile, color, and substructure in the diffuse intracluster light (ICL) in a sample of ten galaxy clusters with a range of mass, morphology, redshift, and density. Deep, wide-field observations for this project were made in two bands at the one meter Swope and 2.5 meter du Pont telescope at Las Campanas Observatory. Careful attention in reduction and analysis was paid to the illumination correction, background subtraction, point spread function determination, and galaxy subtraction. ICL flux is detected in both bands in all ten clusters ranging from 7.6 x 10^{10} to 7.0 x 10^{11} h^{-1} solar luminosities in r and 1.4 x 10^{10} to 1.2 x 10^{11} h^{-1} solar luminosities in the B-band. These fluxes account for 6 to 22% of the total cluster light within one quarter of the virial radius in r and 4 to 21% in the B-band. Average ICL B-r colors range from 1.5 to 2.8 mags when k and evolution corrected to the present epoch. In several clusters we also detect ICL in group environments near the cluster center and up to ~1 h^{-1} Mpc distant from the cluster center. Our sample, having been selected from the Abell sample, is incomplete in that it does not include high redshift clusters with low density, low flux, or low mass, and it does not include low redshift clusters with high flux, mass, or density. This bias makes it difficult to interpret correlations between ICL flux and cluster properties. Despite this selection bias, we do find that the presence of a cD galaxy corresponds to both centrally concentrated galaxy profiles and centrally concentrated ICL profiles. This is consistent with ICL either forming from galaxy interactions at the center, or forming at earlier times in groups and later combining in the center.

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arXiv2005

Diffuse Optical Light in Galaxy Clusters I: Abell 3888

We are undertaking a program to measure the characteristics of the intracluster light (total flux, profile, color, and substructure) in a sample of 10 galaxy clusters with a range of cluster mass, morphology, and redshift. We present here the methods and results for the first cluster in that sample, A3888. We have identified an intracluster light (ICL) component in A3888 in V and r that contains 13\pm5% of the total cluster light and extends to 700h_{70}^{-1}kpc (~0.3 r_{200}) from the center of the cluster. The ICL color in our smallest radial bin is V-r = 0.3 \pm 0.1, similar to the central cluster ellipticals. The ICL is redder than the galaxies at 400 < r < 700h_{70}^{-1}kpc although the uncertainty in any one radial bin is high. Based on a comparison of V-r color with simple stellar models, the ICL contains a component which formed more than 7 Gyr ago (at z > 1), coupled with a high metallicity (1.0Z_{\odot} < Z_{ICL} \la 2.5Z_{\odot}), and a more centralized component which contains stars formed within the past 5 Gyr (at z ~ 1). The profile of the ICL can be roughly fit by a shallow exponential in the outer regions and a steeper exponential in the central region. We also find a concentration of diffuse light around a small group of galaxies 1.4h_{70}^{-1}Mpc from the center of the cluster. In addition, we find 3 low surface brightness features near the cluster center which are blue (V-r = 0.0) and contain a total flux of 0.1M*. Based on these observations and X-ray and galaxy morphology, we suggest that this cluster is entering a phase of significant merging of galaxy groups in the core, whereupon we expect the ICL fraction to grow significantly with the formation of a cD galaxy as well as the in-fall of groups.

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arXiv2005

Detailed Chemical Abundances of Extragalactic Globular Clusters

We outline a method to measure the detailed chemical composition of extragalactic (unresolved) globular clusters (GCs) from echelle spectra of their integrated light. Our goal is to use this method to measure abundance patterns of GCs in distant spiral and elliptical galaxies to constrain their formation histories. To develop this technique we have obtained a ``training set'' of integrated-light spectra of resolved GCs in the Milky Way and LMC by scanning across the clusters during exposures. Our training set also include spectra of individual stars in those GCs from which abundances can be obtained in the normal way to provide a check on our integrated-light results. We present here the preliminary integrated-light analysis of one GC in our training set, NGC 104 (47 Tuc), and outline some of the techniques utilized and problems encountered in that analysis.

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arXiv2005

Corrections of Errors in `The First Detections of the Extragalactic Background Light at 3000, 5500, and 8000 Å. I, II, and III'

We correct errors in Bernstein, Freedman & Madore (2002abc), a series of papers in which we described observations of the optical extragalactic background light (EBL). These errors pertain to the measurement of zodiacal light, given in the second paper of this series. Making these corrections leads to a net decrease of 0.5($\pm$0.6)% in our zodiacal light measurement and a corresponding increase in the inferred extragalactic background light of roughly $0.5 (\pm 0.6) \times 10^{-9}$ ergs sec$^{-1}$ cm$^{-2}$ sr$^{-1}$ Å$^{-1}$. For comparison, the originally--quoted EBL flux at 5500Åwas $2.7 (\pm1.4) \times 10^{-9}$ in the same units ($1σ$ combined systematic and statistical uncertainty). We provide a detailed discussion of these errors and also discuss the evolution of this work prior to the (2002) papers. We note that corrections of the factual errors in our (2002) papers yield a result that is consistent with the results and errors quoted there. However, this is not intended to be a new or updated analysis, and it does not address some methodological objections which have been raised to our prior work.

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arXiv2001

The First Detections of the Extragalactic Background Light at 3000, 5500, and 8000A (II): Measurement of Foreground Zodiacal Light

We present a measurement of the absolute surface brightness of the zodiacal light (3900-5100A) toward a fixed extragalactic target at high ecliptic latitude based on moderate resolution (~1.3A per pixel) spectrophotometry obtained with the du Pont 2.5m telescope at Las Campanas Observatory in Chile. This measurement and contemporaneous Hubble Space Telescope data from WFPC2 and FOS comprise a coordinated program to measure the mean flux of the diffuse extragalactic background light (EBL). The zodiacal light at optical wavelengths results from scattering by interplanetary dust, so that the zodiacal light flux toward any extragalactic target varies seasonally with the position of the Earth. This measurement of zodiacal light is therefore relevant to the specific observations (date and target field) under discussion. To obtain this result, we have developed a technique that uses the strength of the zodiacal Fraunhofer lines to identify the absolute flux of the zodiacal light in the multiple-component night sky spectrum. Statistical uncertainties in the result are 0.6% (1 sigma). However, the dominant source of uncertainty is systematic errors, which we estimate to be 1.1% (1 sigma). We discuss the contributions included in this estimate explicitly. The systematic errors in this result contribute 25% in quadrature to the final error in our coordinated EBL measurement, which is presented in the first paper of this series.

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arXiv2001

The First Detections of the Extragalactic Background Light at 3000, 5500, and 8000A (III): Cosmological Implications

(Abridged) We have used HST WFPC2 and ground-based spectroscopy to measure the integrated extragalactic background light (EBL) at optical wavelengths. We have also computed the integrated light from individual galaxy counts in the images used to measure the EBL and in the Hubble Deep Field. We find that the flux in galaxies as measured by standard galaxy photometry methods has generally been underestimated by about 50%. Further, we find that the total flux in individually detected galaxies is a factor of 2 to 3 less than the EBL at 3000--8000A. We show that a significant fraction of the EBL may come from normal galaxies at z<4, which are simply undetectable as a result of K-corrections and cosmological surface brightness dimming. This is consistent with recent redshift surveys at z<4. In the context of some simple models, we discuss the constraints placed by the EBL on the evolution of the luminosity density at z>1. Based on our optical EBL and published UV and IR EBL measurements, we estimate that the total EBL from 0.1--1000 microns is 100+/-20 nW/m^2/sr. If the total EBL were produced entirely by stellar nucleosynthesis, then we estimate that the total baryonic mass processed through stars is Omega_* = 0.0062 (+/- 0.0022) h^{-2}, which corresponds to 0.33+/-0.12 Omega_B for currently favored values of the baryon density. This estimate is smaller by roughly 7% if 7 h_{0.7} nW/m^2/sr of the total EBL comes from accretion onto central black holes. This estimate of Omega_* suggests that the universe has been enriched to a total metal mass of 0.21(+/-0.13) Z_sun Omega_B. Our estimate is consistent with other measurements of the cumulative metal mass fraction of stars, stellar remnants, and the intracluster medium of galaxy clusters in the local universe.

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arXiv2001

The First Detections of the Extragalactic Background Light at 3000, 5500, and 8000A (I): Results

(Abridged) We present the first detections of the mean flux of the optical extragalactic background light (EBL) at 3000, 5500, and 8000A. Diffuse foreground flux at these wavelengths comes from terrestrial airglow, dust-scattered sunlight (zodiacal light), and dust-scattered Galactic starlight (diffuse Galactic light). We have avoided the brightest of these, terrestrial airglow, by measuring the absolute surface brightness of the night sky from above the Earth's atmosphere using the Wide Field Planetary Camera2 (WFPC2) and Faint Object Spectrograph (FOS), both aboard the Hubble Space Telescope (HST). On the ground, we have used the duPont 2.5 m Telescope at Las Campanas Observatory (LCO) to obtain contemporaneous spectrophotometry of ``blank'' sky in the HST field of view to measure and then subtract foreground zodiacal light from the HST observations. We have minimized the diffuse Galactic light in advance by selecting the HST target field along a line of sight with low Galactic dust column density, and then estimated the low--level Galactic foreground using a simple scattering model and the observed correlation between thermal, 100 micron emission and optical scattered flux from the same dust. In this paper, we describe the coordinated LCO/HST program and the HST observations and data reduction, and present the resulting measurements of the EBL.

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arXiv1999

Galaxy Dynamics from Edge-On Late-type Galaxies

We present first results of a program to study the dynamics of undisturbed bulgeless, low surface density disk galaxies in order to probe the underlying structure of dark matter halos. High resolution H-alpha rotation curves are combined with optical and infrared imaging to place strong limits on the halo profiles. We find noticable variation in the shapes of the rotation curves, in contrast to previous claims. The implied density profiles are still significantly more shallow than profiles derived from most N-body simulations; unlike previous HI observations, beam-smearing cannot significantly affect this result. Based upon stellar mass profiles derived from K' band observations, we derive the angular momentum distribution of the stellar disk and find it to be broader than that of a uniformly rotating solid-body sphere, but remarkably consistent from galaxy-to-galaxy. Finally, based upon K' band surface brightness profiles, we find that low surface density disks must be significantly sub-maximal. Furthermore, maximal disk fits based upon Modified Newtonian Dynamics (MOND) have maximum mass-to-light ratios which are too small to be consistent with stellar population models; without the ability to significantly adjust inclination angles or infrared mass-to-light ratios, this sample presents great difficulties for MOND.

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