arXiv subjects

Judith G. Cohen

Publications and source records attributed to Judith G. Cohen.

At least 91 records · Page 5Linked to original sources

arXiv2002

Abundances in Stars from the Red Giant Branch Tip to Near the Main Sequence Turn Off in M71: III. Abundance Ratios

We present abundance ratios for 23 elements with respect to Fe in a sample of stars with a wide range in luminosity, from luminous giants to stars near the turnoff, in the globular cluster M71. The analyzed spectra, obtained with HIRES at the Keck Observatory, are of high dispersion (R=35,000). We find that the neutron capture, the iron peak and the alpha-element abundance ratios show no trend with Teff, and low scatter around the mean between the top of the RGB and near the main sequence turnoff. The alpha-elements Mg, Ca, Si and Ti are overabundant relative to Fe. The anti-correlation between O and Na abundances, observed in other metal poor globular clusters, is detected in our sample and extends to the main sequence. A statistically significant correlation between Al and Na abundances is observed among the M71 stars in our sample, extending to Mv = +1.8, fainter than the luminosity of the RGB bump in M5. Lithium is varying, as expected, and Zr may be varying from star to star as well. M71 appears to have abundance ratios very similar to M5 whose bright giants were studied by Ivans et al. (2001), but seems to have a smaller amplitude of star-to-star variations at a given luminosity, as might be expected from its higher metallicity. The results of our abundance analysis of 25 stars in M71 provide sufficient evidence of abundance variations at unexpectedly low luminosities to rule out the mixing scenario. Either alone or, even more powerfully, combined with other recent studies of C and N abundances in M71 stars, the existence of such abundance variations cannot be reproduced within the context of our current understanding of stellar evolution.

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arXiv2002

Losing Weight: A KECK Spectroscopic Survey of the Massive Cluster of Galaxies RX J1347-1145

We present a sample of 47 spectroscopically confirmed members of RX J1347-1145, the most luminous X-ray cluster of galaxies discovered to date. With two exceptions, all the galaxies in this sample have red B-R colors and red spectral indices, with spectra similar to old local ellipticals. Using all 47 cluster members, we derive a mean redshift of 0.4509\pm 0.003, and a velocity dispersion of 910\pm130 km/sec, which corresponds to a virial mass of 4.4 x 10^{14} h^{-1} Solar masses with an harmonic radius of 380 h^{-1} kpc. The derived total dynamical mass is marginally consistent with that deduced from the cluster's X-ray emission based on the analysis of ROSAT/ASCA images (Schindler et al. 1997), but not consistent with the more recent X-ray analyses of Allen (2000), Ettori, Allen & Fabian (2001) and Allen, Schmidt & Fabian (2002). Furthermore, the dynamical mass is significantly smaller than that derived from weak lensing (Fischer & Tyson 1997) and from strong lensing (Sahu et al. 1998). We propose that these various discrepant mass estimates may be understood if RX J1347-1145 is the product of two clusters caught in the act of merging in a direction perpendicular to the line of sight, although there is no evidence from the galaxy redshift distribution supporting this hypothesis. Even with this hypothesis, a significant part of the extremely high X-ray luminosity must still arise from non-virialized, presumably shocked, gas. Finally, we report the serendipitous discovery of a lensed background galaxy at z=4.083 which will put strong constraints on the lensing mass determination once its counter-image is securely identified.

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arXiv2002

C and N Abundances in Stars At the Base of the Red Giant Branch in M5

We present an analysis of a large sample of moderate resolution Keck LRIS spectra of subgiant (V \sim 17.2) and fainter stars in the Galactic globular cluster M5 (NGC 5904) with the goal of deriving C and N abundances. Star-to-star stochastic variations with significant range in both [C/Fe] and [N/Fe] are found at all luminosities extending to the bottom of the RGB at M_V \sim +3. Similar variations in CH appear to be present in the main sequence turnoff spectra. There is no sign of a change in the behavior of C and N with evolutionary stage over the full range in luminosity of the RGB and SGB. The C and N abundances appear strongly anti-correlated, as would be expected from the CN-cycle processing of stellar material. Yet the present stars are considerably fainter than the RGB bump, the point at which deep mixing is believed to set in. On this basis, while the observed abundance pattern is consistent with proton capture nucleosynthesis, we infer that the site of the reactions is likely not within the present sample, but rather in a population of more massive (2 -- 5 M(Sun)) now defunct stars. The range of variation of the N abundances is very large and the sum of C+N increases as C decreases. To reproduce this requires the incorporation not only of CN but also of ON-processed material. Furthermore, the existence of this correlation is quite difficult to reproduce with an external mechanism such as ``pollution'' with material processed in a more massive AGB star, which mechanism is fundamentally stochastic in nature. We therefore suggest that although the internal mixing hypothesis has serious flaws,new theoretical insights are needed and it should not be ruled out yet. (abridged)

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arXiv2001

Caltech Faint Galaxy Redshift Survey XVI: The Luminosity Function for Galaxies in the Region of the HDF-North to z = 1.5

We have carried out a study of the luminosity function (henceforth LF) of galaxies in the region of the HDF-North using our very complete redshift catalog. We solve for the LF at four rest frame wavelengths from 0.24 to 2.2mu. We find that the LFs for quiescent galaxies have shallow faint end slopes, while those of galaxies with detectable emission lines have steeper faint end slopes. Furthermore these slopes are independent of redshift out to z=1.05 for each galaxy spectral grouping and agree well with comparable local determinations. We find that galaxies with strong absorption lines become brighter with z with Q \sim 0.6 at all rest frame bands studied here, where Q = Δ{\rm{log}}[L^*(z)]/Δ{z}, while galaxies with detectable emission lines (i.e. star forming galaxies) show a smaller change in L^* with redshift at all bands, Q ~ 0.3, with Q becoming significantly larger at rest frame 2400A. Passive evolution models of galaxies are in reasonable agreement with these results for absorption line dominated galaxies, while plausible star formation histories can reproduce the behavior of the emission line galaxies. We find the co-moving number density and stellar mass in galaxies to be approximately constant out to z~1.05, and with more uncertainty, to z~1.3. The major epoch(s) of star formation and of galaxy formation must have occurred even earlier. (abridged)

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arXiv2001

An abundance analysis for four Red Horizontal Branch Stars in the extremely metal rich globular cluster NGC 6528

We present the results of the first analysis of high dispersion spectra of four red HB stars in the metal rich globular cluster NGC 6528, located in Baade's Window. We find that the mean [Fe/H] for NGC 6528 is +0.07+-0.01 dex (error of the mean), with a star-to-star scatter of sigma = 0.02 dex (4 stars), although the total error is likely to be larger (~0.1 dex) due to systematic errors related to the effective temperature scale and to model atmospheres. This metallicity is somewhat larger than both the mean abundance in the galactic bulge found by McWilliam & Rich (1994) and that found in our previous paper for NGC 6553. However, we find that the spectra of clump stars in NGC 6528 and NGC 6553 are very similar each other, the slightly different metal abundances found being possibly due to the different atmospheric parameters adopted in the two analyses. For NGC 6528 we find excesses for the alpha-process elements Si and Ca ([Si/Fe]=+0.4 and [Ca/Fe]=+0.2), whereas Mn is found to be underabundant ([Mn/Fe]=-0.4). We find a solar abundance of O; however this is somewhat uncertain due to the dependence of the O abundance on the adopted atmospheric parameters and to coupling between C and O abundances in these cool, metal-rich stars. Finally, we find large Na excesses ([Na/Fe]~ +0.4) in all stars examined. Since the present analysis is based on higher quality material, we propose to revise our previous published metal abundance for NGC 6553 to [Fe/H]=$0.06+-0.15.

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arXiv2001

Abundances in Stars from the Red Giant Branch Tip to the Near the Main Sequence in M71: I. Sample Selection, Observing Strategy and Stellar Parameters

We present the sample for an abundance analysis of 25 members of M71 with luminosities ranging from the red giant branch tip to the upper main sequence. The spectra are of high dispersion and of high precision. We describe the observing strategy and determine the stellar parameters for the sample stars using both broad band colors and fits of H$α$ profiles. The derived stellar parameters agree with those from the Yale$^2$ stellar evolutionary tracks to within 50 -- 100K for a fixed log g, which is within the level of the uncertainties.

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arXiv2001

Abundances in Stars from the Red Giant Branch Tip to the Near Main Sequence in M71: II. Iron Abundance

We present [Ffe/H] abundance results that involve a sample of stars with a wide range in luminosity from luminous giants to stars near the turnoff in a globular cluster. Our sample of 25 stars in M71 includes 10 giant stars more luminous than the RHB, 3 horizontal branch stars, 9 giant stars less luminous than the RHB, and 3 stars near the turnoff. We analyzed both Fe I and Fe II lines in high dispersion spectra observed with HIRES at the W. M. Keck Observatory. We find that the [Fe/H] abundances from both Fe I and Fe II lines agree with each other and with earlier determinations. Also the [Fe/H] obtained from Fe I and Fe II lines is constant within the rather small uncertainties for this group of stars over the full range in Teff and luminosity, suggesting that NLTE effects are negligible in our iron abundance determination. In this globular cluster, there is no difference among the mean [Fe/H] of giant stars located at or above the RHB, RHB stars, giant stars located below the RHB and stars near the turnoff.

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arXiv2001

Caltech Faint Galaxy Redshift Survey XV: Classification of Galaxies with 0.2 < z < 1.1 in the Hubble Deep Field (North) and its Flanking Fields

To circumvent the spatial effects of resolution on galaxy classification, the images of 233 objects of known redshift in the Hubble Deep Field (HDF) and its Flanking Fields (FF) that have redshifts in the range 0.20 < z < 1.10 were degraded to the resolution that they would have had if they were all located at a redshift z= 1.00. As in paper XIV of the present series, the effects of shifts in rest wavelength were mitigated by using R-band images for the classification of galaxies with 0.2 < z < 0.6 and I-band images for objects with redshifts 0.6 < z < 1.1. A special effort was made to search for bars in distant galaxies. The present data strongly confirm the previous conclusion that the Hubble tuning fork diagram only provides a satisfactory framework for the classification of galaxies with z ~< 0.3. More distant disk galaxies are often difficult to shoehorn into the Hubble classification scheme. The paucity of barred spirals and of grand-design spirals at large redshifts is confirmed. It is concluded that the morphology of disk galaxies observed at look-back times smaller than 3--4 Gyr differs systematically from that of more distant galaxies viewed at look-back times of 4--8 Gyr. The disks of late-type spirals at z >0.5 are seen to be more chaotic than those of their nearer counterparts. Furthermore the spiral structure in distant early-type spirals appears to be less well-developed than it is in nearby early-galaxies.

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arXiv2001

Calibration of the CH and CN Variations Among Main Sequence Stars in M71 and in M13

An analysis of the CN and CH band strengths measured in a large sample of M71 and M13 main sequence stars by Cohen (1999a,b) is undertaken using synthetic spectra to quantify the underlying C and N abundances. In the case of M71 it is found that the observed CN and CH band strengths are best matched by the {\it{identical}} C/N/O abundances which fit the bright giants, implying: 1) little if any mixing is taking place during red giant branch ascent in M71, and 2) a substantial component of the C and N abundance inhomogeneities is in place before the main sequence turn-off. The unlikelihood of mixing while on the main sequence requires an explanation for the abundance variations which lies outside the present stars (primordial inhomogeneities or intra-cluster self enrichment). For M13 it is shown that the 3883ÅCN bands are too weak to be measured in the spectra for any reasonable set of expected compositions. A similar situation exists for CH as well. However, two of the more luminous program stars do appear to have C abundances considerably greater than those found among the bright giants thereby suggesting deep mixing has taken place on the M13 red giant branch.

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arXiv2001

Caltech Faint Galaxy Redshift Survey XIII: Spectral Energy Distributions for Galaxies in the Region of the Hubble Deep Field North

We introduce a new empirical function for modeling the spectral energy distributions of galaxies. We apply it to a sample of 590 galaxies in the region of the HDF with $z < 1.5$ using multi-color photometry with wide wavelength coverage combined with spectroscopic redshifts from our 93% complete R-selected redshift survey there. We find the following: As expected, galaxies with strong signs of recent star formation (i.e. those which show emission lines) have bluer continua in both the rest frame UV and the optical/near-infrared. The redder galaxies tend to be more luminous. The SEDs of actively star forming galaxies become bluer in the mean in the rest-frame UV at higher redshift, which trend might arise from SED modeling errors. Aside from this, we discern no change with redshift in the relationship between SED characteristics and galaxy spectral type based on the strength of narrow emission and absorption features. There is no evidence in our R-selected sample that supports the existence of a substantial population of very dusty star forming galaxies at z < 1.5. The overall pattern of the L(K) - z relationship suggests that passive evolution at constant stellar mass is a good approximation to the actual behavior of at least the most luminous galaxies in this large sample of galaxies in the region of the HDF out to z \sim 1.5. (abridged)

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arXiv2000

Lost and Found: The Damped Lyman Alpha Absorbers in the QSO OI 363

The galaxy giving rise to the damped Ly$α$ absorbing system in the QSO OI 363 with z=0.221 has been found. A galaxy which is probably associated with the second DLA in this same QSO at z=0.0912 has also been found. Neither galaxy is very luminous, and neither galaxy shows signs of extensive current star formation, a massive disk or lots of gas. The impact parameters for each of the two galaxies with respect to the QSO are reasonable. If most DLA absorbers arise in such low luminosity galaxies, it will be difficult to pick out the correct galaxy giving rise to DLA systems at high redshift within the large projected areal density on the sky of faint galaxies around distant QSOs.

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arXiv2000

The Caltech Faint Galaxy Redshift Survey XII: Clustering of Galaxies

A clustering analysis is performed on two samples of $\sim 600$ faint galaxies each, in two widely separated regions of the sky, including the Hubble Deep Field. One of the survey regions is configured so that some galaxy pairs span angular separations of up to 1 deg. The median redshift is $z_{med}\approx 0.55$. Strong clustering is obvious, with every pencil-beam field containing a handful of narrow redshift-space features, corresponding to galaxy structures with sizes of 5 to 20 Mpc. The structures are not obviously organized on planes, though one prominent, colinear triplet of structures is observed, spanning $\sim 20$ Mpc. This may be evidence of a filament. A galaxy--galaxy correlation function calculation is performed. No significant evolution of clustering (relative to stable clustering) is found in the redshift range 0.3<z<1.0. This is not surprising, since uncertainties in the correlation amplitude estimated from surveys like these are large; field-to-field variations and covariances between data points are both shown to be significant. Consistent with other studies in this redshift range, the galaxy--galaxy correlation length is found to be somewhat smaller than that predicted from local measurements and an assumption of no evolution. Galaxies with absorption-line-dominated spectra show much stronger clustering at distances of <2 Mpc than typical field galaxies. There is some evidence for weaker clustering at intermediate redshift than at low redshift, when the results presented here are compared with surveys of the local Universe. In subsets of the data, the measured pairwise velocity dispersion of galaxies ranges from 200 to $600 km s^{-1}$, depending on the properties of the dominant redshift structures in each subset.

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arXiv2000

Caltech Faint Galaxy Redshift Survey XIV: Galaxy Morphology in the HDF (North) and its Flanking Fields to z=1.2

Morphological classifications are reported for Hubble Space Telescope (HST) images of 241 galaxies in the Hubble Deep Field (HDF) and its Flanking Fields (FF) with measured redshifts in the interval 0.25 < z < 1.2, drawn from a magnitude-limited redshift survey to R = 24.0. The galaxies are divided into three groups with redshifts in the intervals [0.25,0.6], [0.6,0.8], [0.8,1.2]. R606 images from the first group and I814 images from the second and third groups are compared with B-band images of nearby galaxies. All classifications were therefore made at approximately the same rest wavelength. Selection biases are discussed. We corroborate and extend the results of earlier investigations by observing that: Most intermediate and late-type galaxies with $z\gtrsim0.5$ have morphologies that are dramatically different from those of local galaxies and cannot be shoehorned into the Hubble ``tuning fork'' classification scheme. Grand-design spirals appear to be rare or absent for $z\gtrsim0.3$. Many Sa and Sb spirals with $z\gtrsim0.6$ do not exhibit well-defined spiral arms. The arms of distant Sc galaxies appear more chaotic than those of their nearby counterparts. (abridged) On the basis of these and similar observations, it is inferred that the development of pronounced spiral structure was delayed until \~5 Gyr and that most bulges are probably not formed by disintegrating bars. Major morphological changes were still taking place only ~5 Gyr ago even though changes in the integrated light of most galaxies were then much slower than they were ~10 Gyr ago.

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arXiv2000

RX J0911+05: A Massive Cluster Lens at z=0.769

We report the detection of a massive high-redshift cluster of galaxies near the quadruple quasar RX J0911+05, using the LRIS instrument on the Keck-II telescope. The cluster is found to have a mean redshift of =0.7689+/-0.002 and a velocity dispersion of sigma=836{+180-200} km/s, based on redshift measurements for 24 member galaxies. This massive high-redshift cluster is the origin of the unusually large external shear required by lensing models of the quadruple quasar system. We predict the expected time delay depending on the exact contribution of the cluster. A measurement of the time delay and further deep lensing and X-ray observations will unravel useful properties of this serendipitously discovered high-redshift cluster, and may put interesting cosmological constraints on H0.

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arXiv2000

Rotations and Abundances of Blue Horizontal-Branch Stars in Globular Cluster M15

High-resolution optical spectra of eighteen blue horizontal-branch (BHB) stars in the globular cluster M15 indicate that their stellar rotation rates and photospheric compositions vary strongly as a function of effective temperature. Among the cooler stars in the sample, at Teff ~ 8500 K, metal abundances are in rough agreement with the canonical cluster metallicity, and the v sin i rotations appear to have a bimodal distribution, with eight stars at v sin i < 15 km/s and two stars at v sin i ~ 35 km/s. Most of the stars at Teff > 10000 K, however, are slowly rotating, v sin i < 7 km/s, and their iron and titanium are enhanced by a factor of 300 to solar abundance levels. Magnesium maintains a nearly constant abundance over the entire range of Teff, and helium is depleted by factors of 10 to 30 in three of the hotter stars. Diffusion effects in the stellar atmospheres are the most likely explanation for these large differences in composition. Our results are qualitatively very similar to those previously reported for M13 and NGC 6752, but with even larger enhancement amplitudes, presumably due to the increased efficiency of radiative levitation at lower intrinsic [Fe/H]. We also see evidence for faster stellar rotation explicitly preventing the onset of the diffusion mechanisms among a subset of the hotter stars.

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arXiv2000

The Merger Rate to Redshift One from Kinematic Pairs: Caltech Faint Galaxy Redshift Survey XI

The rate of mass accumulation due to galaxy merging depends on the mass, density, and velocity distribution of galaxies in the near neighborhood of a host galaxy. The fractional luminosity in kinematic pairs combines all of these effects in a single estimator which is relatively insensitive to population evolution. Here we use a k-corrected and evolution compensated volume-limited sample drawing about 300 redshifts from CFGRS and 3000 from CNOC2 to measure the rate and redshift evolution of merging. We identify kinematic pairs with projected separations less than either 50 or 100 \hkpc and rest-frame velocity differences of less than 1000\kms. The fractional luminosity in pairs is modeled as f_L(Delta v,r_p,M_r^{ke})(1+z)^{m_L} where [f_L,m_L] are [0.14+/-0.07,0+/-1.4] and [0.37+/-0.7,0.1+/-0.5] for r_p<= 50 and 100\hkpc, respectively (Omega_M=0.2, Omega_Lambda=0.8). The value of m_L is about 0.6 larger if Lambda=0. To convert these redshift space statistics to a merger rate we use the data to derive a conversion factor to physical space pair density, a merger probability and a mean in-spiral time. The resulting mass accretion rate per galaxy (M_1,M_2>= 0.2 M*) is 0.02+/-0.01(1+z)^{0.1+/-0.5} M*~Gyr^{-1}. Present day high-luminosity galaxies therefore have accreted approximately 0.15M* of their mass over the approximately 7 Gyr to redshift one. (abridged)

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arXiv2000

Three-micron imaging of the Hubble Deep Field

Images of the Hubble Deep Field at 3.2 um, taken with the 10-m Keck Telescope, are presented. The images cover a total area of ~2.5 arcmin^2. To a 5-sigma limit of [3.2](total)=17.5 mag (Vega-relative), 11 sources are detected, 9 of which are extragalactic. The integrated galaxy number counts are therefore ~1.3x10^4 deg^{-2} to this depth. The galaxies detected at 3.2 um have a median redshift of z=0.56. All 3.2 um sources have 1.6 um, 1.1 um and visual counterparts, all of fairly regular morphologies; several also have 6.7 um, 15 um, 850 um, 8.5 GHz, or 1.4 GHz counterparts. No sources are found which are either anomalously red or anomalously blue in their H-[3.2] color, and there are significant detections of most of the known near-infrared HDF sources for which detection in these 3.2 um data seemed likely.

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arXiv1999

Clustering in the Caltech Faint Galaxy Redshift Survey

The Caltech Faint Galaxy Redshift Survey has collected $\sim$2000 spectra taken with multi-slit masks using the Low Resolution Imaging Spectrograph at the Keck Observatory (Oke et al 1995) in two widely separated fields on the sky, each $\sim$1 x 1 deg$^2$. Most of these objects are faint field galaxies; about 10% are Galactic stars and about 1% are broad-lined AGNs. I summarize the small scale clustering of this sample as well as the evidence in support of large scale structure.

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