arXiv subjects

Stephen E. Zepf

Publications and source records attributed to Stephen E. Zepf.

88 records · Page 5Linked to original sources

arXiv2000

The Stellar Content of the Halo of NGC 5907 from Deep HST NICMOS Imaging

We present H-band images obtained with NICMOS of a field 75'' (5kpc) above the plane of the disk of the edge-on spiral galaxy NGC 5907. Ground-based observations have shown that NGC 5907 has a luminous halo with a shallow radial profile between 4 and 8 kpc that roughly traces the dark matter distribution of the galaxy deduced from its rotation curve. Our NICMOS observations were designed to resolve bright giants in the halo of NGC 5907 to constrain its stellar composition with the goal of understanding its nature and origin. More than 100 stars are expected in the NICMOS images if the dwarf-to-giant ratio in the halo of NGC 5907 is consistent with that expected from standard stellar initial mass functions, and if ground-based estimates of the distance to NGC 5907 and the integrated colors of its halo are correct. Instead we observe only one candidate giant star. This apparent discrepancy can be resolved by assuming either a significantly larger distance than suggested by several studies, or a halo metallicity much lower than suggested by ground-based colors and as low as metal-poor Galactic globular clusters. If previous distance and halo color estimates for NGC 5907 are correct, our NICMOS results suggest that its extended light is composed of stars that formed with an initial mass function different than that observed locally, leading to a much higher ratio of dwarfs to giants. We describe how these three possible explanations for the absence of bright giants in our NICMOS images of the halo of NGC 5907 might be constrained by future observations.

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arXiv1999

A Cepheid distance to NGC 4258

Distances measured using Cepheid variable stars have been essential for establishing the cosmological distance scale and the value of the Hubble constant. These stars have remained the primary extragalactic distance indicator since 1929 because of the small observed scatter in the relationship between their pulsation period and luminosity, their large numbers, which allow many independent measures of the distance to a galaxy, and the simplicity of the basic physics underlying their variability. Potential systematic uncertainties in the use of the LMC-calibrated Cepheid period-luminosity relation to determine distances using HST are estimated to be 8-10%. Here we describe the results of a search for Cepheids in the nearby galaxy NGC 4258, which has an independently determined geometric distance of 7.2 +/- 0.5 Mpc (Herrnstein et al. 1999). We obtain a Cepheid distance of 8.1 +/- 0.4 (excluding possible systematic errors affecting all HST Cepheid distances) Mpc; there is a 1.3 sigma difference between the two measurements. If the maser-based distance is adopted and other HST Cepheid distances are revised according to our results, the derived value of the Hubble constant would be increased by 12 +/- 9%, and the corresponding age of the Universe would decrease by the same factor.

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arXiv1999

A Cepheid Distance to NGC 4603 in Centaurus

In an attempt to use Cepheid variables to determine the distance to the Centaurus cluster, we have obtained images of NGC 4603 with the Hubble Space Telescope on 9 epochs using WFPC2 and the F555W and F814W filters. This galaxy has been suggested to lie within the ``Cen30'' portion of the cluster and is the most distant object for which this method has been attempted. Previous distance estimates for Cen30 have varied significantly and some have presented disagreements with the peculiar velocity predicted from redshift surveys, motivating this investigation. Using our observations, we have found 61 candidate Cepheid variable stars; however, a significant fraction of these candidates are likely to be nonvariable stars whose magnitude measurement errors happen to fit a Cepheid light curve of significant amplitude for some choice of period and phase. Through a maximum likelihood technique, we determine that we have observed 43 +/- 7 real Cepheids and that NGC 4603 has a distance modulus of 32.61 +0.11/-0.10 (random, 1 sigma) +0.24/-0.25 (systematic, adding in quadrature), corresponding to a distance of 33.3 Mpc. This is consistent with a number of recent estimates of the distance to NGC 4603 or Cen30 and implies a small peculiar velocity consistent with predictions from the IRAS 1.2 Jy redshift survey if the galaxy lies in the foreground of the cluster.

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arXiv1999

HST Imaging of Globular Clusters in the Edge--on Spiral Galaxies NGC 4565 and NGC 5907

We present a study of the globular cluster systems of two edge-on spiral galaxies, NGC4565 and NGC5907, from WFPC2 images in the F450W and F814W filters. The globular cluster systems of both galaxies appear to be similar to the Galactic globular cluster system. In particular, we derive total numbers of globular clusters of N_{GC}(4565)= 204+/-38 {+87}{-53} and N_{GC}(5907)=170+/-41 {+47}{-72} (where the first are statistical, the second potential systematic errors) for NGC4565 and NGC5907, respectively. This determination is based on a comparison to the Milky Way system, for which we adopt a total number of globular clusters of 180+/-20. The specific frequency of both galaxies is S_N~0.6: indistinguishable from the value for the Milky Way. The similarity in the globular cluster systems of the two galaxies is noteworthy since they have significantly different thick disks and bulge-to-disk ratios. This would suggest that these two components do not play a major role in the building up of a globular cluster system around late-type galaxies.

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arXiv1997

Globular Cluster Systems of Elliptical Galaxies

We review the observed properties of globular cluster systems and their implications for models of galaxy formation. Observations show that globular clusters form in gas-rich mergers, and that bimodal metallicity distributions are common in the globular cluster systems of ellipticals, with the metal-poor population more extended than the metal-rich one. These are three of the four predictions of the simple merger model of Ashman & Zepf (1992). The fourth prediction concerns the properties of the globular cluster systems of spirals, and is still to be tested by observation. Adopting Occam's razor, the confirmation of the fundamental predictions of the merger model from both young and old globular cluster systems is strong evidence that typical elliptical galaxies formed from the mergers of spiral galaxies. However, the simplifying assumptions of the Ashman-Zepf merger model limit its applicability to certain complex situations such as the formation of cD galaxies. We conclude this review by introducing new observational and theoretical programs that will further the understanding of the physical mechanisms of globular cluster and galaxy formation.

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arXiv1997

The Absence of Passively Evolving Ellipticals in Deep Optical and Near-Infrared Surveys

The traditional view of elliptical galaxies has been that they formed in a single, rapid burst of star formation at high redshift, and have evolved quiescently since that time. In opposition to this traditional view is evidence that at least some elliptical galaxies have formed from the merger of two disk galaxies. What has not been clear is which process is the dominant formation mechanism for the large majority of elliptical galaxies. This question has significant implications for cosmological models, as different models make different predictions for the formation mechanism and epoch of elliptical galaxies. Here I use deep optical and near-infrared images to show that there are fewer galaxies with very red optical and near-infrared colors than predicted by models in which typical elliptical galaxies have completed their star formation by z >= 5. These observations require that elliptical galaxies have significant star formation at z < 5. This requirement, combined with constraints on lower redshift starbursts from the modest ultraviolet luminosities of galaxies in the Hubble Deep Field and the properties of galaxies from 0 < z < 1, suggests that elliptical galaxies form at moderate redshift in dusty starbursts and/or through the hierarchical merging of smaller objects.

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arXiv1997

The Faint End of the Luminosity Function of Galaxies in Compact Groups

We study the luminosity function of galaxies in Hickson groups using our recent redshift survey of galaxies in and around 17 of these groups. We find that the galaxies in these regions have a luminosity function with M* = -19.5 + 5 log h, and alpha = -1.0, where M* and alpha are the usual parameters in the standard Schechter form of the luminosity function, and the magnitudes are measured in the B band. The formal 95% confidence intervals for M* and alpha range from (-19.3,-0.8), to (-19.7,-1.2) and are highly correlated as is usual for these fits. This luminosity function for galaxies in our Hickson group sample is very similar from that found in large surveys covering a range of environments. These values are also consistent with our earlier estimates based on a photometric analysis with statistical background correction, and do not support previous suggestions of an underabundance of intrinsically faint galaxies in compact groups. We confirm our earlier finding that the fainter galaxies are more diffusely distributed within individual groups than the brighter ones. This can be interpreted either as evidence for mass segregation within the groups or as the result of the selection procedure for Hickson groups.

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arXiv1996

Constraints on Elliptical Galaxy Formation from the Properties of Their Globular Cluster Systems

Globular clusters are valuable fossil records of the formation and evolution of their host galaxies. The color distribution of elliptical galaxy globular cluster systems indicates an episodic formation history, consistent with a merger origin for these galaxies. Spectroscopic studies of globular cluster systems are a promising route to further constrain models of elliptical galaxy formation. They also provide a useful probe of the mass distribution of ellipticals at large radii.

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arXiv1996

Keck Spectroscopy of Objects with Lens-like Morphologies in the Hubble Deep Field

We present spectroscopy from the Keck telescope of three sets of objects in the Hubble Deep Field which have lens-like morphologies. In the case of J123641+621204, which is composed of four objects with similar colors and a mean separation of <= 0.8", we find at least two distinct components at redshifts of z=3.209 and z=3.220 which are separated by 0.5" spatially. Each of these components has narrow Ly-alpha emission, and possibly NV emission and SiIV and CIV in absorption or with a P-Cygni profile. The second case is J123652+621227, which has an arc-like feature offset by 1.8" to the southwest of a red elliptical-like galaxy, and a ``counterimage'' offset 1.4" on the opposite side. We tentatively find a single line at 5301 AA at the spatial position of the counterimage, and no corresponding emission line at the position of the arc. The colors of the counterimage are consistent with the identification of this line as Ly-alpha at z=3.36. The colors of the arc are different than those of the counterimage, and thus both the colors and spectra indicate that this object is unlikely to be a gravitational lens. For a third lensing candidate (J123656+621221), which is a blue arc offset by 0.9" from a red, elliptical-like galaxy, our spectroscopy does not clearly resolve the system spatially, complicating the interpretation of the spectrum. We discuss possible identifications of features in the spectrum, and find that gravitational lensing remains a possibility in this case. We conclude that the frequency of strong gravitational lensing by galaxies in the HDF appears to be very low. This result is difficult to reconcile with the introduction of a cosmological constant to account for the large number of faint blue galaxies via a large volume element at high redshift.

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arXiv1996

On the Effects of Bursts of Massive Star Formation During the Evolution of Elliptical Galaxies

We consider the hypothesis that the formation of elliptical galaxies includes a phase in which star formation is mostly restricted to massive stars, with the bias towards high mass stars increasing with elliptical galaxy mass. The mass fraction of stars in this top-heavy mode of star formation is constrained by requiring the resulting stellar remnants to account for the observed increase in the mass-to-light ratio of ellipticals with increasing galaxy mass. We then consider the implications of this population of massive stars for the intracluster medium and the extragalactic background at various wavelengths. The mass and abundance ratios of metals produced by our proposed population of massive stars are consistent with the observations of the mass and abundance ratios of metals in the hot gas of galaxy clusters for most of the standard range of IMF slopes and SN II yields. The predicted energy density produced by this stellar population approaches current limits on the extragalactic background at both optical wavelengths, into which the ultraviolet radiation of the massive stars is likely to be redshifted, and far-infared wavelengths, at which starlight reprocessed by dust associated with the starburst will be observed. In either case, the background is predicted to be significantly clustered since massive ellipticals are clustered.

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arXiv1996

Globular Cluster Systems as Probes of Elliptical Galaxy Formation

Globular cluster systems provide valuable fossil records of the formation history of their parent galaxies. This review specifically concentrates on using color distributions of the globular cluster systems of elliptical galaxies to distinguish between competing models for the formation of these galaxies. The observational requirements for testing various formation models are discussed, and the results of past and current studies are reviewed. The conclusions of these studies are that most color distributions appear to have two or more peaks, indicating an episodic formation history. This result is consistent with previous predictions of merger models for the formation of elliptical galaxies. Possibilities for improving these results in the future are also presented.

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arXiv1995

Globular Cluster Systems As Distance Indicators: Metallicity Effects On The Luminosity Function

We investigate the universality of the globular cluster luminosity function (GCLF) and the use of this function as an extragalactic distance indicator. Previous studies have found an offset between GCLF distances and those obtained with other techniques. We show that this offset can be understood in terms of a metallicity effect. Specifically, the globular cluster systems used in distance scale studies have traditionally been those around elliptical galaxies. These systems have higher mean metallicities than the Milky Way globular cluster system. Consequently, the peak of the GCLF in the systems around ellipticals is significantly fainter in $B$ and $V$ than the GCLF peak in the Milky Way. We calculate the shift in the peak of the GCLF relative to the Milky Way globulars in $B$, $V$, $R$, $I$ and $J$ for a range of globular cluster metallicities. Applying these corrections, we find good agreement between GCLF distances and those obtained using the surface brightness fluctuations method. The similarity between metallicity-corrected GCLFs suggests that the underlying mass function of globular cluster systems is remarkably constant from one galaxy to another. Our results allow the GCLF to be employed as an improved distance indicator.

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arXiv1995

Spectroscopy of Candidate Young Globular Clusters in NGC 1275

We present spectra of the brightest member of the population of compact blue objects discovered in the peculiar galaxy NGC 1275 by Holtzman et al. using Hubble Space Telescope images. These spectra show strong Balmer absorption lines like those observed in A-type stars, as expected if the object is a young globular cluster. The age estimated from the strength of the Balmer lines is about 0.5 Gyr, although ages ranging from 0.1 Gyr to 0.9 Gyr can not be confidently excluded given current models of stellar populations. If these estimated ages are adopted for the young cluster population of NGC 1275 as a whole, the fading predicted by stellar populations models gives a luminosity function which is consistent with that of the Galactic globular cluster system convolved with the observational selection function for the NGC 1275 system. We also use the equivalent widths of the Mg $b$ and Fe 5270 features to constrain the metallicity of the young cluster. Combining these absorption-line widths with the age estimates from the Balmer lines and stellar population models, we find a metallicity of roughly solar based on the Mg $b$ index and somewhat higher for the Fe 5270 index. The radial velocity of the absorption lines of the cluster spectrum is offset from the emission lines of the galaxy spectrum at the same position by $-130$ \kms, providing further evidence for the identification of the object as a globular cluster, and opening up the future possibility of studying the kinematics of young cluster systems. The discovery of objects with the characteristics of young globular clusters in NGC 1275, which shows evidence of a recent interaction or merger, supports the hypothesis that galaxy interactions and mergers are favorable sites for the formation of globular clusters.

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arXiv1994

Constraints on the Formation History of the Elliptical Galaxy NGC 3923 from the Colors of Its Globular Clusters

We present a study of the colors of globular clusters associated with the elliptical galaxy NGC 3923. Our final sample consists of Washington system C and T_1 photometry for 143 globular cluster candidates with an expected contamination of no more than 10%. We find that the color distribution of the NGC 3923 globular cluster system (GCS) is broad and appears to have at least two peaks. A mixture modeling analysis of the color distribution indicates that a two component model is favored over a single component one at a high level of confidence (>99%). This evidence for more than one population in the GCS of NGC 3923 is similar to that previously noted for the four other elliptical galaxies for which similar data have been published. Furthermore, we find that the NGC 3923 GCS is redder than the GCSs of previously studied elliptical galaxies of similar luminosity. The median metallicity inferred from our (C-T_1)_0 colors is [Fe/H]_{med} = -0.56, with an uncertainty of 0.14 dex arising from all sources of uncertainty in the mean color. This is more metal rich than the median metallicity found for the GCS of M87 using the same method, [Fe/H]_{med} = -0.94. Since M87 is more luminous than NGC 3923, this result points to significant scatter about any trend of higher GCS metallicity with increasing galaxy luminosity. We also show that there is a color gradient in the NGC 3923 GCS corresponding to about -0.5 dex in Delta[Fe/H]Δ(log r). We conclude that the shape of the color distribution of individual GCSs and the variation in mean color among the GCSs of ellipticals are difficult to understand if elliptical galaxies are formed in a single protogalactic

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arXiv1994

Galaxy Formation from Spectroscopy of Extragalactic Globular Cluster Systems

I discuss how spectroscopy of extragalactic globular clusters provides a powerful probe of the formation history and mass distribution of galaxies. One critical area is spectroscopy of objects which have been identified as candidate young globular clusters through HST imaging of galaxy mergers. I discuss how such data can constrain models of globular cluster and galaxy formation. As an example, I present new spectra which confirm the presence of young globular clusters in NGC 1275. A second way wide-field spectroscopy can be used to probe the formation history and mass distribution of galaxies is through spectroscopy of large numbers of globular clusters around elliptical galaxies. Metallicities obtained from such data place strong constraints on models of galaxy formation, and velocities determined from the same data provide kinematical tracers of the mass distribution out to distances of $\sim 100$ kpc.

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arXiv1994

A Recipe for Galaxy Formation Shaun Cole

We present a detailed prescription for how galaxy formation can be modelled in hierarchical theories of structure formation. Our model incorporates the formation and merging of dark matter halos, the shock heating and radiative cooling of baryonic gas gravitationally confined in these halos, the formation of stars regulated by the energy released by evolving stars and supernovae, the merging of galaxies within dark matter halos, and the spectral evolution of the stellar populations that are formed. The procedure that we describe is very flexible and can be applied to any hierarchical clustering theory. We explore the effects of varying the stellar initial mass function, star formation rates and galaxy merging. The results we compare with an extensive range of observational data, including the B and K galaxy luminosity functions, galaxy colours, the Tully-Fisher relation and galaxy number counts.These data strongly constrain the models and enable the relative importance of each of the physical process to be assessed. We present a broadly successful model defined by a plausible choice of parameters. This fiducial model produces a much more acceptable luminosity function than most previous studies. This is achieved through a modest rate of galaxy mergers and strong suppression of star formation in halos of low circular velocity. However, it fails to produce galaxies as red as many observed ellipticals and, compared with the observed Tully-Fisher relation, the model galaxies have circular velocities which are too large. ** uuencoded compressed postscript file containing all text and figures.**

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