arXiv subjects

Stephen E. Zepf

Publications and source records attributed to Stephen E. Zepf.

At least 73 records · Page 4Linked to original sources

arXiv2006

The Formation Histories of Metal-Rich and Metal-Poor Globular Clusters

This review presents the results of ongoing studies of the formation histories of metal-poor and metal-rich globular clusters and their host galaxies. I first discuss the strong observational evidence that the globular cluster systems of most elliptical galaxies have bimodal metallicity distributions. I then focus on new results for metal-poor and metal-rich globular cluster systems. Metal-poor globular clusters are often associated with early structure formation, and I review new constraints on their formation epoch based on the ``bias'' of the number of metal-poor clusters with host galaxy mass. For metal-rich globular clusters, I discuss new results from ongoing optical to near-infrared photometric studies which both confirm an intermediate-age population in NGC 4365 and generally reveal a variety of formation histories for now quiescent ellipticals.

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arXiv2006

Gemini/GMOS Spectra of Globular Clusters in the Virgo Giant Elliptical NGC 4649

NGC 4649 (M60) is one of a handful of giant Virgo ellipticals. We have obtained Gemini/GMOS spectra for 38 GCs associated with this galaxy. Applying the multi-index chi^2 minimisation technique of Proctor & Sansom (2002) with the single stellar population models of Thomas, Maraston & Korn (2004) we derive ages, metallicities and alpha-element abundance ratios. We find several young (2--3 Gyr old) super-solar metallicity GCs, while the majority are old (>10 Gyrs), spanning a range of metallicities from solar to [Z/H]=-2. At least two of these young GCs are at large projected radii of 17-20 kpc. The galaxy itself shows no obvious signs of a recent starburst, interaction or merger. A trend of decreasing alpha-element ratio with increasing metallicity is found.

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arXiv2005

Gemini/GMOS Spectra of Globular Clusters in the Leo Group Elliptical NGC 3379

We have obtained Gemini/GMOS spectra for 22 GCs associated with NGC 3379. We derive ages, metallicities and alpha-element abundance ratios from simple stellar population models using the multi-index chi^2 minimisation method of Proctor & Sansom (2002). All of these GCs are found to be consistent with old ages, i.e. >10 Gyr, with a wide range of metallicities. A trend of decreasing alpha-element abundance ratio with increasing metallicity is indicated. The projected velocity dispersion of the GC system is consistent with being constant with radius. Non-parametric, isotropic models require a significant increase in the mass-to-light ratio at large radii. This result is in contrast to that of Romanowsky et al. (2003) who find a decrease in the velocity dispersion profile as determined from planetary nebulae. Our constant dispersion requires a normal sized dark halo, although without anisotropic models we cannot rigorously determine the dark halo mass. A two-sided chi^2 test over all radii, gives a 2 sigma difference between the mass profile derived from our GCs compared to the PN-derived mass model of Romanowsky et al. (2003). However, if we restrict our analysis to radii beyond one effective radius and test if the GC velocity dispersion is consistently higher, we determine a >3 sigma difference between the mass models, and hence favor the conclusion that NGC 3379 does indeed have dark matter at large radii in its halo. (abridged)

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arXiv2005

The Ages of Globular Clusters in NGC 4365 Revisited with Deep HST Observations

We present new Hubble Space Telescope (HST)-NIC3, near-infrared H-band photometry of globular clusters (GC) around NGC 4365 and NGC 1399 in combination with archival HST-WCPC2 and ACS optical data. We find that NGC 4365 has a number of globular clusters with bluer optical colors than expected for their red optical to near-infrared colors and an old age. The only known way to explain these colors is with a significant population of intermediate-age (2-8 Gyr) clusters in this elliptical galaxy. In contrast, NGC 1399 reveals no such population. Our result for NGC 1399 is in agreement with previous spectroscopic work that suggests that its clusters have a large metallicity spread and are nearly all old. In the literature, there are various results from spectroscopic studies of modest samples of NGC 4365 globular clusters. The spectroscopic data allow for either the presence or absence of a significant population of intermediate-age clusters, given the index uncertainties indicated by comparing objects in common between these studies and the few spectroscopic candidates with optical to near-IR colors indicative of intermediate ages. Our new near-IR data of the NGC 4365 GC system with much higher signal-to-noise agrees well with earlier published photometry and both give strong evidence of a significant intermediate-age component. The agreement between the photometric and spectroscopic results for NGC 1399 and other systems lends further confidence to this conclusion, and to the effectiveness of the near-IR technique.

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arXiv2005

The discovery of X-ray binaries in the Sculptor Dwarf Spheroidal Galaxy

We report the results of a deep Chandra survey of the Sculptor dwarf spheroidal galaxy. We find five X-ray sources with $L_X$ of at least $6\times10^{33}$ ergs/sec with optical counterparts establishing them as members of Sculptor. These X-ray luminosities indicate that these sources are X-ray binaries, as no other known class of Galactic point sources can reach 0.5-8 keV luminosities this high. Finding these systems proves definitively that such objects can exist in an old stellar population without stellar collisions. Three of these objects have highly evolved optical counterparts (giants or horizontal branch stars), as do three other sources whose X-ray luminosities are in the range which includes both quiescent low mass X-ray binaries and the brightest magnetic cataclysmic variables. We predict that large area surveys of the Milky Way should also turn up large numbers of quiescent X-ray binaries.

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arXiv2004

Wide-Field Imaging from Space of Early-Type Galaxies and Their Globular Clusters

Wide-field imaging from space will reveal a wealth of information about the globular cluster systems of any galaxies in the local universe that are observed by such a mission. Individual globular clusters around galaxies in the local universe have compact sizes that are ideal for the excellent spatial resolution afforded by space-based imaging, while systems of these globular clusters have large spatial extent that can only be fully explored by wide-field imaging. One example of the science return from such a study is the determination of the major formation epoch(s) of galaxies from the ages of their globular clusters determined via their optical to near-infrared colors. A second example is determining the sites of metal-poor globular cluster formation from their cosmological bias, which constrains the formation of structures early in the universe.

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arXiv2004

Gemini/GMOS Imaging of Globular Clusters in the Virgo Galaxy NGC 4649 (M60)

We present Sloan g and i imaging from the GMOS instrument on the Gemini North telescope for the globular cluster (GC) system around the Virgo galaxy NGC 4649 (M60). Our three pointings, taken in good seeing conditions, cover an area of about 90 sq. arcmins. We detect 2,151 unresolved sources. Applying colour and magnitude selection criteria to this source list gives 995 candidate GCs that is greater than 90% complete to a magnitude of i = 23.6, with little contamination from background galaxies. We find fewer than half a dozen potential Ultra Compact Dwarf galaxies around NGC 4649. Foreground extinction from the nearby spiral NGC 4647 is limited to be A_V < 0.1. We confirm the bimodality in the GC colour distribution found by earlier work using HST/WFPC2 imaging. As is commonly seen in other galaxies, the red GCs are concentrated towards the centre of the galaxy, having a steeper number density profile than the blue GC subpopulation. The varying ratio of red-to-blue GCs with radius can largely explain the overall GC system colour gradient. The underlying galaxy starlight has a similar density profile slope and colour to the red GCs. This suggests a direct connection between the galaxy field stars and the red GC subpopulation. We estimate a total GC population of 3700 +/- 900, with the uncertainty dominated by the extrapolation to larger radii than observed. This total number corresponds to a specific frequency S_N = 4.1 +/- 1.0. Future work will present properties derived from GMOS spectra of the NGC 4649 GCs.

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arXiv2004

An explanation for metallicity effects on X-ray Binary properties

We show that irradiation induced stellar winds can explain two important metallicity effects in X-ray binaries - the higher numbers and the softer spectra of the X-ray binaries in metal rich globular clusters compared to the metal poor ones. As has been previously noted by Iben, Tutukov and Fedorova, the winds should be stronger at lower metallicity due to less efficient line cooling. This will speed up the evolution of the LMXBs in metal poor clusters, hence reducing their numbers. These winds can also provide extra material near the accreting object which may create an intrinsic absorber to harden the X-ray spectra of the metal poor cluster systems relative to the metal rich ones, as suggested by observations. We outline some additional observational predictions of the model.

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arXiv2003

Microlensing Candidates in M87 and the Virgo Cluster with the Hubble Space Telescope

The position of the giant elliptical galaxy M87 at the center of the Virgo Cluster means that the inferred column density of dark matter associated with both the cluster halo and the galaxy halo is quite large. This system is thus an important laboratory for studying massive dark objects in elliptical galaxies and galaxy clusters by gravitational microlensing, strongly complementing the studies of spiral galaxy halos performed in the Local Group. We have performed a microlensing survey of M87 with the WFPC2 instrument on the Hubble Space Telescope. Over a period of thirty days, with images taken once daily, we discover seven variable sources. Four are variable stars of some sort, two are consistent with classical novae, and one exhibits an excellent microlensing lightcurve, though with a very blue color implying the somewhat disfavored possibility of a horizontal branch source being lensed. Based on sensitivity calculations from artificial stars and from artificial lightcurves, we estimate the expected microlensing rate. We find that the detection of one event is consistent with a dark halo with a 20% contribution of microlensing objects for both M87 and the Virgo Cluster, similar to the value found from observations in the Local Group. Further work is required to test the hypothesized microlensing component to the cluster.

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arXiv2003

The Formation and Evolution of Star Clusters and Galaxies

This paper addresses the questions of what we have learned about how and when dense star clusters form, and what studies of star clusters have revealed about galaxy formation and evolution. One important observation is that globular clusters are observed to form in galaxy mergers and starbursts in the local universe, which both provides constraints on models of globular cluster formation, and suggests that similar physical conditions existed when most early-type galaxies and their globular clusters formed in the past. A second important observation is that globular cluster systems typically have bimodal color distributions. This was predicted by merger models, and indicates an episodic formation history for elliptical galaxies. A third and very recent result is the discovery of large populations of intermediate age globular clusters in several elliptical galaxies through the use of optical to near-infrared colors. These provide an important link between young cluster systems observed in starbursts and mergers and old cluster systems. This continuum of ages of the metal-rich globular cluster systems also indicates that there is no special age or epoch for the formation of the metal-rich globular clusters, which comprise about half of the cluster population. The paper concludes with a brief discussion of recent results on the globular cluster - low-mass X-ray binary connection.

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arXiv2003

Some Constraints On the Effects of Age and Metallicity on the Low Mass X-ray Binary Formation Rate

We have studied the low mass X-ray binary (LMXB) populations within and outside globular clusters (GC) in NGC 4365 and NGC 3115. Using published age and metallicity constraints from optical and IR observations of their GCs, we do not find any evidence for an increase in the LMXB formation rate in the intermediate age cluster population of NGC 4365, as has been proposed in some scenarios of dynamical LMXB formation in GCs. The old, metal-rich, red population of GCs in NGC 3115 on the other hand is {\it at least} three times as efficient at creating LMXBs as the old, metal-poor, blue clusters. These data suggest that the higher formation efficiency of LMXBs in the red GC subsystems of many galaxies is largely a consequence of their higher metallicity. A comparison of the densities of field LMXBs in different galaxies does not reveal an obvious correlation with the age of the field stars as predicted by models in which the LMXB formation rate in the field drops monotonically with time after an initial burst. This suggests that either a significant fraction of the field LMXBs are created in GCs and subsequently injected into the field, or the LMXB formation rate has a more complex time evolution pattern.

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arXiv2002

The Low Mass X-ray Binary-Globular Cluster Connection II: NGC 4472 X-ray Source Properties and Source Catalogs

We present the results of a Chandra/HST study of the point sources of the NGC 4472. We identify 144 X-ray sources, 72 with HST matches. The optical data show 1102 sources, 829 with globular cluster colors. Thirty matches are found - likely to be low mass X-ray binaries in globular clusters, while 42 have no optical counterparts to V~25 and I~24 - likely predominantly LMXBs in the field star population. Thus approximately 40% of the X-ray sources are in globular clusters and ~4% of the globular clusters contain X-ray sources. The blue GC sources may have harder X-ray spectra than the red GC sources. No significant differences are found between the X-ray properties of the field sources and of the GC sources. This study, along with our previous result from Paper I in this series on the similarity of the spatial profile of the field LMXBs, globular cluster LMXBs, and the globular clusters themselves suggest that a significant fraction of the observed low mass X-ray binaries in the field may be created in a globular cluster then ejected into the field by stellar interaction s; however, by comparing the results for NGC 4472 with those in several other galaxies, we find tentative evidence for a correlation be tween the globular cluster specific frequency and the fraction of LMXBs in globular clusters, a correlation which would be most easily explained if some of the field sources were generated in situ. We show that isolated accreting very massive black holes are unlikely to be observable with current X-ray instrumentation and that these sources hence do not contaminate the LMXB population. We discuss the possibility that several equatorial point sources may indicate the presence of a disk wind responsible for the low radiative efficiency observed in the nucleus of this source. (abridged)

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arXiv2002

Extragalactic Globular Clusters in the Near-Infrared II. The Globular Clusters Systems of NGC 3115 and NGC 4365

We combine near-infrared photometry obtained with the VLT/ISAAC instrument and archival HST/WFPC2 optical images to determine VIK magnitudes and colours of globular clusters in two early-type galaxies, NGC 3115 and NGC 4365. The combination of near-IR and optical photometry provides a way to lift the age-metallicity degeneracy. For NGC 3115, the globular cluster colours reveal two major sub-populations, consistent with previous studies. By comparing the V-I, V-K colours of the NGC 3115 globular clusters with SSP models, we find that the colour difference between the two >10 Gyr old major sub-populations is primarily due to a difference in metallicity. We find Δ[Fe/H] = 1.0 +/- 0.3 dex and the blue and red globular cluster sub-populations being coeval within 3 Gyr. In contrast to the NGC 3115 globular clusters, the globular cluster system in NGC 4365 exhibits a more complex age and metallicity structure. We find a significant population of intermediate-age very metal-rich globular clusters along with an old population of both metal-rich and metal-poor clusters. Specifically, we observe a large population of globular clusters with V-K and V-I colours, for which all current SSP models give ages and metallicities in the range ~2-8 Gyr and ~0.5-3 Z_solar, respectively. After 10 Gyr of passive evolution, the intermediate-age globular clusters in NGC 4365 will have colours which are consistent with the very metal-rich population of globular clusters in giant elliptical galaxies, such as M87. Our results for both globular cluster systems are consistent with previous age and metallicity studies of the diffuse galactic light. In addition to the major globular cluster populations in NGC 3115 and NGC 4365 we report on the detection of objects with extremely red colours.

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arXiv2001

Properties of the Outer Halos of Galaxies from the Study of Globular Clusters

This paper reviews some recent work on the properties of the outer halos of galaxies. I particularly focus on recent and upcoming advances made with the study of globular clusters. Globular clusters can be observed out to ~100 kpc from the centers of galaxies, allowing the study of galactic halos well beyond the regions probed by many other techniques such as observations of the integrated light of galaxies. In the few well-studied cases to date, the study of globular cluster systems has provided dynamical evidence for dark matter halos around elliptical galaxies, and demonstrated kinematic differences between different globular cluster populations that shed light on the formation history of their host galaxies.

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arXiv2001

Formation Scenarios for Globular Clusters and Their Host Galaxies

This review focuses on how galaxies and their globular cluster systems form. I first discuss the now fairly convincing evidence that some globular clusters form in galaxy starbursts/mergers. One way these observations are valuable is they place important constraints on the physics of the formation of globular clusters. Moreover, it is natural to associate the typically metal-rich clusters forming in mergers with the substantial metal-rich population of globulars around ellipticals, thereby implying an important role for galaxy mergers in the evolution of elliptical galaxies. I also highlight some new observational efforts aimed at constraining how and when elliptical galaxies and their globular cluster systems formed. These include systematic studies of the number of globular clusters around galaxies as a function of morphological type, studies of the kinematics of globular cluster populations in elliptical galaxies, and a variety of observational programs aimed at constraining the relative ages of globular clusters within galaxies as a function of cluster metallicity. The understanding of the formation of globular cluster systems and their host galaxies has grown dramatically in recent years, and the future looks equally promising.

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arXiv2001

Some Constraints on The Formation of Globular Clusters

We explore the constraints on globular cluster formation provided by the observed conditions in starbursts where globulars are currently forming, and by the observed properties of young and old globular clusters. We note that the pressure in the ISM of starbursts and mergers implies that molecular clouds in these environments have radii similar to those of globular clusters. Such molecular clouds are therefore viable precursors to globular clusters if the star formation efficiency in the clouds is high. A high star formation efficiency may be a consequence of the high density and associated high binding energy and short dynamical timescale of molecular clouds in such environments. We also note that the apparent lack of a mass-radius relationship in young and old globular cluster systems places important constraints on globular cluster formation models. This is because molecular clouds are observed to follow a virial scaling relation between mass and radius. We suggest that a variable star formation efficiency may weaken or eliminate the mass-radius relation of molecular clouds as they fragment to form globular clusters. We attribute the absence of young globular clusters in the disks of ordinary galaxies such as the Milky Way to the relatively low ambient pressures in such systems.

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arXiv2000

A revised Cepheid distance to NGC 4258 and a test of the distance scale

In a previous paper (Maoz et al. 1999), we reported a Hubble Space Telescope (HST) Cepheid distance to the galaxy NGC 4258 obtained using the calibrations and methods then standard for the Key Project on the Extragalactic Distance Scale. Here, we reevaluate the Cepheid distance using the revised Key Project procedures described in Freedman et al. (2001). These revisions alter the zero points and slopes of the Cepheid Period-Luminosity (P-L) relations derived at the Large Magellanic Cloud (LMC), the calibration of the HST WFPC2 camera, and the treatment of metallicity differences. We also provide herein full information on the Cepheids described in Maoz et al. 1999. Using the refined Key Project techniques and calibrations, we determine the distance modulus of NGC 4258 to be 29.47 +/- 0.09 mag (unique to this determination) +/- 0.15 mag (systematic uncertainties in Key Project distances), corresponding to a metric distance of 7.8 +/- 0.3 +/- 0.5 Mpc and 1.2 sigma from the maser distance of 7.2 +/- 0.5 Mpc. We also test the alternative Cepheid P-L relations of Feast (1999), which yield more discrepant results. Additionally, we place weak limits upon the distance to the LMC and upon the effect of metallicity in Cepheid distance determinations.

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arXiv2000

Dynamical Constraints on the Formation of NGC 4472 and Its Globular Clusters

We present new radial velocities for 87 globular clusters around the elliptical galaxy NGC 4472, and combine these with our previously published data to create a data set of velocities for 144 globular clusters around NGC 4472. We utilize this data set to analyze the kinematics of the NGC 4472 globular cluster system. The new data confirms our previous discovery that the metal-poor clusters have significantly higher velocity dispersion than the metal-rich clusters in NGC 4472. We also find very little or no rotation in the more spatially concentrated metal-rich population, with a resulting upper limit for this population of (v/sigma)_{proj} < 0.34 at a 99% confidence level. The very small angular momentum in the metal-rich population requires efficient angular momentum transport during the formation of this population which is spatially concentrated and chemically enriched. Such angular momentum transfer can be provided by galaxy mergers, but has not been achieved in other extant models of elliptical galaxy formation that include dark matter halos. We also calculate the velocity dispersion as a function of radius, and show that it is consistent with roughly isotropic orbits for the clusters and the mass distribution of NGC 4472 inferred from X-ray observations of the hot gas around the galaxy.

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