arXiv subjects

Doug Geisler

Publications and source records attributed to Doug Geisler.

102 records · Page 6Linked to original sources

arXiv2001

Dynamics of the Globular Cluster System Associated with M87 (NGC 4486). II. Analysis

We present a dynamical analysis of the globular cluster system associated with M87 (= NGC 4486), the cD galaxy near the dynamical center of the Virgo cluster. The analysis utilizes a new spectroscopic and photometric database which is described in a companion paper (Hanes et al. 2001). Using a sample of 278 globular clusters with measured radial velocities and metallicities, and new surface density profiles based on wide-field Washington photometry, we study the dynamics of the M87 globular cluster system both globally --- for the entire cluster sample --- and separately --- for the metal-rich and metal-poor globular cluster samples. This constitutes the largest sample of radial velocities for pure Population II tracers yet assembled for any galaxy. We discuss the implications of our findings for models for the formation of giant elliptical galaxies, globular cluster systems, and the Virgo cluster. (ABRIDGED)

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arXiv2001

The Line-of-Sight Depth of Populous Clusters in the Small Magellanic Cloud

We present an analysis of age, metal abundance, and positional data on populous clusters in the Small Magellanic Cloud (SMC) with the ultimate aim of determining the line-of-sight (LOS) depth of the SMC using these clusters as proxies. Our dataset contains 12 objects and is limited to clusters with the highest quality data for which the ages and abundances are best known and can be placed on an internally consistent scale. We have analyzed the variation of the clusters' properties with position on the sky and with line-of-sight depth. Based on this analysis, we draw the following conclusions. 1) The observational data indicates that the eastern side of the SMC (facing the LMC) contains younger and more metal-rich clusters as compared with the western side. This is not a strong correlation because our dataset of clusters is necessarily limited, but it is suggestive and warrants further study. 2) Depending on how the reddening is computed to our clusters, we find a mean distance modulus that ranges from (m-M)_0 = 18.71 +/- 0.06 to (m-M)_0 = 18.82 +/- 0.05. 3) The intrinsic +/- 1-sigma line-of-sight depth of the SMC populous clusters in our study is between ~6 kpc and ~12 kpc depending primarily on whether we adopt the Burstein & Heiles reddenings or those from Schlegel et al. 4) Viewing the SMC as a triaxial galaxy with the Declination, Right Ascension, and LOS depth as the three axes, we find axial ratios of approximately 1:2:4. Taken together, these conclusions largely agree with those of previous investigators and serve to underscore the utility of populous star clusters as probes of the structure of the Small Magellanic Cloud.

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arXiv2000

Globular Clusters as Probes of the Virgo gE NGC 4472

We present radial velocities for 144 globular clusters around the Virgo gE NGC 4472 (M49), and ages and metallicities from co-added spectra of 131 clusters. We confirm our earlier finding that the metal-poor clusters have a significantly higher velocity dispersion than the metal-rich clusters, and we find little or no rotation in the metal-rich clusters. The velocity dispersion profile is consistent with isotropic orbits for the clusters and the NGC 4472 mass distribution inferred from X-ray data. Our sample of clusters spans a metallicity range of -1.6 < [Fe/H] < 0 dex. The metal-poor and metal-rich cluster populations are coeval within the errors, and all clusters are older than 6 Gyr at 95% confidence. Our findings are consistent with a merger origin for NGC 4472, but other elliptical formation models cannot be ruled out.

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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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arXiv2000

Hubble Space Telescope Photometry of the Dwarf Spheroidal Galaxy ESO 410-G005

We present HST WFPC2 imaging of the nearby low-surface-brightness dwarf spheroidal galaxy ESO 410-G005, which has been resolved into stars for the first time. The resulting color-magnitude diagram for about 2500 stars shows a red giant branch branch with a tip at I=(22.4+-0.15), which yields a distance of D=(1.9+-0.2) Mpc. ESO 410-G005 is found to be metal-poor with a mean metallicity of (-1.8+-0.4) dex estimated from its red giant branch. Upper asymptotic giant branch stars appear to be present near the center of the galaxy, indicative of a substantial, centrally concentrated intermediate-age population, unless these objects are artifacts of crowding. Previous studies did not detect ESO 410-G005 in H alpha or in HI. ESO 410-G005 is a probable member of the Sculptor group. Its linear separation from the nearest spiral, NGC 55, is 230 kpc on the sky. The deprojected separation ranges from 340 to 615 kpc depending on the assumed distance of NGC 55. ESO 410 G005 appears to be a relatively isolated dSph within the Sculptor group. Its absolute magnitude, Mv = (-12.1+-0.2) mag, its central surface brightness, mu_V = (22.7+-0.1) mag/arcsec^2, and its mean metallicity, [Fe/H] = (-1.8+-0.4) dex, follow the trend observed for dwarf galaxies in the Local Group. (abridged)

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arXiv2000

Wide Field CCD Surface Photometry of the Giant Elliptical Galaxy NGC 4472 in the Virgo Cluster

We present deep wide field (16'.4 x 16'.4) Washington CT1 CCD surface photometry of the giant elliptical galaxy NGC 4472, the brightest member in the Virgo cluster. Our data cover a wider field than any previous CCD photometry as well as going deeper. Surface brightness profiles of NGC 4472 are not well fit by a single King model, but they can be fit approximately by two King models: with separate models for the inner and outer regions. Surface brightness profiles for the outer region can also be fit approximately by a deVaucouleurs law. There is clearly a negative color gradient within 3' of NGC 4472, in the sense that the color gets bluer with increasing radius. The slope of the color gradient for this region is derived to be $Δμ(C-T_1)$ = -0.08 mag arcsec$^{-2}$ for $Δ\log r =1$, which corresponds to a metallicity gradient of $Δ$ [Fe/H] $= -0.2$ dex. However, the surface color gets redder slowly with increasing radius beyond 3'. A comparison of the structural parameters of NGC 4472 in C and T1 images has shown that there is little difference in the ellipse shapes between isochromes and isophotes. In addition, photometric and structural parameters of NGC 4472 have been determined.

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arXiv1998

HST WFPC2 Color-Magnitude Diagrams of Halo Globular Clusters in M33: Implications for the Early Formation History of the Local Group

We have constructed color-magnitude diagrams (CMDs) for ten globular clusters in the halo of the nearby spiral galaxy M33 based on Hubble Space Telescope Wide-Field Planetary Camera 2 observations in the F555W (~V) and F814W (~I) filters. These data reveal the morphology of the HB and allow us to estimate the cluster metallicity using the shape and color of the red giant branch. The principal result we report herein is that 8 of the 10 clusters possess exclusively red HB morphologies yet their metallicities are as metal-poor as [Fe/H] ~ -1.6. Indeed, these 8 clusters present basically only giant branch clumps reminiscent of intermediate age star clusters in the Magellanic Clouds. In addition, two of the clusters form a second parameter pair which have similar metallicities but very disparate HB types. Under the assumption that cluster age is the global second parameter, the average age of halo globular clusters in M33 appears to be a few Gyr younger than halo clusters in the Milky Way. Using the observed properties of HB stars in M31 and M33 along with published main-sequence turnoff ages for the globular clusters in the Milky Way, LMC, SMC, and the Sagittarius dwarf spheroidal (Sgr), we attempt to sketch the early formation history of these galaxies. This indicates that the Milky Way, M31, M33, the LMC, and Sgr all experienced their first epoch of cluster formation soon after the Big Bang. Three to four Gyr later, the SMC began to form its first generation of clusters; the bulk of the M33 clusters formed later still. We note that the halo clusters in M33 formed over a much longer time period than those in the Milky Way and M31.

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arXiv1998

Ages and Metallicities of Star Clusters and Surrounding Fields in the Outer Disk of the Large Magellanic Cloud

We present Washington system C,T_1 CMDs of 13 star clusters and their surrounding fields which lie in the outer parts of the LMC disk. Ages are determined by means of the magnitude difference between the giant branch clump and the turnoff, while metallicities are derived from the location of the giant and subgiant branches as compared to fiducial star clusters. We find that in most cases the stellar population of each star cluster is quite similar to that of the field where it is embedded. Three particular fields present remarkable properties: (i) The so far unique cluster ESO121-SC03 at ~9 Gyr has a surrounding field which shares the same properties. (ii) The field surrounding the far eastern intermediate age cluster OHSC37 is noteworthy in the sense that we do not detect any evidence of LMC stars. (iii) The fields of SL388 and SL509 present CMDs with a secondary clump ~0.45 mag fainter than the dominant intermediate age clump, suggesting a stellar population component located behind the LMC disk at a distance comparable to that of the SMC. The mean metallicity derived for the intermediate age outer disk clusters is <[Fe/H]>=-0.7 and for their surrounding fields <[Fe/H]>=-0.6. These values are significantly lower than found by Olszewski et al. (1991, AJ, 101, 515) for a sample of clusters of similar age, but are in good agreement with several recent studies. A few clusters stand out in the age--metallicity relation in the sense that they are intermediate age clusters at relatively low metallicity ([Fe/H]~-1).

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arXiv1997

Washington Photometry of the Globular Cluster System of NGC 4472. II. The Luminosity Function and Spatial Structure

We present a comprehensive study of the luminosity function and spatial structure of the globular cluster system of NGC 4472, based on deep wide field Washington CT1 CCD images. The globular cluster luminosity function shows a peak at T1=23.3 +/- 0.1 mag, about 1.5 mags brighter than our 50% completeness limit. We estimate the true distance modulus to NGC 4472 to be (m-M)o = 31.2 +/- 0.2. With our large sample (~ 2000) of bright globular clusters over a wide field, we make a definitive investigation of the spatial structures of the metal-poor and metal-rich cluster populations and find that they are systematically different: (1) The metal-rich clusters are more centrally concentrated than the metal-poor clusters; and (2) The metal-rich clusters are elongated roughly along the major axis of the parent galaxy, while the metal-poor clusters are essentially spherically distributed. In general, the metal-rich clusters closely follow the underlying halo starlight of NGC 4472 in terms of spatial structure and metallicity, while the metal-poor clusters do not. The global value of the specific frequency of the globular clusters in NGC 4472 is estimated to be S_N = 4.7 +/- 0.6. The local specific frequency increases linearly outward from the center of NGC 4472 until ~ 5'.5, beyond which it levels off at S_N ~ 8.5 until the limit of our data at 7'. The specific frequency of both the metal-rich and metal-poor populations shows similar behavior. However, S_N of the metal-poor clusters is about a factor 2 larger than that of the metal-rich clusters in the outer regions. Implications of these results for the origin of the globular clusters in NGC 4472 are discussed.

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arXiv1997

A Search for Old Star Clusters in the Large Magellanic Cloud

We report the first results of a color-magnitude diagram survey of 25 candidate old LMC clusters. For almost all of the sample, it was possible to reach the turnoff region, and in many clusters we have several magnitudes of the main sequence. Age estimates based on the magnitude difference $δT_1$ between the giant branch clump and the turnoff revealed that no new old clusters were found. The candidates turned out to be of intermediate age (1-3 Gyr) We show that the apparently old ages as inferred from integrated UBV colors can be explained by a combination of stochastic effects produced by bright stars and by photometric errors for faint clusters lying in crowded fields. The relatively metal poor candidates from the CaII triplet spectroscopy also turned out to be of intermediate age. This, combined with the fact that they lie far out in the disk, yields interesting constraints regarding the formation and evolution of the LMC disk. We also study the age distribution of intermediate age and old clusters This homogeneous set of accurate relative ages allows us to make an improved study of the history of cluster formation/destruction for ages $>1$Gyr. We confirm previous indications that there was apparently no cluster formation in the LMC during the period from 3-8 Gyr ago, and that there was a pronounced epoch of cluster formation beginning 3 Gyrs ago that peaked at about 1.5 Gyrs ago. Our results suggest that there are few, if any, genuine old clusters in the LMC left to be found.

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arXiv1997

Young Star Clusters in the Dwarf Irregular Galaxy, UGC 7636, Interacting with the Giant Elliptical Galaxy NGC 4472

We present integrated Washington CT1 photometry of 18 bright blue objects discovered in the dwarf galaxy UGC 7636 which is located 5'.5 southeast of the giant elliptical galaxy NGC 4472, the brightest galaxy in the Virgo cluster. Several lines of evidence indicate that UGC 7636 is interacting violently with NGC 4472. These objects are very blue with colors of -0.4 < (C-T1) < 0.6, and their magnitudes are in the range of 20.6 < T1 < 22.9 mag which corresponds to absolute magnitudes of -10.6 < M(T1) < -8.3 mag for a distance modulus of (m-M)o = 31.2. These objects are grouped spatially in three regions: the central region of UGC 7636, the tidal tail region, and the HI cloud region. No such objects were found in the counter tail region. It is concluded that these objects are probably young star clusters which formed < 0.1Gyr ago during the interaction between UGC 7636 and NGC 4472. Surface photometry of UGC 7636 (r < 83") shows that there is a significant excess of blue light along the tidal tail region compared with other regions. The star clusters are bluer than the stellar light in the tidal tail region, indicating that these clusters might have formed later than most stars in the tidal tail region which were formed later than most stars in the main body of the galaxy.

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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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