142 research outputs found
The dynamics and evolution of clusters of galaxies
Research was undertaken to produce a coherent picture of the formation and evolution of large-scale structures in the universe. The program is divided into projects which examine four areas: the relationship between individual galaxies and their environment; the structure and evolution of individual rich clusters of galaxies; the nature of superclusters; and the large-scale distribution of individual galaxies. A brief review of results in each area is provided
Spectroscopy of a Globular Cluster in the Local Group dIrr NGC 6822
We present low-resolution Keck spectroscopy for the globular cluster H VIII
in the Local Group dIrr galaxy NGC 6822. We find the metallicity of the cluster
to be [Fe/H]= -1.58 +/- 0.28 and the age of the cluster to be 3-4 Gyr, slightly
older than but consistent with previous age estimates. H VIII seems to be more
metal-poor than most intermediate-age globular clusters in the Local Group, and
appears most similar to the anomalous Small Magellanic Cloud clusters Lindsay
113 and NGC 339.Comment: 5 pages, including 3 figures. Accepted by MNRAS, uses mn2e.cl
Testing population synthesis models with globular cluster colors
We have measured an extensive set of UBVRIJHK colors for M31 globular
clusters [Barmby et al. 2000]. We compare the predicted simple stellar
population colors of three population synthesis models to the intrinsic colors
of Galactic and M31 globular clusters. The best-fitting models fit the cluster
colors very well -- the weighted mean color offsets are all < 0.05 mag. The
most significant offsets between model and data are in the U and B passbands;
these are not unexpected and are likely due to problems with the spectral
libraries used by the models. The metal-rich clusters ([Fe/H] > -0.8) are best
fit by young (8 Gyr) models, while the metal-poor clusters are best fit by
older (12--16 Gyr) models. If this range of globular cluster ages is correct,
it implies that conditions for cluster formation must have existed for a
substantial fraction of the galaxies' lifetimes.Comment: To appear in ApJ Letters; 8 pages including 3 figures and 1 tabl
Hubble Space Telescope Imaging of the Globular Cluster System around NGC 5846
Bimodal globular cluster metallicity distributions have now been seen in a
handful of large ellipticals. Here we report the discovery of a bimodal
distribution in the dominant group elliptical NGC 5846, using the Hubble Space
Telescope's Wide Field and Planetary Camera 2 (WFPC2). The two peaks are
located at V-I = 0.96 and 1.17, which roughly correspond to metallicities of
[Fe/H] = -1.2 and -0.2 respectively. The luminosity functions of the blue and
red subpopulations appear to be the same, indicating that luminosity does not
correlate with metallicity within an individual galaxy's globular cluster
system. Our WFPC2 data cover three pointings allowing us to examine the spatial
distribution of globular clusters out to 30 kpc (or 2.5 galaxy effective
radii). We find a power law surface density with a very flat slope, and a
tendency for globular clusters to align close to the galaxy minor axis. An
extrapolation of the surface density profile, out to 50 kpc, gives a specific
frequency S_N = 4.3 +/- 1.1. Thus NGC 5846 has a much lower specific frequency
than other dominant ellipticals in clusters but is similar to those in groups.
The central galaxy regions reveal some filamentary dust features, presumably
from a past merger or accretion of a gas-rich galaxy. This dust reaches to the
very nucleus and so provides an obvious source of fuel for the radio core. We
have searched for proto-globular clusters that may have resulted from the
merger/accretion and find none. Finally, we briefly discuss the implications of
our results for globular cluster formation mechanisms.Comment: 22 pages, Latex. To be published in the Astronomical Journal. Full
paper available at http://www.ucolick.org/~forbes/home.htm
Globular Cluster Luminosity Functions and the Hubble Constant from WFPC2 Imaging: The Dominant Group Elliptical NGC 5846
The HST's WFPC2 has several advantages over ground--based observations for
the study of globular cluster luminosity functions (GCLFs) and distance
determination. Here we present WFPC2 data on the globular clusters associated
with NGC 5846. This giant elliptical is the dominant galaxy in a small, compact
group located ~ 13 Mpc beyond the Virgo cluster. We have detected over 1200
globular clusters in three (central, north and south) separate pointings. The
luminosity function in each of these pointings are statistically the same,
indicating that the mean luminosity (mass) does not vary between ~ 3 and 30 kpc
from the galaxy center. This suggests that dynamical friction and bulge
shocking destruction processes are insignificant. We have fit a Gaussian and
t_5 profile to the GCLF (of the combined pointings) and find that it is well
represented by a turnover magnitude of m^0_V = 25.05 +/- 0.10 and a dispersion
of sigma = 1.34 +/- 0.06. Our 50% completeness level is ~ 1 mag fainter than
the turnover. After applying a metallicity correction to the `universal' GCLF
turnover magnitude, we derive a distance modulus of (m--M) = 32.32 +/- 0.23.
For a group velocity V_{CMB} = 1883 +/- 28 km/s, the Hubble constant is H_o =
65 +/- 8 km/s/Mpc.Comment: 12 pages, Plate 1 in JPEG format, Latex, full paper also available at
http://www.ucolick.org/~forbes/home.htm
Erratum: "Groups of Galaxies in the Two Micron All Sky Redshift Survey" (ApJ, 655, 790 [2007])
The SBF Survey of Galaxy Distances. II. Local and Large-Scale Flows
We present analysis of local large scale flows using the Surface Brightness
Fluctuation (SBF) Survey for the distances to 300 early-type galaxies. Our
models of the distribution function of mean velocity and velocity dispersion at
each point in space include a uniform thermal velocity dispersion and spherical
attractors whose position, amplitude, and radial shape are free to vary. Our
fitting procedure performs a maximum likelihood fit of the model to the
observations. We obtain a Hubble constant of Ho = 77 +/- 4 +/- 7 km/s/Mpc, but
a uniform Hubble flow is not acceptable fit to the data. Inclusion of two
attractors, one of whose fit location coincides with the Virgo cluster and the
other whose fit location is slightly beyond the Centaurus clusters nearly
explain the peculiar velocities, but the quality of the fit can be further
improved by the addition of a quadrupole correction to the Hubble flow.
Although the dipole and quadrupole may be genuine manifestations of more
distant density fluctuations, we find evidence that they are more likely due to
non-spherical attractors. We find no evidence for bulk flows which include our
entire survey volume (R < 3000 km/s); our volume is at rest with respect to the
CMB. The fits to the attractors both have isothermal radial profiles (v ~ 1/r)
over a range of overdensity between about 10 and 1, but fall off more steeply
at larger radius. The best fit value for the small scale, cosmic thermal
velocity is 180 +/- 14 km/s.Comment: 37 pages, AASTeX Latex, including 30 Postscript figures, submitted to
Astrophysical Journal, July 2, 199
Thirteen new BL Lacertae objects discovered by an efficient x ray/radio/optical technique
The discovery of 13 serendipitous BL Lac objects in the Einstein IPC Slew Survey by means of x ray/radio vs. x ray/optical color-color diagrams and confirmation by optical spectroscopy are reported. These 13 BL Lacs were discovered using a technique which exploits the characteristic broad band spectra of BL Lacs. New VLA detections provide accurate fluxes (f(6 cm) is approximately 0.5 mJy) and 2 in. positions, facilitating the determination of an optical counterpart. All 13 new BL Lacs show essentially featureless optical spectra. Nine of these lie within the range of colors of known x ray selected BL Lacs. Of the remaining four, one is apparently x ray louder (by a factor of 1.5) or optically quieter (by 0.8 mags); and three are optically louder (by 1-1.3 mags) than x ray selected BL Lacs. Approximately 50 new BL Lacs in total are expected from VLA work and upcoming Australia Telescope observations, yielding a complete Slew Survey sample of approximately 90 BL Lacs
Star Cluster Candidates in M81
We present a catalog of extended objects in the vicinity of M81 based a set
of 24 Hubble Space Telescope Advanced Camera for Surveys (ACS) Wide Field
Camera (WFC) F814W (I-band) images. We have found 233 good globular cluster
candidates; 92 candidate HII regions, OB associations, or diffuse open
clusters; 489 probable background galaxies; and 1719 unclassified objects. We
have color data from ground-based g- and r-band MMT Megacam images for 79
galaxies, 125 globular cluster candidates, 7 HII regions, and 184 unclassified
objects. The color-color diagram of globular cluster candidates shows that most
fall into the range 0.25 < g-r < 1.25 and 0.5 < r-I < 1.25, similar to the
color range of Milky Way globular clusters. Unclassified objects are often
blue, suggesting that many of them are likely to be HII regions and open
clusters, although a few galaxies and globular clusters may be among them.Comment: 35 pages, 11 figures, submitted to A
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