1,172 research outputs found
Globular Cluster Luminosity Functions and the Hubble Constant from WFPC2 Imaging: Galaxies in the Coma I Cloud
The membership of some galaxies in the nearby (d ~ 12 Mpc) Coma I cloud is
uncertain. Here we present globular cluster luminosity functions (GCLFs) from
the HST for two bright ellipticals which may belong to this group. After
fitting the GCLF, we find a turnover magnitude of m_V^0 = 23.23 +/- 0.11 for
NGC 4278 and m_V^0 = 23.07 +/- 0.13 for NGC 4494. Our limiting magnitude is
about two magnitudes fainter than these values, making this data among the most
complete GCLFs published to date. The fitted GCLF dispersions (~ 1.1 mag.) are
somewhat smaller than typical values for other ellipticals. Assuming an
absolute turnover magnitude of M_V^0 = -7.62, and after applying a small
metallicity correction, we derive distance modulii of (m -- M) = 30.61 +/- 0.14
for NGC 4278 and 30.50 +/- 0.15 for NGC 4494. These distance estimates are
compared to other methods, and lie within the published range of values. We
conclude that both galaxies lie at the same distance and are both members of
the Coma I cloud.Comment: 13 pages, Latex. Full paper also available at
http://www.ucolick.org/~forbes/home.htm
Globular Clusters and Galaxy Formation
Globular clusters provide a unique probe of galaxy formation and evolution.
Here I briefly summarize the known observational properties of globular cluster
systems. One re-occurring theme is that the globular cluster systems of spirals
and ellipticals are remarkably similar. Photometry, and the limited spectra
available, are consistent with metal-poor clusters forming before the main
spheroid component is established and the metal-rich ones forming at the same
time as the spheroid in a burst of star formation. These observations are
compared to a model for globular cluster formation in a LCDM hierarchical
universe. One model result reported here is that S_N is determined at early
times and little affected by late epoch mergers.Comment: 2 pages, Latex, 2 figures, To appear in the proceedings of Galaxy
Evolution: Theory and Observations, ed. V. Avila-Reese, C. Firmani, C. Frenk,
C. Allen, RevMexA
The Globular Cluster Systems of Ellipticals and Spirals
In this overview of the key properties of globular cluster (GC) systems I
show that the GCs in elliptical and spiral host galaxies have more in common
than previously thought. After contrasting these properties I briefly comment
on GC formation.Comment: 6 pages, Latex, 4 figures, To appear in the proceedings of IAU Symp.
207, "Extragalactic Star Clusters", eds. Grebel, Geisler, Minnit
Radial Kinematics of Isolated Elliptical Galaxies
Ellipticals in very low density environments are extremely rare but hold
important clues about galaxy formation and evolution. In this paper we continue
our study of isolated elliptical galaxies, presenting results on the radial
stellar kinematics for 13 isolated early-type galaxies. We derive radial
rotation velocity, velocity dispersion and hermite terms to ~1 effective
radius. We observe a dichotomy in kinematic properties similar to that in the
elliptical population as a whole, where low luminosity ellipticals tend to be
rotationally supported. For all galaxies the V/sigma ratio increases with
radius. We find kinematically distinct cores (KDCs), or velocity substructure,
in ~40% of the galaxies for which we have major axis spectra. Such a fraction
is similar to that observed for ellipticals in higher density environments.
Most galaxies in the sample reveal kinematic evidence for a nuclear disk. The
non-relaxed kinematics in several galaxies suggests that they have undergone a
merger or accretion event. Isolated ellipticals generally follow the
fundamental plane defined by cluster ellipticals -- exceptions being those
galaxies with evidence for young stellar populations. Overall, we find isolated
ellipticals have similar kinematic properties to their counterparts in higher
density environments.Comment: 11 pages, 6 figures, MNRAS accepte
High-Precision Dark Halo Virial Masses from Globular Cluster Numbers: Implications for Globular Cluster Formation and Galaxy Assembly
We confirm that the number of globular clusters (GCs), N, is an
excellent tracer of their host galaxy's halo virial mass M. The simple
linear relation M M N fits the
data perfectly from M M to M M. This result is independent of galaxy morphology and
extends statistically into the dwarf galaxy regime with M M, including the extreme ultra diffuse galaxy DF44. As this
correlation does not depend on GC mass it is ideally suited for high-precision
determinations of M. The linearity is most simply explained by
cosmological merging of a high-redshift halo seed population that hosted on
average one GC per M of dark matter. We show that
hierarchical merging is also extremely powerful in restoring a linear
correlation and erasing signatures of even a strong secular evolution of GC
systems. The cosmological merging scenario also implies a strong decline of the
scatter in with increasing virial mass in contrast with the observations that show a roughly constant
scatter, independent of virial mass. This discrepancy can be explained if
errors in determining virial masses from kinematical tracers and gravitational
lensing are on the order of a factor of 2. GCs in dwarf satellite galaxies pose
a serious problem for high-redshift GC formation scenarios; the dark halo
masses of dwarf galaxies hosting GCs therefore might need to be an order of
magnitude larger than currently estimated.Comment: 11 pages, 4 figures, ApJ in pres
Star Formation in Southern Seyfert Galaxies
We have produced radio maps, using the ATCA, of the central regions of six
southern Seyfert 2 galaxies (NGC 1365, 4945, 6221, 6810, 7582, and Circinus)
with circumnuclear star formation, to estimate the relative contribution of
star formation activity compared to activity from the active galactic nucleus
(AGN). The radio morphologies range from extended diffuse structures to compact
nuclear emission, with no evidence, even in the relatively compact sources, for
synchrotron self--absorption. In each case the radio to far--infrared (FIR)
ratio has a value consistent with star formation, and in all but one case the
radio to [FeII] ratio is also consistent with star formation. We derive
supernova rates and conclude that, despite the presence of a Seyfert nucleus in
these galaxies, the radio, FIR, and [FeII] line emission are dominated by
processes associated with the circumnuclear star formation (i.e. supernova
remnants and HII regions) rather than with the AGN.Comment: 26 pages, Latex, 13 figures, submitted to MNRA
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