233 research outputs found
The Spin of M87 as measured from the Rotation of its Globular Clusters
We revisit the kinematical data for 204 globular clusters in the halo of M87.
Beyond 3 r_eff along the major axis of the galaxy light, these globular
clusters exhibit substantial rotation (~ 300 +/- 70 km/s) that translates into
an equally substantial spin (lambda ~ 0.18). The present appearance of M87 is
most likely the product of a single major merger, since this event is best able
to account for so sizable a spin. A rotation this large makes improbable any
significant accretion of material after this merger, since that would have
diluted the rotation signature. We see weak evidence for a difference between
the kinematics of the metal-poor and metal-rich population, in the sense that
the metal-poor globular clusters appear to dominate the rotation. If, as we
suspect, the last major merger event of M87 was mainly dissipationless and did
not trigger the formation of a large number of globular clusters, the kinematic
difference between the two could reflect their orbital properties in the
progenitor galaxies; these differences would be compatible with these
progenitors having formed in dissipational mergers. However, to put strong
kinematic constraints on the origin of the globular clusters themselves is
difficult, given the complex history of the galaxy and its last dominant merger
event.Comment: 20 pages (AAS two column style, including 1 table and 7 figures)
accepted in the AJ (November issue), also available at
http://www.ucolick.org/~mkissler
The ACS Virgo Cluster Survey. XIV. Analysis of Color-Magnitude Relations in Globular Cluster Systems
We examine the correlation between globular cluster (GC) color and magnitude
using HST/ACS imaging for a sample of 79 early-type galaxies (-21.7<M_B<-15.2
mag) with accurate SBF distances from the ACS Virgo Cluster Survey. Using the
KMM mixture modeling algorithm, we find a highly significant correlation,
d(g-z)/dz = -0.037 +- 0.004, between color and magnitude for the subpopulation
of blue GCs in the co-added GC color-magnitude diagram of the three brightest
Virgo galaxies (M49, M87 and M60): brighter GCs are redder than their fainter
counterparts. For the single GC systems of M87 and M60, we find similar
correlations; M49 does not appear to show a significant trend. There is no
correlation between (g-z) and M_z for GCs of the red subpopulation. The
correlation d(g-z)/dg for the blue subpopulation is much weaker than d(g-z)/dz.
Using Monte Carlo simulations, we attribute this to the fact that the blue
subpopulation in M_g extends to higher luminosities than the red subpopulation,
which biases the KMM fits. The correlation between color and M_z thus is a real
effect. This conclusion is supported by biweight fits to the same color
distributions. We identify two environmental dependencies of the
color-magnitude relation: (1) the slope decreases in significance with
decreasing galaxy luminosity; and (2) the slope is stronger for GCs at smaller
galactocentric distances. We examine several mechanisms that might give rise to
the observed color-magnitude relation: (1) presence of contaminators; (2)
accretion of GCs from low-mass galaxies; (3) stochastic effects; (4) capture of
field stars by individual GCs; and (5) GC self-enrichment. We conclude that
self-enrichment and field-star capture, or a combination of these processes,
offer the most promising means of explaining our observations.Comment: 15 pages, 12 figures, accepted for publication in the Astrophysical
Journal. Uses emulateapj.cl
Evidence for the Hierarchical Formation of the Galactic Spheroid
The possibility that the Galactic spheroid was assembled from numerous
chemically-distinct, proto-Galactic fragments is investigated using a
Monte-Carlo technique designed to simulate the chemical evolution of the Galaxy
in hierarchical formation scenarios which involve no gas dissipation. By
comparing the observed and simulated metallicity distributions of Galactic
globular clusters and halo field stars, we estimate the level of fragmentation
in the collapsing proto-Galaxy. Although the formation process is highly
stochastic, the simulations often show good agreement with the observed
metallicity distributions, provided the luminosity function of proto-Galactic
fragments had a power-law form with exponent ~ -2. While this steep slope is
strongly at odds with the presently observed luminosity function of the Local
Group, it is in close agreement with the predictions of semi-analytic and
numerical models of hierarchical galaxy formation. We discuss a number of
possible explanations for this discrepancy. These simulations suggest that the
Galactic halo and its globular cluster system were assembled via the accretion
and disruption of approximately 1000 metal-poor, proto-Galactic fragments by
the dominant Galactic building block: a proto-bulge whose own metal-rich
globular clusters system has been preferentially eroded by dynamical processes.
We argue that the same process (ie, hierarchical growth involving little gas
dissipation) is responsible for the formation of both giant elliptical galaxies
and the bulge-halo components of spiral galaxies. (ABRIDGED).Comment: 20 pages, 9 postscript figures. Accepted for publication in the ApJ,
April 10 2000 issu
The Efficiency of Globular Cluster Formation
(Abridged): The total populations of globular cluster systems (GCSs) are
discussed in terms of their connection to the efficiency of globular cluster
formation---the mass fraction of star-forming gas that was able to form bound
stellar clusters rather than isolated stars or unbound associations---in galaxy
halos. Observed variations in GCS specific frequencies (S_N=N_gc/L_gal), both
as a function of galactocentric radius in individual systems and globally
between entire galaxies, are reviewed in this light. It is argued that trends
in S_N do not reflect any real variation in the underlying efficiency of
cluster formation; rather, they result from ignoring the hot gas in many large
ellipticals. This claim is checked and confirmed in each of M87, M49, and NGC
1399, for which existing data are combined to show that the volume density
profile of globular clusters, rho_cl, is directly proportional to the sum of
(rho_gas+rho_stars) at large radii. The constant of proportionality is the same
in each case: epsilon=0.0026 +/- 0.0005 in the mean. This is identified with
the globular cluster formation efficiency. The implication that epsilon might
have had a universal value is supported by data on the GCSs of 97 early-type
galaxies, on the GCS of the Milky Way, and on the ongoing formation of open
clusters. These results have specific implications for some issues in GCS and
galaxy formation, and they should serve as a strong constraint on more general
theories of star and cluster formation.Comment: 36 pages with 11 figures; accepted for publication in The
Astronomical Journa
Ground-based astrometry with wide field imagers. V. Application to near-infrared detectors: HAWK-I@VLT/ESO
High-precision astrometry requires accurate point-spread function modeling
and accurate geometric-distortion corrections. This paper demonstrates that it
is possible to achieve both requirements with data collected at the high acuity
wide-field K-band imager (HAWK-I), a wide-field imager installed at the Nasmyth
focus of UT4/VLT ESO 8m telescope. Our final astrometric precision reaches ~3
mas per coordinate for a well-exposed star in a single image with a systematic
error less than 0.1 mas. We constructed calibrated astro-photometric catalogs
and atlases of seven fields: the Baade's Window, NGC 6656, NGC 6121, NGC 6822,
NGC 6388, NGC 104, and the James Webb Space Telescope calibration field in the
Large Magellanic Cloud. We make these catalogs and images electronically
available to the community. Furthermore, as a demonstration of the efficacy of
our approach, we combined archival material taken with the optical wide-field
imager at the MPI/ESO 2.2m with HAWK-I observations. We showed that we are able
to achieve an excellent separation between cluster members and field objects
for NGC 6656 and NGC 6121 with a time base-line of about 8 years. Using both
HST and HAWK-I data, we also study the radial distribution of the SGB
populations in NGC 6656 and conclude that the radial trend is flat within our
uncertainty. We also provide membership probabilities for most of the stars in
NGC 6656 and NGC 6121 catalogs and estimate membership for the published
variable stars in these two fields.Comment: 36 pages (included appendix), 13 tables, 35 figures (26 in low
resolution), accepted for publication in Astronomy and Astrophysics. Online
materials will be soon available on CDS. Meanwhile, online materials can be
requested directly to the first autho
AAOmega spectroscopy of 29 351 stars in fields centered on ten Galactic globular clusters
Galactic globular clusters have been pivotal in our understanding of many
astrophysical phenomena. Here we publish the extracted stellar parameters from
a recent large spectroscopic survey of ten globular clusters. A brief review of
the project is also presented. Stellar parameters have been extracted from
individual stellar spectra using both a modified version of the Radial Velocity
Experiment (RAVE) pipeline and a pipeline based on the parameter estimation
method of RAVE. We publish here all parameters extracted from both pipelines.
We calibrate the metallicity and convert this to [Fe/H] for each star and,
furthermore, we compare the velocities and velocity dispersions of the Galactic
stars in each field to the Besan\c{c}on Galaxy model. We find that the model
does not correspond well with the data, indicating that the model is probably
of little use for comparisons with pencil beam survey data such as this.Comment: 6 pages, 5 figures, 4 tables. Accepted for publication in A&A. Data
described in tables will be available on CDS (at
http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/530/A31) once publishe
Normal Globular Cluster Systems in Massive Low Surface Brightness Galaxies
We present the results of a study of the globular cluster systems of 6
massive spiral galaxies, originally cataloged as low surface brightness
galaxies but here shown to span a wide range of central surface brightness
values, including two intermediate to low surface brightness galaxies. We used
the Advanced Camera for Surveys on board HST to obtain photometry in the F475W
and F775W bands and select sources with photometric and morphological
properties consistent with those of globular clusters. A total of 206
candidates were identified in our target galaxies. From a direct comparison
with the Galactic globular cluster system we derive specific frequency values
for each galaxy that are in the expected range for late-type galaxies. We show
that the globular cluster candidates in all galaxies have properties consistent
with globular cluster systems of previously studied galaxies in terms of
luminosity, sizes and color. We establish the presence of globular clusters in
the two intermediate to low surface brightness galaxies in our sample and show
that their properties do not have any significant deviation from the behavior
observed in the other sample galaxies. Our results are broadly consistent with
a scenario in which low surface brightness galaxies follow roughly the same
evolutionary history as normal (i.e. high surface) brightness galaxies except
at a much lower rate, but require the presence of an initial period of star
formation intense enough to allow the formation of massive star clusters.Comment: 14 pages, 6 figures. AJ accepte
The Ringed Spiral Galaxy NGC 4622. I. Photometry, Kinematics, and the Case for Two Strong Leading Outer Spiral Arms
The intriguing nearly face-on southern ringed spiral galaxy NGC 4622, the
first galaxy definitively shown to have leading spiral structure, is revisited
in this paper with new images from the Hubble Space Telescope's WFPC2, together
with ground-based optical and near-IR imaging, and a Fabry-Perot H-alpha
velocity field. The data provide new information on the disk/bulge/halo mix,
rotation curve, star formation in the galaxy, and the sense of winding of its
prominent spiral arms. Previously, we suggested that the weaker, inner single
arm most likely has the leading sense, based on a numerical simulation. Now,
taking advantage of HST resolution and using de Vaucouleurs' standard
extinction and reddening technique to determine the near side of the galaxy's
slightly tilted disk, we come to the more surprising conclusion that the two
strong outer arms have the leading sense. We suggest that this highly unusual
configuration may be the result of a past minor merger or mild tidal encounter.
Possible evidence for a minor merger is found in a short, central dust lane,
although this is purely circumstantial and an unrelated interaction with a
different companion could also be relevant. The leading arms may be allowed to
persist because NGC 4622 is dark halo-dominated (i.e., not ``maximum disk'' in
the inner regions) and displays a significantly rising rotation curve. The new
HST observations also reveal a rich globular cluster system in the galaxy. The
mean color of these clusters is = 1.04 and the specific frequency is
3.40.6. The luminosity function of these clusters confirms the membership
of NGC 4622 in the Centaurus Cluster.Comment: Accepted for publication in the Astronomical Journal, February 2003
issue (LaTex, 28 pages + 38 figures, 3 in color); for gzipped postscript file
see http://bama.ua.edu/~rbuta/ngc4622/paper/paper.htm
SINFONI in the Galactic Center: young stars and IR flares in the central light month
We report 75 milli-arcsec resolution, near-IR imaging spectroscopy within the
central 30 light days of the Galactic Center [...]. To a limiting magnitude of
K~16, 9 of 10 stars in the central 0.4 arcsec, and 13 of 17 stars out to 0.7
arcsec from the central black hole have spectral properties of B0-B9, main
sequence stars. [...] all brighter early type stars have normal rotation
velocities, similar to solar neighborhood stars. We [...] derive improved 3d
stellar orbits for six of these S-stars in the central 0.5 arcsec. Their
orientations in space appear random. Their orbital planes are not co-aligned
with those of the two disks of massive young stars 1-10 arcsec from SgrA*. We
can thus exclude [...] that the S-stars as a group inhabit the inner regions of
these disks. They also cannot have been located/formed in these disks [...].
[...] we conclude that the S-stars were most likely brought into the central
light month by strong individual scattering events. The updated estimate of
distance to the Galactic center from the S2 orbit fit is Ro = 7.62 +/- 0.32
kpc, resulting in a central mass value of 3.61 +/- 0.32 x 10^6 Msun. We
happened to catch two smaller flaring events from SgrA* [...]. The 1.7-2.45 mum
spectral energy distributions of these flares are fit by a featureless, red
power law [...]. The observed spectral slope is in good agreement with
synchrotron models in which the infrared emission comes from [...] radiative
inefficient accretion flow in the central R~10 Rs region.Comment: 50 pages, 10 figures, 2 tables, submitted to ApJ, February 6th, 2005,
abstract abridge
The Rate and Spatial Distribution of Novae in M101 (NGC 5457)
A new multi-epoch H-alpha imaging study of M101 (NGC 5457) has been carried
out as part of a larger campaign to study the rate and stellar population of
extragalactic novae. The survey yielded a total of 13 nova detections from 10
epochs of M101 observations spanning a three year period. After correcting for
the temporal coverage and survey completeness, a global nova rate of
11.7^{+1.9}_{-1.5} per year is found. This value corresponds to a
luminosity-specific nova rate of 1.23\pm0.27 novae per year per 10^{10}
L_{\sun,K} when the K luminosity is derived from the B-K color, or 1.94\pm0.42
novae per year per 10^{10} L_{\sun,K} when the K magnitude from the Two Micron
All Sky Survey is used. These values are consistent with previous estimates by
Shafter et al. that were based on more limited data. The spatial distribution
of the combined nova sample from the present survey and from the earlier
Shafter et al. survey shows that the specific frequency of novae closely
follows the integrated background light of the galaxy.Comment: 22 pages; 4 figures; accepted for publication in the Astrophysical
Journa
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