1,536 research outputs found
VLT Diffraction Limited Imaging and Spectroscopy in the NIR: Weighing the black hole in Centaurus A with NACO
We present high spatial resolution near-infrared spectra and images of the
nucleus of Centaurus A (NGC 5128) obtained with NAOS-CONICA at the VLT. The
adaptive optics corrected data have a spatial resolution of 0.06" (FWHM) in K-
and 0.11" in H-band, four times higher than previous studies. The observed gas
motions suggest a kinematically hot disk which is orbiting a central object and
is oriented nearly perpendicular to the nuclear jet. We model the central
rotation and velocity dispersion curves of the [FeII] gas orbiting in the
combined potential of the stellar mass and the (dominant) black hole. Our
physically most plausible model, a dynamically hot and geometrically thin gas
disk, yields a black hole mass of M_bh = (6.1 +0.6/-0.8) 10^7 M_sun. As the
physical state of the gas is not well understood, we also consider two limiting
cases: first a cold disk model, which completely neglects the velocity
dispersion; it yields an M_bh estimate that is almost two times lower. The
other extreme case is to model a spherical gas distribution in hydrostatic
equilibrium through Jeans equation. Compared to the hot disk model the best-fit
black hole mass increases by a factor of 1.5. This wide mass range spanned by
the limiting cases shows how important the gas physics is even for high
resolution data. Our overall best-fitting black hole mass is a factor of 2-4
lower than previous measurements. With our revised M_bh estimate, Cen A's
offset from the M_bh-sigma relation is significantly reduced; it falls above
this relation by a factor of ~2, which is close to the intrinsic scatter of
this relation. (Abridged)Comment: 12 pages, 14 figures, including minor changes following the referee
report; accepted for publication in The Astrophysical Journa
HST/STIS Spectra of Nuclear Star Clusters in Spiral Galaxies: Dependence of Age and Mass On Hubble Type
(Abridged) We study the nuclear star clusters in spiral galaxies of various
Hubble types using spectra obtained with STIS on-board HST. We observed the
nuclear clusters in 40 galaxies, selected from two previous HST/WFPC2 imaging
surveys. The spectra provide a better separation of cluster light from
underlying galaxy light than is possible with ground-based spectra. To infer
the star formation history, metallicity and dust extinction, we fit weighted
superpositions of single-age stellar population templates to the spectra. The
luminosity-weighted age ranges from 10 Myrs to 10 Gyrs. The stellar populations
of NCs are generally best fit as a mixture of populations of different ages.
This indicates that NCs did not form in a single event, but instead they had
additional star formation long after the oldest stars formed. On average, the
sample clusters in late-type spirals have a younger luminosity-weighted mean
age than those in early-type spirals (log(age/yr) = 8.37+/-0.25 vs.
9.23+/-0.21). The average cluster masses are smaller in late-type spirals than
in early-type spirals (log(M/Msun) = 6.25+/-0.21 vs. 7.63+/-0.24), and exceed
the masses typical of globular clusters. The cluster mass correlates strongly
with both the Hubble type of the host galaxy and the luminosity of its bulge.
The latter correlation has the same slope as the well-known correlation between
supermassive black hole mass and bulge luminosity. The properties of both
nuclear clusters and black holes are therefore intimately connected to the
properties of the host galaxy.Comment: AJ submitted (original submission Nov 30, 2005, present version
includes changes based on referee recommendations). 69 pages, 16 figures, 7
table
Improved photometry of SDSS crowded field images: Structure and dark matter content in the dwarf spheroidal galaxy Leo I
We explore how well crowded field point-source photometry can be accomplished
with SDSS data: We present a photometric pipeline based on DoPhot, and tuned
for analyzing crowded-field images from the SDSS. Using Monte Carlo simulations
we show that the completeness of source extraction is above 80% to i < 21 (AB)
and a stellar surface density of about 200 sq.amin. Hence, a specialized data
pipeline can efficiently be used for e.g. nearby resolved galaxies in SDSS
images, where the standard SDSS photometric package Photo, when applied in
normal survey mode, gives poor results. We apply our pipeline to an area of
about 3.55sq.deg. around the dwarf spheroidal galaxy (dSph) Leo I, and
construct a high S/N star-count map of Leo I via an optimized filter in
color-magnitude space (g,r,i). Although the radial surface-density profile of
the dwarf deviates from the best fit empirical King model towards outer radii,
we find no evidence for tidal debris out to a stellar surface-density of
4*10^(-3) of the central value. We determine the total luminosity of Leo I, and
model its mass using the spherical and isotropic Jeans equation. Assuming that
'mass follows light' we constrain a lower limit of the total mass of the dSph
to be (1.7+/-0.2)*10^7 Msol. Contrary, if the mass in Leo I is dominated by a
constant density dark-matter (DM) halo, then the mass within the central 12' is
(2+/-0.6)*10^8 Msol. This leads to a mass-to-light ratio of >>6 (Ic_sol), and
possibly >75 if the DM halo dominates the mass and extends further out than
12'. In summary, our results show that Leo I is a symmetric, relaxed and bound
system; this supports the idea that Leo I is a dark-matter dominated system.Comment: 13 pages, 11 figures; accepted for publication in A
Spectroscopic Redshifts for Seven Lens Galaxies
We report VLT observations of 11 lensed quasars, designed to measure the
redshifts of their lens galaxies. We successfully determined the redshifts for
seven systems, five of which were previously unknown. The securely measured
redshifts for the lensing galaxies are: HE0047-1756 z=0.408; PMNJ0134-0931
z=0.766; HE0230-2130 z=0.522; HE0435-1223 z=0.455; SDSS0924+021 z=0.393;
LBQS1009-025 z=0.871; and WFIJ2033-472 z=0.658. For four additional systems
(BRI0952-0115, Q1017-207, Q1355-2257 and PMNJ1632-003) we estimate tentative
redshifts based on some features in their spectra.Comment: 8 pages, ApJ, submitte
The Tully-Fisher Relation of Barred Galaxies
We present new data exploring the scaling relations, such as the Tully-Fisher
relation (TFR), of bright barred and unbarred galaxies. A primary motivation
for this study is to establish whether barredness correlates with, and is a
consequence of, virial properties of galaxies. Various lines of evidence
suggest that dark matter is dominant in disks of bright unbarred galaxies at
2.2 disk scale lengths, the point of peak rotation for a pure exponential disk.
We test the hypothesis that the TF plane of barred high surface brightness
galaxies is offset from the mean TFR of unbarred galaxies, as might be expected
if barred galaxies are ``maximal'' in their inner parts. We use existing and
new TF data to search for basic structural differences between barred and
unbarred galaxies. Our new data consist of 2-dimensional Halpha velocity fields
derived from SparsePak integral field spectroscopy (IFS) and V,I-band CCD
images collected at the WIYN Observatory for 14 strongly barred galaxies. We
use WIYN/SparsePak (2-D) velocity fields to show that long-slit (1-D) spectra
yield reliable circular speed measurements at or beyond 2.2 disk scale lengths,
far from any influence of the bar. This enables us to consider line width
measurements from extensive TF surveys which include barred and nonbarred disks
and derive detailed scaling relation comparisons. We find that for a given
luminosity, barred and unbarred galaxies have comparable structural and
dynamical parameters, such as peak velocities, scale lengths, or colors. In
particular, the location of a galaxy in the TF plane is independent of
barredness. In a global dynamical sense, barred and unbarred galaxies behave
similarly and are likely to have, on average, comparable fractions of luminous
and dark matter at a given radius. (abridged)Comment: Accepted for publication in the ApJ (September 1, 2003 issue, v594).
Appendix figures with I-band image and superimposed 2-D velocity field plus
rotation curves must be downloaded separately (due to size constraints) from
http://www.astro.ubc.ca/people/courteau/public/courteau03_TFbars.ps.g
New Observations of the Interstellar Medium in the Lyman Break Galaxy MS 1512-cB58
We present the results of a detailed study of the interstellar medium of MS
1512-cB58, an L* Lyman break galaxy at z = 2.7276, based on new spectral
observations obtained with the Echelle Spectrograph and Imager on the Keck II
telescope at 58 km/s resolution. We focus in particular on the chemical
abundances and kinematics of the interstellar gas and our main findings are as
follows. Even at this relatively early epoch, the ISM of this galaxy is already
highly enriched in elements released by Type II supernovae; the abundances of
O, Mg, Si, P, and S are all about 2/5 of their solar values. In contrast, N and
the Fe-peak elements Mn, Fe, and Ni are underabundant by a factor of about 3.
Based on current ideas of stellar nucleosynthesis, these results can be
understood if most of the metal enrichment in cB58 has taken place within the
last 300 million years, the timescale for the release of N from intermediate
mass stars. cB58 appears to be an example of a galaxy in the process of
converting its gas into stars on a few dynamical timescales; quite possibly we
are witnessing the formation of a galactic bulge or an elliptical galaxy. The
energetic star formation activity has stirred the interstellar medium to high
velocities of up to 1000 km/s. The net effect is a bulk outflow of the ISM at a
speed of 255 km/s and at a rate which exceeds the star formation rate. It is
unclear whether this gas will be lost or retained by the galaxy. We point out
that the chemical and kinematic properties of cB58 are markedly different from
those of most damped Lyman alpha systems at the same redshift.Comment: 38 pages, LaTeX, 9 Postscript Figures. Accepted for publication in
the Astrophysical Journal. Sections 3.3 and 5.3 expanded, and two additional
figures included, following referee's repor
The Broad-Line and Narrow-Line Regions of the LINER NGC 4579
We report the discovery of an extremely broad H-alpha emission line in the
LINER nucleus of NGC 4579. From ground-based observations, the galaxy was
previously known to contain a Type 1 nucleus with a broad H-alpha line of FWHM
= 2300 km/s and FWZI ~ 5000 km/s. New spectra obtained with the Hubble Space
Telescope and a 0.2 arcsec-wide slit reveal an H-alpha component with FWZI ~
18,000 km/s. The line is not obviously double-peaked, but it does possess
shoulders on the red and blue sides which resemble the H-alpha profiles of
double-peaked emitters such as NGC 4203 and NGC 4450. This similarity suggests
that the very broad H-alpha profile in NGC 4579 may represent emission from an
accretion disk. Three such objects have been found recently in two HST programs
which have targeted a total of 30 galaxies, demonstrating that double-peaked or
extremely broad-line emission in LINERs must be much more common than would be
inferred from ground-based surveys. The ratio of the narrow [S II] 6716, 6731
lines shows a pronounced gradient indicating a steep rise in density toward the
nucleus. The direct detection of a density gradient within the inner arcsecond
of the narrow-line region confirms expectations from previous observations of
linewidth-critical density correlations in several LINERs.Comment: 8 pages, includes 3 figures. To appear in The Astrophysical Journa
The structural properties and star formation history of Leo T from deep LBT photometry
We present deep, wide-field g and r photometry of the transition type dwarf
galaxy Leo T, obtained with the blue arm of the Large Binocular Telescope. The
data confirm the presence of both very young (5
Gyr) stars. We study the structural properties of the old and young stellar
populations by preferentially selecting either population based on their color
and magnitude. The young population is significantly more concentrated than the
old population, with half-light radii of 104+-8 and 148+-16 pc respectively,
and their centers are slightly offset. Approximately 10% of the total stellar
mass is estimated to be represented by the young stellar population. Comparison
of the color-magnitude diagram (CMD) with theoretical isochrones as well as
numerical CMD-fitting suggest that star formation began over 10 Gyr ago and
continued in recent times until at least a few hundred Myr ago. The CMD-fitting
results are indicative of two distinct star formation bursts, with a quiescent
period around 3 Gyr ago, albeit at low significance. The results are consistent
with no metallicity evolution and [Fe/H] ~ -1.5 over the entire age of the
system. Finally, the data show little if any sign of tidal distortion of Leo T.Comment: 8 pages, 9 figures, some small textual changes, accepted for
publication in the Astrophysical Journa
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