4,663 research outputs found
The Kinematics of the Ultra-Faint Milky Way Satellites: Solving the Missing Satellite Problem
We present Keck/DEIMOS spectroscopy of stars in 8 of the newly discovered
ultra-faint dwarf galaxies around the Milky Way. We measure the velocity
dispersions of Canes Venatici I and II, Ursa Major I and II, Coma Berenices,
Hercules, Leo IV and Leo T from the velocities of 18 - 214 stars in each galaxy
and find dispersions ranging from 3.3 to 7.6 km/s. The 6 galaxies with absolute
magnitudes M_V < -4 are highly dark matter-dominated, with mass-to-light ratios
approaching 1000. The measured velocity dispersions are inversely correlated
with their luminosities, indicating that a minimum mass for luminous galactic
systems may not yet have been reached. We also measure the metallicities of the
observed stars and find that the 6 brightest of the ultra-faint dwarfs extend
the luminosity-metallicity relationship followed by brighter dwarfs by 2 orders
of magnitude in luminosity; several of these objects have mean metallicities as
low as [Fe/H] = -2.3 and therefore represent some of the most metal-poor known
stellar systems. We detect metallicity spreads of up to 0.5 dex in several
objects, suggesting multiple star formation epochs. Having established the
masses of the ultra-faint dwarfs, we re-examine the missing satellite problem.
After correcting for the sky coverage of the SDSS, we find that the ultra-faint
dwarfs substantially alleviate the discrepancy between the predicted and
observed numbers of satellites around the Milky Way, but there are still a
factor of ~4 too few dwarf galaxies over a significant range of masses. We show
that if galaxy formation in low-mass dark matter halos is strongly suppressed
after reionization, the simulated circular velocity function of CDM subhalos
can be brought into approximate agreement with the observed circular velocity
function of Milky Way satellite galaxies. [slightly abridged]Comment: 22 pages, 15 figures (12 in color), 6 tables, minor revisions in
response to referee report. Accepted for publication in Ap
HST Observations and Models of The Gravitational Lens System MG 0414+0534
Quadruple gravitational lens systems offer the possibility of measuring time
delays for image pairs, microlensing effects, and extinction in distant
galaxies. Observations of these systems may be used to obtain estimates of H_o
and to study the various mass components of lens galaxies at high redshifts.
With HST, we have observed the reddest known gravitational lens system, MG
0414+0534. We used WFPC2/PC1 to obtain deep, high-resolution images with two
filters, F675W and F814W. We present a detailed analysis of all of the
components, as well as macrolens models. Our main results are: (1) confirmation
that MG 0414+0534 is inescapably a gravitational lens system; (2) discovery of
a blue arc connecting the 3 brightest images of the QSO central core; (3)
accurate positions and apparent brightnesses for all 4 known images of the QSO
central core and for the lens galaxy G; (4) a good representation of the
brightness distribution of G by elliptical isophotes with a De Vaucouleurs
profile, characteristic of an elliptical galaxy; (5) models that consist of
simple elliptical potentials and account qualitatively, not quantitatively, for
the HST image positions, arc morphology and radio flux ratios for the images of
the QSO central core; (6) a possible new test to distinguish between reddening
in the host galaxy of the QSO and in the lens galaxy, based on future accurate
measurements of spatial variations in the color of the arc; and (7) the
suggestion that microlensing is a plausible cause for the differences between
the radio and optical flux ratios for the brightest images, A1 and A2. Further
observations and measurements such as of the redshift of the lens galaxy, can
be used fruitfully to study microlensing for this system.Comment: 27 pages, 8 .ps figs, AAS Latex, AJ, in press, Feb 199
Galactic Archeology with 4MOST
4MOST is a new wide-field, high-multiplex spectroscopic survey facility for
the VISTA telescope of ESO. Starting in 2022, 4MOST will deploy more than 2400
fibres in a 4.1 square degree field-of-view using a positioner based on the
tilting spine principle. In this ontribution we give an outline of the major
science goals we wish to achieve with 4MOST in the area of Galactic Archeology.
The 4MOST Galactic Archeology surveys have been designed to address
long-standing and far-reaching problems in Galactic science. They are focused
on our major themes: 1) Near-field cosmology tests, 2) Chemo-dynamical
characterisation of the major Milky Way stellar components, 3) The Galactic
Halo and beyond, and 4) Discovery and characterisation of extremely metal-poor
stars. In addition to a top-level description of the Galactic surveys we
provide information about how the community will be able to join 4MOST via a
call for Public Spectroscopic Surveys that ESO will launch.Comment: To be published in "Rediscovering our Galaxy", IAU Symposium 334,
Eds. C. Chiappini, I. Minchev, E. Starkenburg, M. Valentin
The elusive old population of the dwarf spheroidal galaxy Leo I
We report the discovery of a significant old population in the dwarf
spheroidal (dSph) galaxy Leo I as a result of a wide-area search with the ESO
New Technology Telescope. Studies of the stellar content of Local Group dwarf
galaxies have shown the presence of an old stellar population in almost all of
the dwarf spheroidals. The only exception was Leo I, which alone appeared to
have delayed its initial star formation episode until just a few Gyr ago. The
color-magnitude diagram of Leo I now reveals an extended horizontal branch,
unambiguously indicating the presence of an old, metal-poor population in the
outer regions of this galaxy. Yet we find little evidence for a stellar
population gradient, at least outside R > 2' (0.16 kpc), since the old
horizontal branch stars of Leo I are radially distributed as their more
numerous intermediate-age helium-burning counterparts. The discovery of a
definitely old population in the predominantly young dwarf spheroidal galaxy
Leo I points to a sharply defined first epoch of star formation common to all
of the Local Group dSph's as well as to the halo of the Milky Way.Comment: 4 pages, 3 postscript figures, uses apjfonts.sty, emulateapj.sty.
Accepted for publication in ApJ Letter
X-ray Spectral Properties of Low-Mass X-ray Binaries in Nearby Galaxies
We investigated the X-ray spectral properties of a collection of low-mass
X-ray binaries (LMXBs) within a sample of 15 nearby early-type galaxies
observed with Chandra. We find that the spectrum of the sum of the sources in a
given galaxy is remarkably similar from galaxy to galaxy when only sources with
X-ray luminosities less than 10^39 ergs/s (0.3-10 keV) are considered. Fitting
these lower luminosity sources in all galaxies simultaneously yielded a
best-fit power law exponent of Gamma = 1.56 +/- 0.02 (or kT_brem = 7.3 +/- 0.3
keV). This is the tightest constraint to date on the spectral properties of
LMXBs in external galaxies. There is no apparent difference in the spectral
properties of LMXBs that reside inside and outside globular clusters. We
demonstrate how the uniformity of the spectral properties of LMXBs can lead to
more accurate determinations of the temperature and metallicity of the hot gas
in galaxies. Although few in number in any given galaxy, sources with
luminosities of 1-2 x 10^39 ergs/s are present in 10 of the galaxies. The
spectra of these luminous sources are softer than the spectra of the rest of
the sources, and are consistent with the spectra of Galactic black hole X-ray
binary candidates when they are in their very high state. The spectra of these
sources are very different than those of ultraluminous X-ray sources (ULXs)
found within spiral galaxies, suggesting that the two populations of X-ray
luminous objects have different formation mechanisms. The number of sources
with apparent luminosities above 2 x 10^39 ergs/s is equal to the number of
expected background AGN and thus appear to not be associated with the galaxy,
indicating that very luminous sources are absent or very rare in early-type
galaxies. (Abridged)Comment: 11 pages in emulateapj5 style with 4 embedded Postscript figures; to
be accepted by Ap
Orbits of Globular Clusters in the Outer Galaxy: NGC 7006
We present a proper motion study of the distant globular cluster NGC 7006
based on the measurement of 25 photographic plates spanning a 40-year interval.
The absolute proper motion determined with respect to extragalactic objects is
(-0.96, -1.14) +- (0.35, 0.40) mas/yr. The total space velocity of NGC 7006 in
a Galactocentric rest frame is 279 km/s, placing the cluster on one of the most
energetic orbits (Ra =102 kpc) known to date for clusters within 40-kpc from
the Galactic center. We compare the orbits of four clusters that have
apocentric radii larger than 80 kpc (NGC 5466, NGC 6934, NGC 7006 and Pal 13)
with those of Galactic satellites with well-measured proper motions. These
clusters have orbits that are highly eccentric and of various inclinations with
respect to the Galactic plane. In contrast, the orbits of the Galactic
satellites are of low to moderate eccentricity and highly inclined. Based on
orbit types, chemical abundances and cluster parameters, we discuss the
properties of the hypothetical host systems of the remote globular clusters in
the Searle-Zinn paradigm. It is apparent that clusters such as NGC 5466, NGC
6934 and NGC 7006 formed in systems that more likely resemble the Fornax dSph,
rather than the Sagittarius dSph. We also discuss plausible causes for the
difference found so far between the orbit type of outer halo clusters and that
of Galactic satellites and for the tentative, yet suggestive phase-space
scatter found among outer halo clusters.Comment: 27 pages, 5 figures, to be published in the Astronomical Journa
A trio of new Local Group galaxies with extreme properties
We report on the discovery of three new dwarf galaxies in the Local Group.
These galaxies are found in new CFHT/MegaPrime g,i imaging of the south-western
quadrant of M31, extending our extant survey area to include the majority of
the southern hemisphere of M31's halo out to 150 kpc. All these galaxies have
stellar populations which appear typical of dwarf spheroidal (dSph) systems.
The first of these galaxies, Andromeda XVIII, is the most distant Local Group
dwarf discovered in recent years, at ~1.4 Mpc from the Milky Way (~ 600 kpc
from M31). The second galaxy, Andromeda XIX, a satellite of M31, is the most
extended dwarf galaxy known in the Local Group, with a half-light radius of r_h
~ 1.7 kpc. This is approximately an order of magnitude larger than the typical
half-light radius of many Milky Way dSphs, and reinforces the difference in
scale sizes seen between the Milky Way and M31 dSphs (such that the M31 dwarfs
are generally more extended than their Milky Way counterparts). The third
galaxy, Andromeda XX, is one of the faintest galaxies so far discovered in the
vicinity of M31, with an absolute magnitude of order M_V ~ -6.3. Andromeda
XVIII, XIX and XX highlight different aspects of, and raise important questions
regarding, the formation and evolution of galaxies at the extreme faint-end of
the luminosity function. These findings indicate that we have not yet sampled
the full parameter space occupied by dwarf galaxies, although this is an
essential pre-requisite for successfully and consistently linking these systems
to the predicted cosmological dark matter sub-structure.Comment: 32 pages, 7 figures (ApJ preprint format). Accepted for publication
in Ap
Discovery of an Optical Jet in the BL Lac Object 3C 371
We have detected an optical jet in the BL Lac object 3C 371 that coincides
with the radio jet in this object in the central few kpc. The most notable
feature is a bright optical knot 3 arcsec (4 kpc) from the nucleus that occurs
at the location where the jet apparently changes its direction by ~30 degrees.
The radio, near-infrared and optical observations of this knot are consistent
with a single power-law spectrum with a radio-optical spectral index alpha =
-0.81. One possible scenario for the observed turn is that the jet is
interacting with the material in the bridge connecting 3C 371 to nearby
galaxies and the pressure gradient is deflecting the jet significantly.Comment: 11 pages, LaTeX, 4 figures (1 eps, 3 gifs), accepted for publication
in ApJ Letter
The Draco and Ursa Minor dwarf spheroidals. A comparative study
We present (V,I) photometry of two wide (~25 X 25 arcmin^2) fields centered
on the low surface brightness dwarf spheroidal galaxies Draco and Ursa Minor.
New estimates of the distance to these galaxies are provided
((m-M)_0(UMi)=19.41 \pm 0.12 and (m-M)_0(Dra)=19.84 \pm 0.14) and a comparative
study of their evolved stellar population is presented. We detect for the first
time the RGB-bump in the Luminosity Function of UMi (V_{RGB}^{Bump}=19.40\pm
0.06) while the feature is not detected in Draco. Photometric metallicity
distributions are obtained for the two galaxies and an accurate analysis to
determine the intrinsic metallicity spread is performed by means of artificial
stars experiments. (Abridged) We demonstrate that the inner region of UMi is
significantly structured, at odds with what expected for a system in dynamical
equilibrium. In particular we show that the main density peak of UMi is
off-centered with respect to the center of symmetry of the whole galaxy and it
shows a much lower ellipticity with respect to the rest of the galaxy.
Moreover, UMi stars are shown to be clustered according to two different
characteristic clustering scales, as opposite to Draco, which instead has a
very symmetric and smooth density profile. The possible consequences of this
striking structural difference on our ideas about galaxy formation are briefly
discussed. Combining our distance modulus with the more recent estimates of the
total luminosity of UMi, we find that the mass to light (M/L) ratio of this
galaxy may be as low as M/L ~ 7, a factor 5-10 lower than current estimates.Comment: 31 pages, 18 .ps figures. AAS Latex. emulateapj5.sty macro included.
Fig. 3 is provided in a low res. version. The full resolution version is
available upon request to [email protected]. Accepted by The
Astronomical Journa
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