237 research outputs found
New distances of unresolved dwarf elliptical galaxies in the vicinity of the Local Group
We present Surface Brightness Fluctuation distances of nine early-type dwarf
galaxies and the S0 galaxy NGC 4150 in the Local Volume based on deep B- and
R-band CCD images obtained with the 2.56 metre Nordic Optical Telescope.
Typically, six stellar fields at various galactocentric distances have been
chosen for each galaxy as appropriately free of foreground stars and other
contaminants, and Fourier analysed to determine the distances, which are found
to lie in the range of 3 to 16 Mpc. The SBF method is thus demonstrated to
efficiently measure distances from the ground with mid-aperture telescopes for
galaxies for which only the tip of the red giant branch method in combination
with the Hubble Space Telescope has been available until now. We obtained the
following distance moduli: 28.11 +/- 0.15 mag (or 4.2 +/- 0.3 Mpc) for UGC
1703, 27.61 +/- 0.17 mag (or 3.3 +/- 0.3 Mpc) for KDG 61, 29.00 +/- 0.27 mag
(or 6.3 +/- 0.8 Mpc) for UGCA 200, 27.74 +/- 0.18 mag (or 3.5 +/- 0.3 Mpc) for
UGC 5442, 30.22 +/- 0.17 mag (or 11.1 +/- 0.9 Mpc) for UGC 5944, 30.79 +/- 0.11
mag (or 14.4 +/- 0.7 Mpc) for NGC 4150, 31.02 +/- 0.25 mag (or 16.0 +/- 1.9
Mpc) for BTS 128, 29.27 +/- 0.16 mag (or 7.1 +/- 0.6 Mpc) for UGC 7639, 30.19
+/- 0.23 mag (or 10.9 +/- 1.2 Mpc) for UGC 8799 with an alternative distance of
30.61 +/- 0.26 mag (or 13.2 +/- 1.7 Mpc), and 29.60 +/- 0.20 mag (or 8.3 +/-
0.8 Mpc) for UGC 8882.Comment: 14 pages, 13 figures (figures 1, 2 and 10 are of reduced quality),
accepted for publication in A&
Deep Near-Infrared Surface Photometry and Properties of Local Volume Dwarf Irregular Galaxies
We present deep H-band surface photometry and analysis of 40 Local Volume
galaxies, a sample primarily composed of dwarf irregulars in the Cen A group,
obtained using the IRIS2 detector at the 3.9m Anglo-Australian Telescope. We
probe to a surface brightness of ~25 mag arcsec, reaching a 40 times
lower stellar density than the Two Micron All Sky Survey (2MASS). Employing
extremely careful and rigorous cleaning techniques to remove contaminating
sources, we perform surface photometry on 33 detected galaxies deriving the
observed total magnitude, effective surface brightness and best fitting
S\'ersic parameters. We make image quality and surface photometry comparisons
to 2MASS and VISTA Hemispheric Survey (VHS) demonstrating that deep targeted
surveys are still the most reliable means of obtaining accurate surface
photometry. We investigate the B-H colours with respect to mass for Local
Volume galaxies, finding that the colours of dwarf irregulars are significantly
varied, eliminating the possibility of using optical-NIR colour transformations
to facilitate comparison to the more widely available optical data sets. The
structure-luminosity relationships are investigated for our `clean' sample of
dwarf irregulars. We demonstrate that a significant fraction of the Local
Volume dwarf irregular population have underlying structural properties similar
to both Local Volume and Virgo Cluster dwarf ellipticals. Linear regressions to
structure-luminosity relationships for the Local Volume galaxies and Virgo
Cluster dwarf ellipticals show significant differences in both slope and
scatter around the established trend lines, suggesting that environment might
regulate the structural scaling relationships of dwarf galaxies in comparison
to their more isolated counterparts.Comment: 27 pages, 14 figures, 5 table
Photometric Properties of Six Local Volume Dwarf Galaxies from Deep Near-Infrared Observations
We have obtained deep near-infrared - (1.25 m), - (1.65m)
and -band (2.15 m) imaging for a sample of six dwarf galaxies
(M_B\ga-17 mag) in the Local Volume (LV, D\la10 Mpc). The sample consists
mainly of early-type dwarf galaxies found in various environments in the LV.
Two galaxies (LEDA 166099 and UGCA 200) in the sample are detected in the
near-infrared for the first time. The deep near-infrared images allow for a
detailed study of the photometric and structural properties of each galaxy. The
surface brightness profiles of the galaxies are detected down to the ~ isophote in the - and -bands, and in
the -band. The total magnitudes of the galaxies are derived in the three
wavelength bands. For the brightest galaxies (M_B\la-15.5 mag) in the sample,
we find that the Two Micron All Sky Survey (2MASS) underestimates the total
magnitudes of these systems by up to \la0.5 mag. The radial surface
brightness profiles of the galaxies are fitted with an exponential (for those
galaxies having a stellar disk) or S\'ersic law to derive the structure of the
underlying stellar component. In particular, the effective surface brightness
() and effective radius () are determined from the analytic fits to
the surface brightness profile. The - colours for the galaxies have
been measured to explore the luminosity-metallicity relation for early-type
dwarfs. In addition, the - colours of the galaxies are used to assess
their evolutionary state relative to other galaxy morphologies. The total
stellar masses of the dwarf galaxies are derived from the -band photometric
measurements. These will later be compared to the dynamical mass estimates for
the galaxies to determine their dark matter content.Comment: 14 pages, 8 figures, submitted to MNRA
Discovery of a high-redshift Einstein ring
We report the discovery of a partial Einstein ring of radius 1.48arcsec
produced by a massive (and seemingly isolated) elliptical galaxy. The
spectroscopic follow-up at the VLT reveals a 2L* galaxy at z=0.986, which is
lensing a post-starburst galaxy at z=3.773. This unique configuration yields a
very precise measure of the mass of the lens within the Einstein radius,
(8.3e11 +- 0.4)/h70 Msolar. The fundamental plane relation indicates an
evolution rate of d [log (M/L)B] / dz = -0.57+-0.04, similar to other massive
ellipticals at this redshift. The source galaxy shows strong interstellar
absorption lines indicative of large gas-phase metallicities, with fading
stellar populations after a burst. Higher resolution spectra and imaging will
allow the detailed study of an unbiased representative of the galaxy population
when the universe was just 12% of its current age.Comment: 5 pages, 3 figures, accepted in A&A Le
Detection of neutral hydrogen in early-type dwarf galaxies of the Sculptor Group
We present our results of deep 21 cm line (HI) observations of five early and
mixed-type dwarf galaxies in the nearby Sculptor group using the ATNF 64m
Parkes Radio Telescope. Four of these objects, ESO294-G010, ESO410-G005,
ESO540-G030, and ESO540-G032, were detected in HI with neutral hydrogen masses
in the range of 2-9x10^5 M_{\odot} ( = 0.08, 0.13, 0.16, and
0.18, respectively). These HI masses are consistent with the gas mass expected
from stellar outflows over a large period of time. Higher resolution radio data
from the Australia Telescope Compact Array were further analysed to measure
more accurate positions and the distribution of the HI gas. In the cases of
dwarfs ESO294-G010 and ESO540-G030, we find significant offsets of 290 pc and
460 pc, respectively, between the position of the HI peak flux and the center
of the stellar component. These offsets are likely to have internal cause such
as the winds from star-forming regions. The fifth object, the spatially
isolated dwarf elliptical Scl-dE1, remains undetected at our 3\sigma limit of
22.5 mJy km/s and thus must contain less than 10^5 M_{\odot} of neutral
hydrogen. This leaves Scl-dE1 as the only Sculptor group galaxy known where no
interstellar medium has been found to date. The object joins a list of similar
systems including the Local Group dwarfs Tucana and Cetus that do not fit into
the global picture of the morphology-density relation where gas-rich dwarf
irregulars are in relative isolation and gas-deficient dwarf ellipticals are
satellites of more luminous galaxies.Comment: 21 pages, 4 figures, to be published in AJ (accepted
Tidal Remnants and Intergalactic HII Regions
We report the discovery of two small intergalactic HII regions in the loose
group of galaxies around the field elliptical NGC 1490. The HII regions are
located at least 100 kpc from any optical galaxy but are associated with a
number of large HI clouds that are lying along an arc 500 kpc in length and
that have no optical counterpart on the Digital Sky Survey. The sum of the HI
masses of the clouds is almost 10^10 M_sun and the largest HI cloud is about
100 kpc in size. Deep optical imaging reveals a very low surface brightness
counterpart to this largest HI cloud, making this one of the HI richest optical
galaxies known (M_HI/L_V~200). Spectroscopy of the HII regions indicates that
the abundance in these HII regions is only slightly sub-solar, excluding a
primordial origin of the HI clouds. The HI clouds are perhaps remnants
resulting from the tidal disruption of a reasonably sized galaxy, probably
quite some time ago, by the loose group to which NGC1490 belongs.
Alternatively, they are remnants of the merger that created the field
elliptical NGC1490. The isolated HII regions show that star formation on a very
small scale can occur in intergalactic space in gas drawn from galaxies by
tidal interactions. Many such intergalactic small star formation regions may
exist near tidally interacting galaxies.Comment: To appear in the proceedings of the IAU Symposium #217, Recycling
Intergalactic and Interstellar Matter, eds. P.-A. Duc, J. Braine, and E.
Brinks, 6 pages with low resolution figures. The full paper with high
resolution images can be downloaded from
http://www.astron.nl/~morganti/Papers/cloud.ps.g
Multi-wavelength Hubble Space Telescope photometry of stellar populations in NGC288
We present new UV observations for NGC288, taken with the WFC3 detector on
board the Hubble Space Telescope, and combine them with existing optical data
from the archive to explore the multiple-population phenomenon in this globular
cluster (GC). The WFC3's UV filters have demonstrated an uncanny ability to
distinguish multiple populations along all photometric sequences in GCs, thanks
to their exquisite sensitivity to the atmospheric changes that are tell-tale
signs of second-generation enrichment. Optical filters, on the other hand, are
more sensitive to stellar-structure changes related to helium enhancement. By
combining both UV and optical data we can measure helium variation. We quantify
this enhancement for NGC288 and find that its variation is typical of what we
have come to expect in other clusters.Comment: 15 pages, 5 figures, accepted for publication in Ap
HI Detection in two Dwarf S0 Galaxies in Nearby Groups: ESO384-016 and NGC 59
An \hi survey of 10 dE/dS0 galaxies in the nearby Sculptor and Centaurus A
groups was made using the Australia Telescope Compact Array (ATCA). The
observed galaxies have accurate distances derived by Jerjen et al (1998; 2000b)
using the surface brightness fluctuation technique. Their absolute magnitudes
are in the range . Only two of the ten galaxies were
detected at our detection limit ( \msol for the Centaurus
group and \msol for the Sculptor group), the two dS0
galaxies ESO384-016 in the Centaurus A Group and NGC 59 in the Sculptor Group,
with \hi masses of \msol and \msol respectively. Those two detections were confirmed using the Green
Bank Telescope. These small \hi reservoirs could fuel future generations of low
level star formation and could explain the bluer colors seen at the center of
the detected galaxies. Similarly to what is seen with the Virgo dEs, the two
objects with \hi appear to be on the outskirt of the groups.Comment: 25 pages (11 figures), accepted by A
Internal Dynamics, Structure and Formation of Dwarf Elliptical Galaxies: II. Rotating Versus Non-Rotating Dwarfs
We present spatially-resolved internal kinematics and stellar chemical
abundances for a sample of dwarf elliptical (dE) galaxies in the Virgo Cluster
observed with Keck/ESI. We find that 4 out of 17 dEs have major axis rotation
velocities consistent with rotational flattening, while the remaining dEs have
no detectable major axis rotation. Despite this difference in internal
kinematics, rotating and non-rotating dEs are remarkably similar in terms of
their position in the Fundamental Plane, morphological structure, stellar
populations, and local environment. We present evidence for faint underlying
disks and/or weak substructure in a fraction of both rotating and non-rotating
dEs, but a comparable number of counter-examples exist for both types which
show no evidence of such structure. Absorption-line strengths were determined
based on the Lick/IDS system (Hbeta, Mgb, Fe5270, Fe5335) for the central
region of each galaxy. We find no difference in the line-strength indices, and
hence stellar populations, between rotating and non-rotating dE galaxies. The
best-fitting mean age and metallicity for our 17 dE sample are 5 Gyr and Fe/H =
-0.3 dex, respectively, with rms spreads of 3 Gyr and 0.1 dex. The majority of
dEs are consistent with solar alpha/Fe abundance ratios. By contrast, the
stellar populations of classical elliptical galaxies are, on average, older,
more metal rich, and alpha-enhanced relative to our dE sample. The local
environments of both dEs types appear to be diverse in terms of their proximity
to larger galaxies in real or velocity space within the Virgo Cluster. Thus,
rotating and non-rotating dEs are remarkably similar in terms of their
structure, stellar content, and local environments, presenting a significant
challenge to theoretical models of their formation. (abridged)Comment: 33 pages, 12 figures. To appear in the October 2003 Astronomical
Journal. See http://www.ucolick.org/~mgeha/geha_dE.ps.gz for version with
high resolution figure
Multiple stellar populations in Magellanic Cloud clusters. III. The first evidence of an extended main sequence turn-off in a young cluster: NGC1856
Recent studies have shown that the extended main-sequence turn off (eMSTO) is
a common feature of intermediate-age star clusters in the Magellanic Clouds
(MCs). The most simple explanation is that these stellar systems harbor
multiple generations of stars with an age difference of a few hundred Myrs.
However, while an eMSTO has been detected in a large number of clusters with
ages between ~1-2 Gyrs, several studies of young clusters in both MCs and in
nearby galaxies do not find any evidence for a prolonged star-formation
history, i.e. for multiple stellar generations. These results have suggested
alternative interpretation of the eMSTOs observed in intermediate-age star
clusters. The eMSTO could be due to stellar rotation mimicking an age spread or
to interacting binaries. In these scenarios, intermediate-age MC clusters would
be simple stellar populations, in close analogy with younger clusters.
Here we provide the first evidence for an eMSTO in a young stellar cluster.
We exploit multi-band Hubble Space Telescope photometry to study the ~300-Myr
old star cluster NGC1856 in the Large Magellanic Cloud and detected a broadened
MSTO that is consistent with a prolonged star-formation which had a duration of
about 150 Myrs. Below the turn-off, the MS of NGC1856 is split into a red and
blue component, hosting 33+/-5% and 67+/-5% of the total number of MS stars,
respectively. We discuss these findings in the context of
multiple-stellar-generation, stellar-rotation, and interacting-binary
hypotheses.Comment: 14 pages, 10 figures, accepted for publication in MNRA
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