236 research outputs found
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
A New Galaxy in the Local Group: the Antlia Dwarf Galaxy
We report the discovery of new member of the Local Group in the constellation
of Antlia. Optically the system appears to be a typical dwarf spheroidal galaxy
of type dE3.5 with no apparent young blue stars or unusual features. A
color-magnitude diagram in I, V-I shows the tip of the red giant branch, giving
a distance modulus of 25.3 +/- 0.2 (1.15 Mpc +/- 0.1) and a metallicity of -1.6
+/- 0.3. Although Antlia is in a relatively isolated part of the Local Group it
is only 1.2 degrees away on the sky from the Local Group dwarf NGC3109, and may
be an associated system.Comment: AJ in press, 15 pages, 7 figures, figure 2 in b/w for space saving,
full postscript version available at
http://www.ast.cam.ac.uk/~gkth/antlia-pp.htm
An Observational Limit on the Dwarf Galaxy Population of the Local Group
We present the results of an all-sky, deep optical survey for faint Local
Group dwarf galaxies. Candidate objects were selected from the second Palomar
survey (POSS-II) and ESO/SRC survey plates and follow-up observations performed
to determine whether they were indeed overlooked members of the Local Group.
Only two galaxies (Antlia and Cetus) were discovered this way out of 206
candidates. Based on internal and external comparisons, we estimate that our
visual survey is more than 77% complete for objects larger than one arc minute
in size and with a surface brightness greater than an extremely faint limit
over the 72% of the sky not obstructed by the Milky Way. Our limit of
sensitivity cannot be calculated exactly, but is certainly fainter than 25
magnitudes per square arc second in R, probably 25.5 and possibly approaching
26. We conclude that there are at most one or two Local Group dwarf galaxies
fitting our observational criteria still undiscovered in the clear part of the
sky, and a roughly a dozen hidden behind the Milky Way. Our work places the
"missing satellite problem" on a firm quantitative observational basis. We
present detailed data on all our candidates, including surface brightness
measurements.Comment: 58 pages in AJ manuscript format; some figures at slightly reduced
quality; accepted by the Astronomical Journa
Observational evidence for a connection between supermassive black holes and dark matter haloes
We present new velocity dispersion measurements of sample of 12 spiral
galaxies for which extended rotation curves are available. These data are used
to refine a recently discovered correlation between the circular velocity and
the central velocity dispersion of spiral galaxies. We find a slightly steeper
slope for our larger sample, we confirm the negligible intrinsic scatter on
this correlation, and we find a striking agreement with a corresponding
relation for elliptical galaxies. We combine this correlation with the
well-known MBH-sigma relation to obtain a tight correlation between the
circular velocities of galaxies and the masses of the supermassive black holes
they host. This correlation is the observational evidence for an intimate link
between dark matter haloes and supermassive black holes. Apart from being an
important ingredient for theoretical models of galaxy formation and evolution,
the relation between MBH and circular velocity can serve as a practical tool to
estimate black hole masses in spiral galaxies.Comment: 5 pages, 2 figures, accepted for publication in MNRAS pink page
Discovery of an optical counterpart to the hyperluminous X-ray source in ESO 243-49
The existence of black holes of masses ~ 10^2-10^5 Msun has important
implications for the formation and evolution of star clusters and supermassive
black holes. One of the strongest candidates to date is the hyperluminous X-ray
source HLX1, possibly located in the S0-a galaxy ESO243-49, but the lack of an
identifiable optical counterpart had hampered its interpretation. Using the
Magellan telescope, we have discovered an unresolved optical source with R =
(23.80 +/- 0.25) mag and V = (24.5 +/- 0.3) mag within HLX1's positional error
circle. This implies an average X-ray/optical flux ratio ~ 500. Taking the same
distance as ESO243-49, we obtain an intrinsic brightness M_R = (-11.0 +/- 0.3)
mag, comparable to that of a massive globular cluster. Alternatively, the
optical source is consistent with a main-sequence M star in the Galactic halo
(for example an M4.4 star at ~ 2.5 kpc). We also examined the properties of
ESO243-49 by combining Swift/UVOT observations with stellar population
modelling. We found that the overall emission is dominated by a ~5 Gyr old
stellar population, but the UV emission at ~2000 Ang is mostly due to ongoing
star-formation at a rate of ~ 0.03 Msun/yr. The UV emission is more intense (at
least a 9-sigma enhancement above the mean) North East of the nucleus, in the
same quadrant as HLX1. With the combined optical and X-ray measurements, we put
constraints on the nature of HLX1. We rule out a foreground star and a
background AGN. Two alternative scenarios are still viable. HLX1 could be an
accreting intermediate-mass black hole in a star cluster, which may itself be
the stripped nucleus of a dwarf galaxy that passed through ESO243-49, an event
which might have caused the current episode of star formation. Or, it could be
a neutron star in the Galactic halo, accreting from an M4-M5 donor star.Comment: 7 pages, accepted by MNRAS. Several improvements from Oct 7 version:
stronger evidence of the optical counterpart; more accurate estimate of its
brightness (a factor of 2 brighter than previously estimated); use of a
larger set of Swift/UVOT data to measure the recent star formation rate in
ESO243-49; improved discussion and comparison of the competing scenario
GASP III. JO36: a case of multiple environmental effects at play?
The so-called jellyfish galaxies are objects exhibiting disturbed morphology,
mostly in the form of tails of gas stripped from the main body of the galaxy.
Several works have strongly suggested ram pressure stripping to be the
mechanism driving this phenomenon. Here, we focus on one of these objects,
drawn from a sample of optically selected jellyfish galaxies, and use it to
validate SINOPSIS, the spectral fitting code that will be used for the analysis
of the GASP (GAs Stripping Phenomena in galaxies with MUSE) survey, and study
the spatial distribution and physical properties of gas and stellar populations
in this galaxy. We compare the model spectra to those obtained with GANDALF, a
code with similar features widely used to interpret the kinematic of stars and
gas in galaxies from IFU data. We find that SINOPSIS can reproduce the
pixel-by-pixel spectra of this galaxy at least as good as GANDALF does,
providing reliable estimates of the underlying stellar absorption to properly
correct the nebular gas emission. Using these results, we find strong evidences
of a double effect of ram pressure exerted by the intracluster medium onto the
gas of the galaxy. A moderate burst of star formation, dating between 20 and
500 Myr ago and involving the outer parts of the galaxy more strongly than the
inner regions, was likely induced by a first interaction of the galaxy with the
intracluster medium. Stripping by ram pressure, plus probable gas depletion due
to star formation, contributed to create a truncated ionized gas disk. The
presence of an extended stellar tail on only one side of the disk, points
instead to another kind of process, likely a gravitational interaction by a
fly-by or a close encounter with another galaxy in the cluster.Comment: ApJ in press, 26 pages, 18 figure
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