120 research outputs found
Past and Future of CG J1720-67.8: Constraints from Observations and Models
We discuss the evolution of the peculiar, nearby (z = 0.045), compact galaxy
group CG J1720-67.8, by interpreting a large amount of observational
information on the basis of our recent results from spectrophotometric
evolutionary synthesis models and new N-body/SPH simulations. The group, that
is composed of two spiral galaxies with a mass ratio approximately 4:1 and an
S0 galaxy in a particularly compact configuration, is undergoing an active
pre-merging phase. Several tidal features are signposts of the complex dynamics
of the system. We suggest that the observed structure of the tidal features can
be explained only if all three galaxies are involved in a strong interaction
process.Comment: 5 pages, 3 (degraded) figures. Proc. ESO Workshop "Groups of galaxies
in the nearby Universe", Santiago, Chile, 5-9 Dec. 2005, ESO Astrophysics
Symposia, eds. I. Saviane, V. Ivanov & J. Borissova, Springer-Verla
An uncatalogued optical HII region at the outskirts of the Galaxy
We present NOT optical observations of a clump (l = 127.9435 deg, b = +1.8298
deg), embedded in an extended, irregularly shaped, diffuse optical nebula. This
condensation shows an emission-line spectrum typical of classic HII regions.
Although its location on the sky coincides with a nearby extended photoionized
region recently identified by Cichowolski et al. (2003) in radio data from the
Canadian Galactic Plane Survey (CGPS), the systemic velocity of this ~ 1
arcmin-sized HII region, V_LSR = -7112 km/s, poses it far out in the
Galaxy, beyond the Perseus arm. The location of this region in the Galaxy is
supported by HI structures visible at comparable radial velocity on CGPS data.
We argue that this HII region might belong to an outer Galactic arm. The
emission line ratios of the surrounding extended nebula, whose radial velocity
is consistent with that of the small HII region, are typical of photoionized
gas in the low density limit. Smaller clumps of comparable surface brightness
are visible within the optical boundaries of the extended, faint nebula. After
comparison of the optical data with far infrared and radio observations, we
conclude that this nebula is an HII region, ~ 70 pc in size, probably
photoionized by an association of OB stars and surrounded by a ring of neutral
hydrogen.Comment: 8 pages, 9 figures (some of which degraded due to size constraints),
accepted on 2004/02/12 for publication in Astronomy and Astrophysic
Dynamics and star formation activity of CG J1720-67.8 unveiled through integral field spectroscopy and radio observations
CG J1720-67.8 is an ultra compact group of several galaxies with a low
velocity dispersion, and displaying the hallmarks of mutual interaction and
possible tidal dwarf galaxy formation. In hierarchical models, the system is a
possible precursor to a massive elliptical galaxy. In this paper, we use new
optical integral field spectroscopic and radio observations to investigate the
evolutionary status of the group in more detail: global star-formation rates
are estimated using H and 1.4 GHz radio continuum measurements; H {\sc
i} observations provide an upper limit to the global neutral gas content;
optical broadband colours and spectra provide ages and stellar mass estimates
for the tidal dwarf candidates; the bidimensional H velocity field is
used to trace the kinematics of the group and its members, which are compared
with numerical simulations of galaxy encounters. The observations suggest a
model in which multiple interactions have occurred, with the latest strong
encounter involving at least two major components within the last 200 Myr.
Debris from the encounter fuels ongoing star formation at the global level of
M yr, with self-gravity within the tidal tail
possibly providing a mechanism to enhance the star formation rate of the tidal
dwarf candidates, with bursts of star-formation in clumps of mass M appearing within the last 10 Myr. The amount of time required
for final merging of all group components remains uncertain.Comment: 22 pages, 16 figures. Accepted for publication in MNRA
A New Ultra-dense Group of Obscured Emission-Line Galaxies
We present the discovery of an isolated compact group of galaxies that is
extremely dense (median projected galaxy separation: 6.9 kpc), has a very low
velocity dispersion ( = 67 km s), and where all
observed members show emission lines and are morphologically disturbed. These
properties, together with the lack of spirals and the presence of a prominent
tidal tail make this group one of the most evolved compact groups.Comment: 15 pages,LaTeX, 2figures. A Postscript figure with spectra is
available at ftp://astro.uibk.ac.at/pub/weinberger/ . Accepted for
publication in ApJ Letter
CG J1720-67.8: A Detailed Analysis of Optical and Infrared Properties of a New Ultracompact Group of Galaxies
We present here optical spectroscopy and BVRJHK(s) photometry of the recently
discovered ultra-compact group of galaxies CG J1720-67.8. This work represents
a considerable extension of the preliminary results we presented in a previous
paper. Despite the complicated morphology of the group, a quantitative
morphological classification of the three brightest members of the group is
attempted based on photometric analysis. We find that one galaxy is consistent
with a morphological type S0, while the other two are most probably late-type
spirals that are already losing their identity due tothe interaction process.
Information on the star formation activity and dust content derived from both
spectroscopic data and optical and near-infrared colors are complemented with a
reconstruction of far-infrared (FIR) maps from IRAS raw data. Enhanced star
formation activity is revealed in all the group's members, including the
early-type galaxy and the extended tidal tail, along which several tidal dwarf
galaxy candidates are identified. The metallicity of the gaseous component is
investigated and photoionization models are applied to the three main galaxies
of the group, while a detailed study of the tidal dwarf candidates will appear
in a companion paper. Subsolar metal abundances are found for all the three
galaxies, the highest values being shown by the early-type galaxy (Z ~ 0.5
Zsolar).Comment: Accepted for publication in The Astrophysical Journa
Detection of an optical filament in the Monogem Ring
The Monogem Ring is a huge bright soft X-ray enhancement with a diameter of ~
25\degr. This 0.3 kpc distant structure is a peculiar Galactic supernova
remnant in that it is obviously visible only in X-rays, due to its expansion
into a region of extremely low ambient density: hence, practically no optical
emission or a neutral HI shell was expected to be detectable. - Here we report
on the discovery of a very faint arc-like nebula on a POSS II R film copy, at
the south-eastern borders of the MR. Spectroscopy revealed this filament to
have a very large [SII] 6716+6731/Halpha ratio of up to ~ 1.8,
indicating shock excitation, and a low density of N_e <100 cm^{-3}. There is no
hint of [OIII] emission in the spectra. On deep wide-field direct images in
Halpha and in [SII] the nebula appears as a ~ 20 arcmin long, thin (~ 1
arcmin), structured filament, stretching N-S. We believe that this filament
belongs to the MR and became visible due to the interaction of the expanding
remnant with a mild density increase in the interstellar medium. Only one other
possible optical filament of the MR has been reported in the literature, but no
spectrum was provided.Comment: 6 pages, 4 figures, of which 2 in jpeg format and degraded because of
size constraints. Accepted for publication in A&
Dust-enshrouded star formation in XMM-LSS galaxy clusters
We present an investigation of the dust-enshrouded activity in a sample of
X-ray selected clusters drawn from the XMM-LSS survey in the redshift range z ~
0.05 - 1.05. By taking advantage of the contiguous mid-IR coverage of the
XMM-LSS field by the Spitzer SWIRE legacy survey, we examined the distribution
and number density of mid-IR bright sources out to the cluster periphery and
its dependence on redshift to probe the obscured side of the Butcher-Oemler
effect. Toward intermediate redshift clusters we identified surprisingly high
numbers of bright 24um sources, whose photometric redshifts are compatible with
cluster membership. The stacked surface density profile of 24um sources in
clusters within four redshift bins gives evidence for an excess of bright
mid-IR sources at redshift z 0.4 at cluster-centric radii ~ 200 - 500
kpc. Some traces of excess appear to be present at larger radii as well.Comment: 4 pages, presented at "SF2A-2008: Semaine de l'Astrophysique
Francaise
Candidate Tidal Dwarf Galaxies in the Compact Group CG J1720-67.8
This is the second part of a detailed study of the ultracompact group CG
J1720-67.8: in the first part we have focused the attention on the three main
galaxies of the group and we have identified a number of candidate tidal dwarf
galaxies (TDGs). Here we concentrate on these candidate TDGs. Absolute
photometry of these objects in BVRJHKs bands confirms their relatively blue
colors, as we already expected from the inspection of optical and near-infrared
color maps and from the presence of emission-lines in the optical spectra. The
physical conditions in such candidate TDGs are investigated through the
application of photoionization models, while the optical colors are compared
with grids of spectrophotometric evolutionary synthesis models from the
literature. Although from our data self-gravitation cannot be proved for these
objects, their general properties are consistent with those of other TDG
candidates. Additionally we present the photometry of a few ``knots'' detected
in the immediate surroundings of CG J1720-67.8 and consider the possibility
that these objects might belong to a dwarf population associated with the
compact group.Comment: Accepted for publication in the Astrophysical Journa
A photo-ionized canopy for the shock-excited Criss-Cross Nebula
Aims. We study a new broad well-defined arc of optical nebulosity close to
the cloud-shock interacting Criss-Cross Nebula, derive the basic physical
properties of the former and revise those of the latter, and compare both
objects to simulations of cloud-shock interactions from the literature.
Methods. Deep optical, partly wide-field, images were used to reveal the
intricate morphology and overall extent of the nebulosities. Optical
spectroscopy enabled us to uncover their nature. Results. The two nebulosities
obviously are physically linked, but are of different type; the Criss-Cross
Nebula is, as was shown also in an earlier paper, excited via a slow shock from
the expanding Orion-Eridanus Bubble, but the broad arc is definitely
photo-ionized. The source for ionizing photons appears to be hot gas in this
bubble. Some results of simulations of interactions of SNRs with interstellar
clouds available from the literature bear a striking resemblance to our
nebulae, which appear to represent an example - unrivalled in closeness and
clarity - for an early to medium stage in the destruction of an isolated cloud
over-run by a highly evolved SNR. Thereby the Criss-Cross Nebula is, when seen
from the SNR, the rear disrupted part of the original small cloud, whereas the
arc probably is its yet rather intact front part.Comment: 6 pages, 3 figures. Figures degraded because of size constraints.
Accepted for publication in A&
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