235 research outputs found
Revised positions for the CIG galaxies
We present revised positions for the 1051 galaxies belonging to the
Karachentseva Catalog of Isolated Galaxies (CIG). New positions were calculated
by applying SExtractor to the Digitized Sky Survey CIG fields with a spatial
resolution of 1.2''. We visually checked the results and for 118 galaxies had
to recompute the assigned positions due to complex morphologies (e.g. distorted
isophotes, undefined nuclei, knotty galaxies) or the presence of bright stars.
We found differences between older and newer positions of up to 38'' with a
mean value of 2.96'' relative to SIMBAD and up to 38'' and 2.42'' respectively
relative to UZC. Based on star positions from the APM catalog we determined
that the DSS astrometry of five CIG fields has a mean offset in (RA, Dec) of
(-0.90'',0.93'') with a dispersion of 0.4''. These results have been confirmed
using the 2MASS All-Sky Catalog of Point Sources. The intrinsic errors of our
method combined with the astrometric ones are of the order of 0.5''.Comment: 6 pages, 7 figures. Accepted for publication in A&A. The data can be
downloaded at http://www.iaa.csic.es/AMIGA.htm
Star formation in the warped outer pseudoring of the spiral galaxy NGC 3642
NGC 3642 was classified as a spiral galaxy with three rings and no bar. We
have performed an HI and optical study of this nearly face-on galaxy. We find
that the nuclear ring might in fact be part of an inner one-armed spiral, that
could be driving nuclear accretion and feeding the central activity in the
inner kpc. The inner ring is faint, and the outer ring is a rather ill-defined
pseudoring. Furthermore, the size ratio of the rings is such that they cannot
be due to a single pattern speed linking them together.
The outer pseudoring is peculiar, since it lies in the faint outer parts of
the disk, where star formation is still going on at 1.4 times the optical
radius. Higher HI column densities are associated with these regions and the
atomic gas layer is warped. These perturbations affect only the outer disk,
since the kinematics within the main body conforms well to an ordinary
differentially rotating disk.
We propose here that both nuclear activity and star formation in the warped
outer parts might be linked to the fact that NGC 3642 is located in a rich
environment, where its close neighbors show clear signs of merging. Our
suggestion is that NGC 3642 has captured recently a low-mass, gas-rich dwarf,
and star formation was triggered in this infalling external gas that produced
also a pronounced warp in the gaseous disk.Comment: Accepted for publication in A&A. Full resolution version available at
http://www.iaa.es/~lourdes/3642/H3551.tar.g
DETECTION OF DIFFUSE NEUTRAL INTRAGROUP MEDIUM IN HICKSON COMPACT GROUPS
We present new Green Bank Telescope (GBT) 21 cm neutral hydrogen (H I) observations of a complete distance-limited sample of 22 Hickson Compact Groups (HCGs) with at least four true members. We detected an average H I mass of 8 × 109 M (median = 6 × 109 M ), which is significantly larger than previous single-dish measurements. Consequently, the H I deficiencies for these HCGs have been reduced, although not completely eliminated. Spectral comparison of the GBT data with complementary Very Large Array data shows significant H I excess in the GBT spectra. The observed excess is primarily due to the high surface brightness (HSB) sensitivity of the GBT detecting diffuse, low column density H I in these groups. The excess gas forms a faint diffused neutral medium which is an intermediate stage in the evolution of HSB H I tidal debris in the intragroup medium (IGM) before it is fully ionized. The excess gas mass fraction, (M(H I)GBT – M(H I)VLA)/M(H I)GBT, for our complete sample varies from 5% to 81% with an average of 36% (median = 30%). The excess gas mass fraction is highest in slightly H I deficient groups where the tidal debris has had enough time to evolve. We also find the excess gas content increases with the evolutionary phase of the group described in Verdes-Montenegro et al. Theoretical calculations indicate that an H I cloud of radius ≥ 200 pc would survive in an IGM of 2 × 106 K for more than the typical dynamical lifetime of a group. However, smaller clouds get evaporated and assimilated into the hot IGM in a much shorter timescale
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The VLA H I Observations of Stephan\u27s Quintet (HCG 92)*
Using the Very Large Array, we have made spectral-line and continuum observations of the neutral hydrogen in the direction of the compact group of galaxies Stephan\u27s Quintet. The high-velocity clouds between 5600 and 6600 km s-1, the disk of the foreground galaxy NGC 7320 at 800 km s-1, the extended continuum ridge near the center of the group, and three faint dwarflike galaxies in the surrounding field were imaged with the C, CS, and D configurations. Four of the H I clouds previously detected are confirmed. The two largest H I features are coincident with and concentrated mainly along separate large tidal tails that extend eastward. The most diffuse of the four clouds is resolved into two clumps, one coinciding with tidal features south of NGC 7318A and the other devoid of any detectable stellar or Hα sources. The two compact clouds along the same line of sight have peak emission at luminous infrared and bright Hα sources probably indicative of star-forming activity. The total amount of H I detected at high redshifts is ~1010M. As in previous H I studies of the group, no detectable emission was measured at the positions of any high-redshift galaxies, so that any H I still bound to their disks must be less than 2.4 × 107 M
Evidence for azimuthal variations of the oxygen abundance gradient tracing the spiral structure of the galaxy HCG91c
Context. The distribution of elements in galaxies forms an important
diagnostic tool to characterize the system's formation and evolution. This tool
is however complex to use in practice, as galaxies are subject to a range of
simultaneous physical processes active from pc to kpc scales. This renders
observations of the full optical extent of galaxies down to sub-kpc scales
essential. Aims. Using the WiFeS integral field spectrograph, we previously
detected abrupt and localized variations in the gas-phase oxygen abundance of
the spiral galaxy HCG91c. Here, we follow-up on these observations to map
HCG91c's disk out to ~2Re at a resolution of 600pc, and characterize the
non-radial variations of the gas-phase oxygen abundance in the system. Methods.
We obtained deep MUSE observations of the target under ~0.6 arcsec seeing
conditions. We perform both a spaxel-based and aperture-based analysis of the
data to map the spatial variations of 12+log(O/H) across the disk of the
galaxy. Results. We confirm the presence of rapid variations of the oxygen
abundance across the entire extent of the galaxy previously detected with
WiFeS, for all azimuths and radii. The variations can be separated in two
categories: a) localized and associated with individual HII regions, and b)
extended over kpc scales, and occurring at the boundaries of the spiral
structures in the galaxy. Conclusions. Our MUSE observations suggest that the
enrichment of the interstellar medium in HGC91c has proceeded preferentially
along spiral structures, and less efficiently across them. Our dataset
highlights the importance of distinguishing individual star-forming regions
down to scales of a few 100pc when using integral field spectrographs to
spatially resolve the distribution of oxygen abundances in a given system, and
accurately characterize azimuthal variations and intrinsic scatter.Comment: 12 pages, 11 figures, accepted for publication in A&A. Supplementary
movie assocociated with Fig. 8 is available (until publication) at:
http://www.sc.eso.org/~fvogt/supp_mat/HCG91c/O_gradient.mp
The AMIGA sample of isolated galaxies: VIII. The rate of asymmetric HI profiles in spiral galaxies
(abridged) Measures of the HI properties of a galaxy are among the most
sensitive interaction diagnostic at our disposal. We report here on a study of
HI profile asymmetries (e.g., lopsidedness) in a sample of some of the most
isolated galaxies in the local Universe. This presents us with an excellent
opportunity to quantify the range of intrinsic HI asymmetries and provides us
with a zero-point calibration for evaluating these measurements in less
isolated samples. We characterize the HI profile asymmetries and search for
correlations between HI asymmetry and their environments, as well as their
optical and far infrared (FIR) properties. We use high signal-to-noise global
HI profiles for galaxies in the AMIGA project (http://amiga.iaa.csic.es). We
restrict our study to N=166 galaxies with accurate measures of the HI shape
properties. We quantify asymmetries using a flux ratio parameter. The asymmetry
parameter distribution of our isolated sample is well described by a Gaussian
model. The width of the distribution is sigma=0.13, and could be even smaller
(sigma=0.11) if instrumental errors are reduced. Only 2% of our carefully
vetted isolated galaxies sample show an asymmetry in excess of 3sigma. By using
this sample we minimize environmental effects as confirmed by the lack of
correlation between HI asymmetry and tidal force (one-on-one interactions) and
neighbor galaxy number density. On the other hand, field galaxy samples show
wider distributions and deviate from a Gaussian curve. As a result we find
higher asymmetry rates (~10-20%) in such samples. We find evidence that the
spiral arm strength is inversely correlated with the HI asymmetry. We also find
an excess of FIR luminous galaxies with larger HI asymmetries that may be
spirals associated with hidden accretion events. Our sample presents the
smallest fraction of asymmetric HI profiles compared with any other yet
studied.Comment: 18 pages, 17 figures, accepted for publication in A&
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