59 research outputs found
NGC 4254: An Act of Harassment Uncovered by the Arecibo Legacy Fast ALFA Survey
We present an HI map constructed from the Arecibo Legacy Fast ALFA (ALFALFA)
survey of the surroundings of the strongly asymmetric Virgo cluster Sc galaxy
NGC 4254. Noted previously for its lopsided appearance, rich interstellar
medium, and extradisk HI emission, NGC 4254 is believed to be entering the
Virgo environment for the first time and at high speed. The ALFALFA map clearly
shows a long HI tail extending ~250 kpc northward from the galaxy. Embedded as
one condensation within this HI structure is the object previously identified
as a "dark galaxy": Virgo HI21 (Davies et al. 2004). A body of evidence
including its location within and velocity with respect to the cluster and the
appearance and kinematics of its strong spiral pattern, extra-disk HI and
lengthy HI tail is consistent with a picture of "galaxy harassment" as proposed
by Moore et al. (1996a,b; 1998). The smoothly varying radial velocity field
along the tail as it emerges from NGC 4254 can be used as a timing tool, if
interpreted as resulting from the coupling of the rotation of the disk and the
collective gravitational forces associated with the harassment mechanism.Comment: accepted for publication in Ap.J.(Lett.). higher resolution figure
available at http://egg.astro.cornell.edu/alfalfa/pubs/figs/n4254_f1.ep
The Cold and Hot Gas Content of Fine-Structure E and S0 Galaxies
We investigate trends of the cold and hot gas content of early-type galaxies
with the presence of optical morphological peculiarities, as measured by the
fine-structure index (Sigma). HI mapping observations from the literature are
used to track the cold-gas content, and archival ROSAT PSPC data are used to
quantify the hot-gas content. We find that E and S0 galaxies with a high
incidence of optical peculiarities are exclusively X-ray underluminous and,
therefore, deficient in hot gas. In contrast, more relaxed galaxies with little
or no signs of optical peculiarities span a wide range of X-ray luminosities.
That is, the X-ray excess anticorrelates with Sigma. There appears to be no
similar trend of cold-gas content with either fine-structure index or X-ray
content. The fact that only apparently relaxed E and S0 galaxies are strong
X-ray emitters is consistent with the hypothesis that after strong disturbances
such as a merger hot-gas halos build up over a time scale of several gigayears.
This is consistent with the expected mass loss from stars.Comment: 12 pages, latex, 5 figures. Accepted for publication in A
The origin of type I profiles in cluster lenticulars: an interplay between ram pressure stripping and tidally induced spiral migration
Using N-body + smooth particle hydrodynamics simulations of galaxies falling into a cluster, we study the evolution of their radial density profiles. When evolved in isolation, galaxies develop a type II (down-bending) profile. In the cluster, the evolution of the profile depends on the minimum cluster-centric radius the galaxy reaches, which controls the degree of ram pressure stripping. If the galaxy falls to ∼50 per cent of the virial radius, then the profile remains type II, but if the galaxy reaches down to ∼20 per cent of the virial radius, the break weakens and the profile becomes more type I like. The velocity dispersions are only slightly increased in the cluster simulations compared with the isolated galaxy; random motion therefore cannot be responsible for redistributing material sufficiently to cause the change in the profile type. Instead, we find that the joint action of radial migration driven by tidally induced spirals and the outside-in quenching of star formation due to ram pressure stripping alters the density profile. As a result, this model predicts a flattening of the age profiles amongst cluster lenticulars with type I profiles, which can be observationally tested
Is the far border of the Local Void expanding?
According to models of evolution in the hierarchical structure formation
scenarios, voids of galaxies are expected to expand. The Local Void (LV) is the
closest large void, and it provides a unique opportunity to test
observationally such an expansion. It has been found that the Local Group,
which is on the border of the LV, is running away from the void center at ~260
km/s. In this study we investigate the motion of the galaxies at the far-side
border of the LV to examine the presence of a possible expansion. We selected
late-type, edge-on spiral galaxies with radial velocities between 3000 km/s and
5000 km/s, and carried out HI 21 cm line and H-band imaging observations. The
near-infrared Tully-Fisher relation was calibrated with a large sample of
galaxies and carefully corrected for Malmquist bias. It was used to compute the
distances and the peculiar velocities of the LV sample galaxies. Among the 36
sample LV galaxies with good quality HI line width measurements, only 15
galaxies were selected for measuring their distances and peculiar velocities,
in order to avoid the effect of Malmquist bias. The average peculiar velocity
of these 15 galaxies is found to be -419+208-251 km/s, which is not
significantly different from zero. Due to the intrinsically large scatter of
Tully-Fisher relation, we cannot conclude whether there is a systematic motion
against the center of the LV for the galaxies at the far-side boundary of the
void. However, our result is consistent with the hypothesis that those galaxies
at the far-side boundary have an average velocity of ~260 km/s equivalent to
what is found at the position of the Local Group.Comment: 15 pages, 6 figures, and 4 tables. Accepted for publication in A&
The X-ray nebula around the Seyfert 2 galaxy NGC4388
We report on X-ray emission from the Seyfert 2 galaxy NGC4388 observed with
the Chandra X-ray Observatory. A hard X-ray peak is found at the position of
the active nucleus suggested by optical and radio observations. Extended soft
X-ray emission correlates well with the ionization cone found in optical line
emission. A large soft X-ray extension is found up to 16 kpc to the north of
the galaxy. Photoionized gas with low ionization parameters (xi<3) appears to
be the likely explanation of this emission. The same ionized gas clouds could
be responsible for the optical [OIII] emission. Fe K line emission from cold
material is found to be extended by a few kpc.Comment: 12 pages, one colour figure included, MNRAS in pres
Ram pressure stripping and galaxy orbits: The case of the Virgo cluster
We investigate the role of ram pressure stripping in the Virgo cluster using
N-body simulations. Radial orbits within the Virgo cluster's gravitational
potential are modeled and analyzed with respect to ram pressure stripping. The
N-body model consists of 10000 gas cloud complexes which can have inelastic
collisions. Ram pressure is modeled as an additional acceleration on the clouds
located at the surface of the gas distribution in the direction of the galaxy's
motion within the cluster. We made several simulations changing the orbital
parameters in order to recover different stripping scenarios using realistic
temporal ram pressure profiles. We investigate systematically the influence of
the inclination angle between the disk and the orbital plane of the galaxy on
the gas dynamics. We show that ram pressure can lead to a temporary increase of
the central gas surface density. In some cases a considerable part of the total
atomic gas mass (several 10^8 M_solar) can fall back onto the galactic disk
after the stripping event. A quantitative relation between the orbit parameters
and the resulting HI deficiency is derived containing explicitly the
inclination angle between the disk and the orbital plane. The comparison
between existing HI observations and the results of our simulations shows that
the HI deficiency depends strongly on galaxy orbits. It is concluded that the
scenario where ram pressure stripping is responsible for the observed HI
deficiency is consistent with all HI 21cm observations in the Virgo cluster.Comment: 29 pages with 21 figures. Accepted for publication in Ap
The I-Band Tully-Fisher Relation for Sc Galaxies: 21 Centimeter H I Line Data
A compilation of 21 cm line spectral parameters specifically designed for application of the Tully-Fisher (TF) distance method is presented for 1201 spiral galaxies, primarily field Sc galaxies, for which optical I-band photometric imaging is also available. New H I line spectra have been obtained for 881 galaxies. For an additional 320 galaxies, spectra available in a digital archive have been reexamined to allow application of a single algorithm for the derivation of the TF velocity width parameter. A velocity width algorithm is used that provides a robust measurement of rotational velocity and permits an estimate of the error on that width taking into account the effects of instrumental broadening and signal-to-noise. The digital data are used to establish regression relations between measurements of velocity widths using other common prescriptions so that comparable widths can be derived through conversion of values published in the literature. The uniform H I line widths presented here provide the rotational velocity measurement to be used in deriving peculiar velocities via the TF method
The Arecibo Legacy Fast ALFA Survey: VI. Second HI Source Catalog of the Virgo Cluster Region
We present the third installment of HI sources extracted from the Arecibo
Legacy Fast ALFA extragalactic survey. This dataset continues the work of the
Virgo ALFALFA catalog. The catalogs and spectra published here consist of data
obtained during the 2005 and 2006 observing sessions of the survey. The catalog
consists of 578 HI detections within the range 11h 36m < R.A.(J2000) < 13h 52m
and +08 deg < Dec.(J2000) < +12 deg, and cz_sun < 18000 km/s. The catalog
entries are identified with optical counterparts where possible through the
examination of digitized optical images. The catalog detections can be
classified into three categories: (a) detections of high reliability with S/N >
6.5; (b) high velocity clouds in the Milky Way or its periphery; and (c)
signals of lower S/N which coincide spatially with an optical object and known
redshift. 75% of the sources are newly published HI detections. Of particular
note is a complex of HI clouds projected between M87 and M49 that do not
coincide with any optical counterparts. Candidate objects without optical
counterparts are few. The median redshift for this sample is 6500 km/s and the
cz distribution exhibits the local large scale structure consisting of Virgo
and the background void and the A1367-Coma supercluster regime at cz_sun ~7000
km/s. Position corrections for telescope pointing errors are applied to the
dataset by comparing ALFALFA continuum centroid with those cataloged in the
NRAO VLA Sky Survey. The uncorrected positional accuracy averages
27 arcsec ~(21 arcsec ~median) for all sources with S/N > 6.5 and is of order
~21 arcsec ~(16 arcsec ~median) for signals with S/N > 12. Uncertainties in
distances toward the Virgo cluster can affect the calculated HI mass
distribution.Comment: 25 pages, 1 Table, 8 figures, Accepted by the Astronomical Journa
High Latitude Radio Emission in a Sample of Edge-On Spiral Galaxies
We have mapped 16 edge-on galaxies at 20 cm using the VLA. For 5 galaxies, we
could form spectral index, energy and magnetic field maps. We find that all but
one galaxy show evidence for non-thermal high latitude radio continuum
emission, suggesting that cosmic ray halos are common in star forming galaxies.
The high latitude emission is seen over a variety of spatial scales and in
discrete and/or smooth features. In general, the discrete features emanate from
the disk, but estimates of CR diffusion lengths suggest that diffusion alone is
insufficient to transport the particles to the high latitudes seen (> 15 kpc in
one case). Thus CRs likely diffuse through low density regions and/or are
assisted by other mechanisms (e.g. winds). We searched for correlations between
the prevalence of high latitude radio emission and a number of other
properties, including the global SFR, supernova input rate per unit star
forming, and do not find clear correlations with any of these properties.Comment: 40 pages of text, 3 figures, 6 tables, and an appendix of 21 jpeg
figures (which is a radio continuum catalogue of 17 galaxies). to appear in
A. J. (around January 1999
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