32 research outputs found
Ram pressure stripping of the multiphase ISM in the Virgo cluster spiral galaxy NGC 4438
Ram pressure stripping of the multiphase ISM is studied in the perturbed
Virgo cluster spiral galaxy NGC 4438. This galaxy underwent a tidal interaction
~100 Myr ago and is now strongly affected by ram pressure stripping. Deep VLA
radio continuum observations at 6 and 20 cm are presented. We detect prominent
extraplanar emission to the west of the galactic center, which extends twice as
far as the other tracers of extraplanar material. The spectral index of the
extraplanar emission does not steepen with increasing distance from the galaxy.
This implies in situ re-acceleration of relativistic electrons. The comparison
with multiwavelength observations shows that the magnetic field and the warm
ionized interstellar medium traced by Halpha emission are closely linked. The
kinematics of the northern extraplanar Halpha emission, which is ascribed to
star formation, follow those of the extraplanar CO emission. In the western and
southern extraplanar regions, the Halpha measured velocities are greater than
those of the CO lines. We suggest that the ionized gas of this region is
excited by ram pressure. The spatial and velocity offsets are consistent with a
scenario where the diffuse ionized gas is more efficiently pushed by ram
pressure stripping than the neutral gas. We suggest that the recently found
radio-deficient regions compared to 24 mum emission are due to this difference
in stripping efficiency.Comment: 8 pages, 6 figures, A&A, accepted for publicatio
Ongoing Gas Stripping in the Virgo Cluster Spiral NGC 4522
The Virgo cluster galaxy NGC 4522 is one of the best spiral candidates for
ICM-ISM stripping in action. Optical broadband and H-alpha images from the WIYN
telescope of the highly inclined galaxy reveal a relatively undisturbed stellar
disk and a peculiar distribution of H-alpha emission. Ten percent of the
H-alpha emission arises from extraplanar HII regions which appear to lie within
filamentary structures >3 kpc long above one side of the disk. The filaments
emerge from the outer edge of a disk of bright H-alpha emission which is
abruptly truncated beyond 0.35R(25). Together the truncated H-alpha disk and
extraplanar H-alpha filaments are reminiscent of a bow shock morphology, which
strongly suggests that the interstellar medium (ISM) of NGC 4522 is being
stripped by the gas pressure of the intracluster medium (ICM). The galaxy has a
line-of-sight velocity of 1300 km/sec with respect to the mean Virgo cluster
velocity, and thus is expected to experience a strong interaction with the
intracluster gas. The existence of HII regions apparently located above the
disk plane suggests that star formation is occuring in the stripped gas, and
that newly formed stars will enter the galaxy halo and/or intracluster space.
The absence of HII regions in the disk beyond 0.35R(25), and the existence of
HII regions in the stripped gas suggest that even molecular gas has been
effectively removed from the disk of the galaxy.Comment: to appear in The Astronomical Journal, 16 pages, 5 figures, 1 tabl
Dust Streamers in the Virgo Galaxy M86 from Ram Pressure Stripping of its Companion VCC 882
The giant elliptical galaxy M86 in Virgo has a ~28 kpc long dust trail inside
its optical halo that points toward the nucleated dwarf elliptical galaxy, VCC
882. The trail seems to be stripped material from the dwarf. Extinction
measurements suggest that the ratio of the total gas mass in the trail to the
blue luminosity of the dwarf is about unity, which is comparable to such ratios
in dwarf irregular galaxies. The ram pressure experienced by the dwarf galaxy
in the hot gaseous halo of M86 was comparable to the internal gravitational
binding energy density of the presumed former gas disk in VCC 882. Published
numerical models of this case are consistent with the overall trail-like
morphology observed here. Three concentrations in the trail may be evidence for
the predicted periodicity of the mass loss. The evaporation time of the trail
is comparable to the trail age obtained from the relative speed of the galaxies
and the trail length. Thus the trail could be continuously formed from stripped
replenished gas if the VCC 882 orbit is bound. However, the high gas mass and
the low expected replenishment rate suggest that this is only the first
stripping event. Implications for the origin of nucleated dwarf ellipticals are
briefly discussed.Comment: 22 pages, 7 figures, Astronomical Journal, August 2000, in pres
Low-Level Nuclear Activity in Nearby Spiral Galaxies
We are conducting a search for supermassive black holes (SMBHs) with masses
below 10^7 M_sun by looking for signs of extremely low-level nuclear activity
in nearby galaxies that are not known to be AGNs. Our survey has the following
characteristics: (a) X-ray selection using the Chandra X-ray Observatory, since
x-rays are a ubiquitous feature of AGNs; (b) Emphasis on late-type spiral and
dwarf galaxies, as the galaxies most likely to have low-mass SMBHs; (c) Use of
multiwavelength data to verify the source is an AGN; and (d) Use of the highest
angular resolution available for observations in x-rays and other bands, to
separate nuclear from off-nuclear sources and to minimize contamination by host
galaxy light. Here we show the feasibility of this technique to find AGNs by
applying it to six nearby, face-on spiral galaxies (NGC 3169, NGC 3184, NGC
4102, NGC 4647, NGC 4713, NGC 5457) for which data already exist in the Chandra
archive. All six show nuclear x-ray sources. The data as they exist at present
are ambiguous regarding the nature of the nuclear x-ray sources in NGC 4713 and
NGC 4647. We conclude, in accord with previous studies, that NGC 3169 and NGC
4102 are almost certainly AGNs. Most interestingly, a strong argument can be
made that NGC 3184 and NGC 5457, both of type Scd, host AGNs.Comment: 37 pages, 10 figures, ApJ, in press. Replaced with accepted versio
3C236: Radio Source, Interrupted?
We present new HST STIS/MAMA near-UV images and archival WFPC2 V and R band
images which reveal the presence of four star forming regions in an arc along
the edge of the dust lane in the giant (4 Mpc) radio galaxy 3C236. Two of the
star forming regions are relatively young with ages of order 1E7 yr, while the
other two are older with ages of order 1E8 - 1E9 yr which is comparable to the
estimated age of the giant radio source. Based on dynamical and spectral aging
arguments, we suggest that the fuel supply to the AGN was interrupted for 1E7
yr and has now been restored, resulting in the formation of the inner 2 kpc
scale radio source. This time scale is similar to that of the age of the
youngest of the star forming regions. We suggest that the transport of gas in
the disk is non-steady and that this produces both the multiple episodes of
star formation in the disk as well as the multiple epochs of radio source
activity. If the inner radio source and the youngest star forming region are
related by the same event of gas transport, the gas must be transported from
the hundreds of pc scale to the sub-parsec scale on a time scale of 1E7 yr,
which is similar to the dynamical time scale of the gas on the hundreds of pc
scales
An Atlas of Halpha and R Images and Radial Profiles of 63 Bright Virgo Cluster Spiral Galaxies
Narrow-band Halpha and broadband R images and radial profiles are presented
for 63 bright spiral galaxies in the Virgo Cluster. The sample is complete for
Sb-Scd galaxies with B magnitude less than 12 and inclination less than 75
degrees. Isophotal radii, disk scalelengths, concentration parameters, and
integrated fluxes are derived for the sample galaxies.Comment: 46 pages, 8 figures, including 15 pages of atlas images. Higher
resolution postscript versions of the image and radial profile figures are
available at http://www1.union.edu/~koopmanr/preprints.html . Accepted for
publication in Astrophysical Journal Supplement Serie
The Radio Properties of Composite LINER/HII Galaxies
Arcsec-resolution VLA observations -- newly obtained as well as published --
of 40 nearby galaxies are discussed, completing a study of the radio properties
of a magnitude-limited sample of nearby galaxies of the composite LINER/HII
type. Our results reveal an overall detection rate of at least 25% AGN
candidates among these composite sources. The general properties of these AGN
candidates, as compared to non-AGN composite sources and HII galaxies, are
discussed.Comment: Accepted for publication in ApJ
The Giant Radio Galaxy MRC B0319-454: Circumnuclear Structure of the Host Galaxy ESO248-G10
We present optical and near-infrared images and spectra of ESO248-G10, the
host of giant radio galaxy MRC B0319-454. From near-infrared colours, the
active nucleus is shown to be reddened by hot dust emission or dust extinction.
Star forming regions are identified beyond a radius of 5" (8.1 kpc) where hot
gas is prevalent. The optical spectrum shows gas rotation at speeds of up to
350km/s out to >=32kpc along the radio axis. A model is proposed in which the
giant elliptical is triaxial with the radio axis along the short axis, and the
figure rotation is around the long axis. From the model, the angles of the
principal axes are psi=34 degrees, theta=65 degrees, and phi=19 degrees with
axis ratios of B/A=0.75 and C/A=0.69. A second velocity feature from 5" to 15"
to the north-east of the nucleus is proposed to be a merging gas-rich galaxy
inducing star formation while settling into an orbit about the figure rotation
axis.Comment: Accepted for publication in The Astrophysical Journal. 22 pages, 12
figure
UV to radio centimetric spectral energy distributions of optically-selected late-type galaxies in the Virgo cluster
We present a multifrequency dataset for an optically-selected,
volume-limited, complete sample of 118 late-type galaxies (>= S0a) in the Virgo
cluster. The database includes UV, visible, near-IR, mid-IR, far-IR, radio
continuum photometric data as well as spectroscopic data of Halpha, CO and HI
lines, homogeneously reduced, obtained from our own observations or compiled
from the literature. Assuming the energy balance between the absorbed stellar
light and that radiated in the IR by dust, we calibarte an empirical
attenuation law suitable for correcting photometric and spectroscopic data of
normal galaxies. The data, corrected for internal extinction, are used to
construct the spectral energy distribution (SED) of each individual galaxy, and
combined to trace the median SED of galaxies in various classes of
morphological type and luminosity. Low-luminosity, dwarf galaxies have on
average bluer stellar continua and higher far-IR luminosities per unit galaxy
mass than giant, early-type spirals. If compared to nearby starburst galaxies
such as M82 and Arp 220, normal spirals have relatively similar observed
stellar spectra but 10-100 times lower IR luminosities. The temperature of the
cold dust component increases with the far-IR luminosity, from giant spirals to
dwarf irregulars. The SED are used to separate the stellar emission from the
dust emission in the mid-IR regime. We show that the contribution of the
stellar emission at 6.75 micron to the total emission of galaxies is generally
important, from ~ 80% in Sa to ~ 20% in Sc.Comment: 31 pages, 12 figures, 12 tables. Fig 2 avaliable at
(http://goldmine.mib.infn.it/papers/isosed.html). Accepted for publication on
A&
HST images of B2 radio galaxies: a link between circum-nuclear dust and radio properties?
Almost 60% of the B2 low luminosity radio galaxies have been observed with
the Hubble Space Telescope. We present an analysis of the dust features, which
are often present in the form of circum-nuclear disks or lanes, and show that
there are correlations between radio source and dust properties. It is found
that nearby radio sources in which a jet has been detected tend to have dust
more often than sources without jets; the dust is often in the form of disks or
lanes. Moreover the radio jets are close to perpendicular to the disk or lane
in the weaker radio sources (with P < 10^{24} W/Hz). In stronger sources the
orientation effect appears to be weak or even absent. Also the dust masses
found in the weaker radio sources are smaller than in the stronger ones (log
M/M_sun ~ 3 against 5 respectively). More generally it appears that there is a
correlation between dust mass and total radio power (for those sources in which
dust has been detected); we show that this correlation is not induced by
redshift.Comment: Accepted for publication in A&A, 10 pages, 12 figure