172 research outputs found
Chemistry in isolation: High CCH/HCO+ line ratio in the AMIGA galaxy CIG 638
Multi-molecule observations towards an increasing variety of galaxies have
been showing that the relative molecular abundances are affected by the type of
activity. However, these studies are biased towards bright active galaxies,
which are typically in interaction. We study the molecular composition of one
of the most isolated galaxies in the local Universe where the physical and
chemical properties of their molecular clouds have been determined by intrinsic
mechanisms. We present 3 mm broad band observations of the galaxy CIG 638,
extracted from the AMIGA sample of isolated galaxies. The emission of the J=1-0
transitions of CCH, HCN, HCO+, and HNC are detected. Integrated intensity
ratios between these line are compared with similar observations from the
literature towards active galaxies including starburst galaxies (SB), active
galactic nuclei (AGN), luminous infrared galaxies (LIRG), and GMCs in M33. A
significantly high ratio of CCH with respect to HCN, HCO+, and HNC is found
towards CIG 638 when compared with all other galaxies where these species have
been detected. This points to either an overabundance of CCH or to a relative
lack of dense molecular gas as supported by the low HCN/CO ratio, or both. The
data suggest that the CIG 638 is naturally a less perturbed galaxy where a
lower fraction of dense molecular gas, as well as a more even distribution
could explain the measured ratios. In this scenario the dense gas tracers would
be naturally dimmer, while the UV enhanced CCH, would be overproduced in a less
shielded medium.Comment: Letter accepted for publication in A&
Galaxy Interactions in Compact Groups II: abundance and kinematic anomalies in HCG 91c
Galaxies in Hickson Compact Group 91 (HCG 91) were observed with the WiFeS
integral field spectrograph as part of our ongoing campaign targeting the
ionized gas physics and kinematics inside star forming members of compact
groups. Here, we report the discovery of HII regions with abundance and
kinematic offsets in the otherwise unremarkable star forming spiral HCG 91c.
The optical emission line analysis of this galaxy reveals that at least three
HII regions harbor an oxygen abundance ~0.15 dex lower than expected from their
immediate surroundings and from the abundance gradient present in the inner
regions of HCG 91c. The same star forming regions are also associated with a
small kinematic offset in the form of a lag of 5-10 km/s with respect to the
local circular rotation of the gas. HI observations of HCG 91 from the Very
Large Array and broadband optical images from Pan-STARRS suggest that HCG 91c
is caught early in its interaction with the other members of HCG 91. We discuss
different scenarios to explain the origin of the peculiar star forming regions
detected with WiFeS, and show that evidence point towards infalling and
collapsing extra-planar gas clouds at the disk-halo interface, possibly as a
consequence of long-range gravitational perturbations of HCG 91c from the other
group members. As such, HCG 91c provides evidence that some of the
perturbations possibly associated with the early phase of galaxy evolution in
compact groups impact the star forming disk locally, and on sub-kpc scales.Comment: 25 pages, 21 figures, MNRAS accepted. Until publication of the
article, the interactive component of Figure 4 is available at this URL:
http://www.mso.anu.edu.au/~fvogt/website/misc.htm
A ~ 12 kpc HI extension and other HI asymmetries in the isolated galaxy CIG 340 (IC 2487)
HI kinematic asymmetries are common in late-type galaxies irrespective of
environment, although the amplitudes are strikingly low in isolated galaxies.
As part of our studies of the HI morphology and kinematics in isolated
late-type galaxies we have chosen several very isolated galaxies from the AMIGA
sample for HI mapping. Here we present GMRT 21-cm HI line mapping of CIG 340
which was selected because its integrated HI spectrum has a very symmetric
profile, Aflux = 1.03. Optical images of the galaxy hinted at a warped disk in
contrast to the symmetric integrated HI spectrum profile. Our aim is to
determine the extent to which the optical asymmetry is reflected in the
resolved HI morphology and kinematics. GMRT observations reveal significant HI
morphological asymmetries in CIG 340 despite it's overall symmetric optical
form and highly symmetric HI spectrum. The most notable HI features are: 1) a
warp in the HI disk (with an optical counterpart), 2) the HI north/south flux
ratio = 1.32 is much larger than expected from the integrated HI spectrum
profile and 3) a ~ 45" (12 kpc) HI extension, containing ~ 6% of the detected
HI mass on the northern side of the disk. We conclude that in isolated galaxies
a highly symmetric HI spectrum can mask significant HI morphological
asymmetries. The northern HI extension appears to be the result of a recent
perturbation (10^8 yr), possibly by a satellite which is now disrupted or
projected within the disk. This study provides an important step in our ongoing
program to determine the predominant source of HI asymmetries in isolated
galaxies. For CIG 340 the isolation from major companions, symmetric HI
spectrum, optical morphology and interaction timescales have allowed us to
narrow the possible causes the HI asymmetries and identify tests to further
constrain the source of the asymmetries.Comment: 10 page
The Evolution of Galaxy Disks in Dense Environments - Lessons from Compact Galaxy Groups
Disk galaxies in compact galaxy groups exhibit a remarkable shortfall of
neutral hydrogen compared to both isolated spirals and spirals in more loose
groups, but the origin of this HI deficiency remains unclear. Based on a sample
of highly HI deficient compact galaxy groups, here updated to also include HCG
58 and HCG 93, we summarise the first results of a multi-wavelength campaign
aimed at understanding the processes responsible for modifying the HI content
of galaxy disks in these environments. While tidal stripping, ram pressure
stripping by hot intragroup gas, and star-formation induced strangulation could
individually be affecting the ISM in some of the group members, these processes
each face specific difficulties in explaining the inferred deficiency of HI for
the sample as a whole. A complete picture of the mechanisms driving the ISM
evolution in the disk galaxies of these groups has thus yet to emerge, but
promising avenues for further progress in this field are briefly discussed on
the basis of the present sample.Comment: 6 pages, 3 figures, to appear in online Proc. IAU Symp. 254, "The
Galaxy Disk in Cosmological Context", Eds. J. Andersen, J. Bland-Hawthorn, B.
Nordstroe
Strong Far-IR Cooling Lines, Peculiar CO Kinematics and Possible Star Formation Suppression in Hickson Compact Group 57
We present [C II] and [O I] observations from Herschel and CO(1-0) maps from
the Combined Array for{\dag} Research in Millimeter Astronomy (CARMA) of the
Hickson Compact Group HCG 57, focusing on the galaxies HCG 57a and HCG 57d. HCG
57a has been previously shown to contain enhanced quantities of warm molecular
hydrogen consistent with shock and/or turbulent heating. Our observations show
that HCG 57d has strong [C II] emission compared to L and weak
CO(1-0), while in HCG 57a, both the [C II] and CO(1-0) are strong. HCG 57a lies
at the upper end of the normal distribution of [C II]/CO and [C II]/FIR ratios,
and its far-IR cooling supports a low density warm diffuse gas that falls close
to the boundary of acceptable PDR models. However, the power radiated in the [C
II] and warm H emission have similar magnitudes, as seen in other
shock-dominated systems and predicted by recent models. We suggest that
shock-heating of the [C II] is a viable alternative to photoelectric heating in
violently disturbed diffuse gas. The existence of shocks is also consistent
with peculiar CO kinematics in the galaxy, indicating highly non-circular
motions are present. These kinematically disturbed CO regions also show
evidence of suppressed star formation, falling a factor of 10-30 below normal
galaxies on the Kennicutt-Schmidt relation. We suggest that the peculiar
properties of both galaxies are consistent with a highly dissipative off-center
collisional encounter between HCG 57d and 57a, creating ring-like morphologies
in both systems. Highly dissipative gas-on-gas collisions may be more common in
dense groups because of the likelihood of repeated multiple encounters. The
possibility of shock-induced SF suppression may explain why a subset of these
HCG galaxies have been found previously to fall in the mid-infrared green
valley.Comment: ApJ accepted, 16 pages, 12 figures, 3 table
Non-linear Dependence of L(B) on L(FIR) and M(H2) among Spiral Galaxies and Effects of Tidal Interaction
Through the study of a carefully selected sample of isolated spiral galaxies,
we have established that two important global physical quantities for tracing
star forming activities, L(FIR) and M(H2), have non-linear dependence on
another commonly cited global quantity L(B). Furthermore we show that simple
power law relations can effectively describe these non-linear relations for
spiral galaxies spanning four orders of magnitude in FIR and M(H2) and nearly
three orders of magnitude in L(B). While the existence of non-linear dependence
of M(H2) (assuming a constant CO-to-H2 conversion) and L(FIR) on optical
luminosity L(B) has been previously noted in the literature, an improper
normalization of simple scaling by L(B) has been commonly used in many previous
studies to claim enhanced molecular gas content and induced activities among
tidally interacting and other types of galaxies. We remove these non-linear
effects using the template relations derived from the isolated galaxy sample
and conclude that strongly interacting galaxies do not have enhanced molecular
gas content, contrary to previous claims. With these non-linear relations among
L(B), L(FIR) and M(H2) properly taken into account, we confirm again that the
FIR emission and the star formation efficiency L(FIR)/M(H2) are indeed enhanced
by tidal interactions. Virgo galaxies show the same level of M(H2) and L(FIR)
as isolated galaxies. We do not find any evidence for enhanced star forming
activity among barred galaxies.Comment: 19 pages and 5 figures, requires AAS style files, ApJ, accepte
HI asymmetry in the isolated galaxy CIG 85 (UGC 1547)
We present the results from the Giant Metrewave Radio Telescope (GMRT)
interferometric HI and 20 cm radio continuum observations of CIG 85, an
isolated asymmetric galaxy from the AMIGA (Analysis of the Interstellar Medium
of Isolated GAlaxies) sample. Despite being an isolated galaxy, CIG 85 showed
an appreciable optical and HI spectral asymmetry and therefore was an excellent
candidate for resolved HI studies to understand the reasons giving rise to
asymmetries in isolated galaxies. The galaxy was imaged in HI and 20 cm radio
continuum using the GMRT. For a detailed discussion of the results we also made
use of multi-wavelength data from archival SDSS, GALEX and Halpha imaging. We
find the HI in CIG 85 to have a clumpy, asymmetric distribution which in the NW
part is correlated with optical tail like features, but the HI velocity field
displays a relatively regular rotation pattern. Evaluating all the
observational evidence, we come to a conclusion that CIG 85 is most likely a
case of a disturbed spiral galaxy which now appears to have the morphology of
an irregular galaxy. Although it is currently isolated from major companions,
the observational evidence is consistent with HI asymmetries, a highly
disturbed optical disk and recent increase in star formation having been caused
by a minor merger, remnants of which are now projected in front of the optical
disk. If this is correct, the companion will be fully accreted by CIG 85 in the
near future.Comment: 10 pages, 9 figures, accepted in A&
Circumnuclear stellar population, morphology and environment of Seyfert 2 galaxies: an evolutionary scenario
We investigate the relation between the characteristics of the circumnuclear
stellar population and both the galaxy morphology and the presence of close
companions for a sample of 35 Seyfert 2 nuclei. Fifteen galaxies present
unambiguous signatures of recent episodes of star formation within 300
pc from the nucleus. When we relate this property with the Hubble type of the
host galaxy, we find that the incidence of recent circumnuclear star formation
increases along the Hubble sequence, and seems to be larger than in non-Seyfert
galaxies for the early Hubble types S0 and Sa, but similar to that in
non-Seyfert galaxies for later Hubble types. Both in early-type and late-type
Seyferts, the presence of recent star-formation is related to the galaxy
morphology in the inner few kiloparsecs, as observed in HST images through the
filter F606W by Malkan et al., who has assigned a late ``inner Hubble type'' to
most Seyfert 2s with recent nuclear star-formation. This new classification is
due to the presence of dust lanes and spiral structures in the inner region.
The presence of recent star formation in Seyfert 2 nuclei is also related to
interactions: among the 13 galaxies of the sample with close companions or in
mergers, 9 have recent star formation in the nucleus. These correlations
between the presence of companions, inner morphology and the incidence of
recent star formation suggest an evolutionary scenario in which the interaction
is responsible for sending gas inwards which both feeds the AGN and triggers
star-formation. The starburst then fades with time and the composite Seyfert 2
+ Starburst nucleus evolves to a ``pure'' Seyfert 2 nucleus with an old stellar
population.Comment: 12 pages, 6 figures, 2 table
A Multiwavelength Study of Stephan's Quintet
Stephan's Quintet (SQ) is a compact group that we find in an atypical moment
when a high velocity intruder is passing through it. The intrusion is
particularly interesting because a previous intruder had stripped most of the
gas from the group members. This debris field was shocked in the ongoing
collision with the new intruder. This evolutionary history agrees well with
observations and explains how a strongly interacting system can show low levels
of star formation. We present new multiwavelength data including previously
unpublished ROSAT X-ray, Ha interference filter/FP, ISO MIR/FIR and radio line
and continuum images. These observations and previously published data provide
new insights as well as support for some previous hypotheses. 1) FP and HI
velocities allow us to unambiguously distinguish between gas associated with SQ
and the new intruder.
2) Most detected emission regions are found in the remnant ISM of the NI
which allows us to infer its size and present physical state. 3) The few
emission regions associated with the stripped ISM of SQ include the best
candidate tidal dwarf galaxy. 4) Multiwavelength data suggest that strong
MIR/FIR emission from the Seyfert 2 nucleus of NGC7319 comes from dust heated
directly by a power-law continuum rather than a starburst. 5) The
correspondance between extended X-ray/radio continuum/forbidden optical
emission confirms the existence of a large scale shock in SQ.Comment: In press in AJ. 44 pages, 10 Postscript figures, uses aastex.st
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