653 research outputs found
Scattering of emission lines in galaxy cluster cores: measuring electron temperature
The central galaxies of some clusters can be strong emitters in the
Ly and H lines. This emission may arise either from the
cool/warm gas located in the cool core of the cluster or from the bright AGN
within the central galaxy. The luminosities of such lines can be as high as
erg/s. This emission originating from the core of the
cluster will get Thomson scattered by hot electrons of the intra-cluster medium
(ICM) with an optical depth 0.01 giving rise to very broad ( 15%) features in the scattered spectrum. We discuss the
possibility of measuring the electron density and temperature using information
on the flux and width of the highly broadened line features.Comment: 9 pages, 5 figures, accepted in MNRA
On the reliability of the black hole mass and mass to light ratio determinations with Schwarzschild models
In this Letter, we investigate the claim of Valluri et al. (2003), namely
that the use of Schwarzschild dynamical (orbit-based) models leads to an
indeterminacy regarding the estimation of the free parameters like the central
black hole mass and the stellar mass-to-light ratio of the galaxy under study.
We examine this issue with semi-analytic two-integral models, which are not
affected by the intrinsic degeneracy of three-integral systems. We however
confirm the Valluri et al. result, and observe the so-called widening of the
chi2 contours as the orbit library is expanded. We also show that, although
two-dimensional data coverage help in constraining the orbital structure of the
modelled galaxy, it does not in principle solve the indeterminacy issue, which
mostly originates from the discretization of such an inverse problem. We show
that adding regularization constraints stabilizes the confidence level contours
on which the estimation of black hole mass and stellar mass-to-light ratio are
based. We therefore propose to systematically use regularization as a tool to
prevent the solution to depend on the orbit library. Regularization, however,
introduces an unavoidable bias on the derived solutions. We hope that the
present Letter will trigger some more research directed at a better
understanding of the issues addressed here.Comment: 5 pages, 3 figures, accepted for publication in MNRAS Letter
Assessing the Formation Scenarios for the Double Nucleus of M31 Using Two-Dimensional Image Decomposition
The double nucleus geometry of M31 is currently best explained by the
eccentric disk hypothesis of Tremaine, but whether the eccentric disk resulted
from the tidal disruption of an inbounding star cluster by a nuclear black
hole, or by an m=1 perturbation of a native nuclear disk, remains debatable. I
perform detailed 2-D decomposition of the M31 double nucleus in the Hubble
Space Telescope V-band to study the bulge structure and to address competing
formation scenarios of the eccentric disk. I deblend the double nucleus (P1 and
P2) and the bulge simultaneously using five Sersic and one Nuker components. P1
and P2 appear to be embedded inside an intermediate component (r_e=3.2") that
is nearly spherical (q=0.97+/-m0.02), while the main galaxy bulge is more
elliptical (q=0.81+/-0.01). The spherical bulge mass of 2.8x10^7 M_sol is
comparable to the supermassive black hole mass (3x10^7 M_sol). In the 2-D
decomposition, the bulge is consistent with being centered near the UV peak of
P2, but the exact position is difficult to pinpoint because of dust in the
bulge. P1 and P2 are comparable in mass. Within a radius r=1\arcsec of P2, the
relative mass fraction of the nuclear components is M_BH:M_bulge:P1: P2 =
4.3:1.2:1:0.7, assuming the luminous components have a common mass-to-light
ratio of 5.7. The eccentric disk as a whole (P1+P2) is massive, M ~ 2.1x10^7
M_sol, comparable to the black hole and the local bulge mass. As such, the
eccentric disk could not have been formed entirely out of stars that were
stripped from an inbounding star cluster. Hence, the more favored scenario is
that of a disk formed in situ by an m=1 perturbation, caused possibly by the
passing of a giant molecular cloud, or the passing/accretion of a small
globular cluster.Comment: 19 pages, 8 figures. AJ accepted. For the version of this paper with
high resolution figures, go to:
http://zwicky.as.arizona.edu/~cyp/work/m31.ps.g
A SAURON study of dwarf elliptical galaxies in the Virgo Cluster: kinematics and stellar populations
Dwarf elliptical galaxies (dEs) are the most common galaxy type in nearby
galaxy clusters; even so, many of their basic properties have yet to be
quantified. Here we present the results of our study of 4 Virgo dwarf
ellipticals obtained with the SAURON integral field unit on the William
Herschel Telescope (La Palma, Spain). While traditional long-slit observations
are likely to miss more complicated kinematic features, with SAURON we are able
to study both kinematics and stellar populations in two dimensions, obtaining a
much more detailed view of the mass distribution and star formation histories.
What is visible even in such a small sample is that dEs are not a uniform
group, not only morphologically, but also as far as their kinematic and stellar
population properties are concerned. We find the presence of substructures,
varying degrees of flattening and of rotation, as well as differences in age
and metallicity gradients. We confirm that two of our galaxies are
significantly flattened, yet non-rotating objects, which makes them likely
triaxial systems. The comparison between the dwarf and the giant groups shows
that dEs could be a low-mass extension of Es in the sense that they do seem to
follow the same trends with mass. However, dEs as progenitors of Es seem less
likely as we have seen that dEs have much lower abundance ratios.Comment: 8 pages, 6 figures; to appear in the proceedings of the JENAM 2010
Symposium on Dwarf Galaxies (Lisbon, September 9-10, 2010); minor edits and
references adde
Is NGC 3108 transforming itself from an early to late type galaxy -- an astronomical hermaphrodite?
A common feature of hierarchical galaxy formation models is the process of
"inverse" morphological transformation: a bulge dominated galaxy accretes a gas
disk, dramatically reducing the system's bulge-to-disk mass ratio. During their
formation, present day galaxies may execute many such cycles across the Hubble
diagram. A good candidate for such a "hermaphrodite" galaxy is NGC 3108: a
dust-lane early-type galaxy which has a large amount of HI gas distributed in a
large scale disk. We present narrow band H_alpha and R-band imaging, and
compare the results with the HI distribution. The emission is in two
components: a nuclear bar and an extended disk component which coincides with
the HI distribution. This suggests that a stellar disk is currently being
formed out of the HI gas. The spatial distributions of the H_alpha and HI
emission and the HII regions are consistent with a barred spiral structure,
extending some 20 kpc in radius. We measure an extinction- corrected SFR of
0.42 Msun/yr. The luminosity function of the HII regions is similar to other
spiral galaxies, with a power law index of -2.1, suggesting that the star
formation mechanism is similar to other spiral galaxies. We measured the
current disk mass and find that it is too massive to have been formed by the
current SFR over the last few Gyr. It is likely that the SFR in NGC 3108 was
higher in the past. With the current SFR, the disk in NGC 3108 will grow to be
~6.2x10^9 Msun in stellar mass within the next 5.5 Gyr. While this is
substantial, the disk will be insignificant compared with the large bulge mass:
the final stellar mass disk-to-bulge ratio will be ~0.02. NGC 3108 will fail to
transform into anything resembling a spiral without a boost in the SFR and
additional supply of gas.Comment: 9 pages, 3 figures, accepted for publication in MNRA
How does star formation proceed in the circumnuclear starburst ring of NGC 6951?
Gas inflowing along stellar bars is often stalled at the location of
circumnuclear rings, that form an effective reservoir for massive star
formation and thus shape the central regions of galaxies. However, how exactly
star formation is proceeding within these circumnuclear starburst rings is
subject of debate. Two main scenarios for this process have been put forward:
In the first the onset of star formation is regulated by the total amount of
gas present in the ring with star forming starting once a mass threshold has
reached in a `random' position within the ring like `popcorn'. In the second
star formation preferentially takes place near the locations where the gas
enters the ring. This scenario has been dubbed `pearls-on-a-string'. Here we
combine new optical IFU data covering the full stellar bar with existing
multi-wavelength data to study in detail the 580 pc radius circumnuclear
starburst ring in the nearby spiral galaxy NGC 6951. Using HST archival data
together with Sauron and Oasis IFU data, we derive the ages and stellar masses
of star clusters as well as the total stellar content of the central region.
Adding information on the molecular gas distribution, stellar and gaseous
dynamics and extinction, we find that the circumnuclear ring in NGC 6951 is
~1-1.5 Gyr old and has been forming stars for most of that time. We see
evidence for preferred sites of star formation within the ring, consistent with
the `pearls-on-a-string' scenario, when focusing on the youngest stellar
populations. Due to the ring's longevity this signature is washed out when
older stellar populations are included in the analysis.Comment: accepted for publication in A&A, 15 page
Environmental regulation of cloud and star formation in galactic bars
The strong time-dependence of the dynamics of galactic bars yields a complex
and rapidly evolving distribution of dense gas and star forming regions.
Although bars mainly host regions void of any star formation activity, their
extremities can gather the physical conditions for the formation of molecular
complexes and mini-starbursts. Using a sub-parsec resolution hydrodynamical
simulation of a Milky Way-like galaxy, we probe these conditions to explore how
and where bar (hydro-)dynamics favours the formation or destruction of
molecular clouds and stars. The interplay between the kpc-scale dynamics (gas
flows, shear) and the parsec-scale (turbulence) is key to this problem. We find
a strong dichotomy between the leading and trailing sides of the bar, in term
of cloud fragmentation and in the age distribution of the young stars. After
orbiting along the bar edge, these young structures slow down at the
extremities of the bar, where orbital crowding increases the probability of
cloud-cloud collision. We find that such events increase the Mach number of the
cloud, leading to an enhanced star formation efficiency and finally the
formation of massive stellar associations, in a fashion similar to
galaxy-galaxy interactions. We highlight the role of bar dynamics in decoupling
young stars from the clouds in which they form, and discuss the implications on
the injection of feedback into the interstellar medium, in particular in the
context of galaxy formation.Comment: MNRAS accepte
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