692 research outputs found
Ionized Gas Kinematics and Morphology in Sgr B2 Main on 1000 AU Scales
We have imaged the Sgr B2 Main region with the Very Large Array in the BnA
configuration ( = 0\farcs13) in both the H52 (45.453
GHz) radio recombination line (RRL) and 7 mm continuum emission. At a distance
of 8500 pc, this spatial resolution corresponds to a physical scale of 0.005 pc
(1100 AU). The current observations detect H52 emission in 12
individual ultracompact (UC) and hypercompact (HC) HII regions. Two of the
sources with detected H52 emission have broad
(V50 \kms) recombination lines, and two of the sources
show lines with peaks at more than one velocity. We use line parameters from
the H52 lines and our previous H66 line observations to
determine the relative contribution of thermal, pressure and kinematic
broadening, and electron density. These new observations suggest that pressure
broadening can account for the broad lines in some of the sources, but that gas
motions (e.g. turbulence, accretion or outflow) contribute significantly to the
broad lines in at least one of the sources (Sgr B2 F3).Comment: 10 pages, 2 figure
Broad-line Balmer Decrements in Blue Active Galactic Nuclei
We have investigated the broad-line Balmer decrements (Halpha/Hbeta) for a
large, homogeneous sample of Seyfert 1 galaxies and QSOs using spectroscopic
data obtained in the Sloan Digital Sky Survey. The sample, drawn from the
Fourth Data Release, comprises 446 low redshift (z < 0.35) active galactic
nuclei (AGN) that have blue optical continua as indicated by the spectral
slopes in order to minimize the effect of dust extinction. We find that (i) the
distribution of the intrinsic broad-line Halpha/Hbeta ratio can be well
described by log-Gaussian, with a peak at Halpha/Hbeta=3.06 and a standard
deviation of about 0.03 dex only; (ii) the Balmer decrement does not correlate
with AGN properties such as luminosity, accretion rate, and continuum slope,
etc.; (iii) on average, the Balmer decrements are found to be only slightly
larger in radio-loud sources (3.37) and sources having double-peaked
emission-line profiles (3.27) compared to the rest of the sample. We therefore
suggest that the broad-line Halpha/Hbeta ratio can be used as a good indicator
for dust extinction in the AGN broad-line region; this is especially true for
radio-quiet AGN with regular emission-line profiles, which constitute the vast
majority of the AGN population.Comment: To appear in MNRAS. The data and the fitted parameters for the
decomposed spectral components (continuum, FeII and other emission lines) of
the 446 blue AGNs are available at
http://staff.ustc.edu.cn/~xbdong/Data_Release/blueAGN_DR4
First VLT/X-shooter spectroscopy of early-type stars outside the Local Group
As part of the VLT/X-shooter science verification, we obtained the first
optical medium-resolution spectrum of a previously identified bright O-type
object in NGC55, an LMC-like galaxy at a distance of \sim2.0 Mpc. Based on the
stellar and nebular spectrum, we investigate the nature and evolutionary status
of the central object(s) and its influence on the surrounding interstellar
medium. We conclude that the source, NGC55_C1_31, is a composite object, likely
a stellar cluster, which contains one or several hot (T_eff \simeq 50000 K) WN
stars with a high mass-loss rate (\sim3 \times 10^{-5} M_\odot yr^{-1}) and a
helium-rich composition (N_He/N_H = 0.8). The visual flux is dominated by
OB-type (super)giant stars with T_eff \sim< 35000 K, solar helium abundance
(N_He/N_H = 0.1), and mass-loss rate \sim2 \times 10^{-6} M_\odot yr^{-1}. The
surrounding H II region has an electron density n_e < 10^2 cm^{-3} and an
electron temperature T(OIII) \simeq 11500 \pm 600 K. The oxygen abundance of
this region is [O/H] = 8.18 \pm 0.03 which corresponds to Z = 0.31 \pm 0.04
Z_\odot. We observed no significant gradients in T(OIII), n_e or [O/H] on a
scale of 73 pc extending in four directions from the ionising source. The
properties of the HII region can be reproduced by a CLOUDY model which uses the
central cluster as ionising source, thus providing a self-consistent
interpretation of the data. We also report on the serendipitous discovery of
HeII nebular emission associated with the nearby source NGC55_C2_35, a feature
usually associated with strong X-ray sources.Comment: 12 pages, 10 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Society; the definitive version will be available at
wwww.blackwell-synergy.co
The relationship between [OIII]5007A equivalent width and obscuration in AGN
In this paper we study the relationship between the equivalent width (EW) of
the [OIII]5007A narrow emission line in AGN and the level of obscuration. To
this end, we combine the results of a systematic spectral analysis, both in the
optical and in the X-rays, on a statistically complete sample of ~170 X-ray
selected AGN from the XMM-Newton Bright Serendipitous Source sample (XBS). We
find that the observed large range of [OIII]5007A equivalent widths observed in
the sample (from a few A up to 500A) is well explained as a combination of an
intrinsic spread, probably due to the large range of covering factors of the
Narrow Line Region, and the effect of absorption. The intrinsic spread is
dominant for EW below 40-50A while absorption brings the values of EW up to
~100-150A, for moderate levels of absorption (AV~0.5-2 mag) or up to ~500A for
AV>2 mag. In this picture, the absorption has a significant impact on the
observed EW also in type~1 AGN. Using numerical simulations we find that this
model is able to reproduce the [OIII]5007A EW distribution observed in the XBS
sample and correctly predicts the shape of the EW distribution observed in the
optically selected sample of QSO taken from the SDSS survey.Comment: 7 pages, 5 figures. Accepted for publication in MNRA
Stellar disks of Collisional Ring Galaxies I. New multiband images, Radial intensity and color profiles, and confrontation with N-body simulations
We present new multi-band imaging data in the optical (BVRI and Halpha) and
near infrared bands (JHK) of 15 candidate ring galaxies from the sample of
Appleton & Marston (1997). We use these data to obtain color composite images,
global magnitudes and colors of both the ring galaxy and its companion(s), and
radial profiles of intensity and colors. We find that only nine of the observed
galaxies have multi-band morphologies expected for the classical collisional
scenario of ring formation, indicating the high degree of contamination of the
ring galaxy sample by galaxies without a clear ring morphology. The radial
intensity profiles, obtained by masking the off-centered nucleus, peak at the
position of the ring, with the profiles in the continuum bands broader than
that in the Halpha line. The images as well as the radial intensity and color
profiles clearly demonstrate the existence of the pre-collisional stellar disk
outside the star-forming ring, which is in general bluer than the disk internal
to the ring. The stellar disk seems to have retained its size, with the disk
outside the ring having a shorter exponential scale length as compared to the
values expected in normal spiral galaxies of comparable masses. The rings in
our sample of galaxies are found to be located preferentially at around
half-way through the stellar disk. The most likely reason for this preference
is bias against detecting rings when they are close to the center (they would
be confused with the resonant rings), and at the edge of the disk the gas
surface density may be below the critical density required for star formation.
Most of the observed characteristics point to relatively recent collisions (<80
Myr ago) according to the N-body simulations of Gerber et al. (1996).Comment: To appear in AJ issue of September 2008. High resolution color image
of Figure 2 and other supplementary images are available at
http://www.inaoep.mx/~ydm/rings
Cosmic rays and the primordial gas
One of the most outstanding problems in the gravitational collapse scenario
of early structure formation is the cooling of primordial gas to allow for
small mass objects to form. As the neutral primordial gas is a poor radiator at
temperatures (T\le10^4\unit{K}), molecular hydrogen is needed for further
cooling down to temperatures (T\sim100\unit{K}). The formation of molecular
hydrogen is catalyzed by the presence of free electrons, which could be
provided by the ionization due to an early population of cosmic rays. In order
to investigate this possibility we developed a code to study the effects of
ionizing cosmic rays on the thermal and chemical evolution of primordial gas.
We found that cosmic rays can provide enough free electrons needed for the
formation of molecular hydrogen, and therefore can increase the cooling ability
of such primordial gas under following conditions: A dissociating photon flux
with (F<10^{-18}\unit{erg cm^{-2} Hz^{-1} s^{-1}}), initial temperature of the
gas (\sim10^{3}\unit{K}), total gas number densities (n\ge1\unit{cm^{-3}}),
Cosmic ray sources with (\dot{\epsilon}_{CR}>10^{-33}\unit{erg cm^{-3}
s^{-1}}).Comment: 15 pages, 5 figure
HR Del remnant anatomy using 2-D spectral data and 3-D photoionization shell models
The HR Del nova remnant was observed with the IFU-GMOS at Gemini North. The
spatially resolved spectral data cube was used in the kinematic, morphological
and abundance analysis of the ejecta. The line maps show a very clumpy shell
with two main symmetric structures. The first one is the outer part of the
shell seen in H-alpha, that forms two rings projected in the sky plane. These
ring structures correspond to a closed hourglass shape, first proposed by
Harman and O'Brien (2003). The equatorial emission enhancement is caused by the
superimposed hourglass structures in the line of sight. The second structure
seen only in the [OIII] and [NII] maps is located along the polar directions
inside the hourglass structure. Abundances gradients between the polar caps and
equatorial region were not found. However, the outer part of the shell seems to
be less abundant in Oxygen and Nitrogen than the inner regions. Detailed 2.5D
photoionization modeling of the 3D shell was performed using the mass
distribution inferred from the observations and the presence of mass clumps.
The resulting model grids are used to constrain the physical properties of the
shell as well as the central ionizing source. A sequence of 3D clumpy models
including a disk shaped ionization source is able to reproduce the ionization
gradients between polar and equatorial regions of the shell. Differences
between shell axial ratios in different lines can also be explained by
aspherical illumination. A total shell mass of 9 x 10-4 Msun is derived from
these models. We estimate that 50% to 70% of the shell mass is contained in
neutral clumps with density contrast up to a factor of 30.Comment: 31 pages 12 figures 4 tables title correcte
From cusps to cores: a stochastic model
The cold dark matter model of structure formation faces apparent problems on
galactic scales. Several threads point to excessive halo concentration,
including central densities that rise too steeply with decreasing radius. Yet,
random fluctuations in the gaseous component can 'heat' the centres of haloes,
decreasing their densities. We present a theoretical model deriving this effect
from first principles: stochastic variations in the gas density are converted
into potential fluctuations that act on the dark matter; the associated force
correlation function is calculated and the corresponding stochastic equation
solved. Assuming a power law spectrum of fluctuations with maximal and minimal
cutoff scales, we derive the velocity dispersion imparted to the halo particles
and the relevant relaxation time. We further perform numerical simulations,
with fluctuations realised as a Gaussian random field, which confirm the
formation of a core within a timescale comparable to that derived analytically.
Non-radial collective modes enhance the energy transport process that erases
the cusp, though the parametrisations of the analytical model persist.
In our model, the dominant contribution to the dynamical coupling driving the
cusp-core transformation comes from the largest scale fluctuations. Yet, the
efficiency of the transformation is independent of the value of the largest
scale and depends weakly (linearly) on the power law exponent; it effectively
depends on two parameters: the gas mass fraction and the normalisation of the
power spectrum. This suggests that cusp-core transformations observed in
hydrodynamic simulations of galaxy formation may be understood and parametrised
in simple terms, the physical and numerical complexities of the various
implementations notwithstanding.Comment: Minor revisions to match version to appear in MNRAS; Section~2.3
largely rewritten for clarit
The final two redshifts for radio sources from the equatorial BRL sample
Best, Rottgering and Lehnert (1999, 2000a) defined a new sample of powerful
radio sources from the Molonglo Reference Catalogue, for which redshifts were
compiled or measured for 177 of the 178 objects. For the final object,
MRC1059-010 (3C249), the host galaxy is here identified using near-infrared
imaging, and the redshift is determined from VLT spectroscopy. For one other
object in the sample, MRC0320+053 (4C05.14), the literature redshift has been
questioned: new spectroscopic observations of this object are presented,
deriving a corrected redshift. With these two results, the spectroscopic
completeness of this sample is now 100%.
New redshifts are also presented for PKS0742+10 from the Wall & Peacock 2.7
GHz catalogue, and PKS1336+003 from the Parkes Selected Regions. PKS0742+10
shows a strong neutral hydrogen absorption feature in its Lyman-alpha emission
profile.Comment: 4 pages. LaTeX. Accepted for publication in MNRA
Deep near-infrared imaging of the HE0450-2958 system
The QSO HE0450-2958 and the companion galaxy with which it is interacting,
both ultra luminous in the infrared, have been the subject of much attention in
recent years, as the quasar host galaxy remained undetected. This led to
various interpretations on QSO and galaxy formation and co-evolution, such as
black hole ejection, jet induced star formation, dust obscured galaxy, or
normal host below the detection limit. We carried out deep observations in the
near-IR in order to solve the puzzle concerning the existence of any host. The
object was observed with the ESO VLT and HAWK-I in the near-IR J-band for 8
hours. The images have been processed with the MCS deconvolution method
(Magain, Courbin & Sohy, 1998), permitting accurate subtraction of the QSO
light from the observations. The compact emission region situated close to the
QSO, called the blob, which previously showed only gas emission lines in the
optical spectra, is now detected in our near-IR images. Its high brightness
implies that stars likely contribute to the near-IR emission. The blob might
thus be interpreted as an off-centre, bright and very compact host galaxy,
involved in a violent collision with its companion.Comment: 4 pages, 3 figures, accepted for publication in A&
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