42,877 research outputs found
The Extended Blue Continuum and Line Emission around the Central Radio Galaxy in Abell 2597
We present results from detailed imaging of the centrally dominant radio
elliptical galaxy in the cooling flow cluster Abell 2597, using data obtained
with the Wide Field and Planetary Camera 2 (WFPC2) on the Hubble Space
Telescope (HST). This object is one of the archetypal "blue-lobed" cooling flow
radio elliptical galaxies, also displaying a luminous emission-line nebula, a
compact radio source, and a significant dust lane and evidence of molecular gas
in its center. We show that the radio source is surrounded by a complex network
of emission-line filaments, some of which display a close spatial association
with the outer boundary of the radio lobes. We present a detailed analysis of
the physical properties of ionized and neutral gas associated with the radio
lobes, and show that their properties are strongly suggestive of direct
interactions between the radio plasma and ambient gas. We resolve the blue
continuum emission into a series of knots and clumps, and present evidence that
these are most likely due to regions of recent star formation. We investigate
several possible triggering mechanisms for the star formation, including direct
interactions with the radio source, filaments condensing from the cooling flow,
or the result of an interaction with a gas-rich galaxy, which may also have
been responsible for fueling the active nucleus. We propose that the properties
of the source are plausibly explained in terms of accretion of gas by the cD
during an interaction with a gas-rich galaxy, which combined with the fact that
this object is located at the center of a dense, high-pressure ICM can account
for the high rates of star formation and the strong confinement of the radio
source.Comment: Astrophysical Journal, in press, 34 pages, includes 6 PostScript
figures. Latex format, uses aaspp4.sty and epsf.sty file
The DiskMass Survey. X. Radio synthesis imaging of spiral galaxies
We present results from 21 cm radio synthesis imaging of 28 spiral galaxies
from the DiskMass Survey obtained with the VLA, WSRT, and GMRT facilities. We
detail the observations and data reduction procedures and present a brief
analysis of the radio data. We construct 21 cm continuum images, global HI
emission-line profiles, column-density maps, velocity fields, and
position-velocity diagrams. From these we determine star formation rates
(SFRs), HI line widths, total HI masses, rotation curves, and
azimuthally-averaged radial HI column-density profiles. All galaxies have an HI
disk that extends beyond the readily observable stellar disk, with an average
ratio and scatter of R_{HI}/R_{25}=1.35+/-0.22, and a majority of the galaxies
appear to have a warped HI disk. A tight correlation exists between total HI
mass and HI diameter, with the largest disks having a slightly lower average
column density. Galaxies with relatively large HI disks tend to exhibit an
enhanced stellar velocity dispersion at larger radii, suggesting the influence
of the gas disk on the stellar dynamics in the outer regions of disk galaxies.
We find a striking similarity among the radial HI surface density profiles,
where the average, normalized radial profile of the late-type spirals is
described surprisingly well with a Gaussian profile. These results can be used
to estimate HI surface density profiles in galaxies that only have a total HI
flux measurement. We compare our 21 cm radio continuum luminosities with 60
micron luminosities from IRAS observations for a subsample of 15 galaxies and
find that these follow a tight radio-infrared relation, with a hint of a
deviation from this relation at low luminosities. We also find a strong
correlation between the average SFR surface density and the K-band surface
brightness of the stellar disk.Comment: 22 pages + Appendix, 16 figures + Atlas, 5 tables. Accepted for
publication in Astronomy & Astrophysic
Hubble space telescope STIS spectroscopy of the peculiar nova-like variables BK Lyn, V751 Cygni, and V380 Oph
We obtained Hubble STIS spectra of three nova-like variables: V751 Cygni, V380 Oph, andâthe only confirmed nova-like variable known to be below the period gapâBK Lyn. In all three systems, the spectra were taken during high optical brightness state, and a luminous accretion disk dominates their far-ultraviolet (FUV) light. We assessed a lower limit of the distances by applying the infrared photometric method of Knigge. Within the limitations imposed by the poorly known system parameters (such as the inclination, white dwarf mass, and the applicability of steady state accretion disks) we obtained satisfactory fits to BK Lyn using optically thick accretion disk models with an accretion rate of for a white dwarf mass of Mwd = 1.2M and for Mwd = 0.4M. However, for the VY Scl-type nova-like variable V751 Cygni and for the SW Sex star V380 Oph, we are unable to obtain satisfactory synthetic spectral fits to the high state FUV spectra using optically thick steady state accretion disk models. The lack of FUV spectra information down to the Lyman limit hinders the extraction of information about the accreting white dwarf during the high states of these nova-like systems
On the Origin of [OII] Emission in Red Sequence and Post-starburst Galaxies
We investigate the emission-line properties of galaxies with red rest-frame
colors using spectra from SDSS DR4. Emission lines are detected in more than
half of the red galaxies. We focus on the relationship between two emission
lines commonly used as star formation rate indicators: Ha 6563 and [OII] 3727.
There is a strong bimodality in [OII]/Ha ratio in the full SDSS sample which
closely corresponds to the bimodality in rest-frame color. Nearly all of the
line-emitting red galaxies have line ratios typical of various types of AGN --
most commonly LINERs, a small fraction of transition objects and, more rarely,
Seyferts. Only ~6% of red galaxies display star-forming line ratios. A straight
line in the [OII]-Ha equivalent width plane separates LINER-like galaxies from
other categories. Quiescent galaxies with no detectable emission lines and
LINER-like galaxies combine to form a single, tight red sequence in
color-magnitude-concentration space. [OII] EWs in LINER- and AGN-like galaxies
can be as large as in star-forming galaxies. Thus, unless objects with
AGN/LINER-like line ratios are excluded, [OII] emission cannot be used directly
as a proxy for star formation rate. Lack of [OII] emission is generally used to
indicate lack of star formation when post-starburst galaxies are selected at
high redshift. Our results imply, however, that these samples have been cut on
AGN properties as well as star formation, and therefore may provide seriously
incomplete sets of post-starburst galaxies. Furthermore, post-starburst
galaxies identifed in SDSS by requiring minimal Ha EW generally exhibit weak
but nonzero line emission with ratios typical of AGNs; few of them show
residual star formation. This suggests that most post-starbursts may harbor
AGNs/LINERs.Comment: 21 pages, 15 figures. v2: Added 4 new figures and updated many;
extended text. No conclusions change. v3: minor modifications and figure
updates to match version accepted by Ap
The curvelet transform for image denoising
We describe approximate digital implementations of two new mathematical transforms, namely, the ridgelet transform and the curvelet transform. Our implementations offer exact reconstruction, stability against perturbations, ease of implementation, and low computational complexity. A central tool is Fourier-domain computation of an approximate digital Radon transform. We introduce a very simple interpolation in the Fourier space which takes Cartesian samples and yields samples on a rectopolar grid, which is a pseudo-polar sampling set based on a concentric squares geometry. Despite the crudeness of our interpolation, the visual performance is surprisingly good. Our ridgelet transform applies to the Radon transform a special overcomplete wavelet pyramid whose wavelets have compact support in the frequency domain. Our curvelet transform uses our ridgelet transform as a component step, and implements curvelet subbands using a filter bank of a` trous wavelet filters. Our philosophy throughout is that transforms should be overcomplete, rather than critically sampled. We apply these digital transforms to the denoising of some standard images embedded in white noise. In the tests reported here, simple thresholding of the curvelet coefficients is very competitive with "state of the art" techniques based on wavelets, including thresholding of decimated or undecimated wavelet transforms and also including tree-based Bayesian posterior mean methods. Moreover, the curvelet reconstructions exhibit higher perceptual quality than wavelet-based reconstructions, offering visually sharper images and, in particular, higher quality recovery of edges and of faint linear and curvilinear features. Existing theory for curvelet and ridgelet transforms suggests that these new approaches can outperform wavelet methods in certain image reconstruction problems. The empirical results reported here are in encouraging agreement
A Stellar Population Gradient in VII Zw 403 - Implications for the Formation of Blue Compact Dwarf Galaxies
We present evidence for the existence of an old stellar halo in the Blue
Compact Dwarf galaxy VII Zw 403. VII Zw 403 is the first Blue Compact Dwarf
galaxy for which a clear spatial segregation of the resolved stellar content
into a "core-halo" structure is detected. Multicolor HST/WFPC2 observations
indicate that active star formation occurs in the central region, but is
strikingly absent at large radii. Instead, a globular-cluster-like red giant
branch suggests the presence of an old (> 10 Gyr) and metal poor
(=-1.92) stellar population in the halo. While the vast majority of
Blue Compact Dwarf galaxies has been recognized to possess halos of red color
in ground-based surface photometry, our observations of VII Zw 403 establish
for the first time a direct correspondence between a red halo color and the
presence of old, red giant stars. If the halos of Blue Compact Dwarf galaxies
are all home to such ancient stellar populations, then the fossil record
conflicts with delayed-formation scenarios for dwarfs.Comment: Accepted for publication in the Ap
IRAC Excess in Distant Star-Forming Galaxies: Tentative Evidence for the 3.3m Polycyclic Aromatic Hydrocarbon Feature ?
We present evidence for the existence of an IRAC excess in the spectral
energy distribution (SED) of 5 galaxies at 0.6<z<0.9 and 1 galaxy at z=1.7.
These 6 galaxies, located in the Great Observatories Origins Deep Survey field
(GOODS-N), are star forming since they present strong 6.2, 7.7, and 11.3 um
polycyclic aromatic hydrocarbon (PAH) lines in their Spitzer IRS mid-infrared
spectra. We use a library of templates computed with PEGASE.2 to fit their
multiwavelength photometry and derive their stellar continuum. Subtraction of
the stellar continuum enables us to detect in 5 galaxies a significant excess
in the IRAC band pass where the 3.3 um PAH is expected. We then assess if the
physical origin of the IRAC excess is due to an obscured active galactic
nucleus (AGN) or warm dust emission. For one galaxy evidence of an obscured AGN
is found, while the remaining four do not exhibit any significant AGN activity.
Possible contamination by warm dust continuum of unknown origin as found in the
Galactic diffuse emission is discussed. The properties of such a continuum
would have to be different from the local Universe to explain the measured IRAC
excess, but we cannot definitively rule out this possibility until its origin
is understood. Assuming that the IRAC excess is dominated by the 3.3 um PAH
feature, we find good agreement with the observed 11.3 um PAH line flux arising
from the same C-H bending and stretching modes, consistent with model
expectations. Finally, the IRAC excess appears to be correlated with the
star-formation rate in the galaxies. Hence it could provide a powerful
diagnostic for measuring dusty star formation in z>3 galaxies once the
mid-infrared spectroscopic capabilities of the James Webb Space Telescope
become available.Comment: 25 pages, 4 figures, accepted by Ap
Analysing star cluster populations with stochastic models: the HST/WFC3 sample of clusters in M83
The majority of clusters in the Universe have masses well below 10^5 Msun.
Hence their integrated fluxes and colors can be affected by the random presence
of a few bright stars introduced by stochastic sampling of the stellar mass
function. Specific methods are being developed to extend the analysis of
cluster SEDs into the low-mass regime. In this paper, we apply such a method to
observations of star clusters, in the nearby spiral galaxy M83. We reassess
ages and masses of a sample of 1242 objects for which UBVIHalpha fluxes were
obtained with the HST/WFC3 images. Synthetic clusters with known properties are
used to characterize the limitations of the method. The ensemble of color
predictions of the discrete cluster models are in good agreement with the
distribution of observed colors. We emphasize the important role of the Halpha
data in the assessment of the fraction of young objects, particularly in
breaking the age-extinction degeneracy that hampers an analysis based on UBVI
only. We find the mass distribution of the cluster sample to follow a power-law
of index -2.1 +/-0.2, and the distribution of ages a power-law of index -1.0
+/-0.2 for M > 10^3.5 Msun and ages between 10^7 and 10^9 yr. An extension of
our main method, that makes full use of the probability distributions of age
and mass of the individual clusters, is explored. It produces similar power-law
slopes and will deserve further investigation. Although the properties derived
for individual clusters significantly differ from those obtained with
traditional, non-stochastic models in ~30% of the objects, the first order
aspect of the age and mass distributions are similar to those obtained
previously for this M83 sample in the range of overlap of the studies. We
extend the power-law description to lower masses with better mass and age
resolution and without most of the artifacts produced by the classical method.Comment: accepted for publication in ApJ, 29 pages, 20 figure
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