621 research outputs found
Selecting background galaxies in weak-lensing analysis of galaxy clusters
In this paper, we present a new method to select the faint, background
galaxies used to derive the mass of galaxy clusters by weak lensing.
The method is based on the simultaneous analysis of the shear signal, that
should be consistent with zero for the foreground, unlensed galaxies, and of
the colors of the galaxies: photometric data from the COSMic evOlution Survey
are used to train the color selection. In order to validate this methodology,
we test it against a set of state-of-the-art image simulations of mock galaxy
clusters in different redshift [] and mass
[] ranges, mimicking medium-deep multicolor
imaging observations (e.g. SUBARU, LBT).
The performance of our method in terms of contamination by unlensed sources
is comparable to a selection based on photometric redshifts, which however
requires a good spectral coverage and is thus much more observationally
demanding. The application of our method to simulations gives an average ratio
between estimated and true masses of . As a further test,
we finally apply our method to real data, and compare our results with other
weak lensing mass estimates in the literature: for this purpose we choose the
cluster Abell 2219 (), for which multi-band (BVRi) data are publicly
available.Comment: MNRAS, Accepted 2016 February 2
Far-infrared mapping of the starburst galaxy NGC 253 with ISOPHOT
A 180 micron map and strip maps at 120 and 180 microns were obtained for the
edge-on starburst galaxy NGC 253 with ISOPHOT, the photometer on board the
Infrared Space Observatory. We compare these observations with those obtained
by IRAS at 60 micron and 100 micron and derive the far--infrared spectral
energy distribution at different locations in the galaxy. There is evidence for
the presence of cold dust (T < 20 K) both in the nucleus and in the disk.
Extended emission dominated by cold dust is detected up to 15 arcmin (~ 10 kpc)
along the major and minor axis; its spatial distribution is similar to that
seen in the IRAS and ROSAT PSPC images. The emission along the minor axis is
probably related to large-scale outflows of gas (superwinds) which originate in
the nuclear starburst and maybe to star formation in the halo. The radial
dependence of the dust temperature along the major axis is found using a
radiative transfer code: we show that the dust scale length in the disk is ~
40% larger than that of stars.Comment: 10 pages, 12 figures. Accepted for publication in Astronomy &
Astrophysic
Polarized Broad-Line Emission from Low-Luminosity Active Galactic Nuclei
In order to determine whether unified models of active galactic nuclei apply
to low-luminosity objects, we have undertaken a spectropolarimetric survey of
of LINERs and Seyfert nuclei at the Keck Observatory. The 14 objects observed
have a median H-alpha luminosity of 8x10^{39} erg/s, well below the typical
value of ~10^{41} erg/s for Markarian Seyfert nuclei. Polarized broad H-alpha
emission is detected in three LINERs: NGC 315, NGC 1052, and NGC 4261. Each of
these is an elliptical galaxy with a double-sided radio jet, and the
emission-line polarization in each case is oriented roughly perpendicular to
the jet axis, as expected for the obscuring torus model. NGC 4261 and NGC 315
are known to contain dusty circumnuclear disks, which may be the outer
extensions of the obscuring tori. The detection of polarized broad-line
emission suggests that these objects are nearby, low-luminosity analogs of
obscured quasars residing in narrow-line radio galaxies. The nuclear continuum
of the low-luminosity Seyfert 1 galaxy NGC 4395 is polarized at p = 0.67%,
possibly the result of an electron scattering region near the nucleus.
Continuum polarization is detected in other objects, with a median level of p =
0.36% over 5100-6100 A, but in most cases this is likely to be the result of
transmission through foreground dust. The lack of significant broad-line
polarization in most type 1 LINERs is consistent with the hypothesis that we
view the broad-line regions of these objects directly, rather than in scattered
light.Comment: 28 pages, including 3 tables and 16 figures. Uses the emulateapj
latex style file. Accepted for publication in The Astrophysical Journa
Formation of Large-Scale Obscuring Wall and AGN Evolution Regulated by Circumnuclear Starbursts
By considering the radiative force by a circumnuclear starburst as well as an
AGN, we analyze the equilibrium configuration and the stability of dusty gas in
the circumnuclear regions. It is found that the radiative force by an intensive
starburst can support a stable gaseous wall with a scale-height of several
hundred parsecs. Moreover, by taking the simple stellar evolution in the
starburst into account, we find that the covering factor of the wall decreases
on a time-scale of several yr. The large-scale wall, if formed, works to
obscure the nucleus due to the dust opacity. Hence, it is anticipated that the
index of AGN type tends to shift from higher to lower in several yr
according as the circumnuclear starburst becomes dimmer. On the other hand, if
the AGN itself is brighter than the circumnuclear starburst (e.g. quasar case),
no stable large-scale wall forms. In that case, the AGN is highly probably
identified as type 1. The present mechanism may provide a physical explanation
for the putative correlation between AGN type and host properties that Sy2's
are more frequently associated with circumnuclear starbursts than Sy1's,
whereas quasars are mostly observed as type 1 regardless of star-forming
activity in the host galaxies.Comment: 10 pages, 3 figures, ApJ Letters in pres
Searching for galaxy clusters in the Kilo-Degree Survey
In this paper, we present the tools used to search for galaxy clusters in the
Kilo Degree Survey (KiDS), and our first results. The cluster detection is
based on an implementation of the optimal filtering technique that enables us
to identify clusters as over-densities in the distribution of galaxies using
their positions on the sky, magnitudes, and photometric redshifts. The
contamination and completeness of the cluster catalog are derived using mock
catalogs based on the data themselves. The optimal signal to noise threshold
for the cluster detection is obtained by randomizing the galaxy positions and
selecting the value that produces a contamination of less than 20%. Starting
from a subset of clusters detected with high significance at low redshifts, we
shift them to higher redshifts to estimate the completeness as a function of
redshift: the average completeness is ~ 85%. An estimate of the mass of the
clusters is derived using the richness as a proxy. We obtained 1858 candidate
clusters with redshift 0 < z_c < 0.7 and mass 13.5 < log(M500/Msun) < 15 in an
area of 114 sq. degrees (KiDS ESO-DR2). A comparison with publicly available
Sloan Digital Sky Survey (SDSS)-based cluster catalogs shows that we match more
than 50% of the clusters (77% in the case of the redMaPPer catalog). We also
cross-matched our cluster catalog with the Abell clusters, and clusters found
by XMM and in the Planck-SZ survey; however, only a small number of them lie
inside the KiDS area currently available.Comment: 13 pages, 15 figures. Accepted for publication on Astronomy &
Astrophysic
The far-infrared energy distributions of Seyfert and starburst galaxies in the Local Universe: ISO photometry of the 12 micron active galaxy sample
New far-infrared photometry with ISOPHOT, onboard the Infrared Space
Observatory, is presented for 58 galaxies with homogeneous published data for
another 32 galaxies all belonging to the 12 micron galaxy sample. In total 29
Seyfert 1's, 35 Seyfert 2's and 12 starburst galaxies, about half of the 12
micron active galaxy sample, plus 14 normal galaxies for comparison. The ISO
and the IRAS data are used to define color-color diagrams and spectral energy
distributions (SED). Thermal dust emission at two temperatures (one cold at
15-30K and one warm at 50-70K) can fit the 60-200 micron SED, with a dust
emissivity law proportional to the inverse square of the wavelength. Seyfert
1's and Seyfert 2's are indistinguishable longward of 100 micron, while, as
already seen by IRAS, the former have flatter SEDs shortward of 60 micron. A
mild anti-correlation is found between the [200 - 100] color and the "60 micron
excess". We infer that this is due to the fact that galaxies with a strong
starburst component, and thus a strong 60 micron flux, have a steeper
far-infrared turnover. In non-Seyfert galaxies, increasing the luminosity
corresponds to increasing the star formation rate, that enhances the 25 and 60
micron emission. This shifts the peak emission from around 150 micron in the
most quiescent spirals to shorter than 60 micron in the strongest starburst
galaxies.Comment: Accepted for publication in The Astrophysical Journal AASTeX preprint
with 49 pages and 20 figures Also available at
http://orion.ifsi.rm.cnr.it/publ.htm
A weak lensing analysis of the PLCK G100.2-30.4 cluster
We present a mass estimate of the Planck-discovered cluster PLCK G100.2-30.4,
derived from a weak lensing analysis of deep SUBARU griz images. We perform a
careful selection of the background galaxies using the multi-band imaging data,
and undertake the weak lensing analysis on the deep (1hr) r-band image. The
shape measurement is based on the KSB algorithm; we adopt the PSFex software to
model the Point Spread Function (PSF) across the field and correct for this in
the shape measurement. The weak lensing analysis is validated through extensive
image simulations. We compare the resulting weak lensing mass profile and total
mass estimate to those obtained from our re-analysis of XMM-Newton
observations, derived under the hypothesis of hydrostatic equilibrium. The
total integrated mass profiles are in remarkably good agreement, agreeing
within 1 across their common radial range. A mass is derived for the cluster from our weak lensing
analysis. Comparing this value to that obtained from our reanalysis of
XMM-Newton data, we obtain a bias factor of (1-b) = 0.8 0.1. This is
compatible within 1 with the value of (1-b) obtained by Planck
Collaboration XXIV from their calibration of the bias factor using
newly-available weak lensing reconstructed masses.Comment: 11 pages, 12 figures, accepted for publication on Astronomy &
Astrophysics; updates in affiliation
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