354 research outputs found
What triggers a radio AGN? The intriguing case of PKSB 1718-649
We present new Australia Telescope Compact Array (ATCA) observations of the
young (< 10^2 years) radio galaxy PKS B1718-649. We study the morphology and
the kinematics of the neutral hydrogen (HI) disk (M(HI) = 1.1x 10^10 M(sun),
radius ~ 30 kpc). In particular, we focus on the analysis of the cold gas in
relation to the triggering of the nuclear activity. The asymmetries at the
edges of the disk date the last interaction with a companion to more than 1 Gyr
ago. The tilted-ring model of the HI disk shows that this event may have formed
the disk as we see it now, but that it may have not been responsible for
triggering the AGN. The long timescales of the interaction are incompatible
with the short ones of the radio activity. In absorption, we identify two
clouds with radial motions which may represent a population that could be
involved in the triggering of the radio activity. We argue that PKS B1718-649
may belong to a family of young low-excitation radio AGN where, rather than
through a gas rich merger, the active nuclei (AGN) are triggered by local
mechanisms such as accretion of small gas clouds.Comment: 8 pages, 9 figures, Accepted to A&
The AGN fuelling/feedback cycle in nearby radio galaxies - V. The cold atomic gas of NGC 3100 and its group
We present Australia Compact Telescope Array (ATCA) 21-cm observations of the
nearby low-excitation radio galaxy (LERG) NGC 3100. This is the brightest
galaxy of a loose group and hosts a young ( Myr) radio source. The ATCA
observations reveal for the first time the presence of neutral hydrogen (HI)
gas in absorption in the centre of this radio galaxy, and in emission in two
low-mass galaxies of the group and in a diffuse dark cloud in the proximity of
NGC 3100. The sensitivity to low-column density gas (
cm) allows us to reveal asymmetries in the periphery of most the
HI-detected galaxies, suggesting that tidal interactions may be on-going. The
diffuse cloud does not show a stellar counterpart down to mag/arcsec
and could be the remnant of these interactions. The analysis of the HI
absorption complex in NGC 3100 indicates that the atomic phase of the hydrogen
is distributed as its molecular phase (observed at arcsecond resolution through
several carbon monoxide emission lines). We suggest that the interactions
occurring within the group are causing turbulent cold gas clouds in the
intra-group medium to be slowly accreted towards the centre of NGC 3100. This
caused the recent formation of the cold circum-nuclear disk which is likely
sustaining the young nuclear activity.Comment: 10 pages, 7 figures; submitted to Astronomy & Astrophysic
The MeerKAT Fornax Survey
We present the science case and observations plan of the MeerKAT Fornax
Survey, an HI and radio continuum survey of the Fornax galaxy cluster to be
carried out with the SKA precursor MeerKAT. Fornax is the second most massive
cluster within 20 Mpc and the largest nearby cluster in the southern
hemisphere. Its low X-ray luminosity makes it representative of the environment
where most galaxies live and where substantial galaxy evolution takes place.
Fornax's ongoing growth makes it an excellent laboratory for studying the
assembly of clusters, the physics of gas accretion and stripping in galaxies
falling in the cluster, and the connection between these processes and the
neutral medium in the cosmic web.
We will observe a region of 12 deg reaching a projected distance of 1.5
Mpc from the cluster centre. This will cover a wide range of environment
density out to the outskirts of the cluster, where gas-rich in-falling groups
are found. We will: study the HI morphology of resolved galaxies down to a
column density of a few times 1e+19 cm at a resolution of 1 kpc; measure
the slope of the HI mass function down to M(HI) 5e+5 M(sun); and attempt to
detect HI in the cosmic web reaching a column density of 1e+18 cm at a
resolution of 10 kpc.Comment: Proceedings of Science, "MeerKAT Science: On the Pathway to the SKA",
Stellenbosch, 25-27 May 201
Obscured and unobscured AGN populations in a hard-X-ray selected sample of the XMDS survey
Our goal is to probe the populations of obscured and unobscured AGN
investigating their optical-IR and X-ray properties as a function of X-ray
flux, luminosity and redshift within a hard X-ray selected sample of 136 X-ray
sources in the XMM Medium Deep Survey (XMDS) with wide multiwavelength
coverage. The XMDS area is covered with optical photometry from the VVDS and
CFHTLS surveys and infrared Spitzer data. Based on the X-ray luminosity and
X-ray to optical ratio, 132 sources are likely AGN, of which 122 have
unambiguous optical - IR identification. The observed optical and IR spectral
energy distributions of sources are fitted with AGN/galaxy templates in order
to classify them and compute photometric redshifts. 70% of the AGN are fitted
by a type 2 AGN or a star forming galaxy template and are grouped together in a
single class of ``optically obscured'' AGN. They have ``red'' optical colors
and generally show significant X-ray absorption from X-ray spectra or hardness
ratios (N cm). Sources with SEDs typical of type 1 AGN
have ``blue'' optical colors and exhibit X-ray absorption in about 30% of
cases. We performed a stacking analysis for obscured and type 1 AGN. The
stacked X-ray spectrum of obscured AGN is flatter than that of type 1 AGN and
has an average spectral slope of Gamma = 1.6. The subsample of objects fitted
by a galaxy template has an even harder stacked spectrum, with Gamma = 1.2 -
1.3. The obscured fraction is larger at lower fluxes, lower redshifts and lower
luminosities. X-ray absorption is less common than ``optical'' obscuration and
its incidence is nearly constant with redshift and luminosity. This implies
that X-ray absorption is not necessarily related to optical obscuration.Comment: 33 pages, 21 figures, accepted for publication in A&
Passive galaxies as tracers of cluster environments at z~2
Even 10 billion years ago, the cores of the first galaxy clusters are often
found to host a characteristic population of massive galaxies with already
suppressed star formation. Here we search for distant cluster candidates at z~2
using massive passive galaxies as tracers. With a sample of ~40
spectroscopically confirmed passive galaxies at 1.3<z<2.1, we tune photometric
redshifts of several thousands passive sources in the full 2 sq.deg. COSMOS
field. This allows us to map their density in redshift slices, probing the
large scale structure in the COSMOS field as traced by passive sources. We
report here on the three strongest passive galaxy overdensities that we
identify in the redshift range 1.5<z<2.5. While the actual nature of these
concentrations is still to be confirmed, we discuss their identification
procedure, and the arguments supporting them as candidate galaxy clusters
(likely mid-10^13 M_sun range). Although this search approach is likely biased
towards more evolved structures, it has the potential to select still rare,
cluster-like environments close to their epoch of first appearance, enabling
new investigations of the evolution of galaxies in the context of structure
growth.Comment: 5 pages, 5 figures; A&A Letters, in pres
Spectral Energy Distributions of Hard X-ray selected AGNs in the XMDS Survey
We present the spectral energy distributions (SEDs) of a hard X-ray selected
sample. The sample contains 136 sources with F(2-10 keV)>10^-14 erg/cm^2/s and
132 are AGNs. The sources are detected in a 1 square degree area of the
XMM-Newton-Medium Deep Survey where optical data from the VVDS, CFHTLS surveys,
and infrared data from the SWIRE survey are available. Based on a SED fitting
technique we derive photometric redshifts with sigma(1+z)=0.11 and 6% of
outliers and identify AGN signatures in 83% of the objects. This fraction is
higher than derived when a spectroscopic classification is available. The
remaining 17+9-6% of AGNs shows star-forming galaxy SEDs (SF class). The
sources with AGN signatures are divided in two classes, AGN1 (33+6-1%) and AGN2
(50+6-11). The AGN1 and AGN2 classes include sources whose SEDs are fitted by
type 1 and type 2 AGN templates, respectively. On average, AGN1s show soft
X-ray spectra, consistent with being unabsorbed, while AGN2s and SFs show hard
X-ray spectra, consistent with being absorbed. The analysis of the average SEDs
as a function of X-ray luminosity shows a reddening of the IR SEDs, consistent
with a decreasing contribution from the host galaxy at higher luminosities. The
AGNs in the SF classes are likely obscured in the mid-infrared, as suggested by
their low L(3-20micron)/Lcorr(0.5-10 keV) ratios. We confirm the previously
found correlation for AGNs between the radio luminosity and the X-ray and the
mid-infrared luminosities. The X-ray-radio correlation can be used to identify
heavily absorbed AGNs. However, the estimated radio fluxes for the missing AGN
population responsible for the bulk of the background at E>10 keV are too faint
to be detected even in the deepest current radio surveys.Comment: Accepted for publication in Ap
Observations of the BL Lac Object 3C 66A with STACEE
We present the analysis and results of recent high-energy gamma-ray
observations of the BL Lac object 3C 66A conducted with the Solar Tower
Atmospheric Cherenkov Effect Experiment (STACEE). During the 2003-2004
observing season, STACEE extensively observed 3C 66A as part of a
multiwavelength campaign on the source. A total of 33.7 hours of data was taken
on the source, plus an equivalent-duration background observation. After
cleaning the data set a total of 16.3 hours of live time remained, and a net
on-source excess of 1134 events was seen against a background of 231742 events.
At a significance of 2.2 standard deviations this excess is insufficient to
claim a detection of 3C 66A, but is used to establish flux upper limits for the
source.Comment: Accepted for publication in the Astrophysical Journa
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