160 research outputs found
Multi-band constraints on the nature of emission line galaxies
Our aim is to explore the nature of emission line galaxies by combining
high-resolution observations obtained in different bands to understand which
objects are powered by an Active Galactic Nucleus(AGN). From the spectroscopic
Palomar survey of nearby bright galaxies, we selected a sample of 18 objects
observed with HST, Chandra, and VLA.
No connection is found between X-ray and emission line luminosities from
ground-based data, unlike what is found for brighter AGN. Conversely, a strong
correlation emerges when using the HST spectroscopic data, which are extracted
on a much smaller aperture. This suggests that the HST data better isolate the
AGN component when one is present, while ground-based line measurements are
affected by diffuse emission from the host galaxies.
The sample separates into two populations. The 11 objects belonging to the
first class have an equivalent width of the [OIII] emission line measured from
HST data EW([OIII])>~2 A and are associated with an X-ray nuclear source; in
the second group we find seven galaxies with EW([OIII])<~1 A that generally do
not show any emission related to an active nucleus (emission lines, X-ray, or
radio sources). This latter group includes about half of the Low Ionization
Nuclear Emission-line region (LINERs) or transition galaxies of the sample, all
of which are objects of low [OIII] line luminosity (<~1E38 erg s-1) and low
equivalent width (<~1 A) in ground-based observations. These results strengthen
the suggestion that the EW([OIII]) value is a robust predictor of the nature of
an emission line galaxy.Comment: 6 pages. Accepted for publication in A&
The Chandra view of the 3C/FRI sample of low luminosity radio-galaxies
We present results from Chandra observations of the 3C/FRI sample of low
luminosity radio-galaxies. We detected a power-law nuclear component in 12
objects out of the 18 with available data. In 4 galaxies we detected nuclear
X-ray absorption at a level of about N_H= (0.2-6)e22 cm-2. X-ray absorbed
sources are associated with the presence of highly inclined dusty disks (or
dust filaments projected onto the nuclei) seen in the HST images. This suggests
the existence of a flattened X-ray absorber, but of much lower optical depth
than in classical obscuring tori. We thus have an un-obstructed view toward
most FR~I nuclei while absorption plays only a marginal role in the remaining
objects. Three pieces of evidence support an interpretation for a jet origin
for the X-ray cores: i) the presence of strong correlations between the nuclear
luminosities in the radio, optical and X-ray bands, extending over 4 orders of
magnitude and with a much smaller dispersion (about 0.3 dex) when compared to
similar trends found for other classes of AGNs, pointing to a common origin for
the emission in the three bands; ii) the close similarity of the broad-band
spectral indices with the sub-class of BL Lac objects sharing the same range of
extended radio-luminosity, in accord with the FRI/BL Lacs unified model; iii)
the presence of a common luminosity evolution of spectral indices in both FRI
and BL Lacs. The low luminosities of the X-ray nuclei, regardless of their
origin, strengthens the interpretation of low efficiency accretion in low
luminosity radio-galaxies.Comment: Accepted in A&
The HST view of the innermost narrow line region
We analyze the properties of the innermost narrow line region in a sample of
low-luminosity AGN. We select 33 LINERs (bona fide AGN) and Seyfert galaxies
from the optical spectroscopic Palomar survey observed by HST/STIS. We find
that in LINERs the [NII] and [OI] lines are broader than the [SII] line and
that the [NII]/[SII] flux ratio increases when moving from ground-based to HST
spectra. This effect is more pronounced considering the wings of the lines. Our
interpretation is that, as a result of superior HST spatial resolution, we
isolate a compact region of dense ionized gas in LINERs, located at a typical
distance of about 3 pc and with a gas density of about 10-10 cm,
which we identify with the outer portion of the intermediate line region (ILR).
Instead, we do not observe these kinds of effects in Seyferts; this may be the
result of a stronger dilution from the NLR emission, since the HST slit maps a
larger region in these sources. Alternatively, we argue that the innermost,
higher density component of the ILR is only present in Seyferts, while it is
truncated at larger radii because of the presence of the circumnuclear torus.
The ILR is only visible in its entirety in LINERs because the obscuring torus
is not present in these sources.Comment: 11 pages, 9 figures, A&A in pres
Is there any evidence that ionised outflows quench star formation in type 1 quasars at z<1?
The aim of this paper is to test the basic model of negative AGN feedback.
According to this model, once the central black hole accretes at the Eddington
limit and reaches a certain critical mass, AGN driven outflows blow out gas,
suppressing star formation in the host galaxy and self-regulating black hole
growth. We consider a sample of 224 quasars selected from the SDSS at z<1
observed in the infrared band by Herschel. We evaluate the star formation rate
in relation to several outflow signatures traced by the [OIII]4959,5007 and
[OII]3726,3729 emission lines in about half of the sample with high quality
spectra. Most of the quasars show asymmetric and broad wings in [OIII], which
we interpret as outflow signatures. We separate the quasars in two groups,
``weakly'' and ``strongly'' outflowing, using three different criteria. When we
compare the mean star formation rate in five redshift bins in the two groups,
we find that the SFRs are comparable or slightly larger in the strongly
outflowing quasars. We estimate the stellar mass from SED fitting and the
quasars are distributed along the star formation main sequence, although with a
large scatter. The scatter from this relation is uncorrelated with respect to
the kinematic properties of the outflow. Moreover, for quasars dominated in the
infrared by starburst or by AGN emission, we do not find any correlation
between the star formation rate and the velocity of the outflow, a trend
previously reported in the literature for pure starburst galaxies. We conclude
that the basic AGN negative feedback scenario seems not to agree with our
results. Although we use a large sample of quasars, we did not find any
evidence that the star formation rate is suppressed in the presence of AGN
driven outflows on large scale. A possibility is that feedback is effective
over much longer timescales than those of single episodes of quasar activity.Comment: 18 pages, new version that implements the suggestions of the referee
and matches the AA published versio
The VLT/MUSE view of the central galaxy in Abell 2052. Ionized gas swept by the expanding radio source
We report observations of the radio galaxy 3C317 (at z=0.0345) located at the
center of the Abell cluster A2052, obtained with the VLT/MUSE integral field
spectrograph. The Chandra images of this cluster show cavities in the X-ray
emitting gas, which were produced by the expansion of the radio lobes inflated
by the active galactic nucleus (AGN). Our exquisite MUSE data show with
unprecedented detail the complex network of line emitting filaments enshrouding
the northern X-ray cavity. We do not detect any emission lines from the
southern cavity, with a luminosity asymmetry between the two regions higher
than about 75. The emission lines produced by the warm phase of the
interstellar medium (WIM) enable us to obtain unique information on the
properties of the emitting gas. We find dense gas (up to 270 cm-3) that makes
up part of a global quasi spherical outflow that is driven by the radio source,
and obtain a direct estimate of the expansion velocity of the cavities (265 km
s-1). The emission lines diagnostic rules out ionization from the AGN or from
star-forming regions, suggesting instead ionization from slow shocks or from
cosmic rays. The striking asymmetric line emission observed between the two
cavities contrasts with the less pronounced differences between the north and
south sides in the hot gas; this represents a significant new ingredient for
our understanding of the process of the exchange of energy between the
relativistic plasma and the external medium. We conclude that the expanding
radio lobes displace the hot tenuous phase of the interstellar medium (ISM),
but also impact the colder and denser ISM phases. These results show the
effects of the AGN on its host and the importance of radio mode feedback.Comment: 7 pages, 9 figures, A&A in pres
The accretion mechanism in low-power radio galaxies
We study a sample of 44 low-luminosity radio-loud AGN, which represent a
range of nuclear radio-power spanning 5 orders of magnitude, to unveil the
accretion mechanism in these galaxies. We estimate the accretion rate of gas
associated with their hot coronae by analyzing archival Chandra data, to derive
the deprojected density and temperature profiles in a spherical approximation.
Measuring the jet power from the nuclear radio-luminosity, we find that the
accretion power correlates linearly with the jet power, with an efficiency of
conversion from rest mass into jet power of ~0.012. These results strengthen
and extend the validity of the results obtained by Allen and collaborators for
9 radio galaxies, indicating that hot gas accretion is the dominant process in
FR I radio galaxies across their full range of radio-luminosity.
We find that the different levels of nuclear activity are driven by global
differences in the structure of the galactic hot coronae. A linear relation
links the jet power with the host X-ray surface brightness. This implies that a
substantial change in the jet power must be accompanied by a global change in
its ISM properties, driven for example by a major merger. This correlation
provides a simple widely applicable method to estimate the jet-power of a given
object by observing the intensity of its host X-ray emission.
To maintain the mass flow in the jet, the fraction of gas that crosses the
Bondi radius reaching the accretion disk must be > 0.002. This implies that the
radiative efficiency of the disk must be < 0.005, an indication that accretion
in these objects occurs not only at a lower rate, but also at lower efficiency
than in standard accretion disks.Comment: Accepted for publication in A&
The primordial environment of supermassive black holes (II): deep Y and J band images around the z=6.3 quasar SDSS J1030+0524
Many cosmological studies predict that early supermassive black holes (SMBHs)
can only form in the most massive dark matter halos embedded within large scale
structures marked by galaxy over-densities that may extend up to 10 physical
Mpc. This scenario, however, has not been confirmed observationally, as the
search for galaxy over-densities around high-z quasars has returned conflicting
results. The field around the z=6.28 quasar SDSSJ1030+0524 (J1030) is unique
for multi-band coverage and represents an excellent data legacy for studying
the environment around a primordial SMBH. In this paper we present wide-area
(25x25 arcmin), Y- and J-band imaging of the J1030 field obtained with the near
infrared camera WIRCam at the Canada-France-Hawaii Telescope (CFHT). We built
source catalogues in the Y- and J-band, and matched those with our photometric
catalogue in the r, z, i bands presented in Morselli et al. (2014). We used
these new infrared data together with H and K and Spitzer/IRAC data to refine
our selection of Lyman Break Galaxies (LBGs), extending our selection criteria
to galaxies in the range 25.2<zAB<25.7. We selected 21 robust high-z candidates
in the J1030 field with photometric redshift around 6 and colors i-z>=1.3. We
found a significant asymmetry in the distribution of the high-z galaxies in
J1030, supporting the existence of a coherent large-scale structure around the
quasar. We compared our results with those of Bowler et al. (2015), who adopted
similar LBGs selection criteria, and estimated an over-density of galaxies in
the field of delta = 2.4, which is significant at >4 sigma. The over-density
value and its significance are higher than those found in Morselli et al.
(2014), and we interpret this as evidence of an improved LBG selection.Comment: 13 pages, 8 figures, accepted for publication in A&
The MURALES survey II. Presentation of MUSE observations of 20 3C low-z radio galaxies and first results
We present observations of a complete sub-sample of 20 radio galaxies from
the Third Cambridge Catalog (3C) with redshift <0.3 obtained from VLT/MUSE
optical integral field spectrograph. These data have been obtained as part of
the survey MURALES (a MUse RAdio Loud Emission line Snapshot survey) with the
main goal of exploring the Active Galactic Nuclei (AGN) feedback process in a
sizeable sample of the most powerful radio sources at low redshift. We present
the data analysis and, for each source, the resulting emission line images and
the 2D gas velocity field. Thanks to their unprecedented depth (the median 3
sigma surface brightness limit in the emission line maps is 6X10^-18 erg s-1
cm-2 arcsec-2, these observations reveal emission line structures extending to
several tens of kiloparsec in most objects. In nine sources the gas velocity
shows ordered rotation, but in the other cases it is highly complex. 3C sources
show a connection between radio morphology and emission line properties.
Whereas, in three of the four Fanaroff and Riley Class I radio galaxies (FRIs),
the line emission regions are compact, ~1 kpc in size; in all but one of the
Class II radiogalaxies FRIIs, we detected large scale structures of ionized gas
with a median extent of 17 kpc. Among the FRIIs, those of high and low
excitation show extended gas structures with similar morphological properties,
suggesting that they both inhabit regions characterized by a rich gaseous
environment on kpc scale.Comment: Accepted for publication in A&
The host galaxy/AGN connection. Brightness profiles of early-type galaxies hosting Seyfert nuclei
[ABRIDGED] We recently presented evidence of a connection between the
brightness profiles of nearby early-type galaxies and the properties of the AGN
they host. The radio loudness of the AGN appears to be univocally related to
the host's brightness profile: radio-loud nuclei are only hosted by ``core''
galaxies while radio-quiet AGN are only found in ``power-law'' galaxies. We
extend our analysis here to a sample of 42 nearby (V < 7000 km/s) Seyfert
galaxies hosted by early-type galaxies. We used the available HST images to
study their brightness profiles. Having excluded complex and highly nucleated
galaxies, in the remaining 16 objects the brightness profiles can be
successfully modeled with a Nuker law with a steep nuclear cusp characteristic
of ``power-law'' galaxies (with logarithmic slope 0.51 - 1.07). This result is
what is expected for these radio-quiet AGN based on our previous findings, thus
extending the validity of the connection between brightness profile and radio
loudness to AGN of a far higher luminosity. We explored the robustness of this
result against a different choice of the analytic form for the brightness
profiles, using a Sersic law. In no object could we find evidence of a central
light deficit with respect to a pure Sersic model, the defining feature of
``core'' galaxies in this modeling framework. We conclude that, regardless of
the modeling strategy, the dichotomy of AGN radio loudness can be univocally
related to the host's brightness profile. Our general results can be re-phrased
as ``radio-loud nuclei are hosted by core galaxies, while radio-quiet AGN are
found in non-core galaxies''.Comment: Accepted for publication in A&
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