460 research outputs found
MUSE observations of a changing-look AGN I: The re-appearance of the broad emission lines
Optical changing-look Active Galactic Nuclei (AGN) are a class of sources
that change type within a short timescale of years or decades. This change is
characterised by the appearance or disappearance of broad emission lines, often
associated with dramatic AGN continuum flux changes that are orders of
magnitude larger than those expected from typical AGN variability. In this work
we study for the first time the host galaxy of a changing-look AGN, Mrk 590,
using high spatial resolution optical and near-infrared observations. We
discover that after ~ 10 yr absence, the optical broad emission lines of Mrk
590 have reappeared. The AGN optical continuum flux however, is still ~ 10
times lower than that observed during the most luminous state in the 1990s. The
host galaxy shows a 4.5 kpc radius star-forming ring with knots of ionised and
cold molecular gas emission. Extended ionised and warm molecular gas emission
are detected in the nucleus, indicating that there is a reservoir of gas as
close as 60 pc from the black hole. We observe a nuclear gas spiral between
radii r ~ 0.5 - 2 kpc, which has been suggested as a dynamical mechanism able
to drive the necessary gas to fuel AGN. We also discover blue-shifted and high
velocity dispersion [O III] emission out to a radius of 1 kpc, tracing a
nuclear gas outflow. The gas dynamics in Mrk 590 suggest a complex balance
between gas inflow and outflow in the nucleus of the galaxy.Comment: Accepted for publication in MNRA
Are truncated stellar disks linked to the molecular gas density?
We know that the slope of the radial, stellar light distribution in galaxies
is well described by an exponential decline and this distribution is often
truncated at a break radius (). We don't have a clear understanding for
the origin of these outer truncations and several hypotheses have been proposed
to explain them. We want to test the various theories with direct observations
of the cold molecular gas for a few truncated galaxies in comparison with the
non-truncated ones. The answer to the existence of a possible link between
truncated stellar disks and the molecular gas density cannot be obtained from
CO maps in the literature, because so far there are no galaxies with a clear
truncation observed in CO at high resolution.Comment: 3 pages, 6 figures, to appear in Astrophysics and Space Science
(Apss), special issue of "Science with ALMA: a new era for Astrophysics"
conference, ed. Dr. Bachille
ALMA observations of cool dust in a low-metallicity starburst, SBS0335-052
We present Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 0 Band 7
observations of an extremely metal-poor dwarf starburst galaxy in the Local
Universe, SBS0335-052 (12+log(O/H)~7.2). With these observations, dust is
detected at 870micron (ALMA Band 7), but 87% of the flux in this band is due to
free-free emission from the starburst. We have compiled a spectral energy
distribution (SED) of SBS0335-052 that spans almost 6 orders of magnitude in
wavelength and fit it with a spherical dust shell heated by a single-age
stellar population; the best-fit model gives a dust mass of (3.8+/-0.6)x10^4
Msun. We have also constructed a SED including Herschel archival data for
IZw18, another low-metallicity dwarf starburst (12+log(O/H)=7.17), and fit it
with a similar model to obtain a dust mass of (3.4+/-1.0)x10^2 Msun. Compared
with their atomic gas mass, the dust mass of SBS0335-052 far exceeds the
prediction of a linear trend of dust-to-gas mass ratio with metallicity, while
IZw18 falls far below. We use gas scaling relations to assess a putative
missing gas component in both galaxies and find that the missing, possibly
molecular, gas in SBS0335-052 is a factor of 6 times higher than the value
inferred from the observed HI column density; in IZw18 the missing component is
4 times smaller. Ultimately, despite their similarly low metallicity, the
differences in gas and dust column densities in SBS0335-052 and IZw18 suggest
that metal abundance does not uniquely define star-formation processes. At some
level, self-shielding and the survival of molecules may depend just as much on
gas and dust column density as on metallicity. The effects of low metallicity
may at least be partially compensated for by large column densities in the
interstellar medium.Comment: 15 pages, 11 figures, accepted for publication in A&
Glass forming ability and thermal stability of F-phlogopite based glasses
This paper presents the results of a study that analyses the effect of fluorine content on glass forming ability (GFA), glass stability (GS) and preferred crystallisation mechanism for a series of glasses in the SiO2-Al2O3-MgO-K2O-F system. Three glass compositions, with fluorine contents ranging from 4.50 to 5.70 wt. %, were investigated by differential scanning calorimetry (DSC). The GS was established by estimating different parameters derived from characteristic temperatures of non-isothermal DSC curves, namely, the working range (TTS), reduced glass transition temperature (Tgr), Weinberg (Kw), Hrubÿ (KH) and Lu-Liu (KLL) parameters. The prevalent crystallisation mechanism for each glass was assessed by determining the dissimilarity in crystallisation temperature (Tp) between fine ( 120°C/min) and obtaining amorphous glasses is only possible by fast cooling of the melt. In a subsequent thermal treatment, a volume crystallization mechanism will be prevalent in the process of devitrification of these F-phlogopite based glasses. Nevertheless, the increasing on the fluorine content in the glass composition leads to a variation in the location of the first developed crystals from the internal volume of the glass particle to surface sites. The results established by DSC analyses are verified by the results obtained from field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD).R. Casasola and J. M. Pérez express their gratitude to the Spanish National Research Council (CSIC) for their contract through the JAE Program (JAEPre-08-00456 and JAEDoc-08-00362, respectively), which is co-financed by the European Social Fund. The financial support through the projects MAT 2006-05977 and MAT2013-40477-P is also recognised.Peer reviewe
Molecular depletion times and the CO-to-H2 conversion factor in metal-poor galaxies
Tracing molecular hydrogen content with carbon monoxide in low-metallicity
galaxies has been exceedingly difficult. Here we present a new effort, with
IRAM 30-m observations of 12CO(1-0) of a sample of 8 dwarf galaxies having
oxygen abundances ranging from 12+logO/H=7.7 to 8.4. CO emission is detected in
all galaxies, including the most metal-poor galaxy of our sample (0.1 Zsun); to
our knowledge this is the largest number of 12CO(1-0) detections ever reported
for galaxies with 12+logO/H<=8 (0.2 Zsun) outside the Local Group. We calculate
stellar masses (Mstar) and star-formation rates (SFRs), and analyze our results
by combining our observations with galaxy samples from the literature.
Extending previous results for a correlation of the molecular gas depletion
time, tau(dep), with Mstar and specific SFR (sSFR), we find a variation in
tau(dep) of a factor of 200 or more (from <50 Myr to 10 Gyr) over a spread of
1000 in sSFR and Mstar. We exploit the variation of tau(dep) to constrain the
CO-to-H2 mass conversion factor alpha(CO) at low metallicity, and assuming a
power-law variation find alpha(CO) \propto (Z/Zsun)^1.9, similar to results
based on dust continuum measurements compared with gas mass. By including HI
measurements, we show that the fraction of total gas mass relative to the
baryonic mass is higher in galaxies that are metal poor, of low mass, and of
high sSFR. Finally, comparisons of the data with star-formation models of the
molecular gas phases suggest that, at metallicities Z/Zsun<=0.2, there are some
discrepancies with model predictions.Comment: 18 pages, 15 figures, accepted for publication in A&
ALMA reveals the feeding of the Seyfert 1 nucleus in NGC 1566
We report ALMA observations of CO(3-2) emission in the Seyfert 1 galaxy NGC
1566, at a spatial resolution of 25 pc. Our aim is to investigate the
morphology and dynamics of the gas inside the central kpc, and to probe nuclear
fueling and feedback phenomena. NGC 1566 has a nuclear bar of 1.7 kpc radius
and a conspicuous grand design spiral starting from this radius. The ALMA field
of view, of diameter 0.9 kpc, lies well inside the nuclear bar and reveals a
molecular trailing spiral structure from 50 to 300~pc in size, which is
contributing to fuel the nucleus, according to its negative gravity torques.
The spiral starts with a large pitch angle from the center and then winds up
in a pseudo-ring at the inner Lindblad resonance (ILR) of the nuclear bar.
This is the first time that a trailing spiral structure is clearly seen
driving the gas inwards inside the ILR ring of the nuclear bar. This phenomenon
shows that the massive central black hole has a significant dynamical influence
on the gas, triggering its fueling.
The gaseous spiral is well correlated with the dusty spiral seen through
extinction in HST images, and also with a spiral feature emitting 0.87mm
continuum. This continuum emission must come essentially from cold dust heated
by the interstellar radiation field. The HCN(4-3) and HCO+(4-3) lines were
simultaneously mapped and detected in the nuclear spiral. The HCO+(4-3) line is
3 times stronger than the HCN(4-3), as expected when star formation excitation
dominates over active galactic nucleus (AGN) heating. The CO(3-2)/HCO+(4-3)
integrated intensity ratio is \sim 100.
The molecular gas is in remarkably regular rotation, with only slight
non-circular motions at the periphery of the nuclear spiral arms. These
perturbations are quite small, and no outflow nor AGN feedback is detected.Comment: 11 pages, 16 figures, accepted in Astronomy and Astrophysic
ALMA reveals a warm and compact starburst around a heavily obscured supermassive black hole at z=4.75
We report ALMA Cycle 0 observations at 1.3mm of LESS J033229.4-275619
(XID403), an Ultraluminous Infrared Galaxy at in the Chandra Deep
Field South hosting a Compton-thick QSO. The source is not resolved in our data
at a resolution of 0.75 arcsec, placing an upper-limit of 2.5 kpc to the
half-light radius of the continuum emission from heated-dust. After
deconvolving for the beam size, however, we found a indication of
an intrinsic source size of arcsec (Gaussian FWHM), which would
correspond to kpc. We build the far-IR SED of XID403 by
combining datapoints from both ALMA and Herschel and fit it with a modified
blackbody spectrum. For the first time, we measure the dust temperature
K in this system, which is comparable to what has been
observed in other high-z submillimeter galaxies. The measured star formation
rate is SFR= yr, in agreement with previous
estimates at lower S/N. Based on the measured SFR and source size, we constrain
the SFR surface density to be yrkpc
(yrkpc for kpc). The
compactness of this starburst is comparable to what has been observed in other
local and high-z starburst galaxies. If the gas mass measured from previous
[CII] and CO(2-1) observations at low resolution is confined within the same
dust region, assuming kpc, this would produce a column
density of cm towards the central SMBH,
similar to the column density of cm measured
from the X-rays. Then, in principle, if both gas and dust were confined on
sub-kpc scales, this would be sufficient to produce the observed X-ray column
density without any need of a pc-scale absorber [abridged].Comment: 11 pages, 5 figures. Accepted for publication in A&
ALMA observations of feeding and feedback in nearby Seyfert galaxies: an AGN-driven outflow in NGC 1433
We report ALMA observations of CO(3-2) emission in the Seyfert 2
double-barred galaxy NGC1433, at the unprecedented spatial resolution of
0.5"=24 pc. Our aim is to probe AGN feeding and feedback phenomena through the
morphology and dynamics of the gas inside the central kpc. The CO map, which
covers the whole nuclear region (nuclear bar and ring), reveals a nuclear
gaseous spiral structure, inside the nuclear ring encircling the nuclear
stellar bar.
This gaseous spiral is well correlated with the dusty spiral seen in Hubble
Space Telescope images. The nuclear spiral winds up in a pseudo-ring at 200 pc
radius, which might correspond to the inner ILR. Continuum emission is detected
at 0.87 mm only at the very centre, and its origin is more likely thermal dust
emission than non-thermal emission from the AGN. It might correspond to the
molecular torus expected to exist in this Seyfert 2 galaxy. The HCN(4-3) and
HCO+(4-3) lines were observed simultaneously, but only upper limits are
derived, with a ratio to the CO(3-2) line lower than 1/60 at 3sigma, indicating
a relatively low abundance of very dense gas. The kinematics of the gas over
the nuclear disk reveal rather regular rotation only slightly perturbed by
streaming motions due to the spiral; the primary and secondary bars are too
closely aligned with the galaxy major or minor axis to leave a signature in the
projected velocities. Near the nucleus, there is an intense high-velocity CO
emission feature redshifted to 200 km/s (if located in the plane), with a
blue-shifted counterpart, at 2" (100 pc) from the centre. While the CO spectra
are quite narrow in the centre, this wide component is interpreted as an
outflow involving a molecular mass of 3.6 10^6 Mo and a flow rate 7 Mo/yr. The
flow could be in part driven by the central star formation, but is mainly
boosted by the AGN through its wind or radio jets.Comment: 11 pages, 9 figures, Accepted in Astronomy and Astrophysic
Nucleation and crystallisation kinetics of a Na-fluorrichterite based glass by differential scanning calorimetry (DSC)
The present paper shows the results of a nucleation and crystallisation study of a Na-fluorrichterite glass carried out by dynamic scanning calorimetry (DSC). The kinetic study was performed using different procedures (Kissinger, Matusita–Sakka and Kissinger–Akahira–Sunose (KAS) methods), and the Avrami parameter was determined from the Ozawa and Malek approximations and the Malek equation. The results have indicated the coexistence of surface and bulk crystallisation in the devitrification process of the studied glass. The kinetic study has shown that the activation energy of the crystallisation process is over 400 kJ/mol and that the mechanism proposed is a Johnson–Mehl–Avrami mechanism with n equal to 3, which implies that the crystallisation develops through the three-dimensional growth of crystals. The study of the variation of the activation energy with crystallisation using the KAS method has shown that the crystallisation process undergoes a multiple step mechanism, where the main part of the whole process corresponds to the three-dimensional growth of crystals. The mechanism proposed was confirmed by applying the Pérez-Maqueda et al. criterion.The authors would like to acknowledge Mrs. P. Díaz for the technical support of the experimental work. R. Casasola and J.M. Pérez express their gratitude to the Spanish National Research Council (CSIC) for their contract through the JAE Programme (JAEPre-08-00456 and JAEDoc-08-00362, respectively), co-financed by the European Social Fund.Peer reviewe
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