30 research outputs found
Candidate tidal disruption events from the XMM-Newton Slew Survey
In recent years, giant amplitude X-ray flares have been observed from a
handful of non-active galaxies. The most plausible scenario of these unusual
phenomena is tidal disruption of a star by a quiescent supermassive black hole
at the centre of the galaxy. Comparing the XMM-Newton Slew Survey Source
Catalogue with the ROSAT PSPC All-Sky Survey five galaxies have been detected a
factor of up to 88 brighter in XMM-Newton with respect to ROSAT PSPC upper
limits and presenting a soft X-ray colour. X-ray luminosities of these sources
derived from slew observations have been found in the range 10^41-10^44 erg
s^-1, fully consistent with the tidal disruption model. This model predicts
that during the peak of the outburst, flares reach X-ray luminosities up to
10^45 erg s^-1, which is close to the Eddington luminosity of the black hole,
and afterwards a decay of the flux on a time scale of months to years is
expected. Multi-wavelength follow-up observations have been performed on these
highly variable objects in order to disentangle their nature and to investigate
their dynamical evolution. Here we present sources coming from the XMM-Newton
Slew Survey that could fit in the paradigm of tidal disruption events. X-ray
and optical observations revealed that two of these objects are in full
agreement with that scenario and three other sources that, showing signs of
optical activity, need further investigation within the transient galactic
nuclei phenomena.Comment: 4 pages, 4 figures, A&A accepte
The metallicities of UM151, UM408 and A1228+12 revisited
We present the results of new spectrophotometry and heavy element abundance
determinations for 3 dwarf galaxies UM151, UM408 and A1228+12 (RMB132). These
galaxies have been claimed in the literature to have very low metallicities,
corresponding to log(O/H)+12 < 7.65, that are in the metallicity range of some
candidate local young galaxies. We present higher S/N data for these three
galaxies. UM151 and UM408 have significantly larger metallicities: log(O/H)+12
= 8.5 and 7.93, respectively. For A1228+12 our new log(O/H)+12 = 7.73 is close
to that recalculated from earlier data (7.68). Thus, the rederived
metallicities allow us to remove these objects from the list of galaxies with Z
< 1/20 Z_Sun.Comment: LaTeX, 8 pages with 3 Postscript figures, A&A in pres
The Tully-Fisher relation for S0 galaxies
We present a study of the local B and K-band Tully-Fisher Relation (TFR)
between absolute magnitude and maximum circular speed in S0 galaxies. To make
this study, we have combined kinematic data, including a new high-quality
spectral data set from the Fornax Cluster, with homogeneous photometry from the
RC3 and 2MASS catalogues, to construct the largest sample of S0 galaxies ever
used in a study of the TFR. Independent of environment, S0 galaxies are found
to lie systematically below the TFR for nearby spirals in both optical and
infrared bands. This offset can be crudely interpreted as arising from the
luminosity evolution of spiral galaxies that have faded since ceasing star
formation. However, we also find a large scatter in the TFR. We show that most
of this scatter is intrinsic, not due to the observational uncertainties. The
presence of such a large scatter means that the population of S0 galaxies
cannot have formed exclusively by the above simple fading mechanism after all
transforming at a single epoch. To better understand the complexity of the
transformation mechanism, we have searched for correlations between the offset
from the TFR and other properties of the galaxies such as their structural
properties, central velocity dispersions and ages (as estimated from line
indices). For the Fornax Cluster data, the offset from the TFR relates with the
estimated age of the stars in the individual galaxies, in the sense and of the
magnitude expected if S0 galaxies had passively faded since being converted
from spirals. This correlation implies that a significant part of the scatter
in the TFR arises from the different times at which galaxies began their
transformation.Comment: 17 pages, 11 figures, 3 tables, accepted for publication in MNRA
The Herschel Virgo Cluster Survey. IX. Dust-to-gas mass ratio and metallicity gradients in four Virgo spiral galaxies
Using Herschel data from the Open Time Key Project the Herschel Virgo Cluster
Survey (HeViCS), we investigated the relationship between the metallicity
gradients expressed by metal abundances in the gas phase as traced by the
chemical composition of HII regions, and in the solid phase, as traced by the
dust-to-gas mass ratio. We derived the radial gradient of the dust-to-gas mass
ratio for all galaxies observed by HeViCS whose metallicity gradients are
available in the literature. They are all late type Sbc galaxies, namely
NGC4254, NGC4303, NGC4321, and NGC4501. We examined different dependencies on
metallicity of the CO-to-H conversion factor (\xco), used to transform the
CO observations into the amount of molecular hydrogen. We found that in
these galaxies the dust-to-gas mass ratio radial profile is extremely sensitive
to choice of the \xco\ value, since the molecular gas is the dominant component
in the inner parts. We found that for three galaxies of our sample, namely
NGC4254, NGC4321, and NGC4501, the slopes of the oxygen and of the dust-to-gas
radial gradients agree up to 0.6-0.7R using \xco\ values in the
range 1/3-1/2 Galactic \xco. For NGC4303 a lower value of \xco
10 is necessary. We suggest that such low \xco\ values might be due to a
metallicity dependence of \xco (from close to linear for NGC4254, NGC4321, and
NGC4501 to superlinear for NGC4303), especially in the radial regions
R0.6-0.7R where the molecular gas dominates. On the other hand, the
outer regions, where the atomic gas component is dominant, are less affected by
the choice of \xco, and thus we cannot put constraints on its value.Comment: 13 pages, 8 figures, A&A accepte
Adaptive optics imaging and optical spectroscopy of a multiple merger in a luminous infrared galaxy
(abridged) We present near-infrared (NIR) adaptive optics imaging obtained
with VLT/NACO and optical spectroscopy from the Southern African Large
Telescope (SALT) of a luminous infrared galaxy (LIRG) IRAS 19115-2124. These
data are combined with archival HST imaging and Spitzer imaging and
spectroscopy, allowing us to study this disturbed interacting/merging galaxy,
dubbed the Bird, in extraordinary detail. In particular, the data reveal a
triple system where the LIRG phenomenon is dominated by the smallest of the
components.
One nucleus is a regular barred spiral with significant rotation, while
another is highly disturbed with a surface brightness distribution intermediate
to that of disk and bulge systems, and hints of remaining arm/bar structure. We
derive dynamical masses in the range 3-7x10^10 M_solar for both. The third
component appears to be a 1-2x10^10 M_solar mass irregular galaxy. The total
system exhibits HII galaxy-like optical line ratios and strengths, and no
evidence for AGN activity is found from optical or mid-infrared data. The star
formation rate is estimated to be 190 M_solar/yr. We search for SNe, super star
clusters, and detect 100-300 km/s outflowing gas from the Bird. Overall, the
Bird shows kinematic, dynamical, and emission line properties typical for cool
ultra luminous IR galaxies. However, the interesting features setting it apart
for future studies are its triple merger nature, and the location of its star
formation peak - the strongest star formation does not come from the two major
K-band nuclei, but from the third irregular component. Aided by simulations, we
discuss scenarios where the irregular component is on its first high-speed
encounter with the more massive components.Comment: 24 pages, 16 figures. Accepted MNRAS version, minor corrections only,
references added. Higher resolution version (1.3MB) is available from
http://www.saao.ac.za/~petri/bird/ulirg_bird_highres_vaisanen_v2.pd
Images IV: Strong evolution of the oxygen abundance in gaseous phases of intermediate mass galaxies since z=0.8
Intermediate mass galaxies (logM(Msun)>10) at z~0.6 are the likeliest
progenitors of the present-day numerous population of spirals. There is growing
evidence that they have evolved rapidly since the last 6 to 8 Gyr ago, and
likely have formed a significant fraction of their stellar mass, often showing
perturbed morphologies and kinematics. We have gathered a representative sample
of 88 such galaxies and have provided robust estimates of their gas phase
metallicity. For doing so, we have used moderate spectral resolution
spectroscopy at VLT/FORS2 with unprecedented high S/N allowing to remove biases
coming from interstellar absorption lines and extinction to establish robust
values of R23=([OII]3727 + [OIII]4959,5007)/Hbeta. We definitively confirm that
the predominant population of z~0.6 starbursts and luminous IR galaxies (LIRGs)
are on average, two times less metal rich than the local galaxies at a given
stellar mass. We do find that the metal abundance of the gaseous phase of
galaxies is evolving linearly with time, from z=1 to z=0 and after comparing
with other studies, from z=3 to z=0. Combining our results with the reported
evolution of the Tully Fisher relation, we do find that such an evolution
requires that ~30% of the stellar mass of local galaxies have been formed
through an external supply of gas, thus excluding the close box model. Distant
starbursts & LIRGs have properties (metal abundance, star formation efficiency
& morphologies) similar to those of local LIRGs. Their underlying physics is
likely dominated by gas infall probably through merging or interactions. Our
study further supports the rapid evolution of z~0.4-1 galaxies. Gas exchanges
between galaxies is likely the main cause of this evolution.Comment: 21 pages, 12 figures, A&A, In pres
Physical properties of galaxies and their evolution in the VIMOS VLT Deep Survey. I. The evolution of the mass-metallicity relation up to z~0.9
We derive the mass-metallicity relation of star-forming galaxies up to
, using data from the VIMOS VLT Deep Survey. Automatic measurement of
emission-line fluxes and equivalent widths have been performed on the full
spectroscopic sample. This sample is divided into two sub-samples depending on
the apparent magnitude selection: wide () and deep
). These two samples span two different ranges of stellar
masses. Emission-line galaxies have been separated into star-forming galaxies
and active galactic nuclei using emission line ratios. For the star-forming
galaxies the emission line ratios have also been used to estimate gas-phase
oxygen abundance, using empirical calibrations renormalized in order to give
consistent results at low and high redshifts. The stellar masses have been
estimated by fitting the whole spectral energy distributions with a set of
stellar population synthesis models. We assume at first order that the shape of
the mass-metallicity relation remains constant with redshift. Then we find a
stronger metallicity evolution in the wide sample as compared to the deep
sample. We thus conclude that the mass-metallicity relation is flatter at
higher redshift. The observed flattening of the mass-metallicity relation at
high redshift is analyzed as an evidence in favor of the open-closed model.Comment: 21 pages, revised version submitted to A&
Modeling the spectral energy distribution of ULIRGs I: the radio spectra
As a constraint for new starburst/AGN models of IRAS bright galaxies we
determine the radio spectra of 31 luminous and ultraluminous IRAS galaxies
(LIRGs/ULIRGs). We construct the radio spectra using both new and archival
data. From our sample of radio spectra we find that very few have a straight
power-law slope. Although some sources show a flattening of the radio spectral
slope at high frequencies the average spectrum shows a steepening of the radio
spectrum from 1.4 to 22.5 GHz. This is unexpected because in sources with high
rates of star formation we expect flat spectrum, free-free emission to make a
significant contribution to the radio flux at higher radio frequencies. Despite
this trend the radio spectral indices between 8.4 and 22.5 GHz are flatter for
sources with higher values of the FIR-radio flux density ratio q, when this is
calculated at 8.4 GHz. Therefore, sources that are deficient in radio emission
relative to FIR emission (presumably younger sources) have a larger thermal
component to their radio emission. However, we find no correlation between the
radio spectral index between 1.4 and 4.8 GHz and q at 8.4 GHz. Because the low
frequency spectral index is affected by free-free absorption, and this is a
function of source size for a given mass of ionized gas, this is evidence that
the ionized gas in ULIRGs shows a range of densities. The youngest LIRGs and
ULIRGs are characterized by a larger contribution to their high-frequency radio
spectra from free-free emission. However, the youngest sources are not those
that have the greatest free-free absorption at low radio frequencies. The
sources in which the effects of free-free absorption are strongest are instead
the most compact sources. Although these have the warmest FIR colours, they are
not necessarily the youngest sources.Comment: 16 pages. Submitted to A&A Re-submitted, with aesthetic improvements
to the text and figure
The PN.S Elliptical Galaxy Survey: the dark matter in NGC 4494
We present new Planetary Nebula Spectrograph observations of the ordinary
elliptical galaxy NGC 4494, resulting in positions and velocities of 255 PNe
out to 7 effective radii (25 kpc). We also present new wide-field surface
photometry from MMT/Megacam, and long-slit stellar kinematics from VLT/FORS2.
The spatial and kinematical distributions of the PNe agree with the field stars
in the region of overlap. The mean rotation is relatively low, with a possible
kinematic axis twist outside 1 Re. The velocity dispersion profile declines
with radius, though not very steeply, down to ~70 km/s at the last data point.
We have constructed spherical dynamical models of the system, including Jeans
analyses with multi-component LCDM-motivated galaxies as well as logarithmic
potentials. These models include special attention to orbital anisotropy, which
we constrain using fourth-order velocity moments. Given several different sets
of modelling methods and assumptions, we find consistent results for the mass
profile within the radial range constrained by the data. Some dark matter (DM)
is required by the data; our best-fit solution has a radially anisotropic
stellar halo, a plausible stellar mass-to-light ratio, and a DM halo with an
unexpectedly low central density. We find that this result does not
substantially change with a flattened axisymmetric model.
Taken together with other results for galaxy halo masses, we find suggestions
for a puzzling pattern wherein most intermediate-luminosity galaxies have very
low concentration halos, while some high-mass ellipticals have very high
concentrations. We discuss some possible implications of these results for DM
and galaxy formation.Comment: 29 pages, 17 figures. MNRAS, accepte
What drives the Balmer extinction sequence in spiral galaxies? Clues from the Sloan Digital Sky Survey
Using spectra of normal emission line galaxies from the First Data Release of
the Sloan Digital Sky Survey (SDSS) we have investigated the relations between
the extinction as derived from the \Ha/\Hb emission line
ratio and various global parameters of the galaxies. Our main findings are
that: 1) is linked with the galaxy spectral type and colour,
decreasing from early- to late-type spirals. 2) increases
with increasing metallicity. 3) is larger in galaxies with an
older stellar population. 4) is larger for more luminous
galaxies. 5) The extinction of the stellar light is correlated with both the
extinction of the nebular light and the intrinsic galaxy colours. We propose
phenomenological interpretations of our empirical results. We have also
cross-correlated our sample of SDSS galaxies with the IRAS data base. Due to
the lower redshift limit imposed to our sample and to the detection limit of
IRAS, such a procedure selected only luminous infrared galaxies. We found that
correlations that were shown by other authors to occur between optical and
infrared properties of galaxies disappear when restricting the sample to
luminous infrared galaxies. We also found that the optical properties of the
luminous infrared galaxies in our SDSS sample are very similar to those of our
entire sample of SDSS galaxies. This may be explained by the IRAS luminosity of
the galaxies originating in the regions that formed massive stars less than 1
Myr ago, while the opacity of galaxies as derived from the \Ha/\Hb emission
line ratio is due to diffuse dust.
We show some implications of our empirical results on the determination of
global star formation rates and total stellar masses in normal galaxies.Comment: 15 pages, 13 figures. Accepted for publication in A&A. A PDF file
with better resolution can be downloaded here
http://www.astro.iag.usp.br/~abilio/stasinska.pd