457 research outputs found
Stellar indices and kinematics in Seyfert 1 nuclei
We present spectra of 6 type 1 Seyfert galaxies, 2 Seyfert 2, a starburst
galaxy and a compact narrow line radiogalaxy, taken in two spectral ranges
centered around the near--IR CaII triplet (CaT) (at ~8600 Angstroms), and the
Mgb stellar feature at 5180 Angstroms. We measured the equivalent width (EWs)
of these features and the Fe52 and Fe53 spectral indices. We found that the
strength of the CaT in type 1 Seyfert galaxies with prominent central point
sources, is larger than what would be expected from the observed strength of
the blue indices. This could be explained by the presence of red supergiants in
the nuclei of Seyfert 1 galaxies. On the other hand, the blue indices of these
galaxies could also be diluted by the strong FeII multiplets that can be seen
in their spectra.
We have also measured the stellar and gas velocity dispersions of the
galaxies in the sample. The stellar velocity dispersions were measured using
both, the Mgb and CaT stellar features. The velocity dispersion of the gas in
the narrow line region (NLR) was measured using the strong emission lines
[OIII] 5007, 4959 and [SIII] 9069. We compare the gas and star velocity
dispersions and find that both magnitudes are correlated in Seyfert galaxies.
Most of the Seyfert 1 we observe have stellar velocity dispersion somehow
greater than that of the gas in the NLR.Comment: To appear in MNRAS, 18 pages, 9 figure
High velocity blue-shifted FeII absorption in the dwarf star-forming galaxy PHL293B: Evidence for a wind driven supershell?
X-shooter and ISIS WHT spectra of the starforming galaxy PHL 293B also known
as A2228-00 and SDSS J223036.79-000636.9 are presented in this paper.
We find broad (FWHM = 1000km/s) and very broad (FWZI = 4000km/s) components
in the Balmer lines, narrow absorption components in the Balmer series
blueshifted by 800km/s, previously undetected FeII multiplet (42) absorptions
also blueshifted by 800km/s, IR CaII triplet stellar absorptions consistent
with [Fe/H] < -2.0 and no broad components or blushifted absorptions in the HeI
lines. Based on historical records, we found no optical variability at the 5
sigma level of 0.02 mag between 2005 and 2013 and no optical variability at the
level of 0.1mag for the past 24 years.
The lack of variability rules out transient phenomena like luminous blue
variables or SN IIn as the origin of the blue shifted absorptions of HI and
FeII. The evidence points to either a young and dense expanding supershell or a
stationary cooling wind, in both cases driven by the young cluster wind.Comment: Accepted for publication in MNRAS; 15 pages, 10 figure
Group, field and isolated early-type galaxies II. Global trends from nuclear data
We have derived ages, metallicities and enhanced-element ratios [alpha/Fe]
for a sample of 83 early-type galaxies essentially in groups, the field or
isolated objects. The stellar population properties derived for each galaxy
corresponds to the nuclear r_e/8 aperture extraction. The median age found for
Es is 5.8 +- 0.6 Gyr and the average metallicity is +0.37 +- 0.03 dex. For S0s,
the median age is 3.0 +- 0.6 Gyr and [Z/H] = 0.53 +- 0.04 dex. We compare the
distribution of our galaxies in the Hbeta-[MgFe] diagram with Fornax galaxies.
Our elliptical galaxies are 3-4 Gyr younger than Es in the Fornax cluster. We
find that the galaxies lie in a plane defined by [Z/H] = 0.99 log sigma_0 -
0.46 log Age - 1.60. More massive (larger sigma_0) and older galaxies present,
on average, large [alpha/Fe] values, and therefore, must have undergone shorter
star-formation timescales. Comparing group against field/isolated galaxies, it
is not clear that environment plays an important role in determining their
stellar population history. In particular, our isolated galaxies show ages
differing in more than 8 Gyr. Finally we explore our large spectral coverage to
derive log(O/H) metallicity from the N2 indicator and compare it with
model-dependent [Z/H]. We find that the O/H abundances are similar for all
galaxies, and we can interpret it as if most chemical evolution has already
finished in them.Comment: 23 pages, 17 postscript figures; accepted for publication in the
MNRA
A high excitation HII region in the faint dwarf elliptical galaxy A0951+68
We present the results of BVRI imaging and optical spectroscopy of the dwarf
galaxy A0951+68. The images reveal that, although this galaxy is classified as
a dwarf elliptical, it has some properties that are similar to dwarf irregular
galaxies. It contains two bright knots of emission, one of which is red and
unresolved and the other blue and resolved. The blue knot also shows a high
excitation emission line spectrum. The observed line ratios indicate that this
is an HII region, although with some line ratios that are border-line with
those in AGN. The emission line luminosity is consistent with ionisation by a
single, very luminous O star, or several smaller O stars, but the extended blue
light in the knot shows that this has occurred as part of a substantial recent
star formation event. We find that the metal abundance, while low compared to
typical large galaxies, actually seems to be high for such a low luminosity
dwarf. The position of A0951 in the literature is incorrect and we provide the
correct value.Comment: 8 pages, Latex, 4 encapsulated postscript figures included, 1
separate JPEG figure; to be published in Monthly Notice
Thermal Emission from HII Galaxies: Discovering the Youngest Systems
We studied the radio properties of very young massive regions of star
formation in HII galaxies, with the aim of detecting episodes of recent star
formation in an early phase of evolution where the first supernovae start to
appear. Our sample consists of 31 HII galaxies, characterized by strong
Hydrogen emission lines, for which low resolution VLA 3.5cm and 6cm
observations were obtained. The radio spectral energy distribution has a range
of behaviours; 1) there are galaxies where the SED is characterized by a
synchrotron-type slope, 2) galaxies with a thermal slope, and, 3) galaxies with
possible free-free absorption at long wavelengths. The latter SEDs were found
in a few galaxies and represent a signature of heavily embedded massive star
clusters closely related to the early stages of massive star formation. Based
on the comparison of the star formation rates determined from the recombination
lines and those determined from the radio emission we find that SFR(Ha) is on
average five times higher than SFR(1.4GHz). We confirm this tendency by
comparing the ratio between the observed flux at 20 cm and the expected one,
calculated based on the Ha star formation rates, both for the galaxies in our
sample and for normal ones. This analysis shows that this ratio is a factor of
2 smaller in our galaxies than in normal ones, indicating that they fall below
the FIR/radio correlation. These results suggest that the emission of these
galaxies is dominated by a recent and massive star formation event in which the
first supernovae (SN) just started to explode. We conclude that the systematic
lack of synchrotron emission in those systems with the largest equivalent width
of Hb can only be explained if those are young starbursts of less than 3.5Myr
of age.Comment: Accepted for publication in Ap
Optical Monitoring of Quasars: I. Variability
We present an analysis of quasar variability from data collected during a
photometric monitoring of 50 objects carried out at CNPq/Laboratorio Nacional
de Astrofisica, Brazil, between March 1993 and July 1996. A distinctive feature
of this survey is its photometric accuracy, ~ 0.02 V mag, achieved through
differential photometry with CCD detectors, what allows the detection of faint
levels of variability. We find that the relative variability, delta = sigma /
L, observed in the V band is anti-correlated with both luminosity and redshift,
although we have no means of discovering the dominant relation, given the
strong coupling between luminosity and redshift for the objects in our
sample.We introduce a model for the dependence of quasar variability on
frequency that is consistent with multi-wavelength observations of the nuclear
variability of the Seyfert galaxy NGC 4151. We show that correcting the
observed variability for this effect slightly increases the significance of the
trends of variability with luminosity and redshift. Assuming that variability
depends only on the luminosity, we show that the corrected variability is
anti-correlated with luminosity and is in good agreement with predictions of a
simple Poissonian model. The energy derived for the hypothetical pulses, ~
10^50 erg, agrees well with those obtained in other studies. We also find that
the radio-loud objects in our sample tend to be more variable than the
radio-quiet ones, for all luminosities and redshifts.Comment: 17 pages, 12 figures, accepted for publication in MNRAS (uses MNRAS
style
Optical and X-ray Variability in The Least Luminous AGN, NGC4395
We report the detection of optical and X-ray variability in the least
luminous known Seyfert galaxy, NGC4395. The featureless continuum changed by a
factor of 2 in 6 months, which is typical of more luminous AGN. The largest
variation was seen at shorter wavelengths, so that the spectrum becomes
`harder' during higher activity states. In a one week optical broad band
monitoring program, a 20% change was seen between successive nights. In a 1
month period the spectral shape changed from a power law with spectral index
alpha ~0 (characteristic of quasars) to a spectral index alpha ~2 (as observed
in other dwarf AGN). ROSAT HRI and PSPC archive data show a variable X-ray
source coincident with the galactic nucleus. A change in X-ray flux by a factor
\~2 in 15 days has been observed. When compared with more luminous AGN, NGC4395
appears to be very X-ray quiet. The hardness ratio obtained from the PSPC data
suggests that the spectrum could be absorbed. We also report the discovery of
weak CaIIK absorption, suggesting the presence of a young stellar cluster
providing of the order of 10% of the blue light. Using HST UV archive data,
together with the optical and X-ray observations, we examine the spectral
energy distribution for NGC4395 and discuss the physical conditions implied by
the nuclear activity under the standard AGN model. The observations can be
explained by either an accreting massive black hole emitting at about 10^(-3)
L_(Edd) or by a single old compact SNR with an age of 50 to 500 yr generated by
a small nuclear starburst.Comment: 19 pages, 9 figures, to appear in MNRA
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