39 research outputs found
UV Continuum, Physical Conditions and Filling Factor in Active Galactic Nuclei
The narrow line region of active galaxies is formed by gas clouds surrounded
by a diluted gas. Standard one-dimensional photoionization models are usually
used to model this region in order to reproduce the observed emission lines.
Since the narrow line region is not homogeneous, two major types of models are
used: (a) those assuming a homogeneous gas distribution and a filling factor
less than unity to mimic the presence of the emitting clouds; (b) those based
on a composition of single-cloud models combined in order to obtain the
observed spectra. The first method is largely used but may induce to misleading
conclusions as shown in this paper. The second one is more appropriate, but
requires a large number of observed lines in order to limit the number of
single models used. After discussing the case of an extragalactic HII region,
for which the ionizing radiation spectrum is better known, we show that 1-D
models for the narrow line region with a filling factor less than unit do not
properly mimic the clumpiness, but just simulates an overall lower density.
Multi-cloud models lead to more reliable results. Both models are tested in
this paper, using the emission-line spectra of two well-known Seyfert galaxies,
NGC 4151 and NGC 1068. It is shown that ionizing radiation spectra with a blue
bump cannot be excluded by multi-cloud models, although excluded by Alexander
et al. (1999, 2000)using homogeneous models with a filling factor less than
unity.Comment: 23 pages, 7 figures. Accepted for Publication in Ap
STAR FORMATION ACTIVITY IN A YOUNG GALAXY CLUSTER AT Z=0.866
The galaxy cluster RX J1257+4738 at z = 0.866 is one of the highest redshift clusters with a richness of multi-wavelength data, and is thus a good target to study the star formation-density relation at early epochs. Using a sample of spectroscopically confirmed cluster members, we derive the star-formation rates (SFRs) of our galaxies using two methods: (1) the relation between SFR and total infrared luminosity extrapolated from the observed Spitzer Multiband Imaging Photometer for Spitzer 24 μm imaging data; and (2) spectral energy distribution fitting using the MAGPHYS code, including eight different bands. We show that, for this cluster, the SFR-density relation is very weak and seems to be dominated by the two central galaxies and the SFR presents a mild dependence on stellar mass, with more massive galaxies having higher SFR. However, the specific SFR (SSFR) decreases with stellar mass, meaning that more massive galaxies are forming fewer stars per unit of mass, and thus suggesting that the increase in star-forming members is driven by cluster assembly and infall. If the environment is somehow driving the star formation, one would expect a relation between the SSFR and the cluster centric distance, but that is not the case. A possible scenario to explain this lack of correlation is the contamination by infalling galaxies in the inner part of the cluster, which may be on their initial pass through the cluster center. As these galaxies have higher SFRs for their stellar mass, they enhance the mean SSFR in the center of the cluste
Spectroscopic study of the HII regions in the NGC 1232 galaxy
NGC 1232 is a face-on spiral galaxy that serves as an excellent laboratory
for the study of star formation due to its proximity. Recent studies have
revealed interesting features about this galaxy: X-ray observations suggest
that it recently collided with a dwarf galaxy, however, no apparent remnant is
observed. Here we search for evidence of this collision. We used long-slit
optical spectra in two different positions obtained with the Goodman
spectrograph at the SOAR telescope. We detected 18 HII regions in the
north-south direction and 22 HII regions in the east-west direction and a
background galaxy, NGC 1232B, for which we present the first redshift
measurement and spectral analysis. We used the stellar population fitting
technique to study the underlying stellar population and to subtract it from
the spectra to measure the emission lines. The emission lines were used to
determine the extinction, electron density, chemical abundance, and the
star-formation rate gradient of NGC 1232. As is common in spiral galaxies, we
found a stellar population gradient with older populations at the central
regions and younger ones towards the outskirts, along with a negative oxygen
abundance gradient of -0.16 dex/re. Due to the difficulty of measuring
important emission lines, the number of objects for the abundance gradient is
small, but there is a hint that this galaxy has a broken gradient profile, with
a drop towards the center. If the collision caused any disturbance in the
galaxy, we believe it would be small and hard to detect with a limited number
of objects. From all the other measurements, we found no deviations from a
typical spiral galaxy and no significant difference between different
directions in the galaxy. The stellar population and emission line analysis of
NGC 1232B suggest that it is a starburst galaxy.Comment: 18 pages and 16 figures + 14 pages with 14 figures in the appendix.
Accepted for publication on A&
Techniques for the execution profile analysis and optimization of computational chemistry programs
In this paper we review the basic techniques of performance analysis within the UNIX environment that are relevant in computational chemistry, with particular emphasis on the execution profile using the gprof tool. Two case studies (in ab initio and molecular dynamics calculations) are presented in order to illustrate how execution profiling can be used to effectively identify bottlenecks and to guide source code optimization. Using these profiling and optimization techniques it was possible to obtain significant speedups (of up to 30%) in both cases.465469Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP
Stellar Population in LLAGN.I: Ground-based observations
(Abridge): We present a spectroscopic study of the stellar populations of Low
Luminosity AGN (LLAGN). Our main goal is to determine whether the stars who
live in the innermost (100 pc-scale) regions of these galaxies are in some way
related to the emission line properties, which would imply a link between the
stellar population and the ionization mechanism. High signal to noise, ground
based long-slit spectra in the 3500--5500 A interval were collected for 60
galaxies.Our main findings are: (1) Few LLAGN have a detectable young (< 10
Myr) starburst component, indicating that very massive stars do not contribute
significantly to the optical continuum. In particular, no features due to
Wolf-Rayet stars were convincingly detected. (2) High Order Balmer absorption
lines of HI (HOBLs), on the other hand, are detected in 40% of LLAGN.
These features, which are strongest in 100 Myr--1 Gyr intermediate age
stellar populations, are accompanied by diluted metal absorption lines and
bluer colors than other objects in the sample. (3) These intermediate age
populations are very common (50 %) in LLAGN with relatively weak [OI] emission,
but rare (10 %) in LLAGN with stronger [OI]. This is intriguing since LLAGN
with weak [OI] have been previously hypothesized to be ``transition objects''
in which both an AGN and young stars contribute to the emission-line
excitation. Massive stars, if present, are completely outshone by intermediate
age and old stars in the optical. This happens in at least a couple of objects
where independent UV spectroscopy detects young starbursts not seen in the
optical. (4) Objects with predominantly old stars span the whole range of
[OI]/Halpha values, but (5) sources with significant young and/or intermediate
age populations are nearly all (90%) weak [OI] emitters.Comment: To be published in ApJ (April issue). Paper with full resolution
images is at: http://www.iaa.csic.es/~rosa/preprints/preprints.html
http://www.journals.uchicago.edu/ApJ/journa
Nuclear and Extended Spectra of NGC 1068 - I: Hints from Near-Infrared Spectroscopy
We report the first simultaneous zJHK spectroscopy on the archetypical
Seyfert 2 Galaxy NGC 1068 covering the wavelength region 0.9 to 2.4 micron. The
slit, aligned in the NS direction and centred in the optical nucleus, maps a
region 300 pc in radius at sub-arcsec resolution, with a spectral resolving
power of 360 km s^-1. This configuration allow us to study the physical
properties of the nuclear gas including that of the north side of the
ionization cone, map the strong excess of continuum emission in the K-band and
attributed to dust and study the variations, both in flux and profile, in the
emission lines. Our results show that (1) Mid- to low-ionization emission lines
are splitted into two components, whose relative strengths vary with the
position along the slit and seem to be correlated with the jet. (2) The coronal
lines are single-peaked and are detected only in the central few hundred of
parsecs from the nucleus. (3) The absorption lines indicate the presence of
intermediate age stellar population, which might be a significant contributor
to the continuum in the NIR spectra. (4) Through some simple photoionization
models we find photoionization as the main mechanism powering the emitting gas.
(5) Calculations using stellar features point to a mass concentration inside
the 100 - 200 pc of about 10^10 solar masses.Comment: 19 Pages, 14 figures. Accepted for publication in MNRA