3,555 research outputs found
Semi-empirical analysis of Sloan Digital Sky Survey galaxies III. How to distinguish AGN hosts
We consider the techniques to distinguish normal star forming (NSF) galaxies
and active galactic nuclei (AGN) hosts using optical spectra. The observational
data base is a set of 20000 galaxies extracted from the Sloan Digital Sky
Survey, for which we have determined the emission line intensities after
subtracting the stellar continuum obtained from spectral synthesis. Our
analysis is based on photoionization models computed using the stellar ionizing
radiation predicted by Starburst 99 and, for the AGNs, a broken power-law
spectrum. We explain why, among the four classical emission line diagnostic
diagrams, the [OIII]/Hb vs [NII]/Ha one works best. We show however, that none
of these diagrams is efficient in detecting AGNs in metal poor galaxies, should
such cases exist. We propose a new divisory line between ``pure'' NSF galaxies
and AGN hosts. We also show that a classification into NSF and AGN galaxies
using only [NII]/Ha is feasible and useful. Finally, we propose a new
classification diagram, the DEW diagram, plotting D_n(4000) vs
max(EW[OII],EW[NeIII]). This diagram can be used with optical spectra for
galaxies with redshifts up to z = 1.3, meaning an important progress over
classifications proposed up to now. Since the DEW diagram requires only a small
range in wavelength, it can also be used at even larger redshifts in suitable
atmospheric windows. It also has the advantage of not requiring stellar
synthesis analysis to subtract the stars and of allowing one to see ALL the
galaxies in the same diagram, including passive galaxies.Comment: 14 pages, 9 figures, accepted for publication in MNRAS (replaced on
august 3, 2006, eqs 6 and 7 corrected
Semi-empirical analysis of Sloan Digital Sky Survey galaxies: IV. A nature via nurture scenario for galaxy evolution
We investigate the environmental dependence of stellar population properties
of galaxies in the local universe. Physical quantities related to the stellar
content of galaxies are derived from a spectral synthesis method applied to a
volume-limited sample containing more than 60 thousand galaxies extracted from
the DR4 of the SDSS. Mean stellar ages, mean stellar metallicities and stellar
masses are obtained from this method and used to characterise the stellar
populations of galaxies. The environment is defined by the projected local
galaxy density estimated from a nearest neighbour approach. We recover the star
formation--density relation in terms of the mean light-weighted stellar age,
which is strongly correlated with star formation parameters derived from \Ha.
We find that the age--density relation is distinct when we divide galaxies
according to luminosity or stellar mass. The relation is remarkable for
galaxies in all bins of luminosity. On the other hand, only for an intermediate
stellar mass interval (associated to a transition in galaxy properties) the
relation shows a change in galaxy properties with environment. Such distinct
behaviours are associated to the large stellar masses of galaxies with the same
luminosity in high-density environments. In addition, the well known star
formation--density relation results from the prevalence of massive systems in
high-density environments, independently of galaxy luminosity, with the
additional observed downsizing in galaxy formation, in which the star formation
is shifted from massive galaxies at early times to low-mass galaxies as the
universe evolves. Our results support that a natural path for galaxy evolution
proceeds via a nurture way, in the sense that galaxy evolution is accelerated
in denser environments, that took place mainly at high-redshifts.Comment: 17 pages, 13 figures, revised version accepted for publication in
MNRA
Geo-environmental mapping using physiographic analysis: constraints on the evaluation of land instability and groundwater pollution hazards in the Metropolitan District of Campinas, Brazil
Geo-environmental terrain assessments and territorial zoning are useful tools for the formulation and implementation of environmental management instruments (including policy-making, planning, and enforcement of statutory regulations). They usually involve a set of procedures and techniques for delimitation, characterisation and classification of terrain units. However, terrain assessments and zoning exercises are often costly and time-consuming, particularly when encompassing large areas, which in many cases prevent local agencies in developing countries from properly benefiting from such assessments. In the present paper, a low-cost technique based on the analysis of texture of satellite imagery was used for delimitation of terrain units. The delimited units were further analysed in two test areas situated in Southeast Brazil to provide estimates of land instability and the vulnerability of groundwater to pollution hazards. The implementation incorporated procedures for inferring the influences and potential implications of tectonic fractures and other discontinuities on ground behaviour and local groundwater flow. Terrain attributes such as degree of fracturing, bedrock lithology and weathered materials were explored as indicators of ground properties. The paper also discusses constraints on- and limitations of- the approaches taken
Magnetic reconfiguration of MnAs/GaAs(001) observed by Magnetic Force Microscopy and Resonant Soft X-ray Scattering
We investigated the thermal evolution of the magnetic properties of MnAs
epitaxial films grown on GaAs(001) during the coexistence of
hexagonal/orthorhombic phases using polarized resonant (magnetic) soft X-ray
scattering and magnetic force microscopy. The results of the diffuse satellite
X-ray peaks were compared to those obtained by magnetic force microscopy and
suggest a reorientation of ferromagnetic terraces as temperature rises. By
measuring hysteresis loops at these peaks we show that this reorientation is
common to all ferromagnetic terraces. The reorientation is explained by a
simple model based on the shape anisotropy energy. Demagnetizing factors were
calculated for different configurations suggested by the magnetic images. We
noted that the magnetic moments flip from an in-plane mono-domain orientation
at lower temperatures to a three-domain out-of-plane configuration at higher
temperatures. The transition was observed when the ferromagnetic stripe width L
is equal to 2.9 times the film thickness d. This is in good agreement with the
expected theoretical value of L = 2.6d.Comment: 16 pages in PD
The Evolution of Helium and Hydrogen Ionization Corrections as HII Regions Age
Helium and hydrogen recombination lines observed in low-metallicity,
extragalactic, HII regions provide the data used to infer the primordial helium
mass fraction, Y_P. In deriving abundances from observations, the correction
for unseen neutral helium or hydrogen is usually assumed to be absent; i.e.,
the ionization correction factor is taken to be unity (icf = 1). In a previous
paper (VGS), we revisited the question of the icf, confirming a "reverse"
ionization correction: icf < 1. In VGS the icf was calculated using more nearly
realistic models of inhomogeneous HII regions, suggesting that the published
values of Y_P needed to be reduced by an amount of order 0.003. As star
clusters age, their stellar spectra evolve and so, too, will their icfs. Here
the evolution of the icf is studied, along with that of two, alternate,
measures of the "hardness" of the radiation spectrum. The differences between
the icf for radiation-bounded and matter-bounded models are also explored,
along with the effect on the icf of the He/H ratio (since He and H compete for
some of the same ionizing photons). Particular attention is paid to the amount
of doubly-ionized helium predicted, leading us to suggest that observations of,
or bounds to, He++ may help to discriminate among models of HII regions ionized
by starbursts of different ages and spectra. We apply our analysis to the
Izotov & Thuan (IT) data set utilizing the radiation softness parameter, the
[OIII]/[OI] ratio, and the presence or absence of He++ to find 0.95 < icf <
0.99. This suggests that the IT estimate of the primordial helium abundance
should be reduced by Delta-Y = 0.006 +- 0.002, from 0.244 +- 0.002 to 0.238 +-
0.003.Comment: 27 double-spaced pages, 11 figures, 5 equations; revised to match the
version accepted for publication in the Ap
The Electron Scattering Region in Seyfert Nuclei
The electron scattering region (ESR) is one of important ingredients in
Seyfert nuclei because it makes possible to observe the hidden broad line
region (hereafter HBLR) in some type 2 Seyfert nuclei (hereafter S2s). However,
little is known about its physical and geometrical properties. Using the number
ratio of S2s with and without HBLR, we investigate statistically where the ESR
is in Seyfert nuclei. Our analysis suggests that the ESR is located at radius
between 0.01 pc and 0.1 pc from the central engine. We also
discuss a possible origin of the ESR briefly.Comment: 5 pages and 1 figure. The Astrophysical Journal (Letters), in pres
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