392 research outputs found
The Infrared Emission from the Narrow Line Region
We present models for the mid- and far- infrared emission from the Narrow
Line Region (NLR) of Active Galactic Nuclei (AGN). Using the MAPPINGS III code
we explore the effect of typical NLR parameters on the spectral characteristics
of the IR emission. These include useful IR emission line ratio diagnostic
diagrams for the determination of these parameters, as well as Star
formation--AGN mixing diagnostics. We also examine emission line to continuum
correlations which would assist in separating the IR emission arising from the
NLR from that coming from the inner torus. We find for AGN like NGC 1068 and
NGC 4151 that the NLR only contributes ~10% to the total IRAS 25 mum flux, and
that other components such as a dusty torus are necessary to explain the total
AGN IR emission.Comment: 15 pages, 12 figures, accepted for publication in A&A. Paper with
full resolution figures available at
http://www.mpa-garching.mpg.de/~brent/publications/bgrovesnlrIRpaper.pd
Local Analogs for High-redshift Galaxies: Resembling the Physical Conditions of the Interstellar Medium in High-redshift Galaxies
We present a sample of local analogs for high-redshift galaxies selected in
the Sloan Digital Sky Survey (SDSS). The physical conditions of the
interstellar medium (ISM) in these local analogs resemble those in
high-redshift galaxies. These galaxies are selected based on their positions in
the [OIII]/H versus [NII]/H nebular emission-line diagnostic
diagram. We show that these local analogs share similar physical properties
with high-redshift galaxies, including high specific star formation rates
(sSFRs), flat UV continuums and compact galaxy sizes. In particular, the
ionization parameters and electron densities in these analogs are comparable to
those in galaxies, but higher than those in normal SDSS galaxies
by 0.6~dex and 0.9~dex, respectively. The mass-metallicity
relation (MZR) in these local analogs shows ~dex offset from that in SDSS
star-forming galaxies at the low mass end, which is consistent with the MZR of
the galaxies. We compare the local analogs in this study with those
in other studies, including Lyman break analogs (LBA) and green pea (GP)
galaxies. The analogs in this study share a similar star formation surface
density with LBAs, but the ionization parameters and electron densities in our
analogs are higher than those in LBAs by factors of 1.5 and 3, respectively.
The analogs in this study have comparable ionization parameter and electron
density to the GP galaxies, but our method can select galaxies in a wider
redshift range. We find the high sSFR and SFR surface density can increase the
electron density and ionization parameters, but still cannot fully explain the
difference in ISM condition between nearby galaxies and the local
analogs/high-redshift galaxies.Comment: 13 pages, 11 figures, accepted by Ap
High-mass star formation in southern disk galaxies
As part of a major study of the physical processes of star formation and the evolution of galactic discs, the detailed distribution of high-mass star formation within southern late-type spirals and Magellanic-type galaxies is being measured by means of narrow-band imaging in Ha and the continuum, spectroscopic studies of prominent HII regions identified in the Ha images, and by radio mapping in neutral hydrogen and the continuum. The radio mapping will be undertaken with the Southern Hemisphere's first large, multi-frequency synthesis array, the Australia Telescope. Some optical imaging and spectroscopic data has already been acquired; the optical data and some preliminary results are described
Resolving the electron temperature discrepancies in HII Regions and Planetary Nebulae: kappa-distributed electrons
The measurement of electron temperatures and metallicities in H ii regions
and Planetary Nebulae (PNe) has-for several decades-presented a problem:
results obtained using different techniques disagree. What it worse, they
disagree consistently. There have been numerous attempts to explain these
discrepancies, but none has provided a satisfactory solution to the problem. In
this paper, we explore the possibility that electrons in H ii regions and PNe
depart from a Maxwell-Boltzmann equilibrium energy distribution. We adopt a
"kappa-distribution" for the electron energies. Such distributions are widely
found in Solar System plasmas, where they can be directly measured. This simple
assumption is able to explain the temperature and metallicity discrepancies in
H ii regions and PNe arising from the different measurement techniques. We find
that the energy distribution does not need to depart dramatically from an
equilibrium distribution. From an examination of data from Hii regions and PNe
it appears that kappa ~ 10 is sufficient to encompass nearly all objects. We
argue that the kappa-distribution offers an important new insight into the
physics of gaseous nebulae, both in the Milky Way and elsewhere, and one that
promises significantly more accurate estimates of temperature and metallicity
in these regions.Comment: 16 pages, 11 figures, 2 tables, published in Ap
On the missing 2175 A-bump in the Calzetti extinction curve
Aims. The aim of the paper is to give a physical explanation of the absence of the feature in the Calzetti extinction curve. Methods.We analyze the dust attenuation of a homogeneous source seen through a distant inhomogeneous distant screen. The inhomogeneities are described through an idealized isothermal turbulent medium where the probability distribution function (PDF) of the column density is log-normal. In addition it is assumed that below a certain critical column density the carriers of the extinction bump at 2175 Å are being destroyed by the ambient UV radiation field. Results. Turbulence is found to be a natural explanation not only of the flatter curvature of the Calzetti extinction curve but also of the missing bump provided the critical column density is NH = 1021 cm-2. The density contrast needed to explain both characteristics is well consistent with the Mach number of the cold neutral medium of our own Galaxy which suggests a density contrast s.σp≈6
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