337 research outputs found
ITERA: IDL Tool for Emission-line Ratio Analysis
We present a new software tool to enable astronomers to easily compare
observations of emission line ratios with those determined by photoionization
and shock models, ITERA, the IDL Tool for Emission-line Ratio Analysis. This
tool can plot ratios of emission lines predicted by models and allows for
comparison of observed line ratios against grids of these models selected from
model libraries associated with the tool. We provide details of the libraries
of standard photoionization and shock models available with ITERA, and, in
addition, present three example emission line ratio diagrams covering a range
of wavelengths to demonstrate the capabilities of ITERA. ITERA, and associated
libraries, is available from \url{http://www.brentgroves.net/itera.html}Comment: Accepted for New Astronomy, 3 figures. ITERA tool available to
download from http://www.brentgroves.net/itera.htm
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
Emission-line Diagnostics of Low Metallicity AGN
Current emission-line based estimates of the metallicity of active galactic
nuclei (AGN) at both high and low redshifts indicate that AGN have
predominantly solar to supersolar metallicities. This leads to the question: do
low metallicity AGN exist? In this paper we use photoionization models to
examine the effects of metallicity variations on the narrow emission lines from
an AGN. We explore a variety of emission-line diagnostics that are useful for
identifying AGN with low metallicity gas. We find that line ratios involving
[NII] are the most robust metallicity indicators in galaxies where the primary
source of ionization is from the active nucleus. Ratios involving [SII] and
[OI] are strongly affected by uncertainties in modelling the density structure
of the narrow line clouds. To test our diagnostics, we turn to an analysis of
AGN in the Sloan Digital Sky Survey (SDSS). We find a clear trend in the
relative strength of [NII] with the mass of the AGN host galaxy. The
metallicity of the ISM is known to be correlated with stellar mass in
star-forming galaxies; our results indicate that a similar trend exists for
AGN. We also find that the best-fit models for typical Seyfert narrow line
regions have supersolar abundances. Although there is a mass-dependent range of
a factor of 2-3 in the NLR metallicities of the AGN in our sample, AGN with
sub-solar metallicities are very rare in the SDSS. Out of a sample of ~23000
Seyfert 2 galaxies we find only ~40 clear candidates for AGN with NLR
abundances that are below solar.Comment: Accepted for publication in MNRAS; article with full resolution
figures avaialble at
http://www.mpa-garching.mpg.de/~brent/publications/lowZ.pd
Being WISE I: Validating Stellar Population Models and M/L ratios at 3.4 and 4.6 microns
Using data from the WISE mission, we have measured near infra-red (NIR)
photometry of a diverse sample of dust-free stellar systems (globular clusters,
dwarf and giant early-type galaxies) which have metallicities that span the
range -2.2 < [Fe/H] (dex) < 0.3. This dramatically increases the sample size
and broadens the metallicity regime over which the 3.4 (W1) and 4.6 micron (W2)
photometry of stellar populations have been examined.
We find that the W1 - W2 colors of intermediate and old (> 2 Gyr) stellar
populations are insensitive to the age of the stellar population, but that the
W1 - W2 colors become bluer with increasing metallicity, a trend not well
reproduced by most stellar population synthesis (SPS) models. In common with
previous studies, we attribute this behavior to the increasing strength of the
CO absorption feature located in the 4.6 micron bandpass with metallicity.
Having used our sample to validate the efficacy of some of the SPS models, we
use these models to derive stellar mass-to-light ratios in the W1 and W2 bands.
Utilizing observational data from the SAURON and ATLAS3D surveys, we
demonstrate that these bands provide extremely simple, yet robust stellar mass
tracers for dust free older stellar populations that are freed from many of the
uncertainties common among optical estimators.Comment: 11 pages, 6 figures, submitted to Ap
A morphological segmentation approach to determining bar lengths
Bars are important drivers of galaxy evolution, influencing many physical
processes and properties. Characterising bars is a difficult task, especially
in large-scale surveys. In this work, we propose a novel morphological
segmentation technique for determining bar lengths based on deep learning. We
develop U-Nets capable of decomposing galaxy images into pixel masks
highlighting the regions corresponding to bars and spiral arms. We demonstrate
the versatility of this technique through applying our models to galaxy images
from two different observational datasets with different source imagery, and to
RGB colour and monochromatic galaxy imaging. We apply our models to analyse
SDSS and Subaru HSC imaging of barred galaxies from the NA10 and SAMI
catalogues in order to determine the dependence of bar length on stellar mass,
morphology, redshift and the spin parameter proxy . Based on the
predicted bar masks, we show that the relative bar scale length varies with
morphology, with early type galaxies hosting longer bars. While bars are longer
in more massive galaxies in absolute terms, relative to the galaxy disc they
are actually shorter. We also find that the normalised bar length decreases
with increasing redshift, with bars in early-type galaxies exhibiting the
strongest rate of decline. We show that it is possible to distinguish spiral
arms and bars in monochrome imaging, although for a given galaxy the estimated
length in monochrome tends to be longer than in colour imaging. Our
morphological segmentation technique can be efficiently applied to study bars
in large-scale surveys and even in cosmological simulations.Comment: 22 pages, 18 figures, submitted to MNRA
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