130 research outputs found
Spectral Classification of Galaxies Along the Hubble Sequence
We develop a straightforward and quantitative two-step method for
spectroscopically classifying galaxies from the low signal-to-noise (S/N)
optical spectra typical of galaxy redshift surveys. First, using \chi^2-fitting
of characteristic templates to the object spectrum, we determine the relative
contributions of the old stellar component, the young stellar component, and
various emission line spectra. Then, we classify the galaxy by comparing the
relative strengths of the components with those of galaxies of known
morphological type. In particular, we use the ratios of (1) the emission line
to absorption line contribution, (2) the young to old stellar contribution, and
(3) the oxygen to hydrogen emission line contribution. We calibrate and test
the method using published morphological types for 32 galaxies from the
long-slit spectroscopic survey of Kennicutt (1992) and for 304 galaxies from a
fiber spectroscopic survey of nearby galaxy clusters. From an analysis of a
sample of long-slit spectra of spiral galaxies in two galaxy clusters, we
conclude that the majority of the galaxies observed in the fiber survey are
sufficiently distant that their spectral classification is unaffected by
aperture bias. Our spectral classification is consistent with the morphological
classification to within one type (e.g. E to S0 or Sa to Sb) for \gtsim 80% of
the galaxies. Disagreements between the spectral and morphological
classifications of the remaining galaxies reflect a divergence in the
correspondence between spectral and morphological types, rather than a problem
with the data or method.Comment: 13 pages, uuencoded gzip'ed ps-file that includes 8 of 9 Figures,
accepted for publication in A
A New Hybrid Framework to Efficiently Model Lines of Sight to Gravitational Lenses
In strong gravitational lens systems, the light bending is usually dominated
by one main galaxy, but may be affected by other mass along the line of sight
(LOS). Shear and convergence can be used to approximate the contributions from
less significant perturbers (e.g. those that are projected far from the lens or
have a small mass), but higher order effects need to be included for objects
that are closer or more massive. We develop a framework for multiplane lensing
that can handle an arbitrary combination of tidal planes treated with shear and
convergence and planes treated exactly (i.e., including higher order terms).
This framework addresses all of the traditional lensing observables including
image positions, fluxes, and time delays to facilitate lens modelling that
includes the non-linear effects due to mass along the LOS. It balances accuracy
(accounting for higher-order terms when necessary) with efficiency (compressing
all other LOS effects into a set of matrices that can be calculated up front
and cached for lens modelling). We identify a generalized multiplane mass sheet
degeneracy, in which the effective shear and convergence are sums over the
lensing planes with specific, redshift-dependent weighting factors.Comment: 13 pages, 2 figure
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