22 research outputs found
Redshift Limits of BL Lacertae Objects from Optical Spectroscopy
Context: BL Lacertae objects have been the targets for numerous recent
multiwavelength campaigns, continuum spectral variability studies, and
theoretical spectral and variability modeling. A meaningful interpretation of
the results of such studies requires a reliable knowledge of the objects'
redshifts; however, the redshifts for many are still unknown or uncertain.
Aims: Therefore, we hope to determine or constrain the redshifts of six BL Lac
objects with unknown or poorly known redshifts. Methods: Observations were made
of these objects with the MDM 2.4 m Hiltner telescope. Although no spectral
features were detected, and thus no redshifts could be measured, lower redshift
limits were assigned to the objects based on the expected equivalent widths of
absorption features in their host galaxies. Redshifts were also estimated for
some objects by assuming the host galaxies are standard candles and using host
galaxy apparent magnitudes taken from the literature. Results: The commonly
used redshift of for 1219+285 is almost certainly wrong, while the
redshifts of the other objects studied remain undetermined.Comment: 4 pages, 2 figures. Accepted by A&A Research Note
Cluster Lenses
Clusters of galaxies are the most recently assembled, massive, bound
structures in the Universe. As predicted by General Relativity, given their
masses, clusters strongly deform space-time in their vicinity. Clusters act as
some of the most powerful gravitational lenses in the Universe. Light rays
traversing through clusters from distant sources are hence deflected, and the
resulting images of these distant objects therefore appear distorted and
magnified. Lensing by clusters occurs in two regimes, each with unique
observational signatures. The strong lensing regime is characterized by effects
readily seen by eye, namely, the production of giant arcs, multiple-images, and
arclets. The weak lensing regime is characterized by small deformations in the
shapes of background galaxies only detectable statistically. Cluster lenses
have been exploited successfully to address several important current questions
in cosmology: (i) the study of the lens(es) - understanding cluster mass
distributions and issues pertaining to cluster formation and evolution, as well
as constraining the nature of dark matter; (ii) the study of the lensed objects
- probing the properties of the background lensed galaxy population - which is
statistically at higher redshifts and of lower intrinsic luminosity thus
enabling the probing of galaxy formation at the earliest times right up to the
Dark Ages; and (iii) the study of the geometry of the Universe - as the
strength of lensing depends on the ratios of angular diameter distances between
the lens, source and observer, lens deflections are sensitive to the value of
cosmological parameters and offer a powerful geometric tool to probe Dark
Energy. In this review, we present the basics of cluster lensing and provide a
current status report of the field.Comment: About 120 pages - Published in Open Access at:
http://www.springerlink.com/content/j183018170485723/ . arXiv admin note:
text overlap with arXiv:astro-ph/0504478 and arXiv:1003.3674 by other author