590 research outputs found
On the cosmological distance and redshift between any two objects
We discuss the problem of how to calculate the distance between two
cosmological objects given their redshifts and angular separation on the sky.
Although of a fundamental nature, this problem and its solution seem to lack a
detailed description in the literature. We present a new variant of its
solution and quantitatively assess the most commonly used approximation.Comment: 5 pages, LaTeX, 7 Postscript figures, accepted for publication in
MNRA
Parsec-scale HI absorption structure in a low-redshift galaxy seen against a Compact Symmetric Object
We present global VLBI observations of the 21-cm transition of atomic
hydrogen seen in absorption against the radio source J0855+5751. The foreground
absorber (SDSS~J085519.05+575140.7) is a dwarf galaxy at = 0.026. As the
background source is heavily resolved by VLBI, the data allow us to map the
properties of the foreground HI gas with a spatial resolution of 2pc. The
absorbing gas corresponds to a single coherent structure with an extent
35pc, but we also detect significant and coherent variations, including a
change in the HI optical depth by a factor of five across a distance of
6pc. The large size of the structure provides support for the Heiles &
Troland model of the ISM, as well as its applicability to external galaxies.
The large variations in HI optical depth also suggest that caution should be
applied when interpreting measurements from radio-detected DLAs. In
addition, the distorted appearance of the background radio source is indicative
of a strong jet-cloud interaction in its host galaxy. We have measured its
redshift ( = 0.54186) using optical spectroscopy on the William Herschel
Telescope and this confirms that J0855+5751 is a FRII radio source with a
physical extent of 1kpc and supports the previous identification of this
source as a Compact Symmetric Object. These sources often show absorption
associated with the host galaxy and we suggest that both HI and OH should be
searched for in J0855+5751.Comment: 14 pages and 10 figures. Accepted for publication in MNRA
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Galaxy and mass assembly: the G02 field, Herschel–ATLAS target selection and data release 3
We describe data release 3 (DR3) of the Galaxy And Mass Assembly (GAMA) survey. The GAMA survey is a spectroscopic redshift and multiwavelength photometric survey in three equatorial regions each of 60.0 deg2 (G09, G12, and G15), and two southern regions of 55.7 deg2 (G02) and 50.6 deg2 (G23). DR3 consists of: the first release of data covering the G02 region and of data on H-ATLAS (Herschel – Astrophysical Terahertz Large Area Survey) sources in the equatorial regions; and updates to data on sources released in DR2. DR3 includes 154 809 sources with secure redshifts across four regions. A subset of the G02 region is 95.5 per cent redshift complete to r < 19.8 mag over an area of 19.5 deg2, with 20 086 galaxy redshifts, that overlaps substantially with the XXL survey (X-ray) and VIPERS (redshift survey). In the equatorial regions, the main survey has even higher completeness (98.5 per cent), and spectra for about 75 per cent of H-ATLAS filler targets were also obtained. This filler sample extends spectroscopic redshifts, for probable optical counterparts to HATLAS submillimetre sources, to 0.8 mag deeper (r < 20.6 mag) than the GAMA main survey. There are 25 814 galaxy redshifts for H-ATLAS sources from the GAMA main or filler surveys. GAMA DR3 is available at the survey website (www.gama-survey.org/dr3/)
The Millennium Galaxy Catalogue: The connection between close pairs and asymmetry; implications for the galaxy merger rate
We compare the use of galaxy asymmetry and pair proximity for measuring
galaxy merger fractions and rates for a volume limited sample of 3184 galaxies
with -21 < M(B) -5 log h < -18 mag. and 0.010 < z < 0.123 drawn from the
Millennium Galaxy Catalogue. Our findings are that:
(i) Galaxies in close pairs are generally more asymmetric than isolated
galaxies and the degree of asymmetry increases for closer pairs. At least 35%
of close pairs (with projected separation of less than 20 h^{-1} kpc and
velocity difference of less than 500 km s^{-1}) show significant asymmetry and
are therefore likely to be physically bound.
(ii) Among asymmetric galaxies, we find that at least 80% are either
interacting systems or merger remnants. However, a significant fraction of
galaxies initially identified as asymmetric are contaminated by nearby stars or
are fragmented by the source extraction algorithm. Merger rates calculated via
asymmetry indices need careful attention in order to remove the above sources
of contamination, but are very reliable once this is carried out.
(iii) Close pairs and asymmetries represent two complementary methods of
measuring the merger rate. Galaxies in close pairs identify future mergers,
occurring within the dynamical friction timescale, while asymmetries are
sensitive to the immediate pre-merger phase and identify remnants.
(iv) The merger fraction derived via the close pair fraction and asymmetries
is about 2% for a merger rate of (5.2 +- 1.0) 10^{-4} h^3 Mpc^{-3} Gyr^{-1}.
These results are marginally consistent with theoretical simulations (depending
on the merger time-scale), but imply a flat evolution of the merger rate with
redshift up to z ~1.Comment: 10 pages, 10 figures, emulateapj format. ApJ, accepte
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