Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...311...25w&link_type=abstract
Astronomy and Astrophysics, v.311, p.25-34
Astronomy and Astrophysics
Astrophysics
49
Galaxies: Formation, Quasars: Absorption Lines, Quasars: Individual: Pks0528-250
Scientific paper
In an earlier paper, we reported the detection of Lyαemission from the damped Lyαabsorber at z_abs_=2.811 seen in the spectrum of the quasar PKS0528-250. The source was the nearest to the line of sight to the quasar of three candidates detected in a narrow-band image. We present here a higher S/N spectrum of the first source, as well as spectra of the remaining two sources, and results from a deep broadband image. We confirm Lyαemission from all three sources, and provide redshifts and line widths for each line. Two of the sources display broad lines {DELTA}v>600km/s (FWHM), and combined with the evidence of the shape of the sources this appears to rule out the possibility that the clouds are photoionised by the quasar. The line widths are plausibly explained as the result of resonant scattering of the Lyαphotons escaping through a high column density of Hi, and we conclude that we are probably seeing three regions of star formation (although the possibility that one or more of the sources contains a low-luminosity AGN cannot be excluded). We infer a very low dust to gas ratio for the group, so the continuum magnitudes provide a useful estimate of the SFR in each source. The total SFR for the group is estimated to be a few Msun_/yr. The three Lyα sources are approximately aligned, and we draw attention to the similarity between this arrangement and the filamentary distribution of aggregations of cold gas seen in simulations of galaxy formation. From the estimated gas and dynamical masses, and based on the results of these simulations, we judge that the three sources are likely to merge on a timescale of <10^9^ years. This suggests that at z~3 at least some of the damped Lyα systems are sub-components of galaxies in the process of assembly, rather than fully-formed disks that turn into the stellar disks of spiral galaxies. The total gas mass in the group is estimated to be of the order of 10^10^Msun_. The overall picture that emerges is of star formation in a sub-L_*_ galaxy, at an early stage in its evolution, during the process of assembly.
Moller Palle
Warren Stephen J.
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