2 research outputs found
Radio galaxies at z~2.5: results from Keck spectropolarimetry
In classifying the ensemble of powerful extragalactic radio sources,
considerable evidence has accumulated that radio galaxies and quasars are
orientation-dependent manifestations of the same parent population: massive
spheroidal galaxies containing correspondingly massive black holes. [...] In
this study, we exploit the opportunity to eliminate the quasar glare by
performing sensitive spectropolarimetry with the Keck II telescope of a sample
of radio galaxies with redshifts around 2.5. [...] We show that dust-reflected
quasar light generally dominates the restframe ultraviolet continuum of these
sources and that a highly clumped scattering medium results in almost grey
scattering of the active galactic nucleus photons. The observations, however,
do not exclude a substantial star formation rate averaged over a Gyr of
evolution. The sub-mm reradiation from the scattering dust is likely to
represent only a small fraction (~10%) of the total far infrared luminosity. An
analysis of the emission lines excited in the interstellar medium of the host
galaxy by the hard quasar radiation field reveals evidence of a dramatic
chemical evolution within the spheroid during this epoch. Secondary nitrogen
production in intermediate mass stars produces a characteristic signature in
the NV/CIV and NV/HeII line ratios which has been seen previously in the broad
line region of quasars at similar redshifts. We find intriguing correlations
between the strengths of the Ly alpha and NV emission lines and the degree of
ultraviolet continuum polarization which may represent the dispersal of dust
associated with the chemical enrichment of the spheroid.Comment: 24 pages, 12 figures, accepted for publication in Astronomy &
Astrophysics. Properly formatted for A4 and Letter paper formats ps files
available at http://www.eso.org/~jvernet/research.htm
Investigating the nature of the z~2.8 submillimeter selected galaxy SMM J02399-0136 with VLT spectropolarimetry
We present deep optical spectropolarimety of SMM J02399-0136 (z=2.8) done
with the VLT Antu 8.2 m telescope equipped with FORS1. Moderate continuum and
emission line polarization are measured (P~5%). We do not detect broad lines in
scattered flux as would be expected for a type-2 object but rather a
polarization behaviour similar to BAL quasars. This classification is confirmed
by the detection of both high and low ionization broad absorption troughs and a
very red continuum. We argue that this object shares several properties with
local ULIGs such as Mrk 231 and other ultraluminous infrared Lo-BAL quasars.
However, the fact that the ultraviolet spectrum is dominated by non-stellar
radiation does not prove that the dust that is thermally radiating in the far
infrared is predominantly heated by the AGN. Since the energy that we get in
the far-infrared is precisely that which is removed from the ultraviolet
spectrum, this could mean that the starburst is more dust-enshrouded than the
AGN due to a peculiar dust distribution. The limits we place on the putative
starburst contribution to the restframe ultraviolet continuum together with
constraints on the amount of extinction provides an upper limit to the star
formation rate of about 2000 Msun/yr, consistent with previously claimed high
star formation rate in this object.Comment: 8 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic