313 research outputs found
Rest-frame ultra-violet spectra of massive galaxies at z=3: evidence of high-velocity outflows
Galaxy formation models invoke the presence of strong feedback mechanisms
that regulate the growth of massive galaxies at high redshifts. In this paper
we aim to: (1) confirm spectroscopically the redshifts of a sample of massive
galaxies selected with photometric redshifts z > 2.5; (2) investigate the
properties of their stellar and interstellar media; (3) detect the presence of
outflows, and measure their velocities. To achieve this, we analysed deep,
high-resolution (R~2000) FORS2 rest-frame UV spectra for 11 targets. We
confirmed that 9 out of 11 have spectroscopic redshifts z > 2.5. We also
serendipitously found two mask fillers at redshift z > 2.5, which originally
were assigned photometric redshifts 2.0 < z < 2.5. In the four highest-quality
spectra we derived outflow velocities by fitting the absorption line profiles
with models including multiple dynamical components. We found strongly
asymmetric, high-ionisation lines, from which we derived outflow velocities
ranging from 480 to 1518 km/s. The two galaxies with highest velocity show
signs of AGN. We revised the spectral energy distribution fitting U-band
through 8 micron photometry, including the analysis of a power-law component
subtraction to identify the possible presence of active galactic nuclei (AGN).
The revised stellar masses of all but one of our targets are >1e10 Msun, with
four having stellar masses > 5e10 Msun. Three galaxies have a significant
power-law component in their spectral energy distributions, which indicates
that they host AGN. We conclude that massive galaxies are characterised by
significantly higher velocity outflows than the typical Lyman break galaxies at
z ~ 3. The incidence of high-velocity outflows (~40% within our sample) is also
much higher than among massive galaxies at z < 1, which is consistent with the
powerful star formation and nuclear activity that most massive galaxies display
at z > 2.Comment: 17 pages, 14 figures, Accepted for publication in A&
MOONS: The New Multi-Object Spectrograph for the VLT
MOONS is the new Multi-Object Optical and Near-infrared Spectrograph
currently under construction for the Very Large Telescope (VLT) at ESO. This
remarkable instrument combines, for the first time, the collecting power of an
8-m telescope, 1000 fibres with individual robotic positioners, and both low-
and high-resolution simultaneous spectral coverage across the 0.64-1.8 micron
wavelength range. This facility will provide the astronomical community with a
powerful, world-leading instrument able to serve a wide range of Galactic,
extragalactic and cosmological studies. Construction is now proceeding full
steam ahead and this overview article presents some of the science goals and
the technical description of the MOONS instrument. More detailed information on
the MOONS surveys is provided in the other dedicated articles in this Messenger
issue.STFC
ER
Faint radio-loud quasars: clues to their evolution
The quasar sample selected by cross-correlating the FIRST and the 2dF Quasar
Redshift Surveys allows us to explore, for the first time, the faint end of the
radio and optical luminosity functions up to z = 2.2. We find indications (~3
\sigma) of a negative evolution for these faint sources at z > 1.8, both in
radio and optical bands. This corresponds to a decrement in the space density
of faint quasars of about a factor 2 at z=2.2 and confirms the presence of a
differential evolution for the population of radio-active quasars. The faint
end of both luminosity functions flattens and the comparison with the (optical)
number density of the whole quasar population supports a dependence of the
fraction of radio detected quasars on the optical luminosity. A progressive
decrease in the fraction of quasars in the whole radio source population can be
consistently accounted for within the `receding torus' scenario. The population
of low luminosity quasars, which the FIRST-2dF detects, appears to depart from
the `classical' scheme for radio-loud quasars.Comment: 11 pages, 7 figures, accepted for publication in MNRA
The Radio Loud / Radio Quiet dichotomy: news from the 2dF QSO Redshift Survey
We present a detailed analysis of a sample of radio-detected quasars,
obtained by matching together objects from the FIRST and 2dF Quasar Redshift
Surveys. The dataset consists of 113 sources, spanning a redshift range 0.3 < z
< 2.2, with optical magnitudes 18.25 < b_J < 20.85 and radio fluxes S_{1.4 GHz}
< 1 mJy. These objects exhibit properties such as redshift and colour
distribution in full agreement with those derived for the whole quasar
population, suggestive of an independence of the mechanism(s) controlling the
birth and life-time of quasars of their level of radio emission. The long
debated question of radio-loud (RL)/radio-quiet (RQ) dichotomy is then
investigated for the combined FIRST-2dF and FIRST-LBQS sample, since they
present similar selection criteria. We find the fraction of radio detections to
increase with magnitude from < 3% at the faintest levels up to 20% for the
brightest sources. The classical RL/RQ dichotomy, in which the distribution of
radio-to-optical ratios and/or radio luminosities shows a lack of sources, is
ruled out by our analysis. We also find no tight relationship between optical
and radio luminosities for sources in the considered sample, result that tends
to exclude the mass of the quasar black hole as the physical quantity
associated to the level of radio emission.Comment: 11 pages, 14 figures, accepted by MNRA
MOONS:The New Multi-Object Spectrograph for the VLT
MOONS is the new Multi-Object Optical and Near-infrared Spectrograph currently under construction for the Very Large Telescope (VLT) at ESO. This remarkable instrument combines, for the first time, the collecting power of an 8-m telescope, 1000 fibres with individual robotic positioners, and both low- and high-resolution simultaneous spectral coverage across the 0.64-1.8 micron wavelength range. This facility will provide the astronomical community with a powerful, world-leading instrument able to serve a wide range of Galactic, extragalactic and cosmological studies. Construction is now proceeding full steam ahead and this overview article presents some of the science goals and the technical description of the MOONS instrument. More detailed information on the MOONS surveys is provided in the other dedicated articles in this Messenger issue
A systematic search for very massive galaxies at z > 4
Motivated by the claimed discovery of a very massive galaxy (HUDF-JD2;
M~5x10^11 Msun) at extreme redshift (z = 6.5) within the Hubble Ultra Deep
Field (HUDF) (Mobasher et al. 2005), we have completed a systematic search for
comparably massive galaxies with z > 4 among the 2688 galaxies in our K < 23.5
(AB) catalogue within the CDFS/GOODS-South field. This search was conducted
using redshift estimates based on the recently-completed, uniquely-deep 11-band
imaging in this 125 square arcmin field, ~25 times larger than the NICMOS HUDF.
Initial spectral fitting, based on published catalogue SExtractor photometry,
led us to conclude that at least 2669 of the galaxies in our sample lie at z <
4. We carried out a detailed investigation of the 19 remaining z > 4
candidates, performing aperture photometry on all images, and including
marginal detections and formal non-detections in the fitting process. This led
to the rejection of a further 13 galaxies to lower redshift. Moreover,
subjecting HUDF-JD2 to the same analysis, we find that it lies at z ~ 2.2,
rather than the extreme redshift favoured by Mobasher et al. (2005). The 6
remaining candidates appear to be credible examples of galaxies in the redshift
range z = 4 - 6, with plausible stellar ages. However, refitting with allowance
for extreme values of extinction we find that, even for these objects,
statistically acceptable solutions can be found at z < 3. Moreover, the
recently-released Spitzer MIPS imaging in GOODS-South has revealed that 5 of
our 6 final z > 4 candidates are detected at 24 microns. We conclude that there
is no convincing evidence for any galaxy with M > 3 x 10^11 Msun, and z > 4
within the GOODS-South field (abridged).Comment: 12 pages, 6 figures, submitted to MNRA
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