398 research outputs found
Serendipitous discovery of a projected pair of QSOs separated by 4.5 arcsec on the sky
We present the serendipitous discovery of a projected pair of quasi-stellar
objects (QSOs) with an angular separation of arcsec. The
redshifts of the two QSOs are widely different: one, our programme target, is a
QSO with a spectrum consistent with being a narrow line Seyfert 1 AGN at
. For this target we detect Lyman-, \ion{C}{4}, and
\ion{C}{3]}. The other QSO, which by chance was included on the spectroscopic
slit, is a Type 1 QSO at a redshift of , for which we detect
\ion{C}{4}, \ion{C}{3]} and \ion{Mg}{2}. We compare this system to previously
detected projected QSO pairs and find that only about a dozen previously known
pairs have smaller angular separation.Comment: 4 pages, 3 figures. Accepted for publication in A
The High A(V) Quasar Survey: Reddened quasi-stellar objects selected from optical/near-infrared photometry - II
Quasi-stellar objects (QSOs) whose spectral energy distributions (SEDs) are
reddened by dust either in their host galaxies or in intervening absorber
galaxies are to a large degree missed by optical color selection criteria like
the one used by the Sloan Digital Sky Survey (SDSS). To overcome this bias
against red QSOs, we employ a combined optical and near-infrared color
selection. In this paper, we present a spectroscopic follow-up campaign of a
sample of red candidate QSOs which were selected from the SDSS and the UKIRT
Infrared Deep Sky Survey (UKIDSS). The spectroscopic data and SDSS/UKIDSS
photometry are supplemented by mid-infrared photometry from the Wide-field
Infrared Survey Explorer. In our sample of 159 candidates, 154 (97%) are
confirmed to be QSOs. We use a statistical algorithm to identify sightlines
with plausible intervening absorption systems and identify nine such cases
assuming dust in the absorber similar to Large Magellanic Cloud sightlines. We
find absorption systems toward 30 QSOs, 2 of which are consistent with the
best-fit absorber redshift from the statistical modeling. Furthermore, we
observe a broad range in SED properties of the QSOs as probed by the rest-frame
2 {\mu}m flux. We find QSOs with a strong excess as well as QSOs with a large
deficit at rest-frame 2 {\mu}m relative to a QSO template. Potential solutions
to these discrepancies are discussed. Overall, our study demonstrates the high
efficiency of the optical/near-infrared selection of red QSOs.Comment: 64 pages, 18 figures, 16 pages of tables. Accepted to ApJ
Absorption-selected galaxies trace the low-mass, late-type, star-forming population at
We report on the stellar content, half-light radii and star formation rates
of a sample of 10 known high-redshift () galaxies selected on
strong neutral hydrogen (HI) absorption (log(N(HI)/cm) toward
background quasars. We use observations from the {\it Hubble Space Telescope}
(HST) Wide Field Camera 3 in three broad-band filters to study the spectral
energy distribution(SED) of the galaxies. Using careful quasar point spread
function subtraction, we study their galactic environments, and perform the
first systematic morphological characterisation of such absorption-selected
galaxies at high redshifts. Our analysis reveals complex, irregular hosts with
multiple star-forming clumps. At a spatial sampling of 0.067 arcsec per pixel
(corresponding to 0.55 kpc at the median redshift of our sample), 40% of our
sample requires multiple S\'ersic components for an accurate modelling of the
observed light distributions. Placed on the mass-size relation and the `main
sequence' of star-forming galaxies, we find that absorption-selected galaxies
at high redshift extend known relations determined from deep
luminosity-selected surveys to an order of magnitude lower stellar mass, with
objects primarily composed of star-forming, late-type galaxies. We measure
half-light radii in the range 0.4 to 2.6 kpc based on the
reddest band (F160W) to trace the oldest stellar populations, and stellar
masses in the range 8 to 10
derived from fits to the broad-band SED. Spectroscopic and SED-based star
formation rates are broadly consistent, and lie in the range log(SFR/Myr) 0.0 to 1.7.Comment: 17 pages, Accepted for publication in MNRAS. This revision has minor
text change
Measurement of the branching ratio for beta-delayed alpha decay of 16N
While the 12C(a,g)16O reaction plays a central role in nuclear astrophysics,
the cross section at energies relevant to hydrostatic helium burning is too
small to be directly measured in the laboratory. The beta-delayed alpha
spectrum of 16N can be used to constrain the extrapolation of the E1 component
of the S-factor; however, with this approach the resulting S-factor becomes
strongly correlated with the assumed beta-alpha branching ratio. We have
remeasured the beta-alpha branching ratio by implanting 16N ions in a segmented
Si detector and counting the number of beta-alpha decays relative to the number
of implantations. Our result, 1.49(5)e-5, represents a 24% increase compared to
the accepted value and implies an increase of 14% in the extrapolated S-factor
The VANDELS survey: Dust attenuation in star-forming galaxies at
We present the results of a new study of dust attenuation at redshifts based on a sample of star-forming galaxies from the VANDELS
spectroscopic survey. Motivated by results from the First Billion Years (FiBY)
simulation project, we argue that the intrinsic spectral energy distributions
(SEDs) of star-forming galaxies at these redshifts have a self-similar shape
across the mass range log probed by
our sample. Using FiBY data, we construct a set of intrinsic SED templates
which incorporate both detailed star formation and chemical abundance
histories, and a variety of stellar population synthesis (SPS) model
assumptions. With this set of intrinsic SEDs, we present a novel approach for
directly recovering the shape and normalization of the dust attenuation curve.
We find, across all of the intrinsic templates considered, that the average
attenuation curve for star-forming galaxies at is similar in shape
to the commonly-adopted Calzetti starburst law, with an average
total-to-selective attenuation ratio of . We show that the
optical attenuation () versus stellar mass () relation
predicted using our method is consistent with recent ALMA observations of
galaxies at in the \emph{Hubble} \emph{Ultra} \emph{Deep} \emph{Field}
(HUDF), as well as empirical relations predicted by a
Calzetti-like law. Our results, combined with other literature data, suggest
that the relation does not evolve over the redshift range
, at least for galaxies with log.
Finally, we present tentative evidence which suggests that the attenuation
curve may become steeper at log.Comment: 16 pages, 12 figures, accepted for publication in MNRA
The VANDELS survey: Dust attenuation in star-forming galaxies at
We present the results of a new study of dust attenuation at redshifts based on a sample of star-forming galaxies from the VANDELS
spectroscopic survey. Motivated by results from the First Billion Years (FiBY)
simulation project, we argue that the intrinsic spectral energy distributions
(SEDs) of star-forming galaxies at these redshifts have a self-similar shape
across the mass range log probed by
our sample. Using FiBY data, we construct a set of intrinsic SED templates
which incorporate both detailed star formation and chemical abundance
histories, and a variety of stellar population synthesis (SPS) model
assumptions. With this set of intrinsic SEDs, we present a novel approach for
directly recovering the shape and normalization of the dust attenuation curve.
We find, across all of the intrinsic templates considered, that the average
attenuation curve for star-forming galaxies at is similar in shape
to the commonly-adopted Calzetti starburst law, with an average
total-to-selective attenuation ratio of . We show that the
optical attenuation () versus stellar mass () relation
predicted using our method is consistent with recent ALMA observations of
galaxies at in the \emph{Hubble} \emph{Ultra} \emph{Deep} \emph{Field}
(HUDF), as well as empirical relations predicted by a
Calzetti-like law. Our results, combined with other literature data, suggest
that the relation does not evolve over the redshift range
, at least for galaxies with log.
Finally, we present tentative evidence which suggests that the attenuation
curve may become steeper at log.Comment: 16 pages, 12 figures, accepted for publication in MNRA
The Building the Bridge survey for z=3 Ly-alpha emitting galaxies II: Completion of the survey
(Abridged). We aim at bridging the gap between absorption selected and
emission selected galaxies at z~3 by probing the faint end of the luminosity
function of star-forming galaxies at z~3. We have performed narrow-band imaging
in three fields with intervening QSO absorbers (a damped Ly absorber
and two Lyman-limit systems) using the VLT. We target Ly-alpha at redshifts
2.85, 3.15 and 3.20. We find a consistent surface density of about 10
Ly-alpha-emitters per square arcmin per unit redshift in all three fields down
to our detection limit of about 3x10^41 ergs s^-1. The luminosity function is
consistent with what has been found by other surveys at similar redshifts.
About 85% of the sources are fainter than the canonical limit of R=25.5 for
most Lyman-break galaxy surveys. In none of the three fields do we detect the
emission counterparts of the QSO absorbers. In particular we do not detect the
counterpart of the z=2.85 damped Ly-alpha absorber towards Q2138-4427.
Narrow-band surveys for Ly-alpha emitters are excellent to probe the faint end
of the luminosity function at z~3. There is a very high surface density of this
class of objects. This is consistent with a very steep slope of the faint end
of the luminosity function as has been inferred by other studies. This faint
population of galaxies is playing a central role in the early Universe. There
is evidence that this population is dominating the integrated star-formation
activity, responsible for the bulk of the ionizing photons at z~3 and likely
also responsible for the bulk of the enrichment of the intergalactic medium.Comment: 15 pages, 13 figures. Accepted for publication in A&
Beta-delayed deuteron emission from 11Li: decay of the halo
The deuteron-emission channel in the beta-decay of the halo-nucleus 11Li was
measured at the ISAC facility at TRIUMF by implanting post-accelerated 11Li
ions into a segmented silicon detector. The events of interest were identified
by correlating the decays of 11Li with those of the daughter nuclei. This
method allowed the energy spectrum of the emitted deuterons to be extracted,
free from contributions from other channels, and a precise value for the
branching ratio B_d = 1.30(13) x 10-4 to be deduced for E(c.m.) > 200 keV. The
results provide the first unambiguous experimental evidence that the decay
takes place essentially in the halo of 11Li, and that it proceeds mainly to the
9Li + d continuum, opening up a new means to study of the halo wave function of
11Li.Comment: 4 pages, 3 figure
Implications on obtained data, electronics and DAQ-system by the use of dense detector setups in -decay studies
Star formation in the early universe: beyond the tip of the iceberg
We present late-time Hubble Space Telescope imaging of the fields of six
Swift GRBs lying at 5.0<z<9.5. Our data includes very deep observations of the
field of the most distant spectroscopically confirmed burst, GRB 090423, at
z=8.2. Using the precise positions afforded by their afterglows we can place
stringent limits on the luminosities of their host galaxies. In one case, that
of GRB 060522 at z=5.11, there is a marginal excess of flux close to the GRB
position which may be a detection of a host at a magnitude J(AB)=28.5. None of
the others are significantly detected meaning that all the hosts lie below
L\star at their respective redshifts, with star formation rates SFR<4Mo/yr in
all cases. Indeed, stacking the five fields with WFC3-IR data we conclude a
mean SFR<0.17Mo/yr per galaxy. These results support the proposition that the
bulk of star formation, and hence integrated UV luminosity, at high redshifts
arises in galaxies below the detection limits of deep-field observations.
Making the reasonable assumption that GRB rate is proportional to UV luminosity
at early times allows us to compare our limits with expectations based on
galaxy luminosity functions derived from the Hubble Ultra-Deep Field (HUDF) and
other deep fields. We infer that a luminosity function which is evolving
rapidly towards steeper faint-end slope (alpha) and decreasing characteristic
luminosity (L\star), as suggested by some other studies, is consistent with our
observations, whereas a non-evolving LF shape is ruled out at >90% confidence.
Although it is not yet possible to make stronger statements, in the future,
with larger samples and a fuller understanding of the conditions required for
GRB production, studies like this hold great potential for probing the nature
of star formation, the shape of the galaxy luminosity function, and the supply
of ionizing photons in the early universe.Comment: ApJ in press. 14 pages, 6 figures. (small updates from version 1
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