Quasi-stellar object (QSO) spectral templates are important both to QSO physics and for investigations that use QSOs as probes of intervening gas and dust. However, combinations of various QSO ...samples obtained at different times and with different instruments so as to expand a composite and to cover a wider rest frame wavelength region may create systematic effects, and the contribution from QSO hosts may contaminate the composite. We have constructed a composite spectrum from luminous blue QSOs at 1 < z < 2.1 selected from the Sloan Digital Sky Survey (SDSS). The observations with X-Shooter simultaneously cover ultraviolet (UV) to near-infrared (NIR) light, which ensures that the composite spectrum covers the full rest-frame range from Lyβ to 11 350 Å without any significant host contamination. Assuming a power-law continuum for the composite we find a spectral slope of αλ = 1.70 ± 0.01, which is steeper than previously found in the literature. We attribute the differences to our broader spectral wavelength coverage, which allows us to effectively avoid fitting any regions that are affected either by strong QSO emissions lines (e.g., Balmer lines and complex Fe II blends) or by intrinsic host galaxy emission. Finally, we demonstrate the application of the QSO composite spectrum for evaluating the reddening in other QSOs.
Context. Several issues regarding the nature of dust at high redshift remain unresolved: its composition, its production and growth mechanisms, and its effect on background sources. Aims. We provide ...a more accurate relation between dust depletion levels and dust-to-metals ratio (DTM), and to use the DTM to investigate the origin and evolution of dust in the high-redshift Universe via gamma-ray burst damped Lyman-alpha absorbers (GRB-DLAs). Methods. We use absorption-line measured metal column densities for a total of 19 GRB-DLAs, including five new GRB afterglow spectra from VLT/X-Shooter. We use the latest linear models to calculate the dust depletion strength factor in each DLA. Using these values we calculate total dust and metal column densities to determine a DTM. We explore the evolution of DTM with metallicity, and compare it to previous trends in DTM measured with different methods. Results. We find significant dust depletion in 16 of our 19 GRB-DLAs, yet 18 of the 19 have a DTM significantly lower than the Milky Way. We find that DTM is positively correlated with metallicity, which supports a dominant ISM grain-growth mode of dust formation. We find a substantial discrepancy between the dust content measured from depletion and that derived from the total V-band extinction, AV, measured by fitting the afterglow SED. We advise against using a measurement from one method to estimate that from the other until the discrepancy can be resolved.
ABSTRACT We present rest-frame near-IR (NIR) luminosities and stellar masses for a large and uniformly selected population of gamma-ray burst (GRB) host galaxies using deep Spitzer Space Telescope ...imaging of 119 targets from the Swift GRB Host Galaxy Legacy Survey spanning 0.03 < z < 6.3, and we determine the effects of galaxy evolution and chemical enrichment on the mass distribution of the GRB host population across cosmic history. We find a rapid increase in the characteristic NIR host luminosity between z ∼ 0.5 and z ∼ 1.5, but little variation between z ∼ 1.5 and z ∼ 5. Dust-obscured GRBs dominate the massive host population but are only rarely seen associated with low-mass hosts, indicating that massive star-forming galaxies are universally and (to some extent) homogeneously dusty at high redshift while low-mass star-forming galaxies retain little dust in their interstellar medium. Comparing our luminosity distributions with field surveys and measurements of the high-z mass-metallicity relation, our results have good consistency with a model in which the GRB rate per unit star formation is constant in galaxies with gas-phase metallicity below approximately the solar value but heavily suppressed in more metal-rich environments. This model also naturally explains the previously reported "excess" in the GRB rate beyond z 2; metals stifle GRB production in most galaxies at z < 1.5 but have only minor impact at higher redshifts. The metallicity threshold we infer is much higher than predicted by single-star models and favors a binary progenitor. Our observations also constrain the fraction of cosmic star formation in low-mass galaxies undetectable to Spitzer to be small at z < 4.
Understanding how super-massive black holes form and grow in the early Universe has become a major challenge
since it was discovered that luminous quasars existed only 700 million years after the Big ...Bang
. Simulations indicate an evolutionary sequence of dust-reddened quasars emerging from heavily dust-obscured starbursts that then transition to unobscured luminous quasars by expelling gas and dust
. Although the last phase has been identified out to a redshift of 7.6 (ref.
), a transitioning quasar has not been found at similar redshifts owing to their faintness at optical and near-infrared wavelengths. Here we report observations of an ultraviolet compact object, GNz7q, associated with a dust-enshrouded starburst at a redshift of 7.1899 ± 0.0005. The host galaxy is more luminous in dust emission than any other known object at this epoch, forming 1,600 solar masses of stars per year within a central radius of 480 parsec. A red point source in the far-ultraviolet is identified in deep, high-resolution imaging and slitless spectroscopy. GNz7q is extremely faint in X-rays, which indicates the emergence of a uniquely ultraviolet compact star-forming region or a Compton-thick super-Eddington black-hole accretion disk at the dusty starburst core. In the latter case, the observed properties are consistent with predictions from cosmological simulations
and suggest that GNz7q is an antecedent to unobscured luminous quasars at later epochs.
ABSTRACT
We present a study of the environment of 27 z = 3–4.5 bright quasars from the MUSE Analysis of Gas around Galaxies (MAGG) survey. With medium-depth Multi Unit Spectroscopic Explorer (MUSE) ...observations (4 h on target per field), we characterize the effects of quasars on their surroundings by studying simultaneously the properties of extended gas nebulae and Ly α emitters (LAEs) in the quasar host haloes. We detect extended (up to ≈100 kpc) Ly α emission around all MAGG quasars, finding a very weak redshift evolution between z = 3 and z = 6. By stacking the MUSE datacubes, we confidently detect extended emission of C iv and only marginally detect extended He ii up to ≈40 kpc, implying that the gas is metal enriched. Moreover, our observations show a significant overdensity of LAEs within 300 $\rm km~s^{-1}$ from the quasar systemic redshifts estimated from the nebular emission. The luminosity functions and equivalent width distributions of these LAEs show similar shapes with respect to LAEs away from quasars suggesting that the Ly α emission of the majority of these sources is not significantly boosted by the quasar radiation or other processes related to the quasar environment. Within this framework, the observed LAE overdensities and our kinematic measurements imply that bright quasars at z = 3–4.5 are hosted by haloes in the mass range $\approx 10^{12.0}\small{--}10^{12.5}~\rm M_\odot$.
Abstract
Starting from a summary of detection statistics of our recent X-shooter campaign, we review the major surveys, both space and ground based, for emission counterparts of high-redshift damped ...Ly α absorbers (DLAs) carried out since the first detection 25 yr ago. We show that the detection rates of all surveys are precisely reproduced by a simple model in which the metallicity and luminosity of the galaxy associated to the DLA follow a relation of the form, M
UV = −5 × (M/H + 0.3) − 20.8, and the DLA cross-section follows a relation of the form
$\sigma _{\small {DLA}} \propto L^{0.8}$
. Specifically, our spectroscopic campaign consists of 11 DLAs pre-selected based on their equivalent width of Si ii λ1526 to have a metallicity higher than Si/H > −1. The targets have been observed with the X-shooter spectrograph at the Very Large Telescope to search for emission lines around the quasars. We observe a high detection rate of 64 per cent (7/11), significantly higher than the typical ∼10 per cent for random, H i-selected DLA samples. We use the aforementioned model, to simulate the results of our survey together with a range of previous surveys: spectral stacking, direct imaging (using the ‘double DLA’ technique), long-slit spectroscopy, and integral field spectroscopy. Based on our model results, we are able to reconcile all results. Some tension is observed between model and data when looking at predictions of Ly α emission for individual targets. However, the object-to-object variations are most likely a result of the significant scatter in the underlying scaling relations as well as uncertainties in the amount of dust which affects the emission.
We analyse the redshift evolution of the mass-metallicity relation in a sample of 110 Damped Lyman α absorbers (DLAs) spanning the redshift range z = 0.11-5.06 and find that the zero-point of the ...correlation changes significantly with redshift. The evolution is such that the zero-point is constant at the early phases of galaxy growth (i.e. no evolution) but then features a sharp break at z = 2.6 ± 0.2 with a rapid incline towards lower redshifts such that damped absorbers of identical masses are more metal rich at later times than earlier. The slope of this mass-metallicity correlation evolution is 0.35 ± 0.07 dex per unit redshift.
We compare this result to similar studies of the redshift evolution of emission selected galaxy samples and find a remarkable agreement with the slope of the evolution of galaxies of stellar mass log(M
*/M) 8.5. This allows us to form an observational tie between damped absorbers and galaxies seen in emission.
We use results from simulations to infer the virial mass of the dark matter halo of a typical DLA galaxy and find a ratio (M
vir/M
*) 30.
We compare our results to those of several other studies that have reported strong transition-like events at redshifts around z = 2.5-2.6 and argue that all those observations can be understood as the consequence of a transition from a situation where galaxies were fed more unprocessed infalling gas than they could easily consume to one where they suddenly become infall starved and turn to mainly processing, or re-processing, of previously acquired gas.
We present the results of a new search for bright star-forming galaxies at redshift z ... 7 within the UltraVISTA second data release (DR2) and UKIDSS (UKIRT Infrared Deep Sky Survey) UDS (Ultra Deep ...Survey) DR10 data, which together provide 1.65 deg^sup 2^ of near-infrared imaging with overlapping optical and Spitzer data. Using a full photometric redshift analysis, to identify high-redshift galaxies and reject contaminants, we have selected a sample of 34 luminous (-22.7<M^sub UV^<-21.2) galaxies with 6.5 < z < 7.5. Crucially, the deeper imaging provided by UltraVISTA DR2 confirms all of the robust objects previously uncovered by Bowler et al., validating our selection technique. Our new expanded galaxy sample includes the most massive galaxies known at z ... 7, with M* ... 10^sup 10^ M..., and the majority are resolved, consistent with larger sizes (r^sub ...^ ... 1-1.5 kpc) than displayed by less massive galaxies. From our final robust sample, we determine the form of the bright end of the rest-frame UV galaxy luminosity function (LF) at z ... 7, providing strong evidence that it does not decline as steeply as predicted by the Schechter-function fit to fainter data. We exclude the possibility that this is due to either gravitational lensing, or significant contamination of our galaxy sample by active galactic nuclei (AGN). Rather, our results favour a double power-law form for the galaxy LF at high redshift, or, more interestingly, an LF which simply follows the form of the dark matter halo mass function at bright magnitudes. This suggests that the physical mechanism which inhibits star formation activity in massive galaxies (i.e. AGN feedback or some other form of 'mass quenching') has yet to impact on the observable galaxy LF at z ... 7, a conclusion supported by the estimated masses of our brightest galaxies which have only just reached a mass comparable to the critical 'quenching mass' of M* ... 10^sup 10.2^ M... derived from studies of the mass function of star-forming galaxies at lower redshift. (ProQuest: ... denotes formulae/symbols omitted.)
Abstract
We study a large galaxy sample from the Spitzer Matching Survey of the UltraVISTA ultra-deep Stripes (SMUVS) to search for sources with enhanced
3.6
μ
m
fluxes indicative of strong H
α
...emission at
z
=
3.9
–
4.9
. We find that the percentage of “H
α
excess” sources reaches 37%–40% for galaxies with stellar masses
log
10
(
M
*
/
M
⊙
)
≈
9
–
10
and decreases to
<
20
%
at
log
10
(
M
*
/
M
⊙
)
∼
10.7
. At higher stellar masses, however, the trend reverses, although this is likely due to active galactic nucleus contamination. We derive star formation rates (SFR) and specific SFR (sSFR) from the inferred H
α
equivalent widths of our “H
α
excess” galaxies. We show, for the first time, that the “H
α
excess” galaxies clearly have a bimodal distribution on the SFR–
M
* plane: they lie on the main sequence of star formation (with
log
10
(
sSFR
/
yr
−
1
)
<
−
8.05
) or in a starburst cloud (with
log
10
(
sSFR
/
yr
−
1
)
>
−
7.60
). The latter contains
∼
15
%
of all the objects in our sample and accounts for
>
50
%
of the cosmic SFR density at
z
=
3.9
–
4.9
, for which we derive a robust lower limit of
0.066
M
⊙
yr
−
1
Mpc
−
3
. Finally, we identify an unusual
>
50
σ
overdensity of
z
=
3.9
–
4.9
galaxies within a
0.20
×
0.20
arcmin
2
region. We conclude that the SMUVS unique combination of area and depth at mid-IR wavelengths provides an unprecedented level of statistics and dynamic range that are fundamental to revealing new aspects of galaxy evolution in the young universe.
We present the results of a search for bright (−22.7 ≤ M
UV ≤ −20.5) Lyman-break galaxies at z ≃ 6 within a total of 1.65 deg2 of imaging in the UltraVISTA/Cosmological Evolution Survey (COSMOS) and ...United Kingdom Infrared Telescope Deep Sky Survey (UKIDSS) Ultra Deep Survey (UDS) fields. The deep near-infrared imaging available in the two independent fields, in addition to deep optical (including z
′-band) data, enables the sample of z ≃ 6 star-forming galaxies to be securely detected longward of the break (in contrast to several previous studies). We show that the expected contamination rate of our initial sample by cool Galactic brown dwarfs is ≲3 per cent and demonstrate that they can be effectively removed by fitting brown dwarf spectral templates to the photometry. At z ≃ 6, the galaxy surface density in the UltraVISTA field exceeds that in the UDS by a factor of ≃ 1.8, indicating strong cosmic variance even between degree-scale fields at z > 5. We calculate the bright end of the rest-frame Ultraviolet (UV) luminosity function (LF) at z ≃ 6. The galaxy number counts are a factor of ∼1.7 lower than predicted by the recent LF determination by Bouwens et al. In comparison to other smaller area studies, we find an evolution in the characteristic magnitude between z ≃ 5 and z ≃ 7 of ΔM* ∼ 0.4, and show that a double power law or a Schechter function can equally well describe the LF at z = 6. Furthermore, the bright end of the LF appears to steepen from z ≃ 7 to z ≃ 5, which could indicate the onset of mass quenching or the rise of dust obscuration, a conclusion supported by comparing the observed LFs to a range of theoretical model predictions.
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