16 research outputs found
Gemini Near-infrared Spectroscopy of Luminous z~6 Quasars: Chemical Abundances, Black Hole Masses, and MgII Absorption
We present Gemini near-infrared spectroscopic observations of six luminous
quasars at z=5.86.3. Five of them were observed using Gemini-South/GNIRS,
which provides a simultaneous wavelength coverage of 0.9--2.5 m in cross
dispersion mode. The other source was observed in K band with
Gemini-North/NIRI. We calculate line strengths for all detected emission lines
and use their ratios to estimate gas metallicity in the broad-line regions of
the quasars. The metallicity is found to be supersolar with a typical value of
4 Z_{\sun}, and a comparison with low-redshift observations shows no
strong evolution in metallicity up to z6. The FeII/MgII ratio of the
quasars is 4.9+/-1.4, consistent with low-redshift measurements. We estimate
central BH masses of 10^9 to 10^{10} M_{\sun} and Eddington luminosity ratios
of order unity. We identify two MgII 2796,2803 absorbers with
rest equivalent width W_0^{\lambda2796}>1 \AA at 2.2<z<3 and three MgII
absorbers with W_0^{\lambda2796}>1.5 \AA at z>3 in the spectra, with the two
most distant absorbers at z=4.8668 and 4.8823, respectively. The redshift
number densities (dN/dz) of MgII absorbers with W_0^{\lambda2796}>1.5 \AA are
consistent with no cosmic evolution up to z>4.Comment: 33 pages (including 7 figures and 6 tables), AJ in pres
New insights into the role of AGNs in forming the cluster red sequence
As a considerable investment of time from various telescope facilities were
dedicated toward studying the Spiderweb protocluster at , it so far
remains one of the most extensively studied protocluster. We report here the
latest results in this field, adding a new dimension to previous research on
cluster formation at high redshift. Previous studies have reported a
significant overdensity () of massive H (+ [Nii])
-emitting galaxies in 3700 comoving Mpc. Many of these were previously
considered to be dusty, actively star-forming galaxies, given their rest-frame
optical and infrared features. However, this study argues that a third of them
are more likely to be "passively-evolving" galaxies with low-luminosity active
galactic nuclei (AGNs) rather than star-forming galaxies, given the
multi-wavelength spectral energy distribution (SED) fitting including an AGN
component. For their SED-based star formation rates to be valid, bulk of their
H + [Nii] emission should come from the central AGNs. This difference
in interpretation between this work and past studies, including ours, is
particularly supported by the recent deep Chandra X-ray observation.
Furthermore, we have spectroscopically confirmed a quiescent nature for one of
these AGNs, with its multiple stellar absorption lines but also low ionisation
emission lines. This important update provides new insights into the role of
AGNs in forming the cluster red sequence observed in the present-day universe.Comment: 17 pages, 10 figures, 5 tables, accepted for publication in MNRA
Black hole masses and enrichment of z ~ 6 SDSS quasars
We present sensitive near-infrared spectroscopic observations for a sample of
five z ~ 6 quasars. These are amongst the most distant, currently known quasars
in the universe. The spectra have been obtained using ISAAC at the VLT and
include the CIV, MgII and FeII lines. We measure the FeII/MgII line ratio, as
an observational proxy for the Fe/alpha element ratio. We derive a ratio of
2.7+/-0.8 for our sample, which is similar to that found for lower redshift
quasars, i.e., we provide additional evidence for the lack of evolution in the
FeII/MgII line ratio of quasars up to the highest redshifts. This result
demonstrates that the sample quasars must have undergone a major episode of
iron enrichment in less than one Gyr and star formation must have commenced at
z > 8. The linewidths of the MgII and CIV lines give two estimates for the
black hole masses. A third estimate is given by assuming that the quasars emit
at their Eddington luminosity. The derived masses using these three methods
agree well, implying that the quasars are not likely to be strongly lensed. We
derive central black hole masses of 0.3-5.2 10^9 solar masses. We use the
difference between the redshift of MgII (a proxy for the systemic redshift of
the quasar) and the onset of the Gunn Peterson trough to derive the extent of
the ionized Stromgren spheres around our target quasars. The derived physical
radii are about five Mpc. Using a simple ionization model, the emission of the
central quasars would need of order 10^6-10^8 year to create these cavities in
a surrounding intergalactic medium with a neutral fraction between 0.1 and 1.0.
As the e-folding time scale for the central accreting black hole is on the
order of a few times 10^7 year, it can grow by one e-folding or less within
this time span.Comment: Accepted by ApJ, 15 pages, 8 figure
Dust-free quasars in the early Universe
The most distant quasars known, at redshifts z=6, generally have properties
indistinguishable from those of lower-redshift quasars in the rest-frame
ultraviolet/optical and X-ray bands. This puzzling result suggests that these
distant quasars are evolved objects even though the Universe was only seven per
cent of its current age at these redshifts. Recently one z=6 quasar was shown
not to have any detectable emission from hot dust, but it was unclear whether
that indicated different hot-dust properties at high redshift or if it is
simply an outlier. Here we report the discovery of a second quasar without
hot-dust emission in a sample of 21 z=6 quasars. Such apparently hot-dust-free
quasars have no counterparts at low redshift. Moreover, we demonstrate that the
hot-dust abundance in the 21 quasars builds up in tandem with the growth of the
central black hole, whereas at low redshift it is almost independent of the
black hole mass. Thus z=6 quasars are indeed at an early evolutionary stage,
with rapid mass accretion and dust formation. The two hot-dust-free quasars are
likely to be first-generation quasars born in dust-free environments and are
too young to have formed a detectable amount of hot dust around them.Comment: To be published in Nature on the 18 March 2010
Discovery of an Excess of Halpha Emitters around 4C 23.56 at z=2.48
We report the discovery of a significant excess of candidate Halpha emitters
(HAEs) in the field of the radio galaxy 4C 23.56 at z=2.483. Using the MOIRCS
near-infrared imager on the Subaru Telescope we found 11 candidate
emission-line galaxies to a flux limit of ~7.5 10^-17 erg s-1 cm-2, which is
about 5 times excess from the expected field counts with ~3-sigma significance.
Three of these are spectroscopically confirmed as redshifted Halpha at z=2.49.
The distribution of candidate emitters on the sky is tightly confined to a
1.2-Mpc-radius area at z=2.49, locating 4C 23.56 at the western edge of the
distribution. Analysis of the deep Spitzer MIPS 24 mu m imaging shows that
there is also an excess of faint MIPS sources. All but two of the 11 HAEs are
also found in the MIPS data. The inferred star-formation rate (SFR) of the HAEs
based on the extinction-corrected Halpha luminosity (median SFR >~100 M_solar
yr-1) is similar to those of HAEs in random fields at z~2. On the other hand,
the MIPS-based SFR for the HAEs is on average 3.6 times larger, suggesting the
existence of the star-formation significanly obscured by dust. The comparison
of the Halpha-based star-formation activities of the HAEs in the 4C 23.56 field
to those in another proto-cluster around PKS 1138-262 at z=2.16 reveals that
the latter tend to have fainter Halpha emission despite similar K-band
magnitudes. This suggests that star-formation may be suppressed in the PKS
1138-262 protocluster relative to the 4C 23.56 protocluster. This difference
among the HAEs in the two proto-clusters at z > 2 may imply that some massive
cluster galaxies are just forming at these epochs with some variation among
clusters.Comment: 29 pages, 13 figures, to be published in PASJ Subaru Special Issue
(2011 Mar.
The Spiderweb galaxy: a forming massive cluster galaxy at z~2
We present a deep image of the radio galaxy MRC 1138-262 taken with the
Hubble Space Telescope (HST) at a redshift of z = 2.2. The galaxy is known to
have properties of a cD galaxy progenitor and be surrounded by a 3 Mpc-sized
structure, identified with a protocluster. The morphology shown on the new deep
HST/ACS image is reminiscent of a spider's web. More than 10 individual clumpy
features are observed, apparently star-forming satellite galaxies in the
process of merging with the progenitor of a dominant cluster galaxy 11 Gyr ago.
There is an extended emission component, implying that star formation was
occurring over a 50 times 40 kpc region at a rate of more than 100 M_sun/yr. A
striking feature of the newly named ``Spiderweb galaxy'' is the presence of
several faint linear galaxies within the merging structure. The dense
environments and fast galaxy motions at the centres of protoclusters may
stimulate the formation of these structures, which dominate the faint resolved
galaxy populations in the Hubble Ultra Deep Field. The new image provides a
unique testbed for simulations of forming dominant cluster galaxies.Comment: 5 pages, 2 figures (reduced to grayscale); ApJ Letter
New insights into the role of AGNs in forming the cluster red sequence
As a considerable investment of time from various telescope facilities was dedicated toward studying the Spiderweb protocluster at z = 2.2, it so far remains one of the most extensively studied protocluster. We report here the latest results in this field, adding a new dimension to previous research on cluster formation at high redshift. Previous studies have reported a significant overdensity (ÎŽ ⌠10) of massive Hα (+ [Nâii])-emitting galaxies in 3700 comoving Mpc3. Many of these were previously considered to be dusty, actively star-forming galaxies, given their rest-frame optical and infrared features. However, this study argues that a third of them are more likely to be âpassively evolvingâ galaxies with low-luminosity active galactic nuclei (AGNs) rather than star-forming galaxies, given the multiwavelength spectral energy distribution (SED) fitting including an AGN component. For their SED-based star formation rates to be valid, bulk of their Hα + [Nâii] emission should come from the central AGNs. This difference in interpretation between this work and past studies, including ours, is particularly supported by the recent deep Chandra/X-ray observation. Furthermore, we have spectroscopically confirmed a quiescent nature for one of these AGNs, with its multiple stellar absorption lines but also low-ionization emission lines. This important update provides new insights into the role of AGNs in forming the cluster red sequence observed in the present-day universe
Spectroscopy of Ultra Steep Spectrum Radio Sources
We present optical spectroscopy of 62 objects selected from several samples
of ultra steep spectrum (USS) radio sources. 46 of these are from our primary
catalog, consisting of 669 sources with radio spectral indices alpha < -1.30
(S_nu ~ nu^alpha); this first spectroscopic sub-sample was selected on the
basis of their faint optical and near-IR identifications. Most are identified
as narrow-lined radio galaxies with redshifts ranging from z=0.25 to z=5.19.
Ten objects are at z>3, nearly doubling the number of such sources known to
date. Four of the USS radio sources are identified with quasars, of which at
least three have very red spectral energy distributions. The source TN
J0936-2242 is identified with an extremely red object (ERO, R-K>5); both it and
a close companion are at z=1.479. The spectrum of the ERO closely resembles
that of previously discovered radio galaxies at z~1.5. Five sources show
continuum emission, but fail to show any clear emission or absorption features,
despite integrations of ~1 h with the Keck telescope. We suggest that these
objects could be (i) radio galaxies with faint emission lines in the ``redshift
desert'' at 1.5 <~ z <~ 2.3, (ii) radio galaxies with an obscured AGN, which
are dominated by a stellar continuum observed with insufficient S/N, or (iii)
pulsars. Three radio sources identified with faint objects in the K-band images
remain undetected in 50-90 min spectroscopic integrations with the Keck
telescope, and are possible z>7 candidates.Comment: 59 Pages, including 59 PostScript figures. Accepted for publication
in the Astronomical Journal (March 2001 issue). Tables 5 and 6 will be
available in electronic format on the AJ website or upon reques
MAHALO Deep Cluster Survey II. Characterizing massive forming galaxies in the Spiderweb protocluster at z = 2.2
This paper is the second in a series presenting the results of our deep Hâα-line survey towards protoclusters at z > 2, based on narrow-band imaging with the Subaru Telescope. This work investigates massive galaxies in a protocluster region associated with a radio galaxy (PKS 1138 â 262), the Spiderweb galaxy, at z = 2.2. Our 0.5 mag deeper narrow-band imaging than previous surveys collects a total of 68 Hâα emitters (HAE). Here, 17 out of the 68 are newly discovered protocluster members. First, a very high characteristic stellar mass of Mââ=1011.73 Mâ is measured from a Schechter function fit to the mass distribution of HAEs. Together with the Chandra X-ray data, we find that four out of six massive HAEs (Mâ > 1011 Mâ) show bright X-ray emission, suggesting that they host active galactic nuclei (AGNs). Their mass estimates, therefore, would be affected by the nuclear emission from AGNs. Notably, the X-ray-detected HAEs are likely positioned near the boundary between star-forming and quiescent populations in the rest-frame UVJ plane. Moreover, our deep narrow-band data succeed in probing the bright Hâα (+ [Nâii]) line nebula of the Spiderweb galaxy extending over âŒ100 physical kpc. These results suggest that the massive galaxies in the Spiderweb protocluster are on the way to becoming the bright red sequence objects seen in local galaxy clusters, where AGNs might play an essential role in their quenching processes, though a more statistical database is needed to build a general picture