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Register a new userEvidence for non-evolving FeII/MgII ratios in rapidly accreting z~6 QSOs
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[abridged] Quasars (QSOs) at the highest known redshift (z~6) are unique probes of the early growth of supermassive black holes (BHs). Until now, only the most luminous QSOs have been studied, often one object at a time. Here we present the most extensive consistent analysis to date of z>4 QSOs with observed NIR spectra, combining three new z~6 objects from our ongoing VLT-ISAAC program with nineteen 4<z<6.5 sources from the literature. The new sources extend the existing SDSS sample towards the faint end of the QSO luminosity function. Using a maximum likelihood fitting routine optimized for our spectral decomposition, we estimate the black hole mass (MBH), the Eddington ratio (defined as Lbol/LEdd) and the FeII/MgII line ratio, a proxy for the chemical abundance, to characterize both the central object and the broad line region gas. The QSOs in our sample host BHs with masses of ~10^9 Modot that are accreting close to the Eddington luminosity, consistent with earlier results. We find that the distribution of observed Eddington ratios is significantly different than that of a luminosity-matched comparison sample of SDSS QSOs at lower redshift (0.35<z<2.25): the average <log(Lbol/LEdd)>=-0.37 (Lbol/LEdd~0.43) with a scatter of 0.20 dex for the z>4 sample and the <log(Lbol/LEdd)>=-0.80 (Lbol/LEdd~0.16) with a scatter of 0.24 dex for the 0.35<z<2.25 sample. This implies that, at a given luminosity, the MBH at high-z is typically lower than the average MBH of the lower-redshift population, i.e. the z>4 sources are accreting significantly faster than the lower-redshift ones. We show that the derived FeII/MgII ratios depend sensitively on the performed analysis: our self-consistent, homogeneous analysis significantly reduces the FeII/MgII scatter found in previous studies. The measured FeII/MgII line ratios show no sign of evolution with cosmic time in the redshift range 4<z<6.5 [...]
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We present a study of the prevalence, strength, and kinematics of ultraviolet FeII and MgII emission lines in 212 star-forming galaxies at z = 1 selected from the DEEP2 survey. We find FeII* emission in composite spectra assembled on the basis of different galaxy properties, indicating that FeII* emission is prevalent at z = 1. In these composites, FeII* emission is observed at roughly the systemic velocity. At z = 1, we find that the strength of FeII* emission is most strongly modulated by dust attenuation, and is additionally correlated with redshift, star-formation rate, and [OII] equivalent width, such that systems at higher redshifts with lower dust levels, lower star-formation rates, and larger [OII] equivalent widths show stronger FeII* emission. We detect MgII emission in at least 15% of the individual spectra and we find that objects showing stronger MgII emission have higher specific star-formation rates, smaller [OII] linewidths, larger [OII] equivalent widths, lower dust attenuations, and lower stellar masses than the sample as a whole. MgII emission strength exhibits the strongest correlation with specific star-formation rate, although we find evidence that dust attenuation and stellar mass also play roles in the regulation of MgII emission. Future integral field unit observations of the spatial extent of FeII* and MgII emission in galaxies with high specific star-formation rates, low dust attenuations, and low stellar masses will be important for probing the morphology of circumgalactic gas.
We report on the discovery of strongly variable FeII and MgII absorption lines seen at z=1.48 in the spectra of the z=4.05 GRB 060206 obtained between 4.13 to 7.63 hours (observer frame) after the burst. In particular, the FeII line equivalent width (EW) decayed rapidly from 1.72+-0.25 AA to 0.28+-0.21 AA, only to increase to 0.96+-0.21 AA in a later date spectrum. The MgII doublet shows even more complicated evolution: the weaker line of the doublet drops from 2.05+-0.25 AA to 0.92+-0.32 AA, but then more than doubles to 2.47+-0.41 AA in later data. The ratio of the EWs for the MgII doublet is also variable, being closer to 1:1 (saturated regime) when the lines are stronger and becoming closer to 2:1 (unsaturated regime) when the lines are weaker, consistent with expectations based on atomic physics. We have investigated and rejected the possibility of any instrumental or atmospheric effects causing the observed strong variations. Our discovery of clearly variable intervening FeII and MgII lines lends very strong support to their scenario, in which the characteristic size of intervening patches of MgII ``clouds is comparable to the GRB beam size, i.e, about 10^16 cm. We discuss various implications of this discovery, including the nature of the MgII absorbers, the physics of GRBs, and measurements of chemical abundances from GRB and quasar absorption lines.
We test the recent claim by Hu et al. (2008) that FeII emission in Type 1 AGN shows a systematic redshift relative to the local source rest frame and broad-line Hbeta. We compile high s/n median composites using SDSS spectra from both the Hu et al. sample and our own sample of the 469 brightest DR5 spectra. Our composites are generated in bins of FWHM Hbeta and FeII strength as defined in our 4D Eigenvector 1 (4DE1) formalism. We find no evidence for a systematic FeII redshift and consistency with previous assumptions that FeII shift and width (FWHM) follow Hbeta shift and FWHM in virtually all sources. This result is consistent with the hypothesis that FeII emission (quasi-ubiquitous in type 1 sources) arises from a broad-line region with geometry and kinematics the same as that producing the Balmer lines.
We present observations of the CO[3-2] emission towards two massive and infrared luminous Lyman Break Galaxies at z = 3.21 and z = 2.92, using the IRAM Plateau de Bure Interferometer, placing first constraints on the molecular gas masses (Mgas) of non-lensed LBGs. Their overall properties are consistent with those of typical (Main-Sequence) galaxies at their redshifts, with specific star formation rates ~1.6 and ~2.2 Gyr^(-1), despite their large infrared luminosities L_IR ~2-3 x 10^12 Lsun derived from Herschel. With one plausible CO detection (spurious detection probability of 10^(-3)) and one upper limit, we investigate the evolution of the molecular gas-to-stellar mass ratio (Mgas/M*) with redshift. Our data suggest that the steep evolution of Mgas/M* of normal galaxies up to z~2 is followed by a flattening at higher redshifts, providing supporting evidence for the existence of a plateau in the evolution of the specific star formation rate at z > 2.5.
QSOs have been thought to be important for tracing highly biased regions in the early universe, from which the present-day massive galaxies and galaxy clusters formed. While overdensities of star-forming galaxies have been found around QSOs at 2<z<5, the case for excess galaxy clustering around QSOs at z>6 is less clear. Previous studies with HST have reported the detection of small excesses of faint dropout galaxies in some QSO fields, but these surveys probed a relatively small region surrounding the QSOs. To overcome this problem, we have observed the most distant QSO at z=6.4 using the large field of view of the Suprime-Cam (34 x 27). Newly-installed CCDs allowed us to select Lyman break galaxies (LBG) at z~6.4 more efficiently. We found seven LBGs in the QSO field, whereas only one exists in a comparison field. The significance of this apparent excess is difficult to quantify without spectroscopic confirmation and additional control fields. The Poisson probability to find seven objects when one expects four is ~10%, while the probability to find seven objects in one field and only one in the other is less than 0.4%, suggesting that the QSO field is significantly overdense relative to the control field. We find some evidence that the LBGs are distributed in a ring-like shape centered on the QSO with a radius of ~3 Mpc. There are no candidate LBGs within 2 Mpc from the QSO, i.e., galaxies are clustered around the QSO but appear to avoid the very center. These results suggest that the QSO is embedded in an overdense region when defined on a sufficiently large scale. This suggests that the QSO was indeed born in a massive halo. The central deficit of galaxies may indicate that (1) the strong UV radiation from the QSO suppressed galaxy formation in its vicinity, or (2) that star-formation closest to the QSO occurs mostly in an obscured mode that is missed by our UV selection.
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