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تسجيل مستخدم جديدHow neutral is the intergalactic medium surrounding the redshift z=7.085 quasar ULAS J1120+0641?
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تأليف
James S. Bolton
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The quasar ULAS J1120+0641 at redshift z=7.085 has a highly ionised near zone which is smaller than those around quasars of similar luminosity at z~6. The spectrum also exhibits evidence for a damping wing extending redward of the systemic Lya redshift. We use radiative transfer simulations in a cosmological context to investigate the implications for the ionisation state of the inhomogeneous IGM surrounding this quasar. Our simulations show that the transmission profile is consistent with an IGM in the vicinity of the quasar with a volume averaged HI fraction of f_HI>0.1 and that ULAS J1120+0641 has been bright for 10^6--10^7 yr. The observed spectrum is also consistent with smaller IGM neutral fractions, f_HI ~ 10^-3--10-4, if a damped Lya system in an otherwise highly ionised IGM lies within 5 proper Mpc of the quasar. This is, however, predicted to occur in only ~5 per cent of our simulated sight-lines for a bright phase of 10^6--10^7 yr. Unless ULAS J1120+0641 grows during a previous optically obscured phase, the low age inferred for the quasar adds to the theoretical challenge of forming a 2x10^9 M_sol black hole at this high redshift.
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We present X-ray imaging and spectroscopy of the redshift z=7.084 radio-quiet quasar ULAS J112001.48+064124.3 obtained with Chandra and XMM-Newton. The quasar is detected as a point source with both observatories. The Chandra observation provides a p
recise position, confirming the association of the X-ray source and the quasar, while a sufficient number of photons is detected in the XMM-Newton observation to yield a meaningful X-ray spectrum. In the XMM-Newton observation the quasar has a 2-10 keV luminosity of 4.7+-0.9 times 10^44 erg/s and a spectral slope alpha = 1.6+0.4/-0.3 (where f_nu is proportional to nu^-alpha). The quasar appears to have dimmed in the 15 months between the two observations, with a 2-10 keV luminosity of $1.8+1.0/-0.7 times 10^45 erg/s during the Chandra observation. We derive optical to X-ray spectral slopes alpha_OX of 1.76+-0.07 and 1.54+0.09/-0.08 at the times of the XMM-Newton and Chandra observations respectively, consistent with the range of alpha_OX found in other quasars of comparable ultraviolet luminosity. The very soft X-ray spectrum suggests that the quasar is accreting above the Eddington rate, L/L_Edd = 5+15/-4, compared to L/L_Edd = 1.2+0.6/-0.5 derived from the rest-frame ultraviolet. Super-Eddington accretion would help to reduce the discrepancy between the age of the quasar implied by the small size of the ionized near zone in which it sits (<10^7 years), and the characteristic e-folding time (2.5 times 10^7 years if L/L_Edd=2). Such super-Eddington accretion would also alleviate the challenging constraints on the seed black hole mass provided that the quasar has been rapidly accreting throughout its history. The remnant of an individual population III star is a plausible progenitor if an average L/L_Edd>1.46 has been maintained over the quasars lifetime.
We present new observations of the highest-redshift quasar known, ULAS J1120+0641, redshift $z=7.084$, obtained in the optical, at near-, mid-, and far-infrared wavelengths, and in the sub-mm. We combine these results with published X-ray and radio o
bservations to create the multiwavelength spectral energy distribution (SED), with the goals of measuring the bolometric luminosity $L_{rm bol}$, and quantifying the respective contributions from the AGN and star formation. We find three components are needed to fit the data over the wavelength range $0.12-1000,mu$m: the unobscured quasar accretion disk and broad-line region, a dusty clumpy AGN torus, and a cool 47K modified black body to characterise star formation. Despite the low signal-to-noise ratio of the new long-wavelength data, the normalisation of any dusty torus model is constrained within $pm40%$. We measure a bolometric luminosity $L_{rm bol}=2.6pm0.6times10^{47},$erg$,$s$^{-1}=6.7 pm 1.6times10^{13}L_{odot}$, to which the three components contribute $31%,32%,3%$, respectively, with the remainder provided by the extreme UV $<0.12,mu$m. We tabulate the best-fit model SED. We use local scaling relations to estimate a star formation rate (SFR) in the range $60-270,{rm M}_odot$/yr from the [C$,{scriptsize rm II}$] line luminosity and the $158,mu$m continuum luminosity. An analysis of the equivalent widths of the [C$,{scriptsize rm II}$] line in a sample of $z>5.7$ quasars suggests that these indicators are promising tools for estimating the SFR in high-redshift quasars in general. At the time observed the black hole was growing in mass more than 100 times faster than the stellar bulge, relative to the mass ratio measured in the local universe, i.e. compared to ${M_{rm BH}}/{M_{rm bulge}} simeq 1.4times10^{-3}$, for ULAS J1120+0641 we measure ${dot{M}_{rm BH}}/{dot{M}_{rm bulge}} simeq 0.2$.
We present optical and near-infrared imaging of the field of the z=7.0842 quasar ULAS J112001.48+064124.3 taken with the Hubble Space Telescope. We use these data to search for galaxies that may be physically associated with the quasar, using the Lym
an break technique, and find three such objects, although the detection of one in Spitzer Space Telescope imaging strongly suggests it lies at z~2. This is consistent with the field luminosity function and indicates that there is no excess of >L* galaxies within 1 Mpc of the quasar. A detection of the quasar shortward of the Ly-alpha line is consistent with the previously observed evolution of the intergalactic medium at z>5.5.
We present a search for metal absorption line systems at the highest redshifts to date using a deep (30h) VLT/X-Shooter spectrum of the z = 7.084 quasi-stellar object (QSO) ULAS J1120+0641. We detect seven intervening systems at z > 5.5, with the hig
hest-redshift system being a C IV absorber at z = 6.51. We find tentative evidence that the mass density of C IV remains flat or declines with redshift at z < 6, while the number density of C II systems remains relatively flat over 5 < z < 7. These trends are broadly consistent with models of chemical enrichment by star formation-driven winds that include a softening of the ultraviolet background towards higher redshifts. We find a larger number of weak ( W_rest < 0.3A ) Mg II systems over 5.9 < z < 7.0 than predicted by a power-law fit to the number density of stronger systems. This is consistent with trends in the number density of weak Mg II systems at z = 2.5, and suggests that the mechanisms that create these absorbers are already in place at z = 7. Finally, we investigate the associated narrow Si IV, C IV, and N V absorbers located near the QSO redshift, and find that at least one component shows evidence of partial covering of the continuum source.
We present 1-2 GHz Very Large Array A-configuration continuum observations on the highest redshift quasar known to date, the $z=7.085$ quasar ULAS J112001.48+064124.3. The results show no radio continuum emission at the optical position of the quasar
or its vicinity at a level of $geq 3sigma$ or $23.1 mu$Jy beam$^{-1}$. This $3sigma$ limit corresponds to a rest frame 1.4 GHz luminosity density limit of $L_{ u,1.4,GHz} < 1.76 times 10^{24}$ W Hz$^{-1}$ for a spectral index of $alpha=0$, and $L_{ u,1.4,GHz} < 1.42 times 10^{25}$ W Hz$^{-1}$ for a spectral index of $alpha=-1$. The rest-frame 1.4 GHz luminosity limits are $L_{rad} < 6.43 times 10^6 L_{odot}$ and $L_{rm rad} < 5.20 times 10^7 L_{odot}$ for $alpha=0$ and $alpha=-1$, respectively. The derived limits for the ratio of the rest frame 1.4 GHz luminosity density to the $B$-band optical luminosity density are $Rrlap{}_{1.4}^{*} < 0.53$ and $< 4.30$ for the above noted spectral indices, respectively. Given our upper limits on the radio continuum emission and the radio-to-optical luminosity ratio, we conclude that this quasar is radio-quiet and located at the low end of the radio quiet distribution of high redshift ($z gtrsim 6$) quasars.
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