Junxian Wang

A Classic Type 2 QSO

In the Chandra Deep Field-South 1 Ms exposure, we have found, at redshift 3.700 ± 0.005, the most distant type 2 active galactic nucleus ever detected. It is the source with the hardest X-ray spectrum with redshift z > 3. The optical spectrum has no detected continuum emission to a 3 σ detection limit of ~3 × 10-19 ergs s-1 cm-2 A-1 and shows narrow lines of Lyα, C IV, N V, He II, O VI, [O III], and C III]. Their FWHM line widths have a range of ~700-2300 km s-1 with an average of approximately ~1500 km s-1. The emitting gas is metal-rich (Z 2.5-3 Z☉). In the X-ray spectrum of 130 counts in the 0.5-7 keV band, there is evidence for intrinsic absorption with NH 1024 cm-2. An iron Kα line with rest-frame energy and equivalent width of ~6.4 keV and ~1 keV, respectively, in agreement with the obscuration scenario, is detected at a 2 σ level. If confirmed by our forthcoming XMM-Newton observations, this would be the highest redshift detection of Fe Kα. Depending on the assumed cosmology and the X-ray transfer model, the 2-10 keV rest frame luminosity corrected for absorption is ~1045 ± 0.5 ergs cm-2 s-1, which makes our source a classic example of the long-sought type 2 QSO. From standard population synthesis models, these sources are expected to account for a relevant fraction of the black hole-powered QSO distribution at high redshift.

Spectroscopic properties of the z ≈ 4.5 Lyα emitters

We present Keck Low Resolution Imaging Spectrometer optical spectra of 17 Lyα-emitting galaxies and one Lyman break galaxy at z ≈ 4.5 discovered in the Large Area Lyman Alpha survey. The survey has identified a sample of ~350 candidate Lyα-emitting galaxies at z ≈ 4.5 in a search volume of 1.5 × 106 comoving Mpc3. We targeted 25 candidates for spectroscopy; hence, the 18 confirmations presented herein suggest a selection reliability of 72%. The large equivalent widths (median W ≈ 80 A) but narrow physical widths (Δv 240 A, exceeding the maximum Lyα equivalent width predicted for normal stellar populations. Nonetheless, we find no evidence for He II λ1640 emission in either individual or composite spectra, indicating that although these galaxies are young, they are not truly primitive, Population III objects.

Chandra Deep Field South: The 1 Ms Catalog

We present the main results from our 940 ksec observation of the Chandra Deep Field South (CDFS), using the source catalog described in an accompanying paper (Giacconi et al. 2001). We extend the measurement of source number counts to 5.5e-17 erg/cm^2/s in the soft 0.5-2 keV band and 4.5e-16 erg/cm^2/s in the hard 2-10 keV band. The hard band LogN-LogS shows a significant flattening (slope~=0.6) below ~1e-14 erg/cm^2/s, leaving at most 10-15% of the X-ray background (XRB) to be resolved, the main uncertainty lying in the measurement of the total flux of the XRB. On the other hand, the analysis in the very hard 5-10 keV band reveals a relatively steep LogN-LogS (slope ~=1.3) down to 1e-15 erg/cm^2/s. Together with the evidence of a progressive flattening of the average X-ray spectrum near the flux limit, this indicates that there is still a non negligible population of faint hard sources to be discovered at energies not well probed by Chandra, which possibly contribute to the 30 keV bump in the spectrum of the XRB. We use optical redshifts and identifications, obtained with the VLT, for one quarter of the sample to characterize the combined optical and X-ray properties of the CDFS sample. Different source types are well separated in a parameter space which includes X-ray luminosity, hardness ratio and R-K color. Type II objects, while redder on average than the field population, have colors which are consistent with being hosted by a range of galaxy types. Type II AGN are mostly found at z<~1, in contrast with predictions based on AGN population synthesis models, thus suggesting a revision of their evolutionary parameters.

Effects of Dust Geometry in Lyα Galaxies at z = 4.4*

Equivalent widths (EWs) observed in high-redshift Ly? galaxies could be stronger than the EW intrinsic to the stellar population if dust is present residing in clumps in the interstellar medium (ISM). In this scenario, continuum photons could be extinguished, while the Ly? photons would be resonantly scattered by the clumps, eventually escaping the galaxy. We investigate this radiative transfer scenario with a new sample of six Ly? galaxy candidates in the GOODS CDF-S, selected at -->z = 4.4 with ground-based narrowband imaging obtained at CTIO. Grism spectra from the HST PEARS survey confirm that three objects are at -->z = 4.4, and that another object contains an active galactic nucleus (AGN). If we assume the other five (non-AGN) objects are at -->z = 4.4, they have rest-frame EWs from 47 to 190 ?. We present results of stellar population studies of these objects, constraining their rest-frame UV with HST and their rest-frame optical with Spitzer. Out of the four objects which we analyzed, three objects were best fit to contain stellar populations with ages on the order of 1 Myr and stellar masses from -->3 ? 108 to -->1 ? 109 M?, with dust in the amount of -->A1200 = 0.9?1.8 residing in a quasi-homogeneous distribution. However, one object (with a rest -->EW ~ 150 ?) was best fit by an 800 Myr, -->6.6 ? 109 M? stellar population with a smaller amount of dust ( -->A1200 = 0.4) attenuating the continuum only. In this object, the EW was enhanced ~50% due to this dust. This suggests that large EW Ly? galaxies are a diverse population. Preferential extinction of the continuum in a clumpy ISM deserves further investigation as a possible cause of the overabundance of large-EW objects that have been seen in narrowband surveys in recent years.

A Luminous Lyα-emitting Galaxy at Redshift z = 6.535: Discovery and Spectroscopic Confirmation*

We present a redshift z = 6.535 galaxy discovered by its Lyα emission in a 9180 A narrowband image from the Large Area Lyman Alpha survey. The Lyα line luminosity (1.1 × 1043 ergs s-1) is among the largest known for star-forming galaxies at z ≈ 6.5. The line shows the distinct asymmetry that is characteristic of high-redshift Lyα. The 2 σ lower bound on the observer-frame equivalent width is greater than 530 A. This is hard to reconcile with a neutral intergalactic medium (IGM) unless the Lyα line is intrinsically strong and is emitted from its host galaxy with an intrinsic Doppler shift of several hundred km s-1. If the IGM is ionized, it corresponds to a rest-frame equivalent width greater than 40 A after correcting for Lyα forest absorption. We also present a complete spectroscopic follow-up of the remaining candidates with line flux greater than 2 × 10-17 ergs cm-2 s-1 in our 1200 arcmin2 narrowband image. These include another galaxy with a strong emission line at 9136 A and no detected continuum flux, which, however, is most likely an [O III] λ5007 source at z = 0.824, on the basis of a weak detection of the [O III] λ4959 line.

A Luminosity Function of Lyα-emitting Galaxies at z ≈ 4.5* **

We present a catalog of 59 -->z ? 4.5 Ly?-emitting galaxies spectroscopically confirmed in a campaign of Keck DEIMOS follow-up observations to candidates selected in the Large Area Ly? (LALA) narrowband imaging survey. We targeted 97 candidates for spectroscopic follow-up; by accounting for the variety of conditions under which we performed spectroscopy, we estimate a selection reliability of ~76%. Together with our previous sample of Keck LRIS confirmations, the 59 sources confirmed herein bring the total catalog to 73 spectroscopically confirmed -->z ? 4.5 Ly?-emitting galaxies in the 0.7 deg2 covered by the LALA imaging. As with the Keck LRIS sample, we find that a nonnegligible fraction of the confirmed Ly? lines have rest-frame equivalent widths ( -->W?rest) that exceed the maximum predicted for normal stellar populations: 17%-31% (93% confidence) of the detected galaxies show -->W?rest > 190 ?, and 12%-27% (90% confidence) show -->W?rest > 240 ?. We construct a luminosity function of -->z ? 4.5 Ly? emission lines for comparison to Ly? luminosity functions spanning -->3.1 z ? 3 to -->z ? 6. This result supports the conclusion that the intergalactic medium remains largely reionized from the local universe out to -->z ? 6.5. It is somewhat at odds with the pronounced drop in the cosmic star formation rate density recently measured between -->z ~ 3 and -->z ~ 6 in continuum-selected Lyman-break galaxies, and therefore potentially sheds light on the relationship between the two populations.

Polar outflows in six broad absorption line quasars

Using radio observations by FIRST and NVSS, we build a sample of 151 radio-variable quasars selected from the Sloan Digital Sky Survey Data Release 3 (SDSS DR3). Six (and probably another two) of these objects are classified as broad absorption line (BAL) quasars and show radio flux variations of a few tens of percent within 1.5-5 yr. Such large amplitudes of variations imply brightness temperatures much higher than the inverse Compton limits (10(12) K) in all these BAL quasars, suggesting the presence of relativistic jets beaming toward the observer. The angles between the outflow and the jet are constrained to be less than similar to 20 degrees. Such BAL quasars with polar outflows are beyond the simple unification models of BAL and non-BAL quasars, which hypothesize BAL quasars as otherwise normal quasars seen nearly edge-on.

The X-ray-derived cosmological star formation history and the galaxy X-ray luminosity functions in the chandra deep fields north and south

The cosmological star formation rate in the combined Chandra Deep Fields North and South is derived from our X-ray luminosity function for galaxies in these deep fields. Mild evolution is seen up to redshift order unity with star formation rate ~ (1 + z)2.7. This is the first directly observed normal star-forming galaxy X-ray luminosity function (XLF) at cosmologically interesting redshifts (z > 0). This provides the most direct measure yet of the X-ray-derived cosmic star formation history of the universe. We make use of Bayesian statistical methods to classify the galaxies and the two types of active galactic nuclei (AGNs), finding the most useful discriminators to be the X-ray luminosity, X-ray hardness ratio, and X-ray to optical flux ratio. There is some residual AGN contamination in the sample at the bright end of the luminosity function. Incompleteness slightly flattens the XLF at the faint end of the luminosity function. The XLF has a lognormal distribution and agrees well with the radio and infrared luminosity functions. However, the XLF does not agree with the Schechter luminosity function for the Hα LF, indicating that, as discussed in the text, additional and different physical processes may be involved in the establishment of the lognormal form of the XLF. The agreement of our star formation history points with the other star formation determinations in different wavebands (IR, radio, Hα) gives an interesting constraint on the initial mass function (IMF). The X-ray emission in the Chandra band is most likely due to binary stars, although X-ray emission from nonstellar sources (e.g., intermediate-mass black holes and/or low-luminosity AGNs) remains a possibility. Under the assumption that it is due to binary stars, the overall consistency and correlations between single-star effects and binary-star effects indicate that not only is the one-parameter IMF (M) constant but also the bivariate IMF(M1, M2) must be constant, at least at the high-mass end. Another way to put this, quite simply, is that X-ray observations may be measuring directly the binary-star formation history of the universe. X-ray studies will continue to be useful for probing the star formation history of the universe by avoiding problems of obscuration. Star formation may therefore be measured in more detail by deep surveys with future X-ray missions.

PARTIALLY OBSCURED QUASARS IN THE SLOAN DIGITAL SKY SURVEY EARLY DATA RELEASE

We have compiled a sample of 21 low-redshift (z 0.3), luminous active galactic nuclei (AGNs) with large Balmer decrements (Hα/Hβ > 7) using the galaxy and QSO catalogs of the Sloan Digital Sky Survey Early Data Release. Using this sample we attempt to determine the fraction of quasars with large internal absorption. We find that these AGNs have strong [O III] λ5007 and broad Hα emission, and that starlight dominates the spectra in the blue band, suggesting that these objects are heavily reddened. Their narrow emission line ratios are similar to those of Seyfert 2 galaxies, yet the average [O III] λ5007 emission is ~10 times more luminous. Applying the empirical relation between the optical continuum and the Balmer line luminosity for blue quasars, we find that the intrinsic luminosities of these 21 objects are in the range for quasars. We propose that they are obscured, intermediate-type quasars analogous to type 1.8 and 1.9 Seyfert galaxies. The ratio of these optically selected, intermediate-type quasars to type 1 quasars is found to be around 1, similar to that for local Seyfert galaxies. Preliminary study indicates that most of these quasars are hosted in early-type galaxies.

Ensemble Learning for Independent Component Analysis of Normal Galaxy Spectra

In this paper, we employ a new statistical analysis technique, ensemble learning for independent component analysis (EL-ICA), on the synthetic galaxy spectra from a newly released high-resolution evolutionary model by Bruzual & Charlot. We find that EL-ICA can sufficiently compress the synthetic galaxy spectral library to six nonnegative independent components (ICs), which are good templates for modeling huge amounts of normal galaxy spectra, such as the galaxy spectra in the Sloan Digital Sky Survey (SDSS). Important spectral parameters, such as starlight reddening, stellar velocity dispersion, stellar mass, and star formation histories, can be given simultaneously by the fit. Extensive tests show that the fit and the derived parameters are reliable for galaxy spectra with the typical quality of the SDSS.