Xiaobo Dong

A population of radio-loud narrow-line Seyfert 1 galaxies with blazar-like properties?

Blazars having strong emission lines were found to be associated mostly with broad-line type 1 active galactic nuclei (AGNs). Hitherto, evidence for blazars identified with narrow-line Seyfert 1 (NLS1) AGNs was limited to only a very few individual cases. Here we present a comprehensive study of a sample of 23 genuine radio-loud NLS1 AGNs with radio loudness larger than 100. The radio sources of the sample are ubiquitously compact, unresolved on scales of several arcseconds. Some of these objects show interesting radio to X-ray properties that are unusual to most of the previously known radio-loud NLS1 AGNs, but are reminiscent of blazars. These include flat radio spectra, large-amplitude flux and spectral variability, compact VLBI cores, very high variability brightness temperatures, enhanced optical continuum emission, flat X-ray spectra, and blazar-like spectral energy distributions (SEDs). We interpret them as signatures of the postulated blazar nature of some of the objects, which likely possess at least moderately relativistic jets. Intrinsically, some of them have relatively low radio power and would have been classified as radio-intermediate AGNs. The black hole masses are estimated to be within 10(6)-10(8) M-circle dot, and the Eddington ratios close to unity, as in normal NLS1 AGNs. The results imply that radio-loud AGNs may be powered by black holes of moderate masses (similar to 10(6)-10(7) M-circle dot) accreting at high rates. Some of the objects, despite having strong emission lines, resemble high-energy-peaked BL Lac objects (HBLs) in their SEDs. Given the peculiarities of blazar-like NLS1 AGNs, questions arise as to whether they are plain downsizing extensions of normal radio-loud AGNs, or form a previously unrecognized population.

Broad‐line Balmer decrements in blue active galactic nuclei

We have investigated the broad-line Balmer decrements (H alpha/H beta) for a large, homogeneous sample of Seyfert 1 galaxies and quasi-stellar objects (QSOs) using spectroscopic data obtained in the Sloan Digital Sky Survey. The sample, drawn from the Fourth Data Release, comprises 446 low-redshift (z less than or similar to 0.35) active galactic nuclei (AGN) that have blue optical continua as indicated by the spectral slopes in order to minimize the effect of dust extinction. We find that (i) the distribution of the intrinsic broad-line H alpha/H beta ratio can be well described by log-Gaussian, with a peak at H alpha/H beta = 3.06 and a standard deviation of about 0.03 dex only; (ii) the Balmer decrement does not correlate with AGN properties such as luminosity, accretion rate and continuum slope, etc.; (iii) on average, the Balmer decrements are found to be only slightly larger in radio-loud sources (3.37) and sources having double-peaked emission-line profiles (3.27) compared to the rest of the sample. We therefore suggest that the broad-line H alpha/H beta ratio can be used as a good indicator for dust extinction in the AGN broad-line region; this is especially true for radio-quiet AGN with regular emission-line profiles, which constitute the vast majority of the AGN population.

SDSS J094857.3+002225: A Very Radio Loud, Narrow-Line Quasar with Relativistic Jets?

We report the identification of a very radio loud, narrow-line quasar, SDSS J094857.3+002225, discovered in the Sloan Digital Sky Survey (SDSS) early data release (EDR). With FWHM (Hβ) ≈ 1500 km s-1 and undetected [O III] λλ 4959, 5007 SDSS J094857.3+002225 is consistent with the conventional definition of narrow-line Seyfert 1 galaxies (NLS1s). Its strong optical Fe II emission [R4570 ≡ f(Fe II λ4570)/f(Hβ) ≈ 1.59] is also typical of NLS1s. With a radio loudness RL ≡ (fν,4.85 GHz/fν,B) 1000 and radio power of P4.85 GHz ~ (3.6-5.2) × 1026 W Hz-1, it is the first bona fide, very radio loud NLS1. The inverted radio spectrum and very high brightness temperature (1013 K) derived from variation of the radio flux suggest the presence of a relativistic jet beaming toward the observer in this NLS1.

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.

COEXISTENCE OF GRAVITATIONALLY-BOUND AND RADIATION-DRIVEN C IV EMISSION LINE REGIONS IN ACTIVE GALACTIC NUCLEI

There are mutually contradictory views in the literature of the kinematics and structure of high-ionization line (e. g., C IV) emitting regions in active galactic nuclei (AGNs). Two kinds of broad emission line region (BELR) models have been proposed, outflow and gravitationally-bound BELR, which are supported, respectively, by blueshift of the C IV line and reverberation mapping observations. To reconcile these two apparently different models, we present a detailed comparison study between the C IV and Mg II lines using a sample of AGNs selected from the Sloan Digital Sky Survey. We find that the kinematics of the C IV region is different from that of Mg II, which is thought to be controlled by gravity. A strong correlation is found between the blueshift and asymmetry of the C IV profile and the Eddington ratio. This provides strong observational support for the postulation that the outflow is driven by radiation pressure. In particular, we find robust evidence that the C IV line region is largely dominated by outflow at high Eddington ratios, while it is primarily gravitationally-bounded at low Eddington ratios. Our results indicate that these two emitting regions coexist in most AGNs. The emission strength from these two gases varies smoothly with Eddington ratio in opposite ways. This explanation naturally reconciles the apparently contradictory views proposed in previous studies. Finally, candidate models are discussed which can account for both the enhancement of outflow emission and suppression of normal BEL in AGNs with high Eddington ratios.

Obscured Binary Quasar Cores in SDSS J104807.74+005543.5?

We report the discovery of a possible close binary system of quasars in SDSS J1048+0055. The [O III] λλ4959, 5007 emission lines are clearly double-peaked, and two discrete radio sources with a projected physical separation of ~20 pc are found in the Very Long Baseline Array milliarcsecond resolution image at 8.4 GHz. Each of the [O III] λλ4959, 5007 doublets and Hβ can be well modeled by two Gaussians, and the line ratio [O III] λ5007/Hβ ~ 7 is typical of Seyfert 2 galaxies. No broad component of Hβ was detected, and its [O III] λ5007 luminosity, L ≈ 9.2 × 1042 ergs s-1, is comparable to luminous quasars and is a few ×10 more luminous than typical Seyfert galaxies. One natural interpretation is that SDSS J1048+0055 contains two close quasar-like nuclei, and the broad-line region around them is obscured. Other possible models are also discussed. We suggest that a double-peaked narrow emission line profile may be an effective way of selecting candidates of binary black holes with intermediate separation.

DEPENDENCE OF THE OPTICAL/ULTRAVIOLET VARIABILITY ON THE EMISSION-LINE PROPERTIES AND EDDINGTON RATIO IN ACTIVE GALACTIC NUCLEI

The dependence of the long-term optical/UV variability on the spectral and fundamental physical parameters for radio-quiet active galactic nuclei (AGNs) is investigated. The multi-epoch-repeated photometric scanning data in the Stripe-82 region of the Sloan Digital Sky Survey (SDSS) are exploited for two comparative AGN samples (mostly quasars) selected therein: a broad-line Seyfert 1 (BLS1) type sample and a narrow-line Seyfert 1 (NLS1) type AGN sample within redshifts 0.3-0.8. Their spectral parameters are derived from the SDSS spectroscopic data. It is found that on rest-frame timescales of several years the NLS1-type AGNs show systematically smaller variability compared to the BLS1-type AGNs. In fact, the variability amplitude is found to correlate, though only moderately, with the eigenvector 1 parameters, i.e., the smaller the H beta linewidth, the weaker the [O III] and the stronger the Fe II emission, the smaller the variability amplitude. Moreover, an interesting inverse correlation is found between the variability and the Eddington ratio, which is perhaps more fundamental. The previously known dependence of the variability on luminosity is not significant, and the dependence on black hole mass-as claimed in recent papers and also present in our data-fades out when controlling for the Eddington ratio in the correlation analysis, though these may be partly due to the limited ranges of luminosity and black hole mass of our samples. Our result strongly supports that an accretion disk is likely to play a major role in producing the optical/UV variability.

SDSS J160531.84+174826.1: A dwarf disk galaxy with an intermediate-mass black hole

We report the discovery of a dwarf Seyfert 1 active galactic nucleus (AGN) with a candidate intermediate-mass black hole, hosted by the dwarf galaxy SDSS J160531.84+174826.1 at z = 0.032. A broad component of the H alpha line with FWHM = 781 km s(-1) is detected in its optical spectrum, and a bright, pointlike nucleus is evident from a Hubble Space Telescope (HST) imaging observation. Nonthermal X-ray emission is also detected from the nucleus. The black hole mass, as estimated from the luminosity and width of the broad H alpha component, is about 7 x 10(4) M.. The host galaxy appears to be a disk galaxy with a boxy bulge or nuclear bar. With an absolute magnitude of M-R = -17.8 (M-B = -16.4), it is among the least luminous host galaxies ever identified for a Seyfert 1.

Two extreme double-peaked line emitters in the sloan digital sky survey

Double-peaked Balmer lines have been observed in about 150 active galactic nuclei and were interpreted preferentially as emission from relativistic accretion disks. We report the discovery of extreme double-peaked lines in SDSS J0942+0900 and SDSS J1417+6141. The FWHM of the Hα line, 40,600 km s-1, in the first object is almost twice as large as the broadest previously known. By comparing the line profile with accretion disk models, we find that most of the line flux is emitted from a narrow annulus around 100rg in SDSS J0942+0900, and from a disk with radii between 100rg and 400rg in SDSS J1417+6141. This is the first time that an accretion disk at radii below 100rg has been directly probed through optical emission lines. A certain asymmetry in the disk is required for both objects. Another, much weaker, broad Hα component (Wλ ~ 20 A and FWHM ~ 4000 km s-1) is also detected in both objects. Both objects show weak radio emission and strong high-ionization narrow lines.