AGN's UV and X-ray luminosities in clumpy accretion flows

We consider the fuelling of the central massive black hole in Active Galactic Nuclei, through an inhomogeneous accretion flow. Performing simple analytical treatments, we show that shocks between elements (clumps) forming the accretion flow may account for the UV and X-ray emission in AGNs. In this picture, a cascade of shocks is expected, where optically thick shocks give rise to optical/UV emission, while optically thin shocks give rise to X-ray emission. The resulting blue bump temperature is found to be quite similar in different AGNs. We obtain that the ratio of X-ray luminosity to UV luminosity is smaller than unity, and that this ratio is smaller in massive objects compared to less massive sources. This is in agreement with the observed $L_{X}/L_{UV}$ ratio and suggests a possible interpretation of the $\alpha_{OX}-l_{UV}$ anticorrelation.Comment: 8 pages, 1 figure, accepted for publication in A&

How Common are the Magellanic Clouds?

We introduce a probabilistic approach to the problem of counting dwarf satellites around host galaxies in databases with limited redshift information. This technique is used to investigate the occurrence of satellites with luminosities similar to the Magellanic Clouds around hosts with properties similar to the Milky Way in the object catalog of the Sloan Digital Sky Survey. Our analysis uses data from SDSS Data Release 7, selecting candidate Milky-Way-like hosts from the spectroscopic catalog and candidate analogs of the Magellanic Clouds from the photometric catalog. Our principal result is the probability for a Milky-Way-like galaxy to host N_{sat} close satellites with luminosities similar to the Magellanic Clouds. We find that 81 percent of galaxies like the Milky Way are have no such satellites within a radius of 150 kpc, 11 percent have one, and only 3.5 percent of hosts have two. The probabilities are robust to changes in host and satellite selection criteria, background-estimation technique, and survey depth. These results demonstrate that the Milky Way has significantly more satellites than a typical galaxy of its luminosity; this fact is useful for understanding the larger cosmological context of our home galaxy.Comment: Updated to match published version. Added referenc

X-Ray Properties of Narrow-Line Seyfert 1 Galaxies with Very Small Broad-Line Widths

Narrow-line Seyfert\,1 galaxies (NLS1s) with very small broad-line widths (say, FWHM(\hb) \la 1200\,\kms) represent the extreme type of Seyfert\,1 galaxies that have small black hole masses (\mbh) and/or high Eddington ratios (\redd). Here we study the X-ray properties of a homogeneously and optically selected sample of 13 such objects, termed as very narrow line Seyfert\,1 galaxies (VNLS1s), using archival \xmm\ data. It is found that the Fe K$\alpha$ emission line is at most weak in these objects. A soft X-ray excess is ubiquitous, with the thermal temperatures falling within a strict range of 0.1--0.2\,keV. Our result highlights the puzzling independence of the thermal temperature by extending the relations to even smaller FWHM(\hb), i.e., smaller \mbh\ ($\sim 10^6$ \msun) and/or higher \redd. The excess emission can be modeled by a range of viable models, though the disk reflection and Comptonization models generally give somewhat better fits over the smeared absorption and the $p$-free models. At the Eddington ratios around unity and above, the X-ray spectral slopes in the 2--10\,keV band are systematically flatter than the Risaliti et al.'s predictions of the relationship with \redd\ suggested previously. Short timescale (1--2 hours) X-ray variability is common, which, together with the variability amplitude computed for some of the objects, are supportive of the scenario that NLS1s are indeed AGN with relatively small \mbh.Comment: 11 figures and 4 table. Accepted for publication in the Astrophysical Journa

The Quasar SDSS J105041.35+345631.3: Black Hole Recoil or Extreme Double-Peaked Emitter?

The quasar SDSS J105041.35+345631.3 (z = 0.272) has broad emission lines blueshifted by 3500 km/s relative to the narrow lines and the host galaxy. Such an object may be a candidate for a recoiling supermassive black hole, binary black hole, a superposition of two objects, or an unusual geometry for the broad emission-line region. The absence of narrow lines at the broad line redshift argues against superposition. New Keck spectra of J1050+3546 place tight constraints on the binary model. The combination of large velocity shift and symmetrical H-beta profile, as well as aspects of the narrow line spectrum, make J1050+3546 an interesting candidate for black hole recoil. Other aspects of the spectrum, however, suggest that the object is most likely an extreme case of a ``double-peaked emitter.'' We discuss possible observational tests to determine the true nature of this exceptional object.Comment: 5 pages, 2 figures, LaTeX; substantial revision

Long-Term Profile Variability in Active Galactic Nuclei with Double-Peaked Balmer Emission Lines

An increasing number of Active Galactic Nuclei (AGNs) exhibit broad, double-peaked Balmer emission lines,which represent some of the best evidence for the existence of relatively large-scale accretion disks in AGNs. A set of 20 double-peaked emitters have been monitored for nearly a decade in order to observe long-term variations in the profiles of the double-peaked Balmer lines. Variations generally occur on timescales of years, and are attributed to physical changes in the accretion disk. Here we characterize the variability of a subset of seven double-peaked emitters in a model independent way. We find that variability is caused primarily by the presence of one or more discrete "lumps" of excess emission; over a timescale of a year (and sometimes less) these lumps change in amplitude and shape, but the projected velocity of these lumps changes over much longer timescales (several years). We also find that all of the objects exhibit red peaks that are stronger than the blue peak at some epochs and/or blueshifts in the overall profile, contrary to the expectations for a simple, circular accretion disk model, thus emphasizing the need for asymmetries in the accretion disk. Comparisons with two simple models, an elliptical accretion disk and a circular disk with a spiral arm, are unable to reproduce all aspects of the observed variability, although both account for some of the observed behaviors. Three of the seven objects have robust estimates of the black hole masses. For these objects the observed variability timescale is consistent with the expected precession timescale for a spiral arm, but incompatible with that of an elliptical accretion disk. We suggest that with the simple modification of allowing the spiral arm to be fragmented, many of the observed variability patterns could be reproduced.Comment: 74 pages, 4 tables, 35 figure

Double-Peaked Low-Ionization Emission Lines in Active Galactic Nuclei

We present a new sample of 116 double-peaked Balmer line Active Galactic Nuclei (AGN) selected from the Sloan Digital Sky Survey. Double-peaked emission lines are believed to originate in the accretion disks of AGN, a few hundred gravitational radii (Rg) from the supermassive black hole. We investigate the properties of the candidate disk emitters with respect to the full sample of AGN over the same redshifts, focusing on optical, radio and X-ray flux, broad line shapes and narrow line equivalent widths and line flux-ratios. We find that the disk-emitters have medium luminosities (~10^44erg/s) and FWHM on average six times broader than the AGN in the parent sample. The double-peaked AGN are 1.6 times more likely to be radio-sources and are predominantly (76%) radio quiet, with about 12% of the objects classified as LINERs. Statistical comparison of the observed double-peaked line profiles with those produced by axisymmetric and non-axisymmetric accretion disk models allows us to impose constraints on accretion disk parameters. The observed Halpha line profiles are consistent with accretion disks with inclinations smaller than 50 deg, surface emissivity slopes of 1.0-2.5, outer radii larger than ~2000 Rg, inner radii between 200-800Rg, and local turbulent broadening of 780-1800 km/s. The comparison suggests that 60% of accretion disks require some form of asymmetry (e.g., elliptical disks, warps, spiral shocks or hot spots).Comment: 60 pages, 19 figures, accepted for publication in AJ. For high quality figures and full tables, please see http://astro.princeton.edu/~iskra/disks.htm

The Origin of the 24-micron Excess in Red Galaxies

Observations with the Spitzer Space Telescope have revealed a population of red-sequence galaxies with a significant excess in their 24-micron emission compared to what is expected from an old stellar population. We identify 900 red galaxies with 0.15<z<0.3 from the AGN and Galaxy Evolution Survey (AGES) selected from the NOAO Deep Wide-Field Survey Bootes field. Using Spitzer/MIPS, we classify 89 (~10%) with 24-micron infrared excess (f24>0.3 mJy). We determine the prevalence of AGN and star-formation activity in all the AGES galaxies using optical line diagnostics and mid-IR color-color criteria. Using the IRAC color-color diagram from the IRAC Shallow Survey, we find that 64% of the 24-micron excess red galaxies are likely to have strong PAH emission features in the 8-micron IRAC band. This fraction is significantly larger than the 5% of red galaxies with f24<0.3 mJy that are estimated to have strong PAH emission, suggesting that the infrared emission is largely due to star-formation processes. Only 15% of the 24-micron excess red galaxies have optical line diagnostics characteristic of star-formation (64% are classified as AGN and 21% are unclassifiable). The difference between the optical and infrared results suggest that both AGN and star-formation activity is occurring simultaneously in many of the 24-micron excess red galaxies. These results should serve as a warning to studies that exclusively use optical line diagnostics to determine the dominant emission mechanism in the infrared and other bands. We find that ~40% of the 24-micron excess red galaxies are edge-on spiral galaxies with high optical extinctions. The remaining sources are likely to be red galaxies whose 24-micron emission comes from a combination of obscured AGN and star-formation activity.Comment: ApJ, accepted; 11 pages, 7 figures; corrected reference to IRAC Shallow Survey in abstrac

The stellar content of low redshift radio galaxies from near-infrared spectroscopy

We present medium spectral resolution near-infrared (NIR) HK-band spectra for 8 low redshift (z<0.06) radio galaxies to study the NIR stellar properties of their host galaxies. As a homogeneous comparison sample, we used 9 inactive elliptical galaxies that were observed with similar resolution and wavelength range. The aim of the study is to compare the NIR spectral properties of radio galaxies to those of inactive early-type galaxies and, furthermore, produce the first NIR HK-band spectra for low redshift radio galaxies. For both samples spectral indices of several diagnostic absorption features, SiI(1.589microns), CO(1.619microns), NaI(2.207microns), CaI(2.263microns), CO(>2.29microns), were measured. To characterize the age of the populations, the measured EWs of the absorption features were fitted with the corresponding theoretical evolutionary curves of the EWs calculated by the stellar synthesis model. On average, EW(CO 2.29) of radio galaxies is somewhat greater than that of inactive ellipticals. Most likely, EW(CO 2.29) is not significantly affected by dilution, and thus indicating that elliptical galaxies containing AGN are in a different stage in their evolution than inactive ellipticals. This is also supported by comparing other NIR features, such as CaI and NaI, with each other. Absorption features are consistent with the intermediate age stellar population, suggesting that host galaxies contain both an old and intermediate age components. It is consistent with previous optical spectroscopy studies which have shown evidence on the intermediate age (~2 Gyr) stellar population of radio galaxies, and also in some of the early-type galaxies. The existence of intermediate age population is a link between the star formation episode, possibly induced by interaction or merging event, and the triggering of the nuclear activity.Comment: 10 pages, 7 figure

The properties of Brightest Cluster Galaxies in the SDSS DR6 adaptive matched filter cluster catalogue

We study the properties of Brightest Cluster Galaxies (BCGs) drawn from a catalogue of more than 69000 clusters in the SDSS DR6 based on the adaptive matched filter technique (AMF, Szabo et al., 2010). Our sample consists of more than 14300 galaxies in the redshift range 0.1-0.3. We test the catalog by showing that it includes well-known BCGs which lie in the SDSS footprint. We characterize the BCGs in terms of r-band luminosities and optical colours as well as their trends with redshift. In particular, we define and study the fraction of blue BCGs, namely those that are likely to be missed by either colour-based cluster surveys and catalogues. Richer clusters tend to have brighter BCGs, however less dominant than in poorer systems. 4-9% of our BCGs are at least 0.3 mag bluer in the g-r colour than the red-sequence at their given redshift. Such a fraction decreases to 1-6% for clusters above a richness of 50, where 3% of the BCGs are 0.5 mag below the red-sequence. A preliminary morphological study suggests that the increase in the blue fraction at lower richnesses may have a non-negligible contribution from spiral galaxies. We show that a colour selection based on the g-r red-sequence or on a cut at colour u-r >2.2 can lead to missing the majority of such blue BCGs. We also extend the colour analysis to the UV range by cross-matching our catalogue with publicly available data from Galex GR4 and GR5. We show a clear correlation between offset from the optical red-sequence and the amount of UV-excess. Finally, we cross-matched our catalogue with the ACCEPT cluster sample (Cavagnolo et al., 2009), and find that blue BCGs tend to be in clusters with low entropy and short cooling times. That is, the blue light is presumably due to recent star formation associated to gas feeding by cooling flows. (abridged)Comment: 15 pages, 15 figures, submitted to MNRA

The Mid-IR and X-ray Selected QSO Luminosity Function

We present the J-band luminosity function of 1838 mid-infrared and X-ray selected AGNs in the redshift range 0<z<5.85. These luminosity functions are constructed by combining the deep multi-wavelength broad-band observations from the UV to the mid-IR of the NDWFS Bootes field with the X-ray observations of the XBootes survey and the spectroscopic observations of the same field by AGES. Our sample is primarily composed of IRAC-selected AGNs, targeted using modifications of the Stern et al.(2005) criteria, complemented by MIPS 24 microns and X-ray selected AGNs to alleviate the biases of IRAC mid-IR selection against z~4.5 quasars and AGNs faint with respect to their hosts. This sample provides an accurate link between low and high redshift AGN luminosity functions and does not suffer from the usual incompleteness of optical samples at z~3. We find that the space density of the brightest quasars strongly decreases from z=3 to z=0, while the space density of faint quasars is at least flat, and possibly increasing, over the same redshift range. At z>3 we observe a decrease in the space density of quasars of all brightnesses. We model the luminosity function by a double power-law and find that its evolution cannot be described by either pure luminosity or pure density evolution, but must be a combination of both. Our best-fit model has bright and faint power-law indices consistent with the low redshift measurements based on the 2QZ and 2SLAQ surveys and it generally agrees with the number of bright quasars predicted by other LFs at all redshifts. If we construct the QSO luminosity function using only the IRAC-selected AGNs, we find that the biases inherent to this selection method significantly modify the behavior of phi*(z) only for z<1 and have no significant impact upon the characteristic magnitude M*_J(z).Comment: Corrected minor typo in equations (4) and (6). Accepted for publication in The Astrophysical Journal. 56 pages + 6 tables + 16 figure