ALBERT

Description: This study represents the most sensitive Chandra X-ray point source catalogue of M31. Using 133 publicly available Chandra ACIS-I/S observations totalling ~1 Ms, we detected 795 X-ray sources in the bulge, north-east, and south-west fields of M31, covering an area of 0.6 deg 2 , to a limiting unabsorbed 0.5–8.0 keV luminosity of ~10 34 erg s –1 . In the inner bulge, where exposure is approximately constant, X-ray fluxes represent average values because they were determined from many observations over a long period of time. Similarly, our catalogue is more complete in the bulge fields since monitoring allowed more transient sources to be detected. The catalogue was cross-correlated with a previous XMM–Newton catalogue of M31's D 25 isophote consisting of 1948 X-ray sources, with only 979 within the field of view of our survey. We found 387 (49 per cent) of our Chandra sources (352 or 44 per cent unique sources) matched to within 5 arcsec of 352 XMM–Newton sources. Combining this result with matching done to previous Chandra X-ray sources we detected 259. new sources in our catalogue. We created X-ray luminosity functions (XLFs) in the soft (0.5–2.0 keV) and hard (2.0–8.0 keV) bands that are the most sensitive for any large galaxy based on our detection limits. Completeness-corrected XLFs show a break around 1.3 x 10 37 erg s –1 , consistent with previous work. As in past surveys, we find that the bulge XLFs are flatter than the disc, indicating a lack of bright high-mass X-ray binaries in the disc and an aging population of low-mass X-ray binaries in the bulge.

Description: Thermal mid-infrared emission of quasars requires an obscuring structure that can be modelled as a magneto-hydrodynamic wind in which radiation pressure on dust shapes the outflow. We have taken the dusty-wind models presented by Keating and collaborators that generated quasar mid-infrared spectral energy distributions (SEDs), and explore their properties (such as geometry, opening angle, and ionic column densities) as a function of Eddington ratio and X-ray weakness. In addition, we present new models with a range of magnetic field strengths and column densities of the dust-free shielding gas interior to the dusty wind. We find this family of models – with input parameters tuned to accurately match the observed mid-IR power in quasar SEDs – provides reasonable values of the Type 1 fraction of quasars and the column densities of warm absorber gas, though it does not explain a purely luminosity-dependent covering fraction for either. Furthermore, we provide predictions of the cumulative distribution of E ( B – V ) values of quasars from extinction by the wind and the shape of the wind as imaged in the mid-infrared. Within the framework of this model, we predict that the strength of the near-infrared bump from hot dust emission will be correlated primarily with L / L Edd rather than luminosity alone, with scatter induced by the distribution of magnetic field strengths. The empirical successes and shortcomings of these models warrant further investigations into the composition and behaviour of dust and the nature of magnetic fields in the vicinity of actively accreting supermassive black holes.

Description: Iron low-ionization broad absorption lines (FeLoBALs) trace a rare class of quasar outflows with large column densities and potentially large kinetic energies that might be important for ‘feedback’ to galaxy evolution. In order to probe the physical properties of these outflows, we conducted a multiple-epoch, absorption line variability study of 12 FeLoBAL quasars spanning a redshift range of 0.7 ≤  z  ≤ 1.9 over rest-frame time-scales of ~10 d to 7.6 yr. We detect absorption line variability with ≥8 confidence in 3 out of the 12 sources in our sample over time-scales of ~0.6 to 7.6 yr. Variable wavelength intervals are associated with ground- and excited-state Fe  ii multiplets, the Mg  ii 2796, 2803 doublet, Mg  i 2852, and excited-state Ni  ii multiplets. The observed variability along with evidence of saturation in the absorption lines favours transverse motions of gas across the line of sight as the preferred scenario, and allows us to constrain the outflow distance from the supermassive black hole to be 69, 7, and 60 pc for our three variable sources. In combination with other studies, these results suggest that the outflowing gas in FeLoBAL quasars resides on a range of scales and includes matter within tens of parsecs of the central source.

Description: We present a photometric catalogue of star cluster candidates in Hickson compact groups (HCGs) 7, 31, 42, 59, and 92, based on observations with the Advanced Camera for Surveys and the Wide Field Camera 3 on the Hubble Space Telescope . The catalogue contains precise cluster positions (right ascension and declination), magnitudes, and colours in the BVI filters. The number of detected sources ranges from 2200 to 5600 per group, from which we construct the high-confidence sample by applying a number of criteria designed to reduce foreground and background contaminants. Furthermore, the high-confidence cluster candidates for each of the 16 galaxies in our sample are split into two subpopulations: one that may contain young star clusters and one that is dominated by globular older clusters. The ratio of young star cluster to globular cluster candidates varies from group to group, from equal numbers to the extreme of HCG 31 which has a ratio of 8 to 1, due to a recent starburst induced by interactions in the group. We find that the number of blue clusters with M V  〈 –9 correlates well with the current star formation rate in an individual galaxy, while the number of globular cluster candidates with M V  〈 –7.8 correlates well (though with large scatter) with the stellar mass. Analyses of the high-confidence sample presented in this paper show that star clusters can be successfully used to infer the gross star formation history of the host groups and therefore determine their placement in a proposed evolutionary sequence for compact galaxy groups.

Description: In a single optical spectrum, the quasar narrow-line region (NLR) reveals low-density, photoionized gas in the host galaxy interstellar medium (ISM), while the immediate vicinity of the central engine generates the accretion disc continuum and broad emission lines. To isolate these two components, we construct a library of high-S/N optical composite spectra created from the Sloan Digital Sky Survey Data Release 7. We divide the sample into bins of continuum luminosity and Hβ full width at half-maximum that are used to construct median composites at different redshift steps up to 0.75. We measure the luminosities of the narrow-emission lines [Ne v ] 3427, [Ne iii ] 3870, [O iii ] 5007, and [O ii ] 3728 with ionization potentials (IPs) of 97, 40, 35, and 13.6 eV, respectively. The high IP lines’ luminosities show no evidence of increase with redshift consistent with no evolution in the AGN spectral energy distribution or the host galaxy ISM illuminated by the continuum. In contrast, we find that the [O ii ] line becomes stronger at higher redshifts, and we interpret this as a consequence of enhanced star formation contributing to the [O ii ] emission in host galaxies at higher redshifts. The SFRs estimated from the [O ii ] luminosities show a flatter increase with z than non-AGN galaxies given our assumed AGN contribution to the [O ii ] luminosity. Finally, we confirm an inverse correlation between the strength of the Fe ii 4570 complex and both the [O iii ] equivalent width (though not the luminosity) and the width of the Hβ line as known from the eigenvector 1 correlations.

Description: We report the results of a new 60 ks Chandra X-ray Observatory Advanced CCD Imaging Spectrometer S-array (ACIS-S) observation of the reddened, radio-selected, highly polarized ‘FeLoBAL’ quasar FIRST J1556+3517. We investigated a number of models of varied sophistication to fit the 531-photon spectrum. These models ranged from simple power laws to power laws absorbed by hydrogen gas in differing ionization states and degrees of partial covering. Preferred fits indicate that the intrinsic X-ray flux is consistent with that expected for quasars of similarly high luminosity, i.e. an intrinsic, dereddened and unabsorbed optical to X-ray spectral index of –1.7. We cannot tightly constrain the intrinsic X-ray power-law slope, but find indications that it is flat (photon index  = 1.7 or flatter at a 〉99 per cent confidence for a neutral hydrogen absorber model). Absorption is present, with a column density a few times 10 23 cm –2 , with both partially ionized models and partially covering neutral hydrogen models providing good fits. We present several lines of argument that suggest the fraction of X-ray emissions associated with the radio jet is not large. We combine our Chandra data with observations from the literature to construct the spectral energy distribution of FIRST J1556+3517 from radio to X-ray energies. We make corrections for Doppler beaming for the pole-on radio jet, optical dust reddening and X-ray absorption, in order to recover a probable intrinsic spectrum. The quasar FIRST J1556+3517 seems to be an intrinsically normal radio-quiet quasar with a reddened optical/UV spectrum, a Doppler-boosted but intrinsically weak radio jet and an X-ray absorber not dissimilar from that of other broad absorption line quasars.

Description: Machine learning techniques can provide powerful tools to detect patterns in multidimensional parameter space. We use K-means – a simple yet powerful unsupervised clustering algorithm which picks out structure in unlabelled data – to study a sample of quasar UV spectra from the Quasar Catalog of the 10th Data Release of the Sloan Digital Sky Survey (SDSS-DR10) of Paris et al. Detecting patterns in large data sets helps us gain insights into the physical conditions and processes giving rise to the observed properties of quasars. We use K-means to find clusters in the parameter space of the equivalent width (EW), the blue- and red-half-width at half-maximum (HWHM) of the Mg ii 2800 Å line, the C iv 1549 Å line, and the C iii] 1908 Å blend in samples of broad absorption line (BAL) and non-BAL quasars at redshift 1.6–2.1. Using this method, we successfully recover correlations well-known in the UV regime such as the anti-correlation between the EW and blueshift of the C iv emission line and the shape of the ionizing spectra energy distribution (SED) probed by the strength of He ii and the Si iii] /C iii] ratio. We find this to be particularly evident when the properties of C iii] are used to find the clusters, while those of Mg ii proved to be less strongly correlated with the properties of the other lines in the spectra such as the width of C iv or the Si iii] /C iii] ratio. We conclude that unsupervised clustering methods (such as K-means) are powerful methods for finding ‘natural’ binning boundaries in multidimensional data sets and discuss caveats and future work.

Description: Compact groups of galaxies provide insight into the role of low-mass, dense environments in galaxy evolution because the low velocity dispersions and close proximity of galaxy members result in frequent interactions that take place over extended time-scales. We expand the census of star formation in compact group galaxies by Tzanavaris et al. ( 2010 ) and collaborators with Swift UVOT, Spitzer IRAC and MIPS 24 μm photometry of a sample of 183 galaxies in 46 compact groups. After correcting luminosities for the contribution from old stellar populations, we estimate the dust-unobscured star formation rate (SFR UV ) using the UVOT uvw2 photometry. Similarly, we use the MIPS 24 μm photometry to estimate the component of the SFR that is obscured by dust (SFR IR ). We find that galaxies which are MIR-active (MIR-‘red’), also have bluer UV colours, higher specific SFRs, and tend to lie in H i -rich groups, while galaxies that are MIR-inactive (MIR-‘blue’) have redder UV colours, lower specific SFRs, and tend to lie in H i -poor groups. We find the SFRs to be continuously distributed with a peak at about 1 M  yr –1 , indicating this might be the most common value in compact groups. In contrast, the specific SFR distribution is bimodal, and there is a clear distinction between star-forming and quiescent galaxies. Overall, our results suggest that the specific SFR is the best tracer of gas depletion and galaxy evolution in compact groups.

Description: Mid-infrared (MIR) quasar spectra exhibit a suite of emission features including high ionization coronal lines from the narrow-line region illuminated by the ionizing continuum, broad dust bumps from silicates and graphites, and polycyclic aromatic hydrocarbon (PAH) features from star formation in the host galaxy. However, in Spitzer Infared Spectrograph (IRS) data, few features are detected in most individual spectra because of typically low signal-to-noise ratios (S/N). By generating spectral composites from over 180 IRS observations of Sloan Digital Sky Survey broad-line quasars, we boost the S/N and reveal features in the complex spectra that are otherwise lost in the noise. In addition to an overall composite, we generate composites in three different luminosity bins that span the range of 5.6 μm luminosities of 10 40 –10 46 (erg s –1 ). We detect the high-ionization, forbidden emission lines of [S iv ], [O iv ] and [Ne v ] 14 μm in all templates and PAH features in all but the most luminous template. Ratios of lines with a range of ionization potentials show no evidence for a strong difference in the shape of the 41–97 eV ionizing continuum over this range of luminosities. The scaling of the emission-line luminosities as a function of continuum luminosity is consistent with what is expected, and shows no indication of a ‘disappearing narrow-line region’. The broad 10 and 18 μm silicate features in emission increase in strength with increasing luminosity, and a broad 3–5 μm blackbody consistent with graphite emission at 750 K is evident in the highest luminosity template. We find that the intrinsic quasar continua for all luminosity templates are consistent; apparent differences arise primarily from host galaxy contamination most evident at low luminosity.