arXiv subjects

Christopher D. Fassnacht

Publications and source records attributed to Christopher D. Fassnacht.

68 records · Page 4Linked to original sources

arXiv2011

PTF10iya: A short-lived, luminous flare from the nuclear region of a star-forming galaxy

We present the discovery and characterisation of PTF10iya, a short-lived (dt ~ 10 d, with an optical decay rate of ~ 0.3 mag per d), luminous (M_g ~ -21 mag) transient source found by the Palomar Transient Factory. The ultraviolet/optical spectral energy distribution is reasonably well fit by a blackbody with T ~ 1-2 x 10^4 K and peak bolometric luminosity L_BB ~ 1-5 x 10^44 erg per s (depending on the details of the extinction correction). A comparable amount of energy is radiated in the X-ray band that appears to result from a distinct physical process. The location of PTF10iya is consistent with the nucleus of a star-forming galaxy (z = 0.22405 +/- 0.00006) to within 350 mas (99.7 per cent confidence radius), or a projected distance of less than 1.2 kpc. At first glance, these properties appear reminiscent of the characteristic "big blue bump" seen in the near-ultraviolet spectra of many active galactic nuclei (AGNs). However, emission-line diagnostics of the host galaxy, along with a historical light curve extending back to 2007, show no evidence for AGN-like activity. We therefore consider whether the tidal disruption of a star by an otherwise quiescent supermassive black hole may account for our observations. Though with limited temporal information, PTF10iya appears broadly consistent with the predictions for the early "super-Eddington" phase of a solar-type star disrupted by a ~ 10^7 M_sun black hole. Regardless of the precise physical origin of the accreting material, the large luminosity and short duration suggest that otherwise quiescent galaxies can transition extremely rapidly to radiate near the Eddington limit; many such outbursts may have been missed by previous surveys lacking sufficient cadence.

astro-ph.HE↗

arXiv2011

A Candidate Dual AGN at z=1.175

The X-ray source CXOXBJ142607.6+353351 (CXOJ1426+35), which was identified in a 172 ks Chandra image in the Bootes field, shows double-peaked rest-frame optical/UV emission lines, separated by 0.69" (5.5 kpc) in the spatial dimension and by 690 km s^-1 in the velocity dimension. The high excitation lines and emission line ratios indicate both systems are ionized by an AGN continuum, and the double-peaked profile resembles that of candidate dual AGN. At a redshift of z=1.175, this source is the highest redshift candidate dual AGN yet identified. However, many sources have similar emission line profiles for which other interpretations are favored. We have analyzed the substantial archival data available in this field, as well as acquired near-infrared (NIR) adaptive optics (AO) imaging and NIR slit spectroscopy. The X-ray spectrum is hard, implying a column density of several 10^23 cm^-2. Though heavily obscured, the source is also one of the brightest in the field, with an absorption-corrected 2-10 keV luminosity of ~10^45 erg s^-1. Outflows driven by an accretion disk may produce the double-peaked lines if the central engine accretes near the Eddington limit. However, we may be seeing the narrow line regions of two AGN following a galactic merger. While the AO image reveals only a single source, a second AGN would easily be obscured by the significant extinction inferred from the X-ray data. Understanding the physical processes producing the complex emission line profiles seen in CXOJ1426+35 and related sources is important for interpreting the growing population of dual AGN candidates.

astro-ph.CO↗

arXiv2010

Obscured Starburst Activity in High Redshift Clusters and Groups

Using Spitzer-MIPS 24um imaging and Keck spectroscopy we examine the nature of the obscured star forming population in three clusters and three groups at z~0.9. These six systems are components of the Cl1604 supercluster, the largest structure imaged by Spitzer at redshifts near unity. We find that the average density of 24um-detected galaxies within the Cl1604 clusters is nearly twice that of the surrounding field and that this overdensity scales with the cluster's dynamical state. The 24um-bright members often appear optically unremarkable and exhibit only moderate [OII] line emission due to severe obscuration. Their spatial distribution suggests they are an infalling population, but an examination of their spectral properties, morphologies and optical colors indicate they are not simply analogs of the field population that have yet to be quenched. Using stacked composite spectra, we find the 24um-detected cluster and group galaxies exhibit elevated levels of Balmer absorption compared to galaxies undergoing normal, continuous star formation. A similar excess is not observed in field galaxies with equivalent infrared luminosities, indicating a greater fraction of the detected cluster and group members have experienced a burst of star formation in the recent past compared to their counterparts in the field. Our results suggest that gas-rich galaxies at high redshift experience a temporary increase in their star formation activity as they assemble into denser environments. Using HST-ACS imaging we find that disturbed morphologies are common among the 24um-detected cluster and group members and become more prevalent in regions of higher galaxy density. We conclude that mergers are the dominant triggering mechanism responsible for the enhanced star formation found in the Cl1604 groups, while a mix of harassment and mergers are likely driving the activity of the cluster galaxies.

astro-ph.CO↗

arXiv2010

Cosmic Evolution of Virial and Stellar Mass in Massive Early-Type Galaxies

We measure the average mass properties of a sample of 41 strong gravitational lenses at moderate redshift (z ~ 0.4 - 0.9), and present the lens redshift for 6 of these galaxies for the first time. Using the techniques of strong and weak gravitational lensing on archival data obtained from the Hubble Space Telescope, we determine that the average mass overdensity profile of the lenses can be fit with a power-law profile (Delta_Sigma prop. to R^{-0.86 +/- 0.16}) that is within 1-sigma of an isothermal profile (Delta_Sigma prop. to R^{-1}) with velocity dispersion sigma_v = 260 +/- 20 km/s. Additionally, we use a two-component de Vaucouleurs+NFW model to disentangle the total mass profile into separate luminous and dark matter components, and determine the relative fraction of each component. We measure the average rest frame V-band stellar mass-to-light ratio (Upsilon_V = 4.0 +/- 0.6 h M_sol/L_sol) and virial mass-to-light ratio (tau_V = 300 +/- 90 h M_sol/L_sol) for our sample, resulting in a virial-to-stellar mass ratio of M_vir/M_* = 75 +/- 25. Finally, we compare our results to a previous study using low redshift lenses, to understand how galaxy mass profiles evolve over time. We investigate the evolution of M_vir/M_*(z) = alpha(1+z)^{beta}, and find best fit parameters of alpha = 51 +/- 36 and beta = 0.9 +/- 1.8, constraining the growth of virial to stellar mass ratio over the last ~7 Gigayears. We note that, by using a sample of strong lenses, we are able to constrain the growth of M_vir/M_*(z) without making any assumptions about the IMF of the stellar population.

astro-ph.CO↗

arXiv2010

Adaptive Optics Observations of B0128+437: A Low-Mass, High-Redshift Gravitational Lens

We use high-resolution adaptive optics (AO) imaging on the Keck II telescope to study the gravitational lens B0128+437 in unprecedented detail, allowing us to resolve individual lensed quasar components and, for the first time, detect and measure properties of the lensing galaxy. B0128+437 is a small separation lens with known flux-ratio and astrometric anomalies. We discuss possible causes for these anomalies, including the presence of substructure in the lensing galaxy, propagation effects due to dust and a turbulent interstellar medium, and gravitational microlensing. This work on B0128 demonstrates that AO will be an essential tool for studying the many new small-separation lenses expected from future surveys.

astro-ph.CO↗

arXiv2010

Evidence for the accelerated expansion of the Universe from weak lensing tomography with COSMOS

We present a tomographic cosmological weak lensing analysis of the HST COSMOS Survey. Applying our lensing-optimized data reduction, principal component interpolation for the ACS PSF, and improved modelling of charge-transfer inefficiency, we measure a lensing signal which is consistent with pure gravitational modes and no significant shape systematics. We carefully estimate the statistical uncertainty from simulated COSMOS-like fields obtained from ray-tracing through the Millennium Simulation. We test our pipeline on simulated space-based data, recalibrate non-linear power spectrum corrections using the ray-tracing, employ photometric redshifts to reduce potential contamination by intrinsic galaxy alignments, and marginalize over systematic uncertainties. We find that the lensing signal scales with redshift as expected from General Relativity for a concordance LCDM cosmology, including the full cross-correlations between different redshift bins. For a flat LCDM cosmology, we measure sigma_8(Omega_m/0.3)^0.51=0.75+-0.08 from lensing, in perfect agreement with WMAP-5, yielding joint constraints Omega_m=0.266+0.025-0.023, sigma_8=0.802+0.028-0.029 (all 68% conf.). Dropping the assumption of flatness and using HST Key Project and BBN priors only, we find a negative deceleration parameter q_0 at 94.3% conf. from the tomographic lensing analysis, providing independent evidence for the accelerated expansion of the Universe. For a flat wCDM cosmology and prior w in [-2,0], we obtain w<-0.41 (90% conf.). Our dark energy constraints are still relatively weak solely due to the limited area of COSMOS. However, they provide an important demonstration for the usefulness of tomographic weak lensing measurements from space. (abridged)

astro-ph.CO↗

arXiv2009

LSST Science Book, Version 2.0

A survey that can cover the sky in optical bands over wide fields to faint magnitudes with a fast cadence will enable many of the exciting science opportunities of the next decade. The Large Synoptic Survey Telescope (LSST) will have an effective aperture of 6.7 meters and an imaging camera with field of view of 9.6 deg^2, and will be devoted to a ten-year imaging survey over 20,000 deg^2 south of +15 deg. Each pointing will be imaged 2000 times with fifteen second exposures in six broad bands from 0.35 to 1.1 microns, to a total point-source depth of r~27.5. The LSST Science Book describes the basic parameters of the LSST hardware, software, and observing plans. The book discusses educational and outreach opportunities, then goes on to describe a broad range of science that LSST will revolutionize: mapping the inner and outer Solar System, stellar populations in the Milky Way and nearby galaxies, the structure of the Milky Way disk and halo and other objects in the Local Volume, transient and variable objects both at low and high redshift, and the properties of normal and active galaxies at low and high redshift. It then turns to far-field cosmological topics, exploring properties of supernovae to z~1, strong and weak lensing, the large-scale distribution of galaxies and baryon oscillations, and how these different probes may be combined to constrain cosmological models and the physics of dark energy.

astro-ph.IM↗

arXiv2009

No Evidence of Quasar-Mode Feedback in a Four-Way Group Merger at z~0.84

We report on the results of a Chandra search for evidence of triggered nuclear activity within the Cl0023+0423 four-way group merger at z ~ 0.84. The system consists of four interacting galaxy groups in the early stages of hierarchical cluster formation and, as such, provides a unique look at the level of processing and evolution already under way in the group environment prior to cluster assembly. We present the number counts of X-ray point sources detected in a field covering the entire Cl0023 structure, as well as a cross-correlation of these sources with our extensive spectroscopic database. Both the redshift distribution and cumulative number counts of X-ray sources reveal little evidence to suggest that the system contains X-ray luminous active galactic nuclei (AGNs) in excess to what is observed in the field population. If preprocessing is under way in the Cl0023 system, our observations suggest that powerful nuclear activity is not the predominant mechanism quenching star formation and driving the evolution of Cl0023 galaxies. We speculate that this is due to a lack of sufficiently massive nuclear black holes required to power such activity, as previous observations have found a high late-type fraction among the Cl0023 population. It may be that disruptive AGN-driven outflows become an important factor in the preprocessing of galaxy populations only during a later stage in the evolution of such groups and structures when sufficiently massive galaxies (and central black holes) have built up, but prior to hydrodynamical processes stripping them of their gas reservoirs.

astro-ph.CO↗

arXiv2009

Serendipitous Discovery of an Overdensity of Lyman-Alpha Emitters at z~4.8 in the Cl1604 Supercluster Field

We present results of a spectroscopic search for Lyman-alpha emitters (LAEs) in the Cl1604 supercluster field using the extensive spectroscopic Keck/DEIMOS database taken as part of the Observations of Redshift Evolution in Large Scale Environments (ORELSE) survey. A total of 12 slitmasks were observed and inspected in the Cl1604 field, spanning a survey volume of 1.365x10^4 co-moving Mpc^3. We find a total of 17 high redshift (4.39 < z < 5.67) LAE candidates down to a limiting flux of 1.9x10^(-18) ergs/s/cm (~0.1L* at z~5), 13 of which we classify as high quality. The resulting LAE number density is nearly double that of LAEs found in the Subaru deep field at z~4.9 and nearly an order of magnitude higher than in other surveys of LAEs at similar redshifts, an excess that is essentially independent of LAE luminosity. We also report on the discovery of two possible LAE group structures at z~4.4 and z~4.8 and investigate the effects of cosmic variance of LAEs on our results. Fitting a simple truncated single Gaussian model to a composite spectrum of the 13 high quality LAE candidates, we find a best-fit stellar velocity dispersion of 136 km/s. Additionally, we see modest evidence of a second peak in the composite spectrum, possibly caused by galactic outflows, as well as evidence for a non-trivial Lyman-alpha escape fraction. We find an average LAE star formation rate density (SFRD) of ~5x10^(-3) M_solar/yr/Mpc^3 with moderate evidence for negative evolution in the LAE SFRD from z~4.6 to z~5.7. We measure a best-fit luminosity function generally consistent with measurements from other surveys at similar epochs. Finally, we investigate any possible effects from weak or strong gravitational lensing induced by the foreground supercluster, finding that our LAE candidates are minimally affected by lensing processes.

astro-ph.CO↗

arXiv2009

Estimating cosmological parameters from future gravitational lens surveys

Upcoming ground and space based observatories such as the DES, the LSST, the JDEM concepts and the SKA, promise to dramatically increase the size of strong gravitational lens samples. A significant fraction of the systems are expected to be time delay lenses. Many of the existing lensing degeneracies become less of an issue with large samples since the distributions of a number of parameters are predictable, and can be incorporated into an analysis, thus helping to lessen the degeneracy. Assuming a mean galaxy density profile that does not evolve with redshift, a Lambda-CDM cosmology, and Gaussian distributions for bulk parameters describing the lens and source populations, we generate synthetic lens catalogues and examine the relationship between constraints on the Omega_m - Omega_Lambda plane and H_0 with increasing lens sample size. We find that, with sample sizes of ~400 time delay lenses, useful constraints can be obtained for Omega_m and Omega_Lambda with approximately similar levels of precision as from the best of other methods. In addition, sample sizes of ~100 time delay systems yield estimates of H_0 with errors of only a couple of percent, exceeding the level of precision from current best estimates such as the HST Key Project. We note that insufficient prior knowledge of the lens samples employed in the analysis, via under or overestimates in the mean values of the sample distributions, results in broadening of constraints. This highlights the need for sound prior knowledge of the sample before useful cosmological constraints can be obtained from large time delay samples (abridged).

astro-ph.CO↗

arXiv2008

Properties of Galaxies Hosting X-ray Selected Active Galactic Nuclei in the Cl1604 Supercluster at z=0.9

To investigate the role of feedback from Active Galactic Nuclei (AGN) in driving the evolution of their host galaxies, we have carried out a study of the environments and optical properties of galaxies harboring X-ray luminous AGN in the Cl1604 supercluster at z~0.9. Making use of Chandra, HST/ACS and Keck/DEIMOS observations, we examine the integrated colors, morphologies and spectral properties of nine moderate-luminosity (L_x ~ 10^43 erg s^-1) type 2 Seyferts detected in the Cl1604 complex. We find that the AGN are predominantly hosted by luminous spheroids and/or bulge dominated galaxies which have colors that place them in the valley between the blue cloud and red sequence in color-magnitude space, consistent with predictions that AGN hosts should constitute a transition population. Half of the hosts have bluer overall colors as a result of blue resolved cores in otherwise red spheroids and a majority show signs of recent or pending interactions. We also find a substantial number exhibit strong Balmer absorption features indicative of post-starburst galaxies, despite the fact that we detect narrow [OII] emission lines in all of the host spectra. If the [OII] lines are due in part to AGN emission, as we suspect, then this result implies that a significant fraction of these galaxies (44%) have experienced an enhanced level of star formation within the last ~1 Gyr which was rapidly suppressed. Overall we find that the properties of the nine host galaxies are generally consistent with a scenario in which recent interactions have triggered both increased levels of nuclear activity and an enhancement of centrally concentrated star formation, followed by a rapid truncation of the latter, possibly as a result of feedback from the AGN itself. [Abridged]

astro-ph↗

arXiv2008

Automated detection of galaxy-scale gravitational lenses in high resolution imaging data

Lens modeling is the key to successful and meaningful automated strong galaxy-scale gravitational lens detection. We have implemented a lens-modeling "robot" that treats every bright red galaxy (BRG) in a large imaging survey as a potential gravitational lens system. Using a simple model optimized for "typical" galaxy-scale lenses, we generate four assessments of model quality that are used in an automated classification. The robot infers the lens classification parameter H that a human would have assigned; the inference is performed using a probability distribution generated from a human-classified training set, including realistic simulated lenses and known false positives drawn from the HST/EGS survey. We compute the expected purity, completeness and rejection rate, and find that these can be optimized for a particular application by changing the prior probability distribution for H, equivalent to defining the robot's "character." Adopting a realistic prior based on the known abundance of lenses, we find that a lens sample may be generated that is ~100% pure, but only ~20% complete. This shortfall is due primarily to the over-simplicity of the lens model. With a more optimistic robot, ~90% completeness can be achieved while rejecting ~90% of the candidate objects. The remaining candidates must be classified by human inspectors. We are able to classify lens candidates by eye at a rate of a few seconds per system, suggesting that a future 1000 square degree imaging survey containing 10^7 BRGs, and some 10^4 lenses, could be successfully, and reproducibly, searched in a modest amount of time. [Abridged]

astro-ph↗

arXiv2008

Chandra Observations of the Cl1604 Supercluster at z=0.9: Evidence for an Overdensity of Active Galactic Nuclei

We present the results of Chandra observations of the Cl1604 supercluster at z~0.9. The system is the largest structure mapped at redshifts approaching unity, containing at least eight spectroscopically confirmed galaxy clusters and groups. Using two 50-ksec ACIS-I pointings we examine both the X-ray point source population and the diffuse emission from individual clusters in the system. We find a 2.5σexcess of point sources detected in the hard band (2-10 keV) relative to the number of sources found in blank fields observed by Chandra. No such excess is observed in the soft band (0.5-2 keV). The hard-band source density is 1.47 times greater than that of a blank field, in agreement with the previously reported correlation between overdensity amplitude and cluster redshift. Using a maximum likelihood technique we have matched 112 of the 161 detected X-ray point sources to optical counterparts and found 15 sources that are associated with the supercluster. All 15 sources have rest-frame luminosities consistent with emission from active galactic nuclei (AGN). We find that the supercluster AGN largely avoid the densest regions of the system and are instead distributed on the outskirts of massive clusters or within poorer clusters and groups. We have also detected diffuse emission from two of the eight clusters and groups in the system, clusters Cl1604+4304 and Cl1604+4314. The systems have bolometric luminosities of 1.43x10^44 and 8.20x10^43 h70^-2 erg s^-1 and gas temperatures of 3.50 (+1.82-1.08) and 1.64 (+0.65-0.45) keV, respectively. Using updated velocity dispersions, we compare the properties of these systems to the cluster scaling relations followed by other X-ray and optically selected galaxy clusters at high redshift.

astro-ph↗

arXiv2007

The Evolution of Galaxies in X-ray Luminous Groups

We investigate the galaxy populations in seven X-ray selected, intermediate-redshift groups (0.2<z<0.6). Overall, the galaxy populations in these systems are similar to those in clusters at the same redshift; they have large fractions of early-type galaxies (f_e~70%) and small fractions of galaxies with significant star formation (f_[OII]~30%). We do not observe a strong evolution in the galaxy populations from those seen in X-ray luminous groups at low-redshift. Both f_e and f_[OII] are correlated with radius but do not reach the field value out to ~r_500. However, we find significant variation in the galaxy populations between groups with some groups having field-like populations. Comparisons between the morphological and spectral properties of group galaxies reveal both gas-poor mergers and a population of passive spirals. Unlike low-redshift, X-ray emitting groups, in some of these groups the brightest galaxy does not lie at the center of the X-ray emission, and in several of the groups that do have a central BGG, the BGG has multiple components. These groups appear to represent a range of evolutionary stages in the formation of the BGG. Some groups have relatively large central galaxy densities, and one group contains a string of seven bright galaxies within a radius of 200 kpc that have a lower velocity dispersion than the rest of the system. None of the central galaxies, including those with multiple components, have significant [OII] emission. These observations support a scenario in which BGGs are formed relatively late through gas-poor mergers.

astro-ph↗