Melville P. Ulmer

A Tidal Disruption Flare in A1689 from an Archival X-ray Survey of Galaxy Clusters

Theory suggests that a star making a close passage by a supermassive black hole at the center of a galaxy can under most circumstances be expected to emit a giant flare of radiation as it is disrupted and a portion of the resulting stream of shock-heated stellar debris falls back onto the black hole itself. We examine the first results of an ongoing archival survey of galaxy clusters using Chandra and XMM-Newton-selected data and report a likely tidal disruption flare from SDSS J131122.15-012345.6 in A1689. The flare is observed to vary by a factor of 30 over at least two years to have maximum LX (0.3-3.0 keV) 5 × 1042 erg s–1 and to emit as a blackbody with kT ~ 0.12 keV. From the galaxy population as determined by existing studies of the cluster, we estimate a tidal disruption rate of 1.2 × 10–4 galaxy–1 yr–1 if we assume a contribution to the observable rate from galaxies whose range of luminosities corresponds to a central black hole mass (M •) between 106 and 108 M ☉.

A weak lensing study of the Coma cluster

Due to observational constraints, dark matter determinations in nearby clusters based on weak lensing are still extremely rare, in spite of their importance for the determination of cluster properties independent of other methods. We present a weak lensing study of the Coma cluster (redshift 0.024) based on deep images obtained at the CFHT. After obtaining photometric redshifts for the galaxies in our field based on deep images in the u (1x1 deg2), and in the B, V, R and I bands (42x52), allowing us to eliminate foreground galaxies, we apply weak lensing calculations on shape measurements performed in the u image. We derive a map of the mass distribution in Coma, as well as the radial shear profile, and the mass and concentration parameter at various radii. We obtain M_200c = 5.1+4.3-2.1 x10^14 Msun and c_200c=5.0+3.2-2.5, in good agreement with previous measurements. With deep wide field images it is now possible to analyze nearby clusters with weak lensing techniques, thus opening a broad new field of investigation.

A tidal flare candidate in Abell 1795

As part of our ongoing archival X-ray survey of galaxy clusters for tidal flares, we present evidence of an X-ray transient source within 1 arcmin of the core of Abell 1795. The extreme variability (a factor of nearly 50), luminosity (textgreater 2 x 10(42) erg s(-1)), long duration (textgreater 5 yr) and supersoft X-ray spectrum (textless 0.1 keV) are characteristic signatures of a stellar tidal disruption event according to theoretical predictions and to existing X-ray observations, implying a massive greater than or similar to 10(5) M-circle dot black hole at the centre of that galaxy. The large number of X-ray source counts (similar to 700) and long temporal baseline (similar to 12 yr with Chandra and XMM-Newton) make this one of the best sampled examples of any tidal flare candidate to date. The transient may be the same EUV source originally found contaminating the diffuse intracluster medium observations of Bowyer, Berghofer and Korpela, which would make it the only tidal flare candidate with reported EUV observations and implies an early source luminosity 1-2 orders of magnitude greater. If the host galaxy is a cluster member then it must be a dwarf galaxy, an order of magnitude less massive than the quiescent galaxy Henize 2-10 which hosts a massive black hole that is difficult to reconcile with its low mass. The unusual faintness of the host galaxy may be explained by tidal stripping in the cluster core.

Galaxy structure searches by photometric redshifts in the CFHTLS

Context. Counting clusters is one of the methods to constrain cosmological parameters, but has been limited up to now both by the redshift range and by the relatively small sizes of the homogeneously surveyed areas. Aims. In order to enlarge publicly available optical cluster catalogs, in particular at high redshift, we have performed a systematic search for clusters of galaxies in the Canada France Hawaii Telescope Legacy Survey (CFHTLS). Methods. We considered the deep 2, 3 and 4 CFHTLS Deep fields (each 1 × 1 deg 2 ), as well as the wide 1, 3 and 4 CFHTLS Wide fields. We used the Le Phare photometric redshifts for the galaxies detected in these fields with magnitude limits of i = 25 and 23 for the Deep and Wide fields respectively. We then constructed galaxy density maps in photometric redshift bins of 0.1 based on an adaptive kernel technique and detected structures with SExtractor at various detection levels. In order to assess the validity of our cluster detection rates, we applied a similar procedure to galaxies in Millennium simulations. We measured the correlation function of our cluster candidates. We analyzed large scale properties and substructures, including filaments, by applying a minimal spanning tree algorithm both to our data and to the Millennium simulations. Results. We detected 1200 candidate clusters with various masses (minimal masses between 1.0 × 10 13 and 5.5 × 10 13 and mean masses between 1.3 × 10 14 and 12.6 × 10 14 M ⊙ ) in the CFHTLS Deep and Wide fields, thus notably increasing the number of known high redshift cluster candidates. We found a correlation function for these objects comparable to that obtained for high redshift cluster surveys. We also show that the CFHTLS deep survey is able to trace the large scale structure of the universe up to z ≥ 1. Our detections are fully consistent with those made in various CFHTLS analyses with other methods. We now need accurate mass determinations of these structures to constrain cosmological parameters. Conclusions. We have shown that a search for galaxy clusters based on density maps built from galaxy catalogs in photometric redshift bins is successful and gives results comparable to or better than those obtained with other methods. By applying this technique to the CFHTLS survey we have increased the number of known optical high redshift cluster candidates by a large factor, an important step towards using cluster counts to measure cosmological parameters.

Intracluster light in clusters of galaxies at redshifts 0.4 < z < 0.8

Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, 382 Via Pueblo Mall, Stanford, CA 94305-4060,USAAccepted . Received ; Draft printed: September 21, 2011ABSTRACTContext.The study of intracluster light (ICL) can help us to understand the mechanisms taking place in galaxy clusters, andto place constraints on the cluster formation history and physical properties. However, owing to the intrinsic faintness of ICLemission, most searches and detailed studies of ICL have been limited to redshifts z< 0.4.Aims.To help us extend our knowledge of ICL properties to higher redshifts and study the evolution of ICL with redshift, wesearch for ICL in a subsample of ten clusters detected by the ESO Distant Cluster Survey (EDisCS), at redshifts 0.4 <z< 0.8,that are also part of our DAFT/FADA Survey.Methods.We analyze the ICL by applying the OV WAV package, a wavelet-based technique, to deep HST ACS images in theF814W filter and to V-band VLT/FORS2 images of three clusters. Detection levels are assessed as a function of the diffuse lightsource surface brightness using simulations.Results. In the F814W filter images, we detect diffuse light sources in all the clusters, with typical sizes of a few tens of kpc(assuming that they are at the cluster redshifts). The ICL detected by stacking the ten F814W images shows an 8σ detection inthe source center extending over a ∼50×50 kpc

Galaxy cluster searches based on photometric redshifts in the four CFHTLS Wide fields

Context. Cosmological parameters can be constrained by counting clusters of galaxies as a function of mass and redshift and by considering regions of the sky sampled as deeply and as homogeneously as possible. Aims. Several methods for detecting clusters in large imaging surveys have been developed, among which the one used here, which is based on detecting structures. This method was first applied to the Canada France Hawaii Telescope Legacy Survey (CFHTLS) Deep 1 field by Mazure et al. (2007, A&A, 467, 49), then to all the Deep and Wide CFHTLS fields available in the T0004 data release by Adami et al. (2010, A&A, 509, A81). The validity of the cluster detection rate was estimated by applying the same procedure to galaxies from the Millennium simulation. Here we use the same method to analyse the full CFHTLS Wide survey, based on the T0006 data release. Methods. Our method is based on the photometric redshifts computed with Le Phare for all the galaxies detected in the Wide fields, limited to magnitudes z � ≤ 22.5. We constructed galaxy density maps in photometric redshift bins of 0.1 based on an adaptive kernel technique, detected structures with SExtractor at various detection levels, and built cluster catalogues by applying a minimal spanning tree algorithm. Results. In a total area of 154 deg 2 , we have detected 4061 candidate clusters at 3σ or above (6802 at 2σ and above), in the redshift range 0.1 ≤ z ≤ 1.15, with estimated mean masses between 1.3 × 10 14 and 12.6 × 10 14 M� . This catalogue of candidate clusters will be available at the CDS. We compare our detections with those made in various CFHTLS analyses with other methods. By stacking a subsample of clusters, we show that this subsample has typical cluster characteristics (colour−magnitude relation, galaxy luminosity function). We also confirm that the cluster-cluster correlation function is comparable to the one obtained for other cluster surveys and analyse large-scale filamentary galaxy distributions. Conclusions. We have increased the number of known optical high-redshift cluster candidates by a large factor, an important step towards obtaining reliable cluster counts to measure cosmological parameters. The clusters that we detect behave as expected if they are located at the intersection of filaments by which they are fed.

Structure and substructure analysis of DAFT/FADA galaxy clusters in the [0.4–0.9] redshift range

Context. The DAFT/FADA survey is based on the study of similar to 90 rich (masses found in the literature textgreater2 x 10(14) M-circle dot) and moderately distant clusters (redshifts 0.4 textless z textless 0.9), all with HST imaging data available. This survey has two main objectives: to constrain dark energy (DE) using weak lensing tomography on galaxy clusters and to build a database (deep multi-band imaging allowing photometric redshift estimates, spectroscopic data, X-ray data) of rich distant clusters to study their properties. Aims. We analyse the structures of all the clusters in the DAFT/FADA survey for which XMM-Newton and/or a sufficient number of galaxy redshifts in the cluster range are available, with the aim of detecting substructures and evidence for merging events. These properties are discussed in the framework of standard cold dark matter (Lambda CDM) cosmology. Methods. In X-rays, we analysed the XMM-Newton data available, fit a beta-model, and subtracted it to identify residuals. We used Chandra data, when available, to identify point sources. In the optical, we applied a Serna & Gerbal (SG) analysis to clusters with at least 15 spectroscopic galaxy redshifts available in the cluster range. We discuss the substructure detection efficiencies of both methods. Results. XMM-Newton data were available for 32 clusters, for which we derive the X-ray luminosity and a global X-ray temperature for 25 of them. For 23 clusters we were able to fit the X-ray emissivity with a beta-model and subtract it to detect substructures in the X-ray gas. A dynamical analysis based on the SG method was applied to the clusters having at least 15 spectroscopic galaxy redshifts in the cluster range: 18 X-ray clusters and 11 clusters with no X-ray data. The choice of a minimum number of 15 redshifts implies that only major substructures will be detected. Ten substructures were detected both in X-rays and by the SG method. Most of the substructures detected both in X-rays and with the SG method are probably at their first cluster pericentre approach and are relatively recent infalls. We also find hints of a decreasing X-ray gas density profile core radius with redshift. Conclusions. The percentage of mass included in substructures was found to be roughly constant with redshift values of 5-15%, in agreement both with the general CDM framework and with the results of numerical simulations. Galaxies in substructures show the same general behaviour as regular cluster galaxies; however, in substructures, there is a deficiency of both late type and old stellar population galaxies. Late type galaxies with recent bursts of star formation seem to be missing in the substructures close to the bottom of the host cluster potential well. However, our sample would need to be increased to allow a more robust analysis.

Deep spectroscopy of the MV ~-14.8 host galaxy of a tidal disruption flare in A1795

A likely tidal disruption of a star by the intermediate-mass black hole (IMBH) of a dwarf galaxy was recently identified in association with Abell 1795. Without deep spectroscopy for this very faint object, however, the possibility of a more massive background galaxy or even a disk-instability flare from a weak AGN could not be dismissed. We have now obtained 8 hours of Gemini spectroscopy which unambiguously demonstrate that the host galaxy is indeed an extremely low-mass (M⋆ � 3×10 8 M⊙) galaxy in Abell 1795, comparable to the least-massive galaxies determined to host IMBHs via other studies. We find that the spectrum is consistent with the X-ray flare being due to a tidal disruption event rather than an AGN flare. We also set improved limits on the black hole mass (log[M•/M⊙] � 5.3 5.7) and infer a 15-year X-ray variability of a factor of > 10 4 . The confirmation of this galaxy-black hole system provides a glimpse into a population of galaxies that is otherwise difficultto study, due to the galaxies’ low masses and intrinsic faintness, but which may be important contributors to the tidal disruption rate.

The evolution of the cluster optical galaxy luminosity function between z=0.4 and 0.9 in the DAFT/FADA survey

We compute optical galaxy luminosity functions (GLFs) in the B, V, R, and I rest-frame bands for one of the largest medium-to-high-redshift (0.4 0.1 at 0.65 < z < 0.90. The blue GLFs have a steeper faint end (alpha(blue) ~ -1.6) than the red GLFs, that appears to be independent of redshift. For the full cluster sample, blue and red GLFs intersect at M(V) = -20, M(R) = -20.5, and M(I) = -20.3. A study of how galaxy types evolve with redshift shows that late type galaxies appear to become early types between z ~ 0.9 and today. Finally, the faint ends of the red GLFs of more massive clusters appear to be richer than less massive clusters, which is more typical of the lower redshift behaviour. Our results indicate that our clusters form at redshifts higher than z = 0.9 from galaxy structures that already have an established red sequence. Late type galaxies then appear to evolve into early types, enriching the red-sequence between this redshift and today. This effect is consistent with the evolution of the faint end slope of the red-sequence and the galaxy type evolution that we find. Finally, faint galaxies accreted from the field environment at all redshifts might have replaced the blue late type galaxies that converted into early types, explaining the lack of evolution in the faint end slopes of the blue GLFs.

A review of UV detectors for astrophysics: past, present, and future

This is an update of previous review by Ulmer in 2005 of UV detectors and UV astrophysics. The developments have been slow to come, both observationally and in terms of instrument development. Since it is redundant to devote significant space to repeating the previous review, we concentrate on updated science drivers based on recent NASA project studies such as the Large UV Optical Space Telescope (LUVO or LST) as well as several new areas of instrument advancement. We discuss recent advances in GaN-based avalanche photodiodes, complementary metal-oxide-semiconductor (CMOS) and low temperature devices (LTDs, operating near 50 mK) called Microwave Kinetic Inductance Detectors (MKIDs).