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Astrophysics and Astronomy

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University of Massachusetts Amherst

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2005

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Articles 1 - 27 of 27

Full-Text Articles in Physical Sciences and Mathematics

The Galaxy Angular Correlation Functions And Power Spectrum From The Two Micron All Sky Survey, Ah Maller, Dh Mcintosh, N Katz, Md Weinberg Jan 2005

The Galaxy Angular Correlation Functions And Power Spectrum From The Two Micron All Sky Survey, Ah Maller, Dh Mcintosh, N Katz, Md Weinberg

Astronomy Department Faculty Publication Series

We calculate the angular correlation function of galaxies in the Two Micron All Sky Survey. We minimize the possible contamination by stars, dust, seeing, and sky brightness by studying their cross correlation with galaxy density and limiting the galaxy sample accordingly. We measure the correlation function at scales between 1' < θ < 18° using a half-million galaxies. We find that a best-fit power law to the correlation function has a slope of -0.79 ± 0.02 and an amplitude at 1° of 0.10 ± 0.01 in the range 1'-2fdg5. However, there are statistically significant oscillations around this power law. The largest oscillation occurs at about 0fdg8, corresponding to 700 h-1 kpc at the median redshift of our survey, as expected in halo occupation distribution descriptions of galaxy clustering. In addition, there is a break in the power-law shape of the correlation function at θ > 2fdg5. Our results are in good agreement with other measurements of the angular correlation function. We invert the angular correlation function using singular value decomposition to measure the three-dimensional power spectrum and find that it too is in good agreement with previous measurements. A dip seen in the power spectrum …

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Bar-Induced Evolution Of Dark Matter Cusps, K Holley-Bockelmann, M Weinberg, N Katz Jan 2005

Bar-Induced Evolution Of Dark Matter Cusps, K Holley-Bockelmann, M Weinberg, N Katz

Astronomy Department Faculty Publication Series

The evolution of a stellar bar transforms not only the galactic disc, but also the host dark matter halo. We present high-resolution, fully self-consistent N-body simulations that clearly demonstrate that dark matter halo central density cusps flatten as the bar torques the halo. This effect is independent of the bar formation mode and occurs even for rather short bars. The halo and bar evolution is mediated by resonant interactions between orbits in the halo and the bar pattern speed, as predicted by linear Hamiltonian perturbation theory. The bar lengthens and slows as it loses angular momentum, a process that occurs …

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How Do Galaxies Get Their Gas?, D Keres, N Katz, Dh Weinberg, R Dave Jan 2005

How Do Galaxies Get Their Gas?, D Keres, N Katz, Dh Weinberg, R Dave

Astronomy Department Faculty Publication Series

We examine the temperature history of gas accreted by forming galaxies in smoothed particle hydrodynamics simulations. About half of the gas follows the track expected in the conventional picture of galaxy formation, shock heating to roughly the virial temperature of the galaxy potential well (T∼ 106 K for a Milky Way type galaxy) before cooling, condensing and forming stars. However, the other half radiates its acquired gravitational energy at much lower temperatures, typically T < 105 K, and the histogram of maximum gas temperatures is clearly bimodal. The ‘cold mode’ of gas accretion dominates for low-mass galaxies (baryonic …

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Theoretical Models Of The Halo Occupation Distribution: Separating Central And Satellite Galaxies, Z Zheng, Aa Berlind, Dh Weinberg, Aj Benson, Cm Baugh, S Cole, R Dave, Cs Frenk, N Katz, Cg Lacey Jan 2005

Theoretical Models Of The Halo Occupation Distribution: Separating Central And Satellite Galaxies, Z Zheng, Aa Berlind, Dh Weinberg, Aj Benson, Cm Baugh, S Cole, R Dave, Cs Frenk, N Katz, Cg Lacey

Astronomy Department Faculty Publication Series

The halo occupation distribution (HOD) describes the relation between galaxies and dark matter at the level of individual dark matter halos. The properties of galaxies residing at the centers of halos differ from those of satellite galaxies because of differences in their formation histories. Using a smoothed particle hydrodynamics (SPH) simulation and a semianalytic (SA) galaxy formation model, we examine the separate contributions of central and satellite galaxies to the HOD, more specifically to the probability P(N|M) that a halo of virial mass M contains N galaxies of a particular class. In agreement with earlier …

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Interpreting The Relationship Between Galaxy Luminosity, Color, And Environment, Aa Berlind, Mr Blanton, Dw Hogg, Dh Weinberg, R Dave, Dj Eisenstein, N Katz Jan 2005

Interpreting The Relationship Between Galaxy Luminosity, Color, And Environment, Aa Berlind, Mr Blanton, Dw Hogg, Dh Weinberg, R Dave, Dj Eisenstein, N Katz

Astronomy Department Faculty Publication Series

We study the relationship between galaxy luminosity, color, and environment in a cosmological simulation of galaxy formation. Using a simple prescription to assign colors and luminosities to simulated galaxies, we compare the predicted relationship with that observed for SDSS galaxies and find that the model successfully predicts most of the qualitative features seen in the data, but also shows some interesting differences. Specifically, the simulation predicts that the local density around bright red galaxies is a strong increasing function of luminosity, but does not depend much on color at fixed luminosity. Moreover, we show that these trends are due to …

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Pre-Heating By Pre-Virialization And Its Impact On Galaxy Formation, Hj Mo, Xh Yang, Fc Van Den Bosch, N Katz, I Zehavi, T Tripp, Dv Bowen, Kr Sembach, Eb Jenkins, Bd Savage, P Richter Jan 2005

Pre-Heating By Pre-Virialization And Its Impact On Galaxy Formation, Hj Mo, Xh Yang, Fc Van Den Bosch, N Katz, I Zehavi, T Tripp, Dv Bowen, Kr Sembach, Eb Jenkins, Bd Savage, P Richter

Astronomy Department Faculty Publication Series

We use recent observations of the H i mass function to constrain galaxy formation. The data conflict with the standard model where most of the gas in a low-mass dark matter halo is assumed to settle into a disc of cold gas that is depleted by star formation and supernova-driven outflows until the disc becomes gravitationally stable. Assuming a star formation threshold density supported by both theory and observations, this model predicts H i masses that are much too large. The reason is simple: supernova feedback requires star formation, which in turn requires a high surface density for the gas. …

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Measuring The Halo Mass Of Z Similar To 3 Damped Ly Alpha Absorbers From The Absorber-Galaxy Cross-Correlation, N Bouche, Jp Gardner, N Katz, Dh Weinberg, R Dave, Jd Lowenthal Jan 2005

Measuring The Halo Mass Of Z Similar To 3 Damped Ly Alpha Absorbers From The Absorber-Galaxy Cross-Correlation, N Bouche, Jp Gardner, N Katz, Dh Weinberg, R Dave, Jd Lowenthal

Astronomy Department Faculty Publication Series

We test the reliability of a method to measure the mean halo mass of absorption-line systems such as damped Lyα absorbers (DLAs). The method is based on measuring the ratio of the cross-correlation between DLAs and galaxies to the autocorrelation of the galaxies themselves, which is (in linear theory) the ratio of their bias factor . We show that the ratio of the projected cross- and autocorrelation functions [wdg(rθ)/wgg(rθ)] is also the ratio of their bias factor, irrespective of the galaxy distribution, provided that one uses the same …

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Simulation Of Soft X-Ray Emission Lines From The Missing Baryons, Tt Fang, Rac Croft, Wt Sanders, J Houck, R Dave, N Katz, Dh Weinberg, L Hernquist Jan 2005

Simulation Of Soft X-Ray Emission Lines From The Missing Baryons, Tt Fang, Rac Croft, Wt Sanders, J Houck, R Dave, N Katz, Dh Weinberg, L Hernquist

Astronomy Department Faculty Publication Series

We study the soft X-ray emission (0.1-1 keV) from the warm-hot intergalactic medium (WHIM) in a hydrodynamic simulation of a cold dark matter universe. Our main goal is to investigate how such emission can be explored with a combination of imaging and spectroscopy and to motivate future X-ray missions. We first present high-resolution images of the X-ray emission in several energy bands in which emission from different ion species dominates. We pick three different areas to study the high-resolution spectra of X-rays from the WHIM: (1) a galaxy group, (2) a filament, and (3) an underluminous region. By taking into …

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Supernova Blast Waves In Low-Density Hot Media: A Mechanism For Spatially Distributed Heating, S Tang, Qd Wang Jan 2005

Supernova Blast Waves In Low-Density Hot Media: A Mechanism For Spatially Distributed Heating, S Tang, Qd Wang

Astronomy Department Faculty Publication Series

Most supernovae are expected to explode in low-density hot media, particularly in galactic bulges and elliptical galaxies. The remnants of such supernovae, although difficult to detect individually, can be profoundly important in heating the media on large scales. We characterize the evolution of this kind of supernova remnant, based on analytical approximations and hydrodynamic simulations. We generalize the standard Sedov solution to account for both temperature and density effects of the ambient media. Although cooling can be neglected, the expansion of such a remnant deviates quickly from the standard Sedov solution and asymptotically approaches the ambient sound speed as the …

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Extra-Planar Diffuse Hot Gas Around Normal Disk Galaxies, Qd Wang Jan 2005

Extra-Planar Diffuse Hot Gas Around Normal Disk Galaxies, Qd Wang

Astronomy Department Faculty Publication Series

I review results from Chandra observations of nearby normal edge-on galaxies (Sd to Sa types). These galaxies have a broad range of star formation rate, but none of them is dominated by a nuclear starburst. The galaxies are all in directions of low Galactic foreground absorption (NHI ≤ 4 × 1020 cm−2). Extra-Planar diffuse soft X-ray emission is detected unambiguously from all the galaxies, except for N4244 (Sd), which is low in both stellar mass and star formation rate. The thermal nature of the X-ray-emitting gas is well established, although its chemical and ionization states …

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Diffuse X-Rays From The Arches And Quintuplet Clusters, G Rockefeller, Cl Fryer, F Melia, Qd Wang Jan 2005

Diffuse X-Rays From The Arches And Quintuplet Clusters, G Rockefeller, Cl Fryer, F Melia, Qd Wang

Astronomy Department Faculty Publication Series

The origin and initial mass function of young stellar clusters near the Galactic center are still poorly understood. Two of the more prominent ones, the Arches and Quintuplet clusters, may have formed from a shock-induced burst of star formation, given their similar age and proximity to each other. Their unusual mass distribution, however, may be evidence of a contributing role played by other factors, such as stellar capture from regions outside the clusters themselves. Diffuse X-ray emission from these sources provides us with a valuable, albeit indirect, measure of the stellar mass-loss rate from their constituents. Using recent data acquired …

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An Infrared Imaging Survey Of The Faint Chandra Sources Near The Galactic Centre, Rm Bandyopadhyay, Jca Miller-Jones, Km Blundell, Fe Bauer, Ph Podsiadlowski, Aj Gosling, Qd Wang, E Pfahl, S Rappaport Jan 2005

An Infrared Imaging Survey Of The Faint Chandra Sources Near The Galactic Centre, Rm Bandyopadhyay, Jca Miller-Jones, Km Blundell, Fe Bauer, Ph Podsiadlowski, Aj Gosling, Qd Wang, E Pfahl, S Rappaport

Astronomy Department Faculty Publication Series

We present near-infrared imaging of a sample of the faint, hard X-ray sources discovered in the 2001 Chandra Advanced CCD Imaging Spectrometer (ACIS)-I survey towards the Galactic Centre (GC) by Wang et al. These ∼800 discrete sources represent an important and previously undetected population within the Galaxy. From our Very Large Telescope observations of 77 X-ray sources, we identify candidate K-band counterparts to 75 per cent of the Chandra sources in our sample. The near-infrared magnitudes and colours of the majority of candidate counterparts are consistent with highly reddened stars, indicating that most of the Chandra sources are likely …

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Atomic And Molecular Gas In Colliding Galaxy Systems. I. The Data, D Iono, Min Yun, Ptp Ho Jan 2005

Atomic And Molecular Gas In Colliding Galaxy Systems. I. The Data, D Iono, Min Yun, Ptp Ho

Astronomy Department Faculty Publication Series

We present H I and CO (1-0) interferometric observations of 10 comparable-mass interacting systems obtained at the Very Large Array (VLA) and the Owens Valley Radio Observatory (OVRO) millimeter array. The primary intent of this study is to investigate the response of cold gas during the early stages of collision of massive disk galaxies. The sample sources are selected based on their luminosity (MB ≤ -19), projected separation (5-40 kpc), and single-dish CO (1-0) content (SCO ≥ 20 Jy km s-1). These selection criteria result in a sample that primarily consists of systems in …

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Hcn And Hco+ Observations Of The Galactic Circumnuclear Disk, Mh Christopher, Nz Scoville, Sr Stolovy, Min Yun Jan 2005

Hcn And Hco+ Observations Of The Galactic Circumnuclear Disk, Mh Christopher, Nz Scoville, Sr Stolovy, Min Yun

Astronomy Department Faculty Publication Series

We present high spatial resolution (51 × 27) Owens Valley Radio Observatory (OVRO) millimeter array observations of HCN (J = 1-0) and HCO+ (J = 1-0) emission in the inner 3 pc of the Galaxy (0.04 pc ~ 1''). The HCN and HCO+ emission of the circumnuclear disk (CND) is distributed in a well-defined ring with a peak at a radius of 1.6 pc. The observed radial velocities are generally consistent with rotation at ~110 km s-1 (except along the western edge of the CND). The HCO+/HCN emission ratio is typically ~0.4 but …

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Probing The Local Group Medium Toward Markarian 421 With Chandra And The Far Ultraviolet Spectroscopic Explorer, Rj Williams, S Mathur, F Nicastro, M Elvis, Jj Drake, T Fang, F Fiore, Y Krongold, Qd Wang, Y Yao Jan 2005

Probing The Local Group Medium Toward Markarian 421 With Chandra And The Far Ultraviolet Spectroscopic Explorer, Rj Williams, S Mathur, F Nicastro, M Elvis, Jj Drake, T Fang, F Fiore, Y Krongold, Qd Wang, Y Yao

Astronomy Department Faculty Publication Series

We report the detection of highly ionized gas at z ~ 0 seen in resonant UV and X-ray absorption lines toward the z = 0.03 blazar Mrk 421. A total of 13 X-ray and three UV lines were measured (or upper limits derived), including three lines in the O VII K series and Kα transitions from neon, carbon, and nitrogen. From the three O VII lines we derive a 2 σ Doppler parameter constraint of 24 km s-1 < b < 55 km s-1. The FUSE spectrum shows strong Galactic low-velocity O VI λ1032 absorption and a possible weak O VI high-velocity …

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X-Ray Absorption Line Spectroscopy Of The Galactic Hot Interstellar Medium, Y Yao, Qd Wang Jan 2005

X-Ray Absorption Line Spectroscopy Of The Galactic Hot Interstellar Medium, Y Yao, Qd Wang

Astronomy Department Faculty Publication Series

We present an X-ray absorption line spectroscopic study of the large-scale hot interstellar medium (HISM) in the Galaxy. We detect Ne IX Kα absorption lines in the Chandra grating spectra of seven Galactic low-mass X-ray binaries. Three of these sources also show absorption of O VII Kα, O VII Kβ, and/or O VIII Kα. Both the centroid and width of the lines are consistent with a Galactic HISM origin of the absorption. By jointly fitting the multiple lines, accounting for line saturation, and assuming the collisional ionization equilibrium, we estimate the average absorbing gas temperature as ~(2.4 ± 0.3) × …

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Warm-Hot Gas In And Around The Milky Way: Detection And Implications Of Ovii Absorption Toward Lmc X-3, Qd Wang, Y Yao, Tm Tripp, Tt Fang, W Cui, F Nicastro, S Mathur, Rj Williams, L Song, R Croft Jan 2005

Warm-Hot Gas In And Around The Milky Way: Detection And Implications Of Ovii Absorption Toward Lmc X-3, Qd Wang, Y Yao, Tm Tripp, Tt Fang, W Cui, F Nicastro, S Mathur, Rj Williams, L Song, R Croft

Astronomy Department Faculty Publication Series

X-ray absorption lines of highly ionized species such as O VII at about zero redshift have been firmly detected in the spectra of several active galactic nuclei. However, the location of the absorbing gas remains a subject of debate. To separate the Galactic and extragalactic contributions to the absorption, we have obtained Chandra LETG-HRC and Far Ultraviolet Spectroscopic Explorer observations of the black hole X-ray binary LMC X-3. We clearly detect the O VII Kα absorption line with an equivalent width of 20(14, 26) mÅ (90% confidence range). The Ne IX Kα absorption line is also detected, albeit marginally. A …

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Ly Alpha Flux Power Spectrum And Its Covariance, H Zhan, R Dave, D Eisenstein, N Katz Jan 2005

Ly Alpha Flux Power Spectrum And Its Covariance, H Zhan, R Dave, D Eisenstein, N Katz

Astronomy Department Faculty Publication Series

We analyse the flux power spectrum and its covariance using simulated Lyα forests. We find that pseudo-hydro techniques are good approximations of hydrodynamical simulations at high redshift. However, the pseudo-hydro techniques fail at low redshift because they are insufficient for characterizing some components of the low-redshift intergalactic medium, notably the warm–hot intergalactic medium. Hence, to use the low-redshift Lyα flux power spectrum to constrain cosmology, one would need realistic hydrodynamical simulations. By comparing (one-dimensional) mass statistics with flux statistics, we show that the non-linear transform between density and flux quenches the fluctuations so that the flux power spectrum is much …

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The Large Asymmetric Hi Envelope Of The Isolated Galaxy Ngc 864 (Cig 96), D Espada, A Bosma, L Verdes-Montenegro, E Athanassoula, S Leon, J Sulentic, Min Yun Jan 2005

The Large Asymmetric Hi Envelope Of The Isolated Galaxy Ngc 864 (Cig 96), D Espada, A Bosma, L Verdes-Montenegro, E Athanassoula, S Leon, J Sulentic, Min Yun

Astronomy Department Faculty Publication Series

We present an HI synthesis imaging study of NGC 864 (CIG 96), a spiral galaxy well isolated from similarly sized companions, yet presenting an intriguing asymmetry in its integral HI spectrum. The asymmetry in the HI profile is associated with a strong kinematical perturbation in the gaseous envelope of the galaxy, where at one side the decay of the rotation curve is faster than Keplerian. We detect a small (M(HI) = 5 106 ) galaxy with a faint optical counterpart at ~80 kpc projected distance from NGC 864. This galaxy is probably not massive enough to have …

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Optical Identification Of Infrared Space Observatory Far-Infrared Sources In The Lockman Hole Using A Deep Very Large Array 1.4 Ghz Continuum Survey, S Oyabu, Min Yun, T Murayama, Db Sanders, K Kawara, Y Taniguchi, S Veilleux, H Okuda, H Matsuhara, Ll Cowie, Y Sato, K Wakamatsu, Y Sofue Jan 2005

Optical Identification Of Infrared Space Observatory Far-Infrared Sources In The Lockman Hole Using A Deep Very Large Array 1.4 Ghz Continuum Survey, S Oyabu, Min Yun, T Murayama, Db Sanders, K Kawara, Y Taniguchi, S Veilleux, H Okuda, H Matsuhara, Ll Cowie, Y Sato, K Wakamatsu, Y Sofue

Astronomy Department Faculty Publication Series

By exploiting the far-infrared (FIR) and radio correlation, we have performed a likelihood-ratio analysis to identify optical counterparts to the FIR sources that have been found in an area of ~0.9 deg2 during the Infrared Space Observatory (ISO) deep FIR survey in the Lockman Hole. New ground-based observations have been conducted to build up the catalogs of radio and optical objects, which include a deep Very Large Array (VLA) observation at 1.4 GHz, optical R- and I-band imaging with the Subaru 8 m and University of Hawaii 2.2 m telescopes, and optical spectroscopy with the …

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Building Galaxies With Simulations, R Dave, K Finlator, L Hernquist, N Katz, D Keres, C Papovich, Dh Weinberg Jan 2005

Building Galaxies With Simulations, R Dave, K Finlator, L Hernquist, N Katz, D Keres, C Papovich, Dh Weinberg

Astronomy Department Faculty Publication Series

We present an overview of some of the issues surrounding current models of galaxy formation, highlighting recent insights obtained from cosmological hydrodynamic simulations. Detailed examination of gas accretion processes show a hot mode of gas cooling from near the halo's virial temperature, and a previously underappreciated cold mode where gas flows in along filaments on dynamical timescales, emitting its energy in line radiation. Cold mode dominates in systems with halo masses slightly smaller than the Milky Way and below, and hence dominates the global accretion during the heydey of galaxy formation. This rapid accretion path enables prompt assembly of massive …

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Highly Clumpy Structure Of The Thermal Composite Supernova Remnant 3c 391 Unveiled By Chandra, Y Chen, Y Su, Po Slane, Qd Wang Jan 2005

Highly Clumpy Structure Of The Thermal Composite Supernova Remnant 3c 391 Unveiled By Chandra, Y Chen, Y Su, Po Slane, Qd Wang

Astronomy Department Faculty Publication Series

The nature of the internal thermal X-ray emission seen in “thermal composite” supernova remnants is still uncertain. Chandra observation of the 3C391 shows a southeast–northwest elongated morphology and unveils a highly clumpy structure of the remnant. Detailed spatially resolved spectral analysis for the small-scale features reveals normal metal abundance and uniform temperature for the interior gas. The properties of the hot gas comparatively favor the cloudlet evaporation model as a main mechanism for the “thermal composite” X-ray appearance, though radiative rim and thermal conduction may also be effective. A faint protrusion is found in Si and S lines out of …

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Exploring The Nature Of Weak Chandra Sources Near The Galactic Centre, Rm Bandyopadhyay, Jca Miller-Jones, Km Blundell, Fe Bauer, Ph Podsiadlowski, Qd Wang, S Rappaport, E Pfahl Jan 2005

Exploring The Nature Of Weak Chandra Sources Near The Galactic Centre, Rm Bandyopadhyay, Jca Miller-Jones, Km Blundell, Fe Bauer, Ph Podsiadlowski, Qd Wang, S Rappaport, E Pfahl

Astronomy Department Faculty Publication Series

We present results from the first near-IR imaging of the weak X-ray sources discovered in the Chandra/ACIS-I survey (Wang et al. 2002) towards the Galactic Centre (GC). These ~800 discrete sources, which contribute significantly to the GC X-ray emission, represent an important and previously unknown population within the Galaxy. From our VLT observations we will identify likely IR counterparts to a sample of the hardest sources, which are most likely X-ray binaries. With these data we can place constraints on the nature of the discrete weak X-ray source population of the GC. ©2005 American Institute of Physics

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Black Hole X-Ray Binaries Lmc X–1 And X–3: Observations Confront Spectral Models, Y Yao, Qd Wang, Sn Zhang Jan 2005

Black Hole X-Ray Binaries Lmc X–1 And X–3: Observations Confront Spectral Models, Y Yao, Qd Wang, Sn Zhang

Astronomy Department Faculty Publication Series

We present a comprehensive spectral analysis of black hole X-ray binaries, LMC X–1 and X–3, based on BeppoSAX observations. We test both the multicolour disc plus power-law (MCD+PL) model and a newly developed Monte Carlo simulation-based model for a Comptonized MCD (CMCD) with either a spherical or a slab-like corona, by comparing the inferred parameters with independent direct measurements. While all models give an adequate description of the spectra, we find a significant discrepancy between the MCD+PL inferred X-ray-absorbing gas column density and the absorption-edge measurement based on dispersed X-ray spectra. The MCD+PL fits to the LMC X–1 spectra also …

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An Xmm–Newton And Chandra Study Of The Starburst Galaxy Ic 10, Qd Wang, Ke Whitaker, R Williams Jan 2005

An Xmm–Newton And Chandra Study Of The Starburst Galaxy Ic 10, Qd Wang, Ke Whitaker, R Williams

Astronomy Department Faculty Publication Series

We present an X-ray study of our nearest starburst galaxy IC 10, based on XMM–Newton and Chandra observations. A list of 73 XMM–Newton and 28 Chandra detections of point-like X-ray sources in the field is provided; a substantial fraction of them are likely stellar objects in the Milky Way due to the low Galactic latitude location of IC 10. The brightest source in the IC 10 field, X-1, has a mean 0.3–8.0 keV luminosity of ∼1.2 × 1038 erg s−1 and shows a large variation by a factor of up to ∼6 on time-scales of ∼104 s …

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A Deep Radio Survey Of Abell 2125. Ii. Accelerated Galaxy Evolution During A Cluster-Cluster Merger, Fn Owen, Mj Ledlow, Wc Keel, Qd Wang, Ge Morrison Jan 2005

A Deep Radio Survey Of Abell 2125. Ii. Accelerated Galaxy Evolution During A Cluster-Cluster Merger, Fn Owen, Mj Ledlow, Wc Keel, Qd Wang, Ge Morrison

Astronomy Department Faculty Publication Series

Using our extensive radio, optical, near-IR, and X-ray imaging and spectroscopy, we consider the reason for the unusually large number of radio-detected galaxies, mostly found outside the cluster core, in Abell 2125 (z = 0.2465, richness class 4). With 20 cm VLA data, we detect continuum emission from 90 cluster members. The multiwavelength properties of these galaxies suggest that most of the radio emission is due to an enhanced star formation rate. The recent dynamical study of Miller et al. suggests that A2125 is undergoing a major cluster-cluster merger, with our view within 30° of the merger axis and …

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Unusual X-Ray Transients In The Galactic Centre, M Sakano, Rs Warwick, A Decourchelle, Qd Wang Jan 2005

Unusual X-Ray Transients In The Galactic Centre, M Sakano, Rs Warwick, A Decourchelle, Qd Wang

Astronomy Department Faculty Publication Series

We report the discovery in the Galactic Centre region of two hard X-ray sources, designated as XMM J174457−2850.3 and XMM J174544−2913.0, which exhibit flux variations in the 2–10 keV band in excess of a factor of 100 in observations spanning roughly a year. In both cases the observed hydrogen column density is consistent with a location near to the Galactic Centre, implying peak X-ray luminosities of ∼5 × 1034 erg s-1. These objects may represent a new population of transient source, with properties very different from those of the much more luminous Galactic Centre transients associated with …

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