arXiv subjects

Helen R. Russell

Publications and source records attributed to Helen R. Russell.

At least 19 recordsLinked to original sources

arXiv2026

Lynx2030 Science Analysis Group: Final Report

The Lynx2030 Science Analysis Group (SAG) was convened to reassess the scientific goals and technical drivers of the Lynx mission concept amid a rapidly evolving astrophysics landscape. Building on the original Lynx Concept Study, the SAG examined how recent discoveries, emerging facilities, and advances in instrumentation influence the scientific opportunities for a next-generation flagship X-ray observatory. Through focused working groups, the SAG investigated the scientific impact of enhanced capabilities: (i) improved angular resolution, (ii) broader bandpass coverage, (iii) an enhanced microcalorimeter, (iv) new capabilities and observing modes, and (v) larger fields of view. Across a broad range of topics, from the formation of the first black holes and the evolution of galaxies to the baryon cycle, compact objects, stellar explosions, multi-messenger astrophysics, and the dynamic high-energy Universe, the SAG finds that the scientific motivation for a Lynx-class observatory remains compelling and, in many areas, has significantly strengthened over the past decade, prominently through JWST's discovery of the "Little Red Dots", likely massive accreting black holes in infant galaxies whose nature is fundamentally an X-ray question. This report shows that modest extensions beyond the original Lynx design reference mission can unlock transformative science while preserving the observatory's core architecture. Powerful current and future facilities such as Roman, Rubin, JWST, SKA, ngVLA, LISA, and NewAthena highlight the unique role a high-angular-resolution, high-throughput X-ray observatory would play in the multi-wavelength and multi-messenger ecosystem of the 2030s and beyond. The findings of the Lynx2030 SAG confirm Lynx's central vision: an unprecedented view of the hot and energetic Universe, enabling discoveries that will define high-energy astrophysics in the coming decades.

astro-ph.IM↗

arXiv2026

Mapping gas accretion and stellar kinematics to sub-kiloparsec scales in NGC 4696 with JWST/NIRSpec

We present JWST/NIRSpec IFU spectroscopy of the central $618\times618$ pc$^2$ ($\sim3''\times3''$) of NGC 4696, the BCG in the Centaurus cluster. Leveraging the $\sim0.1''$ ($20.6$ pc) pixel size of JWST, we resolve a compact circumnuclear rotating disk (radius of $\sim120$ pc) traced by Pa$\alpha$ and H$_2$ 1$-$0 S(1) emission, which allows a reassessment of the AGN position based on the kinematic centre of this disk. A central Pa$\alpha$ velocity dispersion reaching $\sigma\sim449$ km s$^{-1}$ implies a SMBH mass of $\sim10^9$ M$_\odot$, corresponding to a sphere of influence of $r_\mathrm{inf}\sim60$ pc, resolved by our observations. Position-velocity diagrams reveal an increase from $\sim-200$ to $\sim600$ km s$^{-1}$ on scales of $\sim150$ pc (a gradient of $4.7$ km s$^{-1}$ pc$^{-1}$) and an accretion rate of $\sim18$ M$_\odot$ yr$^{-1}$ feeding the CND. The Pa$\alpha$ emission shows a double-component in the core, with a high-dispersion redshifted component reaching $\sigma\sim600$ km s$^{-1}$. In contrast, MUSE H$\alpha$ observations covering the $\sim10$ kpc-scale filamentary structure recover only weak velocity gradients ($\lesssim150$ km s$^{-1}$) within $\sim300\times300$ pc$^2$ and do not resolve the disk due to larger PSFs and pixel sizes. ALMA CO(2-1) data reveal only compact molecular clumps within a $\sim410\times410$ pc$^2$ region, with no extended counterpart to the structures traced by Pa$\alpha$ and H$_2$ 1$-$0 S(1). Stellar kinematics show a smooth velocity field and broad dispersion profile, clearly decoupled from both the multiphase gas and the hot ICM probed by XRISM. These results provide a direct, spatially resolved view of gas dynamics within the inner few hundred parsecs, demonstrating the power of JWST/NIRSpec to probe SMBH feeding.

astro-ph.GA↗

arXiv2026

JWST reveals how black holes are fed: kiloparsec-scale multiphase filaments feed sub-kiloparsec circumnuclear disks

The Centaurus cluster is one of the most important archetypes of radio-mode AGN feedback, with its central galaxy, NGC 4696, launching powerful jets that inflate X-ray cavities and regulate cooling and star formation. NGC 4696 lies within a spectacular multiphase nebula of filaments extending over tens of kiloparsecs and spanning six decades in temperature, from hot (10^8 K) X-ray-emitting plasma to cold molecular gas. Owing to its proximity, Hubble Space Telescope H_alpha imaging reveals a striking S-shaped ionized-gas swirl within the black hole's sphere of influence - the first such structure identified in a cluster core. Here we present the first JWST observations of NGC 4696 with NIRSpec, probing the inner 618 pc * 618 pc at 10 pc resolution. These data reveal that the ionized swirl is a rotating, multiphase circumnuclear disk (CND) physically and kinematically connected to the larger-scale filamentary network. This provides the long-sought missing link between kiloparsec-scale cooling flows and black hole accretion on <100 pc scales. Strikingly, the observed morphology and kinematics are reproduced by tailored magnetohydrodynamic simulations, in which filamentary gas condenses from the hot atmosphere, loses angular momentum, and feeds a rotating CND that mediates accretion onto the black hole. A similar structure in NGC 1275, the Perseus cluster's central galaxy, together with our results on NGC 4696 - two prototypical radio-mode AGN feedback systems - points to a common mechanism: multiphase filaments transport gas from cluster scales down to the vicinity of the black hole via a CND, closing the AGN feedback loop and establishing a physically grounded framework for self-regulated galaxy evolution.

astro-ph.GA↗

arXiv2023

Overview of the Advanced X-ray Imaging Satellite (AXIS)

The Advanced X-ray Imaging Satellite (AXIS) is a Probe-class concept that will build on the legacy of the Chandra X-ray Observatory by providing low-background, arcsecond-resolution imaging in the 0.3-10 keV band across a 450 arcminute$^2$ field of view, with an order of magnitude improvement in sensitivity. AXIS utilizes breakthroughs in the construction of lightweight segmented X-ray optics using single-crystal silicon, and developments in the fabrication of large-format, small-pixel, high readout rate CCD detectors with good spectral resolution, allowing a robust and cost-effective design. Further, AXIS will be responsive to target-of-opportunity alerts and, with onboard transient detection, will be a powerful facility for studying the time-varying X-ray universe, following on from the legacy of the Neil Gehrels (Swift) X-ray observatory that revolutionized studies of the transient X-ray Universe. In this paper, we present an overview of AXIS, highlighting the prime science objectives driving the AXIS concept and how the observatory design will achieve these objectives.

astro-ph.IM↗

arXiv2022

Radio jet-ISM interaction and positive radio-mechanical feedback in Abell 1795

We present XSHOOTER observations with previous ALMA, MUSE and $HST$ observations to study the nature of radio-jet triggered star formation and the interaction of radio jets with the interstellar medium in the brightest cluster galaxy (BCG) in the Abell 1795 cluster. Using $HST$ UV data we determined an ongoing star formation rate of 9.3 M$_\odot$ yr$^{-1}$. The star formation follows the global Kennicutt-Schmidt law, however, it has a low efficiency compared to circumnuclear starbursts in nearby galaxies with an average depletion time of $\sim$1 Gyr. The star formation and molecular gas are offset by $\sim1$ kpc indicating that stars have decoupled from the gas. We detected an arc of high linewidth in ionized gas where electron densities are elevated by a factor of $\sim$4 suggesting a shock front driven by radio jets or peculiar motion of the BCG. An analysis of nebular emission line flux ratios suggests that the gas is predominantly ionized by star formation with a small contribution from shocks. We also calculated the velocity structure function (VSF) of the ionized and molecular gases using velocity maps to characterize turbulent motion in the gas. The ionized gas VSF suggests that the radio jets are driving supersonic turbulence in the gas. Thus radio jets can not only heat the atmosphere on large scales and may quench star formation on longer timescales while triggering star formation in positive feedback on short timescales of a few million years.

astro-ph.GA↗

arXiv2022

Molecular Flows in Contemporary Active Galaxies and the Efficacy of Radio-Mechanical Feedback

Molecular gas flows are analyzed in 14 cluster galaxies (BCGs) centered in cooling hot atmospheres. The BCGs contain $10^{9}-10^{11}~\rm M_\odot$ of molecular gas, much of which is being moved by radio jets and lobes. The molecular flows and radio jet powers are compared to molecular outflows in 45 active galaxies within $z<0.2$. We seek to understand the relative efficacy of radio, quasar, and starburst feedback over a range of active galaxy types. Molecular flows powered by radio feedback in BCGs are $\sim$10--1000 times larger in extent compared to contemporary galaxies hosting quasar nuclei and starbursts. Radio feedback yields lower flow velocities but higher momenta compared to quasar nuclei, as the molecular gas flows in BCGs are usually $\sim$10--100 times more massive. The product of the molecular gas mass and lifting altitude divided by the AGN or starburst power -- a parameter referred to as the lifting factor -- exceeds starbursts and quasar nuclei by two to three orders of magnitude, respectively. When active, radio feedback is generally more effective at lifting gas in galaxies compared to quasars and starburst winds. The kinetic energy flux of molecular clouds generally lies below and often substantially below a few percent of the driving power. We find tentatively that star formation is suppressed in BCGs relative to other active galaxies, perhaps because these systems rarely form molecular disks that are more impervious to feedback and are better able to promote star formation.

astro-ph.GA↗

arXiv2022

Testing the Limits of AGN Feedback and the Onset of Thermal Instability in the Most Rapidly Star Forming Brightest Cluster Galaxies

We present new, deep, narrow- and broad-band Hubble Space Telescope observations of seven of the most star-forming brightest cluster galaxies (BCGs). Continuum-subtracted [O II] maps reveal the detailed, complex structure of warm ($T \sim 10^4$ K) ionized gas filaments in these BCGs, allowing us to measure spatially-resolved star formation rates (SFRs) of ~60-600 Msun/yr. We compare the SFRs in these systems and others from the literature to their intracluster medium (ICM) cooling rates (dM/dt), measured from archival Chandra X-ray data, finding a best-fit relation of log(SFR) = (1.67+/-0.17) log(dM/dt) + (-3.25+/-0.38) with an intrinsic scatter of 0.39+/-0.09 dex. This steeper-than-unity slope implies an increasingly efficient conversion of hot ($T \sim 10^7$ K) gas into young stars with increasing dM/dt, or conversely a gradual decrease in the effectiveness of AGN feedback in the strongest cool cores. We also seek to understand the physical extent of these multiphase filaments that we observe in cluster cores. We show, for the first time, that the average extent of the multiphase gas is always smaller than the radii at which the cooling time reaches 1 Gyr, the tcool/tff profile flattens, and that X-ray cavities are observed. This implies a close connection between the multiphase filaments, the thermodynamics of the cooling core, and the dynamics of X-ray bubbles. Interestingly, we find a one-to-one correlation between the average extent of cool multiphase filaments and the radius at which the cooling time reaches 0.5 Gyr, which may be indicative of a universal condensation timescale in cluster cores.

astro-ph.GA↗

arXiv2021

New constraints on light Axion-Like Particles using Chandra Transmission Grating Spectroscopy of the powerful cluster-hosted quasar H1821+643

Axion-Like Particles (ALPs) are predicted by several Beyond the Standard Model theories, in particular, string theory. In the presence of an external magnetic field perpendicular to the direction of propagation, ALPs can couple to photons. Therefore, if an X-ray source is viewed through a magnetised plasma, such as a luminous quasar in a galaxy cluster, we may expect spectral distortions that are well described by photon-ALP oscillations. We present a $571 \ \mathrm{ks}$ combined High and Low Energy Transmission Grating (HETG/LETG) Chandra observation of the powerful radio-quiet quasar H1821+643, hosted by a cool-core cluster at redshift $0.3$. The spectrum is well described by a double power-law continuum and broad$+$narrow iron line emission typical of type-1 Active Galactic Nuclei (AGN), with remaining spectral features $< 2.5\%$. Using a cell-based approach to describe the turbulent cluster magnetic field, we compare our spectrum with photon-ALP mixing curves for 500 field realisations assuming that the thermal-to-magnetic pressure ratio remains constant up to the virial radius. At $99.7\%$ credibility and taking $β= 100$, we exclude all couplings $g_\mathrm{aγ} > 6.3 \times 10^{-13} \ {\mathrm{GeV}}^{-1}$ for most ALP masses $< 10^{-12} \ \mathrm{eV}$. Our results are moderately more sensitive to constraining ALPs than the best previous result from Chandra observations of the Perseus cluster, albeit with a less constrained field model. We reflect on the promising future of ALP studies with bright AGN embedded in rich clusters, especially with the upcoming Athena mission.

astro-ph.HE↗

arXiv2021

Suppressed cooling and turbulent heating in the core of X-ray luminous clusters RXCJ1504.1-0248 and Abell 1664

We present the analysis of XMM-Newton observations of two X-ray luminous cool core clusters, RXCJ1504.1-0248 and Abell 1664. The Reflection Grating Spectrometer reveals a radiative cooling rate of $180\pm 40\, \rm M_{\odot}\rm\,yr^{-1}$ and $34\pm 6\, \rm M_{\odot}\rm\,yr^{-1}$ in RXCJ1504.1-0248 and Abell 1664 for gas above 0.7 keV, respectively. These cooling rates are higher than the star formation rates observed in the clusters, and support simultaneous star formation and molecular gas mass growth on a timescale of 3$\times 10^8$ yr or longer. At these rates, the energy of the X-ray cooling gas is inadequate to power the observed UV/optical line-emitting nebulae, which suggests additional strong heating. No significant residual cooling is detected below 0.7 keV in RXCJ1504.1-0248. By simultaneously fitting the first and second order spectra, we place an upper limit on turbulent velocity of 300 km$\rm s^{-1}$ at 90 per cent confidence level for the soft X-ray emitting gas in both clusters. The turbulent energy density is considered to be less than 8.9 and 27 per cent of the thermal energy density in RXCJ1504.1-0248 and Abell 1664, respectively. This means it is insufficient for AGN heating to fully propagate throughout the cool core via turbulence. We find the cool X-ray component of Abell 1664 ($\sim$0.8 keV) is blueshifted from the systemic velocity by 750$^{+800}_{-280}$ km$\rm s^{-1}$. This is consistent with one component of the molecular gas in the core and suggests a similar dynamical structure for the two phases. We find that an intrinsic absorption model allows the cooling rate to increase to $520\pm 30\, \rm M_{\odot}\rm\,yr^{-1}$ in RXCJ1504.1-0248.

astro-ph.HE↗

arXiv2020

Astrophysical limits on very light axion-like particles from Chandra grating spectroscopy of NGC 1275

Axions/axion-like particles (ALPs) are a well motivated extension of the Standard Model and are generic within String Theory. The X-ray transparency of the intracluster medium (ICM) in galaxy clusters is a powerful probe of light ALPs (with mass $<10^{-11}\,{\rm eV}$); as X-ray photons from an embedded or background source propagate through the magnetized ICM, they may undergo energy-dependent quantum mechanical conversion into ALPs (and vice versa), imprinting distortions on the X-ray spectrum. We present Chandra data for the active galactic nucleus NGC1275 at the center of the Perseus cluster. Employing a 490ks High-Energy Transmission Gratings (HETG) exposure, we obtain a high-quality 1-9keV spectrum free from photon pileup and ICM contamination. Apart from iron-band features, the spectrum is described by a power-law continuum, with any spectral distortions at the $<3\%$ level. We compute photon survival probabilities as a function of ALP mass $m_a$ and ALP-photon coupling constant $g_{aγ}$ for an ensemble of ICM magnetic field models, and then use the NGC1275 spectrum to constraint the $(m_a, g_{aγ})$-plane. Marginalizing over magnetic field realizations, the 99.7% credible region limits the ALP-photon coupling to $g_{aγ}<6-8\times 10^{-13}\, {\rm GeV}^{-1}$ (depending upon magnetic field model) for masses $m_a<1\times 10^{-12}\,{\rm eV}$. These are the most stringent limit to date on $g_{aγ}$ for these light ALPs, and have already reached the sensitivity limits of next-generation helioscopes and light-shining-through-wall experiments. We highlight the potential of these studies with the next-generation X-ray observatories Athena and Lynx, but note the critical importance of advances in relative calibration of these future X-ray spectrometers.

hep-ph↗

arXiv2019

The Advanced X-ray Imaging Satellite

Much of the baryonic matter in the Universe, including the most active and luminous sources, are best studied in the X-ray band. Key advances in X-ray optics and detectors have paved the way for the Advanced X-ray Imaging Satellite (AXIS), a Probe-class mission that is a major improvement over Chandra, which has generated a steady stream of important discoveries for the past 2 decades. AXIS can be launched in the late 2020s and will transform our understanding in several major areas of astrophysics, including the growth and fueling of supermassive black holes, galaxy formation and evolution, the microphysics of cosmic plasmas, the time-variable universe, and a wide variety of cutting-edge studies. Relative to Chandra, the AXIS PSF is nearly twice as sharp on-axis; its field of view for subarcsecond imaging 70 times larger by area; its effective area at 1 keV is 10 times larger. The low-Earth orbit ensures a low and stable detector background, resulting in 50 times greater sensitivity than Chandra for extended sources. AXIS has a rapid repointing response with operations similar to Swift, but is 100 times more sensitive for time-domain science. These capabilities open up a vast discovery space and complement the next generation of astronomical observatories. A high-spectral-resolution mission (Athena) operating at the same time as a high-angular-resolution mission (AXIS) greatly increases the range of scientific discovery. AXIS will use lightweight X-ray optics made of thin single-crystal silicon mirrors developed at NASA Goddard. The detector array builds on a long legacy of X-ray CCD and provides improved photon localization, much faster readout time, and broader energy band. The estimated mission costs are consistent with the $1B Probe mission cost guideline.

astro-ph.HE↗

arXiv2019

Searching for cool and cooling X-ray emitting gas in 45 galaxy clusters and groups

We present a spectral analysis of cool and cooling gas in 45 cool-core clusters and groups of galaxies obtained from Reflection Grating Spectrometer (RGS) XMM-$Newton$ observations. The high-resolution spectra show FeXVII emission in many clusters, which implies the existence of cooling flows. The cooling rates are measured between the bulk Intracluster Medium (ICM) temperature and 0.01 keV and are typically weak, operating at less than a few tens of $\rm M_{\odot}\rm yr^{-1}$ in clusters, and less than 1 $\rm M_{\odot}\rm yr^{-1}$ in groups of galaxies. They are 10-30% of the classical cooling rates in the absence of heating, which suggests that AGN feedback has a high level of efficiency. If cooling flows terminate at 0.7 keV in clusters, the associated cooling rates are higher, and have a typical value of a few to a few tens of $\rm M_{\odot}\rm yr^{-1}$. Since the soft X-ray emitting region, where the temperature $kT<1$ keV, is spatially associated with H$α$ nebulosity, we examine the relation between the cooling rates above 0.7 keV and the H$α$ nebulae. We find that the cooling rates have enough energy to power the total UV-optical luminosities, and are 5 to 50 times higher than the observed star formation rates for low luminosity objects. In 4 high luminosity clusters, the cooling rates above 0.7 keV are not sufficient and an inflow at a higher temperature is required. Further residual cooling below 0.7 keV indicates very low complete cooling rates in most clusters.

astro-ph.HE↗

arXiv2018

Revealing a Highly-Dynamic Cluster Core in Abell 1664 with Chandra

We present new, deep (245 ks) Chandra observations of the galaxy cluster Abell 1664 ($z = 0.1283$). These images reveal rich structure, including elongation and accompanying compressions of the X-ray isophotes in the NE-SW direction, suggesting that the hot gas is sloshing in the gravitational potential. This sloshing has resulted in cold fronts, at distances of 55, 115 and 320 kpc from the cluster center. Our results indicate that the core of A1664 is highly disturbed, as the global metallicity and cooling time flatten at small radii, implying mixing on large scales. The central AGN appears to have recently undergone a mechanical outburst, as evidenced by our detection of cavities. These cavities are the X-ray manifestations of radio bubbles inflated by the AGN, and may explain the motion of cold molecular CO clouds previously observed with ALMA. The estimated mechanical power of the AGN, using the minimum energy required to inflate the cavities as a proxy, is $P_{\rm cav} = (1.1 \pm 1.0) \times 10^{44} $ erg s$^{-1}$, which may be enough to drive the molecular gas flows, and offset the cooling luminosity of the ICM, at $L_{\rm cool} = (1.90 \pm0.01)\times 10^{44}$ erg s$^{-1}$. This mechanical power is orders of magnitude higher than the measured upper limit on the X-ray luminosity of the central AGN, suggesting that its black hole may be extremely massive and/or radiatively inefficient. We map temperature variations on the same spatial scale as the molecular gas, and find that the most rapidly cooling gas is mostly coincident with the molecular gas reservoir centered on the BCG's systemic velocity observed with ALMA and may be fueling cold accretion onto the central black hole.

astro-ph.CO↗

arXiv2018

AGN feedback in the Phoenix cluster

Active galactic nuclei (AGN) release a huge amount of energy into the intracluster medium (ICM) with the consequence of offsetting cooling and star formation (AGN feedback) in the centers of cool core clusters. The Phoenix cluster is among the most massive clusters of galaxies known in the Universe. It hosts a powerful starburst of several hundreds of Solar masses per year and a large amount of molecular gas in the center. In this work we use the high-resolution Reflection Grating Spectrometer (RGS) on board XMM-Newton to study the X-ray emitting cool gas in the Phoenix cluster and heating-cooling balance. We detect for the first time evidence of O VIII and Fe XXI-XXII emission lines, the latter demonstrating the presence of gas below 2 keV. We find a cooling rate of 350 (-200,+250) Msun/year below 2 keV (at the 90% confidence level), which is consistent with the star formation rate in this object. This cooling rate is high enough to produce the molecular gas found in the filaments via instabilities during the buoyant rising time. The line broadening indicates that the turbulence (~ 300 km/s or less) is below the level required to produce and propagate the heat throughout the cool core. This provides a natural explanation to the coexistence of large amounts of cool gas, star formation and a powerful AGN in the core. The AGN activity may be either at a young stage or in a different feedback mode, due to a high accretion rate.

astro-ph.HE↗

arXiv2018

A Galaxy-Scale Fountain of Cold Molecular Gas Pumped by a Black Hole

We present ALMA and MUSE observations of the Brightest Cluster Galaxy in Abell 2597, a nearby (z=0.0821) cool core cluster of galaxies. The data map the kinematics of a three billion solar mass filamentary nebula that spans the innermost 30 kpc of the galaxy's core. Its warm ionized and cold molecular components are both cospatial and comoving, consistent with the hypothesis that the optical nebula traces the warm envelopes of many cold molecular clouds that drift in the velocity field of the hot X-ray atmosphere. The clouds are not in dynamical equilibrium, and instead show evidence for inflow toward the central supermassive black hole, outflow along the jets it launches, and uplift by the buoyant hot bubbles those jets inflate. The entire scenario is therefore consistent with a galaxy-spanning "fountain", wherein cold gas clouds drain into the black hole accretion reservoir, powering jets and bubbles that uplift a cooling plume of low-entropy multiphase gas, which may stimulate additional cooling and accretion as part of a self-regulating feedback loop. All velocities are below the escape speed from the galaxy, and so these clouds should rain back toward the galaxy center from which they came, keeping the fountain long-lived. The data are consistent with major predictions of chaotic cold accretion, precipitation, and stimulated feedback models, and may trace processes fundamental to galaxy evolution at effectively all mass scales.

astro-ph.GA↗

arXiv2017

A New Bound on Axion-Like Particles

Axion-like particles (ALPs) and photons can quantum mechanically interconvert when propagating through magnetic fields, and ALP-photon conversion may induce oscillatory features in the spectra of astrophysical sources. We use deep (370 ks), short frame time Chandra observations of the bright nucleus at the centre of the radio galaxy M87 in the Virgo cluster to search for signatures of light ALPs. The absence of substantial irregularities in the X-ray power-law spectrum leads to a new upper limit on the photon-ALP coupling, $g_{aγ}$: using a conservative model of the cluster magnetic field consistent with Faraday rotation measurements from M87 and M84, we find $g_{a γ} < 2.6\times10^{-12}$ GeV$^{-1}$ at 95% confidence level for ALP masses $m_a \leq 10^{-13}$ eV. Other consistent magnetic field models lead to stronger limits of $g_{a γ} \lesssim 1.1$--$1.5 \times 10^{-12}$ GeV$^{-1}$. These bounds are all stronger than the limit inferred from the absence of a gamma-ray burst from SN1987A, and rule out a substantial fraction of the parameter space accessible to future experiments such as ALPS-II and IAXO.

hep-ph↗

arXiv2016

The Mass Distribution of the Unusual Merging Cluster Abell 2146 from Strong Lensing

Abell 2146 consists of two galaxy clusters that have recently collided close to the plane of the sky, and it is unique in showing two large shocks on $\textit{Chandra X-ray Observatory}$ images. With an early stage merger, shortly after first core passage, one would expect the cluster galaxies and the dark matter to be leading the X-ray emitting plasma. In this regard, the cluster Abell 2146-A is very unusual in that the X-ray cool core appears to lead, rather than lag, the Brightest Cluster Galaxy (BCG) in their trajectories. Here we present a strong lensing analysis of multiple image systems identified on $\textit{Hubble Space Telescope}$ images. In particular, we focus on the distribution of mass in Abell 2146-A in order to determine the centroid of the dark matter halo. We use object colours and morphologies to identify multiple image systems; very conservatively, four of these systems are used as constraints on a lens mass model. We find that the centroid of the dark matter halo, constrained using the strongly lensed features, is coincident with the BCG, with an offset of $\approx$ 2 kpc between the centres of the dark matter halo and the BCG. Thus from the strong lensing model, the X-ray cool core also leads the centroid of the dark matter in Abell 2146-A, with an offset of $\approx$ 30 kpc.

astro-ph.CO↗

arXiv2016

The Distribution of Dark and Luminous Matter in the Unique Galaxy Cluster Merger Abell 2146

Abell 2146 ($z$ = 0.232) consists of two galaxy clusters undergoing a major merger. The system was discovered in previous work, where two large shock fronts were detected using the $\textit{Chandra X-ray Observatory}$, consistent with a merger close to the plane of the sky, caught soon after first core passage. A weak gravitational lensing analysis of the total gravitating mass in the system, using the distorted shapes of distant galaxies seen with ACS-WFC on $\textit{Hubble Space Telescope}$, is presented. The highest peak in the reconstruction of the projected mass is centred on the Brightest Cluster Galaxy (BCG) in Abell 2146-A. The mass associated with Abell 2146-B is more extended. Bootstrapped noise mass reconstructions show the mass peak in Abell 2146-A to be consistently centred on the BCG. Previous work showed that BCG-A appears to lag behind an X-ray cool core; although the peak of the mass reconstruction is centred on the BCG, it is also consistent with the X-ray peak given the resolution of the weak lensing mass map. The best-fit mass model with two components centred on the BCGs yields $M_{200}$ = 1.1$^{+0.3}_{-0.4}$$\times$10$^{15}$M$_{\odot}$ and 3$^{+1}_{-2}$$\times$10$^{14}$M$_{\odot}$ for Abell 2146-A and Abell 2146-B respectively, assuming a mass concentration parameter of $c=3.5$ for each cluster. From the weak lensing analysis, Abell 2146-A is the primary halo component, and the origin of the apparent discrepancy with the X-ray analysis where Abell 2146-B is the primary halo is being assessed using simulations of the merger.

astro-ph.CO↗