arXiv subjects

Emily M. Silich

Publications and source records attributed to Emily M. Silich.

8 recordsLinked to original sources

arXiv2026

Measuring the Temperature of Extremely Hot Shock-Heated Gas in the Major Merger MACS J0717.5+3745 With Relativistic Corrections to the Sunyaev-Zel'dovich Effect

The conversion of gravitational potential to kinetic energy results in an intracluster medium (ICM) gas with a characteristic temperature near 10 keV in the most massive galaxy clusters. X-ray observations, primarily from Chandra and XMM-Newton, have revealed a wealth of information about the thermodynamics of this gas. However, two regimes remain difficult to study with current instruments: superheated gas well above 10 keV generated by shocks from major mergers, and distant systems strongly impacted by cosmological dimming. Relativistic corrections to the Sunyaev-Zel'dovich effect (rSZe) produce a fractional spectral distortion in the cosmic microwave background at submillimeter and millimeter wavelengths that could offer a complementary probe of both high-temperature and high-redshift ICM gas. Here we describe multiband measurements of the rSZe, including observations from the Fourier Transform Spectrometer on the Herschel-SPIRE instrument, that constrain the ICM thermodynamics of the major merger MACS J0717.5+3745. Within the seven observed lines of sight, we find an average temperature of $T_{\mathrm{rSZe}}=15.1^{+3.8}_{-3.3}$ keV which is consistent with the values obtained from X-ray measurements of the same regions, with $T_{\mathrm{Chandra}}=18.0^{+1.1}_{-1.1}$ keV and $T_{\mathrm{XMM}}=13.9^{+0.9}_{-0.9}$ keV. This work demonstrates that the rSZe signal can be detected with moderate spectral resolution submillimeter data, while also establishing the utility of such measurements for probing superheated regions of the ICM.

astro-ph.GA↗

arXiv2026

An Inverse-Compton-Boosted Cool Core Unifies Perseus's Radio and X-ray Halos

Perseus is the brightest X-ray strong cool-core (SCC) cluster, with a bright central radio and $γ$-ray source plus low-frequency radio mini and giant halos. It is the archetype of the cooling flow (CF) problem, with X-rays implying mass cooling rates orders-of-magnitude larger than observed in other channels. Recent work suggested that ancient ($\gtrsim$\,Gyr-old) cosmic ray (CR) halos (ACRHs), injected by the central source, would produce thermal-like soft X-ray inverse-Compton (CR-IC) emission 'boosting' the CC and alleviating the CF problem. We examine Perseus and show that a simple model of CRs injected by NGC 1275 (+satellites) simultaneously accounts for the excess CF luminosity and minihalo. The models reproduce Perseus's soft X-ray surface brightness and X-ray inferred density/temperature/pressure/metallicity/cooling time/mass deposition rates; $γ$-ray spectra; extended hard X-rays; and radio surface brightness and spectral index data, from kpc-Mpc. These also reproduce independent constraints on magnetic field strengths and mass/potential models. The evolution of the minihalo spectral index and surface brightness are predicted by an aging population of CRs boosting the apparent SCC luminosity via CR-IC, and match well the observed hard X-ray slopes. The 'giant' low-frequency halo can be predicted by the sum of ACRHs around satellites distributed throughout the cluster, dominating diffuse synchrotron at $\gtrsim 100\,$kpc. Re-acceleration is neither needed nor important in these models, and implied CR transport speeds are consistent with buoyant advection. Previous claims of upper limits to non-thermal X-rays and CR pressure relied on strong assumptions which are not valid at the CR energies of interest, e.g. a power-law spectrum of CRs. This could resolve many historical puzzles about Perseus, and makes new predictions for future observations.

astro-ph.HE↗

arXiv2026

Strong Evidence for Cosmic Ray-Supported $\sim$L$^{\ast}$ Galaxy Halos via X-ray \& tSZ Constraints

Many state-of-the-art galaxy simulations featuring traditional feedback modes have significant challenges producing enough extended soft X-ray ($\sim 0.5-2$ keV) emission at R $\sim 0.5-1$ R$_{\rm vir}$ observed around galaxies with stellar masses M$_{\rm \ast} \lesssim 10^{11} \rm M_\odot$, without violating galaxy mass function constraints. Moreover, thermal Sunyaev-Zel'dovich (tSZ) measurements probing the thermal pressure of similar galaxies indicate it is orders-of-magnitude lower than predictions from simple halo hydrodynamics and many hydrodynamical simulations. We demonstrate that these constraints can be met congruously with a large non-thermal pressure contribution in the form of cosmic rays (CRs) from SNe and/or AGN, which lowers the tSZ signal while CR leptons produce plentiful soft X-rays via inverse Compton scattering of the CMB. The combination of these two observations is far more constraining on the pressure budget of galactic halos than either alone -- if these novel tSZ and X-ray observations are borne out by future studies, then taken together they reveal \textit{the strongest evidence for CR support in halos to date}. Conversely, it is very difficult to produce the extended X-rays via traditional thermal emission without increasing the overall thermal pressure and thus tSZ signal in tandem, making these tensions even worse. Finally, tSZ \& X-rays together unlock a novel observational method to constrain halo CR pressure relative to thermal pressure, with implications for CR transport parameters and AGN feedback energetics across various galaxy mass scales. Taking the currently observed constraints at M$_{\rm halo} \sim 10^{\rm 12} \rm M_\odot$ imply the halo CR pressure must at least be equal to the gas thermal pressure.

astro-ph.GA↗

arXiv2025

Cosmic Rays Masquerading as Cool Cores: An Inverse-Compton Origin for Cool Core Cluster Emission

X-ray bright cool-core (CC) clusters contain luminous radio sources accelerating cosmic ray (CR) leptons at prodigious rates. Near the acceleration region, high-energy leptons produce synchrotron (mini)halos and sometimes observable gamma rays, but these leptons have short lifetimes and so cannot propagate far from sources without some rejuvenation. However, low-energy (~0.1-1 GeV) CRs should survive for >Gyr, potentially reaching ~100 kpc before losing energy via inverse-Compton (IC) scattering of CMB photons to keV X-ray energies, with remarkably thermal X-ray spectra. In groups/clusters, this will appear similar to relatively 'cool' gas in cluster cores (i.e. CCs). In lower-mass (e.g. Milky Way/M31) halos, analogous CR IC emission will appear as hot (super-virial) gas at outer CGM radii, explaining recent diffuse X-ray observations. We show that for plausible (radio/gamma-ray observed) lepton injection rates, the CR-IC emission could contribute significantly to the X-ray surface brightness (SB) in CCs, implying that CC gas densities may have been overestimated and alleviating the cooling flow problem. A significant IC contribution to diffuse X-ray emission in CC clusters also explains the tight correlation between the X-ray 'cooling luminosity' and AGN/cavity/jet power, because the apparent CC emission is itself driven by the radio source. Comparing observed Sunyaev Zeldovich to X-ray inferred pressures at $\ll 100$ kpc in CCs represents a clean test of this scenario, and existing data appears to favor significant CR-IC. A significant IC contribution also implies that X-ray inferred gas-phase metallicities have been underestimated in CCs, potentially explaining the discrepancy between X-ray (sub-Solar) and optical/UV (super-Solar) observed metallicities in the central ~10 kpc of nearby CCs. We also discuss the model's connection to observations of multiphase gas in clusters.

astro-ph.HE↗

arXiv2025

Exploring a cosmic ray inverse-Compton origin to the SZ-to-X-ray pressure deficit in the cool core cluster ZwCl 3146

We explore the possibility that inverse-Compton (IC) scattering of cosmic microwave background photons by $\sim$GeV cosmic rays (CRs) injected by the central active galactic nucleus (AGN) in cool core (CC) clusters produces a non-negligible continuum-like X-ray signal that is easily misinterpreted as intracluster medium (ICM) thermal bremsstrahlung continuum. This is particularly relevant to the cooling flow problem--the lack of star formation relative to X-ray-inferred ICM cooling rates. Using ZwCl 3146, a relaxed CC system at $z = 0.291$, we compare pressure profiles derived via X-rays and the thermal Sunyaev-Zel'dovich (SZ) effect. While SZ measurements probe only thermal ICM electrons, additional CR-IC emission would appear to boost the X-ray-inferred pressure. Relative to unity, we measure a $\simeq30\%$ decrement in $P_{SZ}/P_X$ within 100 kpc of the ZwCl 3146 center at a statistical significance of $\simeq 3.3σ$, consistent with predicted deficits from CR-IC contamination in reasonable models of central AGN-driven CR injection. X-ray spectral fits of a two-component model with thermal ICM and CR-IC emission are consistent with CR-IC as the cause of this deficit. We test alternative explanations and systematics that could drive such a decrement, with the leading order systematics associated with halo triaxiality. Collectively, these systematics are unlikely to produce a $P_{SZ}/P_X$ decrement $\gtrsim10\%$. While our results establish that non-negligible CR-IC emission is plausible in ZwCl 3146, we stress that more detailed studies of larger cluster samples are required to robustly assess whether CR-IC is relevant to the cooling flow problem.

astro-ph.HE↗

arXiv2025

X-ray emission signatures of galactic feedback in the hot circumgalactic medium: predictions from cosmological hydrodynamical simulations

Little is currently known about the physical properties of the hot circumgalactic medium (CGM) surrounding massive galaxies. Next-generation X-ray observatories will enable detailed studies of the hot CGM in emission. To support these future efforts, we make predictions of the X-ray emission from the hot CGM using a sample of 28 $\sim$Milky Way-mass disk galaxies at $z=0$ from seven cosmological hydrodynamical simulation suites incorporating a wide range of galactic feedback prescriptions. The X-ray surface brightness (XSB) morphology of the hot CGM varies significantly across simulations. XSB-enhanced outflows and bubble-like structures are predicted in many galaxies simulated with AGN feedback and in some stellar-feedback-only galaxies, while other galaxies exhibit more isotropic XSB distributions at varying brightnesses. Galaxies simulated without cosmic ray physics exhibit radial XSB profiles with similar shapes ($\propto r^{-3}$ within $20-200$ kpc), with scatter about this slope likely due to underlying feedback physics. The hot CGM kinematics also differ substantially: velocity maps reveal signatures of bulk CGM rotation and high-velocity biconical outflows, particularly in simulations incorporating AGN feedback. Some stellar-feedback-only models also generate similar AGN-like outflows, which we postulate is due to centrally-concentrated star formation. Simulations featuring AGN feedback frequently produce extended temperature enhancements in large-scale galactic outflows, while simulations incorporating cosmic ray physics predict the coolest CGM due to pressure support being provided by cosmic rays rather than hot CGM. Individually-resolved X-ray emission lines further distinguish hot CGM phases, with lower-energy lines (e.g., O VII) largely tracing volume-filling gas, and higher-energy lines (e.g., Fe XVII) highlighting high-velocity feedback-driven outflows.

astro-ph.GA↗

arXiv2024

ICM-SHOX. Paper I: Methodology overview and discovery of a gas--dark matter velocity decoupling in the MACS J0018.5+1626 merger

Galaxy cluster mergers are rich sources of information to test cluster astrophysics and cosmology. However, cluster mergers produce complex projected signals that are difficult to interpret physically from individual observational probes. Multi-probe constraints on the gas and dark matter cluster components are necessary to infer merger parameters that are otherwise degenerate. We present ICM-SHOX (Improved Constraints on Mergers with SZ, Hydrodynamical simulations, Optical, and X-ray), a systematic framework to jointly infer multiple merger parameters quantitatively via a pipeline that directly compares a novel combination of multi-probe observables to mock observables derived from hydrodynamical simulations. We report a first application of the ICM-SHOX pipeline to MACS J0018.5+1626, wherein we systematically examine simulated snapshots characterized by a wide range of initial parameters to constrain the MACS J0018.5+1626 merger geometry. We constrain the epoch of MACS J0018.5+1626 to the range $0$--$60$ Myr post-pericenter passage, and the viewing angle is inclined $\approx 27$--$40$ degrees from the merger axis. We obtain constraints for the impact parameter ($\lesssim 250$ kpc), mass ratio ($\approx 1.5$--$3.0$), and initial relative velocity when the clusters are separated by 3 Mpc ($\approx 1700$--3000 km s$^{-1}$). The primary and secondary clusters initially (at 3 Mpc) have gas distributions that are moderately and strongly disturbed, respectively. We discover a velocity space decoupling of the dark matter and gas distributions in MACS J0018.5+1626, traced by cluster-member galaxy velocities and the kinematic Sunyaev-Zel'dovich effect, respectively. Our simulations indicate this decoupling is dependent on the different collisional properties of the two distributions for particular merger epochs, geometries, and viewing angles.

astro-ph.CO↗

arXiv2024

Improved Constraints on Mergers with SZ, Hydrodynamical simulations, Optical, and X-ray (ICM-SHOX). Paper II: Galaxy cluster sample overview

Galaxy cluster mergers are representative of a wide range of physics, making them an excellent probe of the properties of dark matter and the ionized plasma of the intracluster medium. To date, most studies have focused on mergers occurring in the plane of the sky, where morphological features can be readily identified. To allow study of mergers with arbitrary orientation, we have assembled multi-probe data for the eight-cluster ICM-SHOX sample sensitive to both morphology and line of sight velocity. The first ICM-SHOX paper (Silich+2023) provided an overview of our methodology applied to one member of the sample, MACS J0018.5+1626, in order to constrain its merger geometry. That work resulted in an exciting new discovery of a velocity space decoupling of its gas and dark matter distributions. In this work, we describe the availability and quality of multi-probe data for the full ICM-SHOX galaxy cluster sample. These datasets will form the observational basis of an upcoming full ICM-SHOX galaxy cluster sample analysis.

astro-ph.CO↗