arXiv subjects

Isabel S. Sands

Publications and source records attributed to Isabel S. Sands.

8 recordsLinked to original sources

arXiv2026

The Galactic Center Gamma-Ray Excess and the Look-Elsewhere Effect

The origin of the excess of GeV gamma-rays in the Milky Way's galactic center observed by Fermi-LAT has been an open question for nearly two decades. The two leading interpretations of this signal are that it is produced either by annihilating dark matter (DM), or arises from unresolved point sources, such as a novel population of millisecond pulsars. In both scenarios, uncertainties and limitations in the modeling of galactic diffuse gamma-ray emission, such as the steady-state approximation of cosmic ray (CR) propagation and assumption of an axisymmetric distribution of CRs, complicate the interpretation of the galactic center excess (GCE). Simulations with full magnetohydrodynamics (MHD) and CR transport can provide a new avenue for understanding the astrophysical phenomena that drive galactic diffuse gamma-ray emission. We utilize MHD simulations of Milky Way analogs with time-dependent, spectrally-resolved CR transport to reconstruct diffuse gamma-ray emission. Comparing instantaneous emission to a time-averaged baseline, we find residuals up to several times larger than the Fermi GCE, with a wide range of spectral shapes and spatial morphologies. At some simulation snapshots, the spectrum and morphology of the residuals resemble the Fermi GCE. Our results suggest that the Fermi GCE may be a manifestation of the look-elsewhere effect- a transient fluctuation in diffuse emission from stellar sources, observed at an inopportune moment- and motivate the development of new Fermi-LAT analysis templates that account for time-dependent CR transport.

astro-ph.HE↗

arXiv2026

The Virgo Cluster as a Case Study for Ancient Cosmic Ray Halos in Radio and X-rays

Virgo is the nearest cool core (CC) galaxy cluster, and is archetypal of the class of X-ray weak CCs. Despite the X-ray continuum implying a large cooling rate ($\gtrsim 10\,{\rm M_{\odot}\,yr^{-1}}$), measurements of cooler gas phases, dust, and star formation show almost no cooling outside the M87 nucleus. It is also known that M87$^{\ast}$ is a prodigious source of cosmic ray (CR) leptons observed in radio and $γ$-rays, and recent radio observations have shown that the M87 radio halo is low-frequency and its spectral index steepens rapidly away from the injection region+jets, consistent with an aging, low-energy CR population. We demonstrate that coupling X-ray and radio to newer Faraday rotation limits on the magnetic field, this implies the CRs within the CC radius ($\lesssim 40$kpc) must contribute significantly to the total pressure and soft X-ray emission, through a combination of Coulomb heating the gas and inverse-Compton (IC) scattering microwave background photons to $\sim$keV. We show this can explain the apparent correspondence between jet/radio power and X-ray cooling luminosity, and why the X-ray inferred cooling rate appears much larger than otherwise observed. This would also explain why the central X-ray inferred metallicity at $\lesssim 10\,$kpc appears to be suppressed relative to the UV/optical inferred metallicity (in a wavelength-dependent manner). Compared to more extreme radio+X-ray sources like Perseus, we find the observations require a softer 'escaping' CR spectrum (escaping or accelerated outside $\gtrsim 1\,$kpc from M87$^{\ast}$).

astro-ph.HE↗

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↗

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

Fermi Bubbles Without AGN: Gamma-Ray Bubbles in MHD Galaxy Formation Simulations with Full Cosmic Ray Spectra

For the first time, we show in MHD simulations with cosmological initial conditions that bi-lobed gamma-ray outflows similar to the Fermi bubbles can form from star formation and supernova feedback, without involvement from active galactic nuclei (AGN). We use simulations run with full MHD and dynamical, on-the-fly multi-species cosmic ray transport in MeV-TeV energy bins to model gamma-ray emission in Milky Way-mass spiral galaxies from neutral pion decay, relativistic non-thermal Bremsstrahlung, and inverse Compton scattering. We find that these gamma-ray outflows are present in all three Milky-Way mass simulated galaxies. The amplitude, shape, and the composition of the gamma-ray spectrum of these bubbles fluctuates over time, with lepton-dominated and hadron-dominated phases. Spectra in which there is O(1) more gamma-ray flux from inverse Compton scattering than neutral pion decay are a good fit to the measured Fermi-LAT spectrum. Additionally, these simulations predict multi-wavelength features in soft x-rays and synchrotron radio, potentially providing new observational signatures that can connect the circumgalactic medium to cosmic ray physics and activity in the galactic center.

astro-ph.HE↗

arXiv2025

Galactic Center Gamma-Ray Emission in MHD Galaxy Formation Simulations with Full Cosmic Ray Spectra

The Milky Way's galactic center is a highly dynamical, crowded environment. Gamma ray observations of this region, such as the excess of GeV scale gamma rays observed by Fermi LAT, have been of tremendous interest to both the high energy astrophysics and particle physics communities. However, nearly all past studies of gamma ray emission make simplifying assumptions about cosmic ray (CR) propagation that may not be valid in the galactic center. Recent numerical breakthroughs now enable fully time dependent dynamical evolution of CRs in magnetohydrodynamic simulations with resolved, multi phase small scale structure in the interstellar medium (ISM), allowing self consistent comparisons to the Milky Way cosmic ray spectrum. We model diffuse gamma ray emission from cosmic ray interactions for a set of Feedback in Realistic Environments (FIRE) simulations of Milky Way mass galaxies run with spectrally resolved cosmic ray spectra for multiple species at MeV to TeV energies. We find that the galactic center gamma ray spectrum can vary by order of magnitude amounts in normalization, and by approx. 10 percent in spectral slope at high energies, driven by both injection from highly variable star formation and losses from variable structure in the turbulent ISM. Gamma ray emission from inverse Compton scattering and relativistic nonthermal Bremsstrahlung is particularly variable on Myr timescales. We argue that features of the observed Milky Way gamma ray spectrum may arise from such transient phenomena in gamma rays produced from CR interactions.

astro-ph.HE↗

arXiv2025

Diverse dark matter profiles in FIRE dwarfs: black holes, cosmic rays and the cusp-core enigma

Dwarf galaxies have historically posed challenges to the cold dark matter (CDM) model and, while many of the so-called 'dwarf galaxy problems' have been mitigated by incorporating baryonic processes, the observed diversity of dwarf galaxy rotation curves remains a contentious topic. Meanwhile, the growing observational samples of active galactic nuclei (AGN) in dwarf galaxies have prompted a paradigm shift in our understanding of dwarf galaxy evolution, traditionally thought to be regulated by stellar feedback. In this study, we explore the potential role of AGN feedback in shaping dark matter distributions and increasing the diversity of dwarf galaxy rotation curves, using a new suite of cosmological zoom-in simulations of dwarf galaxies with the FIRE-3 model. Our findings indicate that the presence of active black holes (BHs) in dwarf galaxies can lead to diverse outcomes, ranging from cuspier to more core-like profiles. This variability arises from the dual role of BHs in providing additional feedback and regulating the extent of stellar feedback. Consistent with previous research, we find that AGN feedback is most impactful when cosmic ray (CR) modelling is included, with CRs from any source significantly influencing dark matter profiles. Overall, our results highlight that the interplay between stellar feedback, BHs, and CRs produces a broad spectrum of dark matter density profiles, which align with observed correlations between rotation curve shapes and baryonic dominance. This underscores the importance of including the full range of baryonic processes in dwarf galaxy simulations to address the persistent 'small-scale challenges' to the CDM paradigm.

astro-ph.GA↗

arXiv2024

Confronting the Diversity Problem: The Limits of Galaxy Rotation Curves as a tool to Understand Dark Matter Profiles

While galaxy rotation curves provide one of the most powerful methods for measuring dark matter profiles in the inner regions of rotation-supported galaxies, at the dwarf scale there are factors that can complicate this analysis. Given the expectation of a universal profile in dark matter-only simulations, the diversity of observed rotation curves has become an often-discussed issue in Lambda Cold Dark Matter cosmology on galactic scales. We analyze a suite of Feedback in Realistic Environments (FIRE) simulations of $10^{10}-10^{12}$ $M_\odot$ halos with standard cold dark matter, and compare the true circular velocity to rotation curve reconstructions. We find that, for galaxies with well-ordered gaseous disks, the measured rotation curve may deviate from true circular velocity by at most 10% within the radius of the disk. However, non-equilibrium behavior, non-circular motions, and non-thermal and non-kinetic stresses may cause much larger discrepancies of 50% or more. Most rotation curve reconstructions underestimate the true circular velocity, while some reconstructions transiently over-estimate it in the central few kiloparsecs due to dynamical phenomena. We further demonstrate that the features that contribute to these failures are not always visibly obvious in HI observations. If such dwarf galaxies are included in galaxy catalogs, they may give rise to the appearance of "artificial" rotation curve diversity that does not reflect the true variation in underlying dark matter profiles.

astro-ph.GA↗