arXiv subjects

Annika H. G. Peter

Publications and source records attributed to Annika H. G. Peter.

At least 19 recordsLinked to original sources

arXiv2026

The Merian Survey: A Statistical Census of Bright Satellites of Sub-Milky Way-Mass Galaxies

Satellite populations provide sensitive tests of the galaxy-halo connection, but studies have largely focused on Milky Way (MW)-mass host galaxies or a small number of nearby dwarf galaxy hosts. Here we present a statistical census of bright, star-forming satellite galaxies around sub-MW-mass hosts using the first data release of the Merian Survey. Our sample consists of 357 hosts with stellar mass $10^{9.5} 3$. Satellite abundance increases with host stellar mass across the sub-MW regime, extending the observed trend from dwarf to MW-mass galaxies and broadly agreeing with predictions from the TNG50 simulation. The radial distribution of satellites around sub-MW-mass hosts is similar to that around MW-mass hosts and broadly consistent with TNG50. Together, these results extend our empirical view of satellite populations across the previously sparsely sampled transition between dwarf and MW-mass hosts.

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arXiv2026

Galaxies Below the Fundamental Metallicity Relation Are Morphologically Disturbed: Merian H$α$ Morphologies and DESI Metallicities

Empirical results have revealed a connection between a galaxy's metallicity, mass, and star formation rate, known as the fundamental metallicity relation (FMR), in which metallicity increases with stellar mass and decreases with SFR. Physical interpretations of the FMR suggest that actively star-forming galaxies are fueled by pristine gas, likely obtained via IGM accretion or minor mergers. Here we demonstrate the value of galaxy morphology as an additional axis in the FMR. Using medium-band imaging from the Merian Survey, we construct spatially resolved maps of stellar continuum and H$α$ emission for a large sample of galaxies with DESI DR1 spectra, reaching stellar masses down to $10^8$ M$_\odot$. We measure non-parametric morphological parameters from the emission maps and derive gas-phase oxygen abundances from the spectra using strong-line calibrations. We fit a series of simple linear models to explore correlations among the metallicity, stellar mass, sSFR, and morphological parameters. We find that galaxies that are more metal-poor than predicted by the FMR appear particularly disturbed. At fixed stellar mass and sSFR, metallicity is inversely correlated with the asymmetry of both the continuum and H$α$ morphology. The observed trends -- and the existence of particularly metal-poor, morphologically disturbed, highly star-forming galaxies -- are consistent with external burst triggering. Our results indicate that a galaxy's morphology is linked to its metallicity and star formation history, providing an additional observational probe of the physical drivers of elevated star-formation in low-mass galaxies.

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arXiv2026

Feeding the Void: Co-evolution of the SIDM-seeded Black Hole and the dark halo after core-collapse

While the gravothermal collapse of self-interacting dark matter (SIDM) halos provides a compelling mechanism for seeding supermassive black holes (BHs), the post-collapse macroscopic co-evolution of the BH and its host halo remains largely unexplored. In this paper, we introduce a novel N-body framework to dynamically model the dark matter accretion onto the central BH. By bracketing the accretion rate with two complementary phenomenological schemes---a `multi-round core-collapse' scenario and a `continuous accretion' scenario---we establish bounds on the BH mass growth trajectory. We find that the BH mass reaches $5\%-12\%$ of the total halo mass within $\lesssim2$ Gyr post-collapse. Concurrently, this sustained mass depletion drives a `gravothermal supply chain' that forces outer dark matter inward, severely depleting the inner halo density ($\lesssim 2r_s$). This unique structural imprint qualitatively matches the recently detected anomaly in the strong lens JVAS B1938+666 (detection V), where the best-fit lens model consists of an unresolved central point mass and a shallow, uniform-surface-density extended disk, offering a distinct observational signature for SIDM-seeded BHs.

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arXiv2026

Bypassed Core Formation in Milky Way-Mass SIDM Halos: Implications for the Local Group Past-Pericenter Scenario

We consider a scenario in which the Milky Way (MW) and M31 have had a previous pericentric passage, and investigate its compatibility with self-interacting dark matter (SIDM). Using initial conditions sampled from Local Group (LG) analogues in the IllustrisTNG simulation, we perform controlled re-simulations of the MW-M31 orbit, evolving the system under both standard cold dark matter (CDM) and various SIDM cross-sections. We find that the deep baryonic potential of the MW preconditions the halo's thermal structure, establishing an initial negative temperature gradient. This drives SIDM halos to bypass the standard core-formation phase and enter immediate core-collapse, resulting in monotonically increasing central densities. In full orbital simulations, the compact stellar component (disk/bulge) of the MW analog remains robust against tidal disruption for pericenter distances as close as $r_{\rm peri}\lesssim20$ kpc during an encounter at cosmic time $\sim8$ Gyr. The diffuse stellar halo is comparatively more susceptible, facing disruption for $r_{\rm peri}\lesssim100$ kpc. Our results demonstrate a dichotomy in structural evolution: the compact disk/bulge is sensitive to intrinsic SIDM thermodynamics but dynamically robust against the pericenter encounter, whereas the diffuse stellar halo is largely independent of the specific SIDM model but more vulnerable to orbital tidal disruptions.

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arXiv2026

Morphologies of SAGAbg low-mass galaxies in Legacy Survey multi-band imaging: dependence on stellar masses, star-formation rates and low-redshift evolution

The optical morphologies of low-mass galaxies can be used to directly trace their assembly and constrain models of galaxy evolution. We select a sample of 6211 low-mass ($7\lesssim {\rm log}(M_{\ast}/M_{\odot})\lesssim 10$) star-forming galaxies from the SAGAbg catalog that has high-completeness at low-redshifts ($z<0.1$). We obtain their galaxy maps in the $griz$ - bands of the Legacy Surveys and apply STATMORPH to calculate non-parametric morphological measures including the Gini index, $M_{20}$ measure, and CAS parameters. We study how resolution and signal-to-noise affect the morphology measures and find that the bulge strength measurements are the most reliable. The sequence in Gini$-M_{20}$ space is directly linked to the star-forming sequence of galaxies and is dominated by Sb/Sc/Ir morphologies. The $g$-band light distributions are the least concentrated among all the bands. The systematic trends of $M_{20}$ with respect to stellar mass and GALEX NUV-derived specific star formation rate (sSFR) strongly indicate that the galaxies with flatter light profiles are less massive with higher sSFR and vice-versa. We statistically infer that star-forming low-mass galaxies predominantly have disk morphologies with bulges becoming more prominent in quenched systems at higher masses (${\rm log}(M_{\ast}/M_{\odot})\gtrsim 9$).

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arXiv2026

Mapping dark matter in the Bullet Cluster using JWST imaging and spectroscopy

We present an updated gravitational lens model of the Bullet cluster (1E 0657-56) by combining JWST NIRCam imaging and NIRSpec spectroscopy. Although previous lens models relied on many multiply imaged galaxies, only six systems had spectroscopic redshifts prior to this work. Our lens model is constrained by a catalogue of 135 secure multiple images from 27 background galaxies with spectroscopic redshifts, uniformly covering both subclusters and a wide redshift range of 0.9 - 6.7. We also provide a catalogue of 199 multiple image candidates. We modelled the cluster with Lenstool and incorporated several large-scale haloes, cluster members, the intracluster gas, and group-scale haloes surrounding the cluster core, motivated by spectroscopic studies of cluster member kinematics. We describe the main cluster component with a complex, elongated double-peaked distribution, and the subcluster with a single large-scale halo aligning closely with the brightest cluster galaxy ($4_{-2}^{+3}$ kpc). The uncertainty of the displacement has been improved threefold thanks to the addition of JWST systems. The addition of group-scale substructures, roughly following the two axes of cluster assembly, improves the fit to the multiple image positions and provides a physically motivated alternative to constant shear. Our lens model shows the closest agreement with previous studies in aperture mass profiles at $\sim60$ kpc from the brightest cluster galaxies (BCGs), but exhibits significant differences in the detailed mass distribution as a result of different lens-modelling strategies and adopted constraints. The differences are reflected in small but spatially coherent deviations between the new spectroscopic redshifts and redshifts predicted by earlier lens models.

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arXiv2026

MARVELously Dark: the density profile evolution of dwarf halos in velocity-dependent SIDM

Self-interacting dark matter (SIDM) with a sufficiently large cross section has been shown to naturally produce constant dark matter (DM) cores, as well as core-collapse, at the centers of dwarf halos on cosmic timescales, potentially reducing tensions with observation. Here, we present halos from a new dark matter only (DMO) cosmological (SIDM) simulation: Ms.Marvel DMO with a velocity-dependent self-interaction cross section with $σ/m_\text{max} = 50$ cm$^2$/g at $v_\text{max} = 35$ km/s. We compare these to the CDM suite of Storm simulations including both DMO and dark matter + hydrodynamics runs, in order to test core-formation (and core-collapse) across different dark matter models. We show that Ms.Marvel DMO can reproduce core slopes consistent with observations of isolated dwarf galaxies and more massive ($\text{M}_{vir} \gtrsim 10^{10} M_{\odot}$) CDM dwarf halos that include stellar feedback from the matched CDM run (Storm CDM+baryons). We identify nine Ms.Marvel SIDM DMO halos in the core-collapse phase of gravothermal evolution with halo masses below $2\times 10^9 M_{\odot}$. We find that using core slope to measure the core-collapse timescales of Ms.Marvel DMO halos agrees well with predicted collapse times estimated with the parametric model for SIDM halos introduced by Yang et al.(2023). Additionally, compared to central density, core slope is less sensitive to both the radius of measurement and halo merger history. These results indicate that the slope of the inner DM density profile more cleanly differentiates core-collapsed versus core-forming halos than central density amplitude.

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arXiv2026

The Sensitivity of Substructure Lensing to SIDM Core-collapse Model Variation

Strong gravitational lensing has emerged as a powerful probe of dark matter substructure, and shows particularly strong promise as a test of self-interacting dark matter (SIDM). The compact halos produced by SIDM can leave distinct imprints on lensing observations, but the core-collapse timeline for subhalos is difficult to model accurately. This difficulty is an obstacle to accurate substructure lensing predictions, where small variations in core-collapsing subhalos can lead to significant differences in the lensing power. To quantify this problem and inform future lensing analyses, we test various methods of modeling core-collapsing halos and show the effect of each variation on the two-point correlation function of the effective deflection field's divergence and curl. Our tests include smoothly evolving density profiles versus instantaneously collapsing halos, probabilistic collapse versus individual halo evolution, and variation of the initial and final density profile parameters. We find that the two-point correlation function is sensitive to most of these variations at small length scales, but the detectability of these differences will depend on the observational probe.

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arXiv2026

$V/σ$ Trends with Mass for Dwarf Galaxies from the Marvelous Massive Dwarfs Suite

Galaxy formation scenarios can be interpreted through galaxy morphology and the level of rotational versus pressure support, quantified through the ratio of a galaxy's rotation speed to its velocity dispersion: $V/σ$. Observational studies of dwarf galaxies find that $V/σ$ does not strongly depend on environment, and may weakly depend on galaxy mass, which could shift our understanding of how dwarf galaxies form. We utilize the Marvelous Massive Dwarfs suite to examine whether $V/σ$ depends on mass in simulations, and understand how this varies for different baryonic components of the galaxy: HI gas, young stars ($<$ 1 Gyr) and old stars ($>$ 1 Gyr). We use a simulation sample of 67 isolated dwarf galaxies with M$_\star=10^6-10^9$ M$_\odot$ and produce line-of-sight maps for rotation speed and dispersion for different viewing angles of each galaxy. We find that $V/σ$ increases with mass, and that HI gas and young stars are more rotation-supported ($V/σ\approx 1-13$) while old stars are more dispersion-supported ($V/σ\approx 0.2-5$). This result is consistent with the scenario where young stars are born from dynamically cold gas in the interstellar medium and undergo dynamical heating over time. We quantify the effects of spatial resolution in observational determinations of $V/σ$ and find that existing observations using old stars may underestimate the intrinsic $V/σ$. We find a correlation between $V/σ_\mathrm{HI,global}$ and HI line profile shape that is qualitatively similar to previous simulation results, but we find higher $V/σ_\mathrm{HI,global}$ compared to prior work which found values $\lesssim 2$ for most galaxies in this mass range. Our results motivate future work to examine $V/σ$ and dwarf galaxy formation with different kinematic tracers of the galaxy.

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arXiv2026

First Observations of Solar Halo Gamma Rays Over a Full Solar Cycle

We analyze 15 years of Fermi-LAT data and produce a detailed model of the Sun's inverse-Compton scattering emission (solar halo), which is powered by interactions between ambient cosmic-ray electrons and positrons with sunlight. By developing a novel analysis method to analyze moving sources, we robustly detect the solar halo at energies between 31.6 MeV and 100 GeV, and angular extensions up to 45$^\circ$ from the Sun, providing new insight into spatial regions where there are no direct measurements of the galactic cosmic-ray flux. The large statistical significance of our signal allows us to sub-divide the data and provide the first $γ$-ray probes into the time-variation and azimuthal asymmetry of the solar modulation potential, finding time-dependent changes in solar modulation both parallel and perpendicular to the ecliptic plane. Our results are consistent with (but with independent uncertainties from) local cosmic-ray measurements, unlocking new probes into both astrophysical and beyond-standard-model processes near the solar surface.

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arXiv2026

Convection-Driven Multi-Scale Magnetic Fields Determine the Observed Solar-Disk Gamma Rays

The solar disk is a continuous source of GeV--TeV gamma rays. The emission is thought to originate from hadronic Galactic cosmic rays (GCRs) interacting with the gas in the photosphere and uppermost convection zone after being reflected by solar magnetic fields. Despite this general understanding, existing theoretical models have yet to match observational data. At the photosphere and the uppermost convection zone, granular convection drives a multi-scale magnetic field, forming a larger-scale filamentary structure while also generating turbulence-scale Alfvén wave turbulence. Here, we demonstrate that the larger-scale filamentary field shapes the overall gamma-ray emission spectrum, and the Alfvén wave turbulence is critical for further suppressing the gamma-ray emission spectrum below $\sim 100$~GeV. For a standard Alfvén wave turbulence level, our model's predicted spectrum slope from 1~GeV to 1~TeV is in excellent agreement with observations from Fermi-LAT and HAWC, an important achievement. The predicted absolute flux is a factor of 2--5 lower than the observed data; we outline future directions to resolve this discrepancy. The key contribution of our work is providing a new theoretical framework for using solar disk gamma-ray observations to probe hadronic GCR transport in the lower solar atmosphere.

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arXiv2025

Astrophysical Tests of Dark Matter Self-Interactions

Self-interacting dark matter (SIDM) arises generically in scenarios for physics beyond the Standard Model that have dark sectors with light mediators or strong dynamics. The self-interactions allow energy and momentum transport through halos, altering their structure and dynamics relative to those produced by collisionless dark matter. SIDM models provide a promising way to explain the diversity of galactic rotation curves, and they form a predictive and versatile framework for interpreting astrophysical phenomena related to dark matter. This review provides a comprehensive explanation of the physical effects of dark matter self-interactions in objects ranging from galactic satellites (dark and luminous) to clusters of galaxies and the large-scale structure. The second major part describes the methods used to constrain SIDM models including current constraints, with the aim of advancing tests with upcoming galaxy surveys. This part also provides a detailed review of the unresolved small-scale structure formation issues and concrete ways to test simple SIDM models. The review is rounded off by a discussion of the theoretical motivation for self-interactions, degeneracies with baryonic and gravitational effects, extensions to the single-component elastic-interactions SIDM framework, and future observational and theoretical prospects.

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arXiv2025

The Merian Survey: A Statistical Census of Bright Satellites of Milky Way Analogs

We present a statistical census of bright, star-forming satellite galaxies around Milky Way (MW) analogs using the first data release of the Merian Survey. Our sample consists of 393 MW analogs with stellar masses $10^{10.5} < M_{\star, \rm host} < 10^{10.9} M_\odot$ at redshifts $0.07 < z < 0.09$, all central galaxies of their own dark matter halos. Using photometric selection -- including magnitude, color, angular size, photometric redshift, and size-mass cuts -- we identify 793 satellite candidates around these 393 hosts. Our selection leverages two medium-band filters targeting H$α$ and [O \textsc{iii}] emission, enabling a nearly complete sample of star-forming, Magellanic Clouds-like satellites with $M_{\star, \rm sat} \gtrsim 10^{8} M_\odot$. We find that $\sim80\%$ of hosts have 0-3 massive satellites, and $13\pm4\%$ have two satellites (similar to the MW). Satellite abundance correlates with total stellar mass, and we provide significantly improved statistics for the most massive satellites at $\log_{10}[M_{\star, \rm sat}/M_{\odot}] \gtrsim 10$. The completeness-corrected radial distribution is less centrally concentrated than an NFW profile. In contrast, the Milky Way satellites are more centrally concentrated than the 50\% richest Merian systems, but are broadly consistent with the 50\% most centrally concentrated Merian systems. Our results highlight the power of medium-band photometry for satellite identification and provide a key benchmark for studying satellite quenching, environmental effects, and hierarchical galaxy formation.

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arXiv2025

Star Formation under a Cosmic Microscope: Highly magnified z = 11 galaxy behind the Bullet Cluster

We present measurements of stellar population properties of a newly discovered spectroscopically confirmed $z=11.10^{+0.11}_{-0.26}$, gravitationally lensed galaxy, using JWST NIRSpec PRISM spectroscopy and NIRCam imaging. The arc is highly magnified by the Bullet Cluster (magnification factor $μ=14.0^{+6.2}_{-0.3}$. It contains three star-forming components of which one is barely resolved and two are unresolved, giving intrinsic sizes of $\lesssim 10pc$. The clumps also contain ~50% of the total stellar mass. The galaxy formed the majority of its stars ~150Myr ago (by z~14). The spectrum shows a pronounced damping wing, typical for galaxies deep in the reionisation era and indicating a neutral IGM at this line of sight. The intrinsic luminosity of the galaxy is $0.086^{+0.008}_{-0.030} L^*$ (with $L^*$ being the characteristic luminosity for this redshift), making it the lowest luminosity spectroscopically confirmed galaxy at $z>10$ discovered to date.

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arXiv2025

A deep dive down the broad-line region: permitted OI, CaII and FeII emission in an AGN Little Red Dot at z=5.3

We present a spectroscopic analysis of a broad-line active galactic nucleus (AGN) selected as little red dot at $z = 5.3$ behind the Bullet cluster (Bz5.3), based on JWST/NIRCam and NIRSpec data. The detection of strong FeII, OI, and CaII triplet emission lines, along with the evidence of broad Balmer lines, provides strong evidence of a broad-line region (BLR) and an accreting supermassive black hole. Notably, we report the first detection of the $\lambda1304$ bump (i.e., blend of OI$λ$1304 and SiII) at high redshift, a feature commonly seen in local AGNs but not yet reported in the early Universe. The OI$λ$1304/$\lambda8446$ photon ratio provides an independent measurement of dust attenuation in galaxies. In Bz5.3, this ratio is highly suppressed (0.1--0.3), implying significant internal dust extinction, with estimated dust attenuation $A_V \sim 0.4$--$1.0$. We identify Ly$β$ fluorescence as the dominant excitation mechanism of the low-ionization lines, with additional contributions from collisional excitation. High OI$λ$8446 equivalent width and weak OI$λ$7774 support this interpretation. The detection of iron emission, whether from broad permitted or narrow forbidden lines, supports the presence of a stratified BLR, as also recently proposed in local LRDs. Photoionization modeling of OI$λ$8446 and CaII further suggests the coexistence of multiple gas phases with distinct densities and ionization states, highlighting the complexity of the BLR. Bz5.3 thus offers a rare window into early AGN activity and BLR physics at early times.

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arXiv2025

Dwarf diversity in $Λ$CDM with baryons

Observed rotation curves of dwarf galaxies exhibit significant diversity at fixed halo mass, challenging galaxy formation within the Cold Dark Matter (CDM) model. Previous cosmological galaxy formation simulations with baryonic physics fail to reproduce the full diversity of rotation curves, suggesting either that there is a flaw in baryonic feedback models, or that an alternative to CDM must be invoked. In this work, we use the Marvelous Massive Dwarf zoom-in simulations, a suite of high-resolution dwarf simulations with $M_{200}~\sim 10^{10}-10^{11}$ ${\rm M}_{\odot}$ and $M_{*}\sim 10^{7}-10^{9}$ ${\rm M}_{\odot}$, designed to target the mass range where galaxy rotation curve diversity is maximized, i.e., between $V_{\rm max} \sim 70-100~ {\rm km/s}$. We add to this a set of low-mass galaxies from the Marvel Dwarf Zoom Volumes to extend the galaxy mass range to lower values. Our fiducial star formation and feedback models produce simulated dwarfs with a broader range of rotation curve shapes, similar to observations. These are the first simulations that can both create dark matter cores via baryonic feedback, reproducing the slower rising rotation curves, while also allowing for compact galaxies and steeply rising rotation curves. Our simulated dwarfs also reproduce the observed size$-M_*$ relation, including scatter, producing both extended and compact dwarfs for the first time in simulated field dwarfs. We explore star formation and feedback models and conclude that previous simulations may have had feedback that was too strong to produce compact dwarfs.

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arXiv2025

Till the core collapses: the evolution and properties of self-interacting dark matter subhalos

One of the hottest questions in the cosmology of self-interacting dark matter (SIDM) is whether scatterings can induce detectable core-collapse in halos by the present day. Because gravitational tides can accelerate core-collapse, the most promising targets to observe core-collapse are satellite galaxies and subhalo systems. However, simulating small subhalos is computationally intensive, especially when subhalos start to core-collapse. In this work, we present a hierarchical framework for simulating a population of SIDM subhalos, which reduces the computation time to linear order in the total number of subhalos. With this method, we simulate substructure lensing systems with multiple velocity-dependent SIDM models, and show how subhalo evolution depends on the SIDM model, subhalo mass and orbits. We find that an SIDM cross section of $\gtrsim 200$ cm$^2$/g at velocity scales relevant for subhalos' internal heat transfer is needed for a significant fraction of subhalos to core-collapse in a typical lens system at redshift $z=0.5$, and that core-collapse has unique observable features in lensing. We show quantitatively that core-collapse in subhalos is typically accelerated compared to field halos, except when the SIDM cross section is non-negligible ($\gtrsim \mathcal{O}(1)$ cm$^2$/g) at subhalos' orbital velocities, in which case evaporation by the host can delay core-collapse. This suggests that substructure lensing can be used to probe velocity-dependent SIDM models, especially if line-of-sight structures (field halos) can be distinguished from lens-plane subhalos. Intriguingly, we find that core-collapse in subhalos can explain the recently reported ultra-steep density profiles of substructures found by lensing with the \emph{Hubble Space Telescope}

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arXiv2025

Diversity and universality: evolution of dwarf galaxies with self-interacting dark matter

Dark matter halos with self-interacting dark matter (SIDM) experience a unique evolutionary phenomenon, in that their central regions eventually collapse to high density through the runaway gravothermal process after initially forming a large and low-density core. When coupled with orbital evolution, this is expected to naturally produce a large diversity in dark-matter halos' inner mass distribution, potentially explaining the diversity problem of dwarf galaxies. However, it remains unknown how the diversity in SIDM dark-matter halos propagates to the more easily observed luminous matter at the center of the halo, especially the stellar component. In this work, we use idealized N-body simulations with two species of particles (dark matter and stars) to study the response of the stellar properties of field and satellite dwarf galaxies to SIDM evolution and orbital effects on their halos. Galaxies' stellar components, including galaxy size, mass-to-light ratio, and stellar velocity dispersion, display a much larger scatter in SIDM than the standard cold dark matter (CDM) model. Importantly, we find signs of universality in the evolution pathways, or "tidal tracks", of SIDM dwarf satellites, which are physically interpretable and potentially parameterizable. This type of tidal-track model can be layered onto larger-scale, cosmological simulations to reconstruct the evolution of populations of SIDM dwarfs in cases where high-resolution simulations of galaxies are otherwise prohibitively expensive.

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