Ethan O. Nadler’s research while affiliated with University of Southern California and other places

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Publications (126)


Testing the parametric model for self-interacting dark matter using matched halos in cosmological simulations
  • Article

February 2025

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8 Reads

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1 Citation

Physics of the Dark Universe

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Ethan O. Nadler

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Hai-Bo Yu


A semi-analytic model for decaying dark matter halos

January 2025

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1 Read

Decaying dark matter (DDM) affects the evolution of cosmic structure relative to standard cold, collisionless, stable dark matter (CDM). We introduce a new semi-analytic model for the effects of two-body DDM on halo structure and subhalo populations. In this scenario, cold parent dark matter particles decay into less massive daughter particles plus dark radiation with a lifetime comparable to the age of the universe. Our DDM model is implemented in the open-source software Galacticus\texttt{Galacticus} and accounts for heating (due to the velocity kicks imparted on daughter particles) and mass loss (due to the parent--daughter mass splitting). We show that decays flatten and reduce the amplitude of halos' inner density profiles. These effects make DDM subhalos susceptible to tidal disruption, which we show yields a mass-dependent suppression of the subhalo mass function relative to CDM. Our predictions for DDM density profiles, velocity dispersion profiles, and subhalo populations are consistent with results from isolated and cosmological DDM N-body simulations. Thus, our model enables efficient and accurate exploration of DDM parameter space and will be useful for deriving constraints from upcoming small-scale structure observations.


The GD-1 Stellar Stream Perturber as a Core-collapsed Self-interacting Dark Matter Halo

January 2025

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1 Read

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2 Citations

The Astrophysical Journal Letters

The GD-1 stellar stream exhibits spur and gap structures that may result from a close encounter with a dense substructure. When interpreted as a dark matter subhalo, the perturber is denser than predicted in the standard cold dark matter (CDM) model. In self-interacting dark matter (SIDM), however, a halo could evolve into a phase of gravothermal collapse, resulting in a higher central density than its CDM counterpart. We conduct high-resolution controlled N -body simulations to show that a collapsed SIDM halo could account for the GD-1 perturber's high density. We model a progenitor halo with a mass of 3 × 10 ⁸ M ⊙ , motivated by a cosmological simulation of a Milky Way analog, and evolve it in the Milky Way's tidal field. For a cross section per mass of σ / m ≈ 30–100 cm ² g ⁻¹ at V max ~ 10 km s − 1 , the enclosed mass of the SIDM halo within the inner 10 pc can be increased by more than 1 order of magnitude compared to its CDM counterpart, leading to a good agreement with the properties of the GD-1 perturber. Our findings indicate that stellar streams provide a novel probe into the self-interacting nature of dark matter.


COZMIC. III. Cosmological Zoom-in Simulations of SIDM with Suppressed Initial Conditions

December 2024

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3 Reads

Ethan O. Nadler

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Rui An

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[...]

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Vera Gluscevic

We present eight cosmological dark matter (DM)--only zoom-in simulations of a Milky Way-like system that include suppression of the linear matter power spectrum P(k), and/or velocity-dependent DM self-interactions, as the third installment of the COZMIC suite. We consider a model featuring a massive dark photon that mediates DM self-interactions and decays into massless dark fermions. The dark photon and dark fermions suppress linear matter perturbations, resulting in dark acoustic oscillations in P(k), which ultimately affect dwarf galaxy scales. The model also features a velocity-dependent elastic self-interaction between DM particles (SIDM), with a cross section that can alleviate small-scale structure anomalies. For the first time, our simulations test the impact of P(k) suppression on gravothermal evolution in an SIDM scenario that leads to core collapse in (sub)halos with present-day virial masses below 109 M\approx 10^9~M_{\mathrm{\odot}}. In simulations with P(k) suppression and self-interactions, the lack of low-mass (sub)halos and the delayed growth of structure reduce the fraction of core-collapsed systems relative to SIDM simulations without P(k) suppression. In particular, P(k) suppression that saturates current warm DM constraints almost entirely erases core collapse in isolated halos. Models with less extreme P(k) suppression produce core collapse in 20%\approx 20\% of subhalos and 5%\approx 5\% of isolated halos above 108 M10^8~M_{\mathrm{\odot}}, and also increase the abundance of extremely low-concentration isolated low-mass halos relative to SIDM. These results reveal a complex interplay between early and late-universe DM physics, revealing new discovery scenarios in the context of upcoming small-scale structure measurements.


Figure 4. Quenched fractions as a function of stellar mass. SAGA satellites, corrected for incompleteness, are shown in both panels as solid green circles with Poisson error bars. Left: We plot the uncorrected confirmed satellite quenched fractions (open green squares), which agree with the completeness-corrected values for the Gold sample (gold bar) but increasingly deviate toward the Silver sample (gray bar). The 1σ system-to-system scatter for the 101 individual SAGA systems is shown by the light green shaded region. A subsample of 18 SAGA hosts in Local Group-like systems is shown as the dotted green line and is statistically indistinguishable from the full sample. Black stars represent MW satellites. Right: We compare to the ELVES Survey (Carlsten et al. 2022), which classifies satellites using a color-only criterion. We plot the full sample of 28 ELVES hosts (orange open squares) and a subsample of 14 ELVES hosts matching the SAGA MW criteria (solid orange squares, −23 > M K > − 24.6).
Figure 8. The SFR NUV (left) and sSFR NUV (right) as a function of stellar mass. SAGA star-forming satellites are plotted as gray circles. Triangles indicate NUV upper limits. Solid blue squares are binned medians, excluding upper limits. The SAGA background (SAGAbg) galaxy sample (z < 0.035) is plotted as dashed black line. The green line (Leroy et al. 2019) is fit at higher stellar mass and plotted down to the stellar mass where it is well-measured. An extrapolation to lower stellar mass would disagree with SAGA samples. The SAGA satellites and background sample agree well with the LVL sample (orange line) from Dale et al. (2023). These samples are all fit in the same stellar mass regime.
Figure 9. The BPT (left) and [S II]-BPT (right) diagram for SAGA satellites color-coded by stellar mass. The SAGA background sample (z < 0.035 and M å < 10 10 M e ) is shown by the gray contour-scatter plots. We restrict both samples to galaxies with S/N > 4 in all lines, but also plot satellites with a lower threshold 3 > S/N > 4 for comparison (triangles). Regions of the BPT where emission is dominated by star formation (SF), AGN, or composite (Comp) are labeled. No high-S/N SAGA satellites fall within the AGN region as defined by Kewley et al. (2006, solid line) or Kauffmann et al. (2003, dotted line). The single SAGA satellite AGN confirmed via radio continuum does not fall in the optical AGN region (red circle). The SAGA satellite population roughly follows the background distribution.
The SAGA Survey. IV. The Star Formation Properties of 101 Satellite Systems around Milky Way–mass Galaxies
  • Article
  • Full-text available

November 2024

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3 Reads

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2 Citations

The Astrophysical Journal

We present the star-forming properties of 378 satellite galaxies around 101 Milky Way analogs in the Satellites Around Galactic Analogs (SAGA) Survey, focusing on the environmental processes that suppress or quench star formation. In the SAGA stellar mass range of 10 ⁶⁻¹⁰ M ⊙ , we present quenched fractions, star-forming rates, gas-phase metallicities, and gas content. The fraction of SAGA satellites that are quenched increases with decreasing stellar mass and shows significant system-to-system scatter. SAGA satellite quenched fractions are highest in the central 100 kpc of their hosts and decline out to the virial radius. Splitting by specific star formation rate (sSFR), the least star-forming satellite quartile follows the radial trend of the quenched population. The median sSFR of star-forming satellites increases with decreasing stellar mass and is roughly constant with projected radius. Star-forming SAGA satellites are consistent with the star formation rate–stellar mass relationship determined in the Local Volume, while the median gas-phase metallicity is higher and median H i gas mass is lower at all stellar masses. We investigate the dependence of the satellite quenched fraction on host properties. Quenched fractions are higher in systems with larger host halo mass, but this trend is only seen in the inner 100 kpc; we do not see significant trends with host color or star formation rate. Our results suggest that lower-mass satellites and satellites inside 100 kpc are more efficiently quenched in a Milky Way–like environment, with these processes acting sufficiently slowly to preserve a population of star-forming satellites at all stellar masses and projected radii.

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Figure B1. The f Q (v Mpeak ) functions of the best-fit UM-SAGA (solid) and UM DR1 (dashed) models. The different colors represent the model relations at different redshifts. The new low-mass quenching mostly affects halos with v Mpeak  100 km s −1 .
The SAGA Survey. V. Modeling Satellite Systems around Milky Way–Mass Galaxies with Updated UniverseMachine

November 2024

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4 Reads

The Astrophysical Journal

Environment plays a critical role in shaping the assembly of low-mass galaxies. Here, we use the U niverse M achine (UM) galaxy–halo connection framework and Data Release 3 of the Satellites Around Galactic Analogs (SAGA) Survey to place dwarf galaxy star formation and quenching into a cosmological context. UM is a data-driven forward model that flexibly parameterizes galaxy star formation rates (SFRs) using only halo mass and assembly history. We add a new quenching model to UM, tailored for galaxies with m ⋆ ≲ 10 ⁹ M ⊙ , and constrain the model down to m ⋆ ≳ 10 ⁷ M ⊙ using new SAGA observations of 101 satellite systems around Milky Way (MW)–mass hosts and a sample of isolated field galaxies in a similar mass range from the Sloan Digital Sky Survey. The new best-fit model, “UM-SAGA,” reproduces the satellite stellar mass functions, average SFRs, and quenched fractions in SAGA satellites while keeping isolated dwarfs mostly star-forming. The enhanced quenching in satellites relative to isolated field galaxies leads the model to maximally rely on halo assembly to explain the observed environmental quenching. Extrapolating the model down to m ⋆ ∼ 10 6.5 M ⊙ yields a quenched fraction of ≳30% for isolated field galaxies and ≳80% for satellites of MW-mass hosts at this stellar mass. Spectroscopic surveys can soon test this specific prediction to reveal the relative importance of internal feedback, cessation of mass and gas accretion, satellite-specific gas processes, and reionization for the evolution of faint low-mass galaxies.


The SAGA Survey. III. A Census of 101 Satellite Systems around Milky Way–mass Galaxies

November 2024

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3 Reads

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7 Citations

The Astrophysical Journal

We present Data Release 3 (DR3) of the Satellites Around Galactic Analogs (SAGA) Survey, a spectroscopic survey characterizing satellite galaxies around Milky Way (MW)-mass galaxies. The SAGA Survey DR3 includes 378 satellites identified across 101 MW-mass systems in the distance range of 25–40.75 Mpc, and an accompanying redshift catalog of background galaxies (including about 46,000 taken by SAGA) in the SAGA footprint of 84.7 deg ² . The number of confirmed satellites per system ranges from zero to 13, in the stellar mass range of 10 ⁶⁻¹⁰ M ⊙ . Based on a detailed completeness model, this sample accounts for 94% of the true satellite population down to M ⋆ = 10 7.5 M ⊙ . We find that the mass of the most massive satellite in SAGA systems is the strongest predictor of satellite abundance; one-third of the SAGA systems contain LMC-mass satellites, and they tend to have more satellites than the MW. The SAGA satellite radial distribution is less concentrated than the MW's, and the SAGA quenched fraction below 10 8.5 M ⊙ is lower than the MW's, but in both cases, the MW is within 1 σ of SAGA system-to-system scatter. SAGA satellites do not exhibit a clear corotating signal as has been suggested in the MW/M31 satellite systems. Although the MW differs in many respects from the typical SAGA system, these differences can be reconciled if the MW is an older, slightly less massive host with a recently accreted LMC/SMC system.


SAGAbg. II. The Low-mass Star-forming Sequence Evolves Significantly between 0.05 < z < 0.21

November 2024

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1 Read

The Astrophysical Journal

The redshift-dependent relation between galaxy stellar mass and star formation rate (SFR), known as the star-forming sequence (SFS), is a key observational yardstick for galaxy assembly. We use the SAGAbg-A sample of background galaxies from the Satellites Around Galactic Analogs (SAGA) Survey to model the low-redshift evolution of the low-mass SFS. The sample is comprised of 23,258 galaxies with H α -based SFRs spanning 6 < log 10 ( M ⋆ / [ M ⊙ ] ) < 10 and z < 0.21 ( t < 2.5 Gyr). Although it is common to bin or stack galaxies at z ≲ 0.2 for galaxy population studies, the difference in lookback time between z = 0 and z = 0.21 is comparable to the time between z = 1 and z = 2. We develop a model to account for both the physical evolution of low-mass SFS and the selection function of the SAGA Survey, allowing us to disentangle redshift evolution from redshift-dependent selection effects across the SAGAbg-A redshift range. Our findings indicate significant evolution in the SFS over the last ∼2.5 Gyr, with a rising normalization: 〈 SFR ( M ⋆ = 10 8.5 M ⊙ ) 〉 ( z ) = 1.24 − 0.23 + 0.25 z − 1.47 − 0.03 + 0.03 . We also identify the redshift limit at which a static SFS is ruled out at the 95% confidence level, which is z = 0.05 based on the precision of the SAGAbg-A sample. Comparison with cosmological hydrodynamic simulations reveals that some contemporary simulations underpredict the recent evolution of the low-mass SFS. This demonstrates that the recent evolution of the low-mass SFS can provide new constraints on the assembly of the low-mass Universe and highlights the need for improved models in this regime.


COZMIC. II. Cosmological Zoom-in Simulations with Fractional non-CDM Initial Conditions

November 2024

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1 Read

We present 24 cosmological dark matter (DM)--only zoom-in simulations of a Milky Way (MW) analog with initial conditions appropriate for scenarios where non-cold DM is a fraction of the total DM abundance (f-NCDM models), as the second installment of the COZMIC suite. We initialize our simulations using transfer functions, TfNCDM(k)PfNCDM(k)/PCDM(k)T_{\mathrm{f-NCDM}}(k)\equiv\sqrt{P_{\mathrm{f-NCDM}}(k)/P_{\mathrm{CDM}}(k)} (where P(k) is the linear matter power spectrum), with an initial suppression similar to thermal-relic warm DM (WDM) followed by a constant-amplitude plateau. We simulate suppression wave numbers [22.8, 32.1, 41.8, 52.0, 57.1, 95.3] Mpc1[22.8,~ 32.1,~ 41.8,~ 52.0,~ 57.1,~ 95.3]~\mathrm{Mpc}^{-1}, corresponding to thermal-relic WDM masses mWDM[3, 4, 5, 6, 6.5, 10] keVm_{\mathrm{WDM}}\in [3,~ 4,~ 5,~ 6,~ 6.5,~ 10]~\mathrm{keV}, and plateau amplitudes δ[0.2, 0.4, 0.6, 0.8]\delta\in [0.2,~ 0.4,~ 0.6,~ 0.8]. We model the subhalo mass function in terms of the suppression wave number and δ\delta. Integrating these models into a forward model of the MW satellite galaxy population yields new limits on f-NCDM scenarios, with suppression wave numbers greater than 46 and 40 Mpc1 40~\mathrm{Mpc}^{-1} for δ=0.2\delta=0.2, 0.4, respectively, at 95%95\% confidence. The current data do not constrain δ>0.4\delta>0.4. We map these limits to scenarios where a fraction fWDMf_{\mathrm{WDM}} of DM behaves as a thermal relic, which yields the following bounds on cosmologies with a mixture of WDM and CDM: mWDM>3.6, 4.1, 4.6, 4.9, 5.4 keVm_{\mathrm{WDM}}>3.6,~ 4.1,~ 4.6,~ 4.9,~ 5.4~\mathrm{keV} for fWDM=0.5, 0.6, 0.7, 0.8, 0.9f_{\mathrm{WDM}}=0.5,~ 0.6,~ 0.7,~ 0.8,~ 0.9, respectively, at 95%95\% confidence. The current data do not constrain WDM fractions fWDM<0.5f_{\mathrm{WDM}}<0.5. Our results affirm that low-mass halo abundances are sensitive to partial suppression in P(k), indicating the possibility of using galactic substructure to reconstruct P(k) on small scales.


Citations (47)


... However, this suppression does not increase at small scales, where we observe a flattening similar to ther < 1 scenarios. This is important because small-scale suppression of the matter power spectrum is expected to impact the distribution of low-mass DM halos and dwarf galaxies around the Milky Way [13,25,29]. The faintest observed dwarf galaxies correspond to wavenumbers of k ∼ 10 − 100 h/Mpc, which goes beyond the typical constraining power of WL data. ...

Reference:

Can $\nu$DM interactions solve the $S_8$ discrepancy?
Dark matter coupled to radiation: Limits from the Milky Way satellites
  • Citing Article
  • January 2025

Physical Review D

... Since exotic dark matter seems to be highly localized around MACHOs, it is very possible that clusters of these WilloWISPs could be roaming the cosmos, quiescently contributing the majority of the dark matter's missing mass since the era of the CMB and first stars (Cann et al. 2018;Hütsi et al. 2023). There has even been evidence of such a structure that has no other bright tracers besides the "Spur and Gap of the GD-1 Stellar Stream" in the Milky Way Galaxy (Bonaca et al. 2019;Zhang et al. 2025). If the recently discovered dark dwarf galaxies (Xu et al. 2023), Nube (Montes et al. 2023) and Ursa Major III (UNIONS-1) (Smith et al. 2024), are found to be mostly comprised of quiescent black holes, then the culprit of the spur and gap in GD-1 is likely a similar cluster of WilloWISPs. ...

The GD-1 Stellar Stream Perturber as a Core-collapsed Self-interacting Dark Matter Halo
  • Citing Article
  • January 2025

The Astrophysical Journal Letters

... Additionally, among classical dwarfs, ones belonging to the MW and M31 are generally quiescent, while the ELVES and SAGA surveys of satellites around MW-mass analogs have found quenched fractions 1σ lower than that of the Local Group (Y.-Y. Mao et al. 2024). This is in stark contrast to the M81 group. ...

The SAGA Survey. III. A Census of 101 Satellite Systems around Milky Way–mass Galaxies

The Astrophysical Journal

... With these ingredients, we run eight new high-resolution cosmological DM-only zoom-in simulations of a MW analog from the Milky Way-est suite (Buch et al. 2024), in WSIDM models. 1 Our simulations are part of the COsmological ZooM-in simulations with Initial Conditions beyond CDM (COZMIC) suite, which includes over 100 beyond-CDM simulations with ICs for warm, fuzzy, and baryon-scattering DM (Nadler et al. 2024a, hereafter Paper I), and for models with a fractional non-CDM component (Paper II; . We show that the severity of P(k) suppression influences whether SIDM core collapse occurs or not. ...

Milky Way-est: Cosmological Zoom-in Simulations with Large Magellanic Cloud and Gaia–Sausage–Enceladus Analogs

The Astrophysical Journal

... We analyze the Symphony subhalo catalogs extracted using the Rockstar [64] halo finder. Recent work by Mansfield et al. [65] has identified 15-40% more subhalos within the virial radius when compared to the Rockstar results, increasing to 35-120% within r < r v /4. Another possibility may be 2). ...

symfind : Addressing the Fragility of Subhalo Finders and Revealing the Durability of Subhalos

The Astrophysical Journal

... On the one hand, some present-day properties (e.g. luminosity function, quenched fraction) of the brightest MW satellites are similar to other satellite systems * NASA Hubble Fellow around MW-mass hosts in the Local Volume (e.g., Chiboucas et al. 2013;Carlsten et al. 2022;Mao et al. 2021;Danieli et al. 2023;Mao et al. 2024;Geha et al. 2024). On the other hand, there are well-documented differences in the properties (e.g. ...

The SAGA Survey. III. A Census of 101 Satellite Systems around Milky Way-mass Galaxies

... Several observational probes have been proposed to determine the abundance of DM subhalos in this mass range, including ultra-faint dwarf galaxies (e.g. Nadler et al. (2024)), strong gravitational lensing (e.g. Keeley et al. (2024)) and stellar streams (e.g. ...

Forecasts for Galaxy Formation and Dark Matter Constraints from Dwarf Galaxy Surveys

The Astrophysical Journal

... In future work, we plan to compare UM-SAGA predictions with the SAGA-bg (E. Kado-Fong et al. 2024) sample, a background galaxy spectroscopic sample generated in the process of identifying SAGA satellites. This sample obviously shares similar completeness characteristics as SAGA satellites and goes down to brighter limiting M å compared to the Local Volume field galaxies mentioned above. ...

SAGAbg. I. A Near-unity Mass-loading Factor in Low-mass Galaxies via Their Low-redshift Evolution in Stellar Mass, Oxygen Abundance, and Star Formation Rate

The Astrophysical Journal

... Volume (e.g., D. Martínez-Delgado et al. 2022;M. G. Jones et al. 2023;K. B. W. McQuinn et al. 2023aK. B. W. McQuinn et al. , 2023b. Paired with advances in numerical simulations (e.g., C. Wheeler et al. 2015;M. Ricotti et al. 2016;F. Munshi et al. 2019;E. Applebaum et al. 2021) and semianalytical modeling (e.g., V. Manwadkar & A. V. Kravtsov 2022;N. Ahvazi et al. 2024;S. Weerasooriya et al. 2023), the large statistical sample of satellites built by these search efforts has not only largely alleviated concerns of tension with ΛCDM (J. S. Bullock & M. Boylan-Kolchin 2017; S. Y. Kim et al. 2018;A. Drlica-Wagner et al. 2020; L. V. Sales et al. 2022) but also enabled wide-ranging and detailed tests of gala ...

A comprehensive model for the formation and evolution of the faintest Milky Way dwarf satellites
  • Citing Article
  • March 2024

Monthly Notices of the Royal Astronomical Society