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1. The Effects of Magnetic Fields and Outflow Feedback on the Shape and Evolution of the Density Probability Distribution Function in Turbulent Star-forming Clouds

   https://doi.org/10.3847/1538-4357/ac4be3  

   Appel, Sabrina M.; Burkhart, Blakesley; Semenov, Vadim A.; Federrath, Christoph; Rosen, Anna L. (March 2022, The Astrophysical Journal)

   Abstract Using a suite of 3D hydrodynamical simulations of star-forming molecular clouds, we investigate how the density probability distribution function (PDF) changes when including gravity, turbulence, magnetic fields, and protostellar outflows and heating. We find that the density PDF is not lognormal when outflows and self-gravity are considered. Self-gravity produces a power-law tail at high densities, and the inclusion of stellar feedback from protostellar outflows and heating produces significant time-varying deviations from a lognormal distribution at low densities. The simulation with outflows has an excess of diffuse gas compared to the simulations without outflows, exhibits an increased average sonic Mach number, and maintains a slower star formation rate (SFR) over the entire duration of the run. We study the mass transfer between the diffuse gas in the lognormal peak of the PDF, the collapsing gas in the power-law tail, and the stars. We find that the mass fraction in the power-law tail is constant, such that the stars form out of the power-law gas at the same rate at which the gas from the lognormal part replenishes the power law. We find that turbulence does not provide significant support in the dense gas associated with the power-law tail. When including outflows and magnetic fields in addition to driven turbulence, the rate of mass transfer from the lognormal to the power law, and then to the stars, becomes significantly slower, resulting in slower SFRs and longer depletion times. 

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2. The Odd Dark Matter Halos of Isolated Gas-rich Ultradiffuse Galaxies

   https://doi.org/10.3847/1538-4357/ac8875  

   Kong, Demao; Kaplinghat, Manoj; Yu, Hai-Bo; Fraternali, Filippo; Mancera Piña, Pavel E. (September 2022, The Astrophysical Journal)

   Abstract We analyze circular velocity profiles of seven ultradiffuse galaxies (UDGs) that are isolated and gas-rich. Assuming that the dark matter halos of these UDGs have a Navarro–Frenk–White (NFW) density profile or a Read density profile (which allows for constant-density cores), the inferred halo concentrations are systematically lower than the cosmological median, even as low as −0.6 dex (about 5σaway) in some cases. Alternatively, similar fits can be obtained with a density profile that scales roughly as 1/r²for radii larger than a few kiloparsecs. Both solutions require the radius where the halo circular velocity peaks ( $R_{\max }$) to be much larger than the median expectation. Surprisingly, we find an overabundance of such large- $R_{\max }$halos in the IllustrisTNG dark-matter-only simulations compared to the Gaussian expectation. These halos form late and have higher spins compared to median halos of similar masses. The inner densities of the most extreme among these late-forming halos are higher than their NFW counterparts, leading to a ∼1/r²density profile. However, the two well-resolved UDGs in our sample strongly prefer lower dark matter densities in the center than the simulated ones. Comparing to IllustrisTNG hydrodynamical simulations, we also find a tension in getting both low enough circular velocities and high enough halo mass to accommodate the measurements. Our results indicate that the gas-rich UDGs present a significant challenge for galaxy formation models. 

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3. The onset of gravothermal core collapse in velocity-dependent self-interacting dark matter subhaloes

   https://doi.org/10.1093/mnras/stab1725  

   Turner, Hannah C; Lovell, Mark R; Zavala, Jesús; Vogelsberger, Mark (June 2021, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT It has been proposed that gravothermal collapse due to dark matter self-interactions (i.e. self-interacting dark matter, SIDM) can explain the observed diversity of the Milky Way (MW) satellites’ central dynamical masses. We investigate the process behind this hypothesis using an N-body simulation of a MW-analogue halo with velocity-dependent SIDM (vdSIDM) in which the low-velocity self-scattering cross-section, $$\sigma _{\rm T}/m_{\rm x}$$, reaches 100 cm2 g−1; we dub this model the vd100 model. We compare the results of this simulation to simulations of the same halo that employ different dark models, including cold dark matter (CDM) and other, less extreme SIDM models. The masses of the vd100 haloes are very similar to their CDM counterparts, but the values of their maximum circular velocities, Vmax, are significantly higher. We determine that these high Vmax subhaloes were objects in the mass range [5 × 106, 1 × 108] M⊙ at z = 1 that undergo gravothermal core collapse. These collapsed haloes have density profiles that are described by single power laws down to the resolution limit of the simulation, and the inner slope of this density profile is approximately −3. Resolving the ever decreasing collapsed region is challenging, and tailored simulations will be required to model the runaway instability accurately at scales <1 kpc. 

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4. A Physically Motivated Framework to Compare the Merger Timescales of Isolated Low- and High-mass Galaxy Pairs Across Cosmic Time

   https://doi.org/10.3847/1538-4357/ad7bad  

   Chamberlain, Katie; Patel, Ekta; Besla, Gurtina; Torrey, Paul; Rodriguez-Gomez, Vicente (October 2024, The Astrophysical Journal)

   Abstract The merger timescales of isolated low-mass pairs (10⁸<M_*< 5 × 10⁹M_⊙) on cosmologically motivated orbits have not yet been studied in detail, though isolated high-mass pairs (5 × 10⁹<M_*< 10¹¹M_⊙) have been studied extensively. It is common to apply the same separation criteria and expected merger timescales of high-mass pairs to low-mass systems, however, it is unclear if their merger timescales are similar, or if they evolve similarly with redshift. We use the Illustris TNG100 simulation to quantify the merger timescales of isolated low-mass and high-mass major pairs as a function of cosmic time, and explore how different selection criteria impact the mass and redshift dependence of merger timescales. In particular, we present a physically motivated framework for selecting pairs via a scaled separation criterion, wherein pair separations are scaled by the virial radius of the primary’s Friends-of-Friends (FoF) group halo (r_sep< 1R_vir). Applying these scaled separation criteria yields equivalent merger timescales for both mass scales at all redshifts. Alternatively, static physical separation selections applied equivalently to all galaxy pairs at all redshifts lead to a difference in merger rate of up to ∼1 Gyr between low- and high-mass pairs, particularly forr_sep< 150 kpc. As a result, applying the same merger timescales to physical-separation-selected pairs will lead to a bias that systematically overpredicts low-mass galaxy merger rates. 

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5. The Distant Milky Way Halo from the Southern Hemisphere: Characterization of the LMC-induced Dynamical Friction Wake*

   https://doi.org/10.3847/1538-4357/adbf08  

   Cavieres, Manuel; Chanamé, Julio; Navarrete, Camila; Ordenes-Briceño, Yasna; Garavito-Camargo, Nicolás; Besla, Gurtina; Hempel, Maren; Vivas, A Katherina; Gómez, Facundo (April 2025, The Astrophysical Journal)

   Abstract The infall of the Large Magellanic Cloud (LMC) into the Milky Way’s halo impacts the distribution of stars and dark matter (DM) in our Galaxy. Mapping the observational consequences of this encounter can inform us about the properties of both galaxies, details of their interaction, and possibly distinguish between different DM models.N-body simulations predict a localized overdensity trailing the LMC’s orbit both in baryonic and DM, known as the wake. We collected wide-field, deep near-infrared, and optical photometry using VIRCAM and DECam across four fields along the expected wake, covering the sky region expected to span most of its predicted density contrast. We identify over 400 stars comprising two different tracers, near main-sequence turnoff stars and red giants, which map the halo between 60 and 100 kpc, deriving stellar halo densities as a function of sky position and Galactocentric radius. We detect (1) a break in the halo radial density profile at 70 kpc not seen in northern halo studies and (2) a clear halo overdensity starting also at 70 kpc, with density contrast increasing steadily toward the expected current location of the wake. If this overdensity is the LMC wake, its peak density contrast is as pronounced as the most massive LMC model considered. Contamination from unidentified substructures may bias our wake detections, so wider-area surveys with similar depth are needed for confirmation. 

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