Etude des relations entre la masse stellaire et la masse de halo de matière noire dans les galaxies à disque
Details:
Background Galaxies are the building blocks of our universe, and understanding their formation and evolution is central to contemporary astrophysics. Detailed studies of the dynamics of galaxies and of the distribution of baryons within them has led over the last decades to the establishment of scaling relations that are key constraints. Many of these relations link the galaxies’ light (from stars, cold neutral gas and hot ionised gas) to the gravitational field, providing insights into galaxy formation processes and possibly the nature of dark matter. However, most of these relations focus on either the total baryonic masses of the galaxies (e.g. baryonic Tully-Fisher relation; McGaugh et al. 2000, ApJL, 533, L99) or their stellar contents (e.g. stellar-to-halo mass relation; Posti et al. 2019, A&A, 629, A59). Despite its crucial role in the star-formation process, little effort has been devoted to connect the cold gas to the dark matter of disc galaxies. An accurate assessment of the relation between neutral hydrogen (HI) mass and dark matter halo mass (HIHMR) in disc galaxies can only be achieved through the detailed modelling of individual galaxies, a method more direct than the others used previously (e.g. galaxy clusters, abundance matching, spectral HI stacking).
Recent Research Consequently, I recently focused on studying the rotation curves of disc galaxies, to in turn investigate the less explored HIHMR. I exploited the SPARC (Spitzer Photometry and Accurate Rotation Curves) sample of 175 isolated disc galaxies (Lelli et al. 2016, AJ, 152, 157) with near-infrared (3.6 μm) imaging and HI rotation curves, combined with the analogous LITTLE THINGS (Local Irregulars That Trace Luminosity Extremes, The HI Nearby Galaxy Survey) sample of 15 dwarf galaxies. The SPARC database is the largest sample of galaxies with both extended HI rotation curves and near-infrared photometry (minimising variations of the stellar mass-to-light ratio). I used two analytic dark matter profiles to study the rotation curves and investigate the HIHMR relation. I showed for the first time that the HI-to-halo mass ratio of nearby rotationally-supported disc galaxies is approximately constant (with a ratio of ≈ 1.25%) across the entire galaxy mass range, and that massive discs (M⋆ 10^10 M⊙) hosting active galactic nuclei (AGN) follow exactly the same relation. This is inconsistent with past observational and numerical simulation results of similar galaxies, that suggest a break of the relation at both the low- and high-mass ends. My new results thus reveal that isolated, rotationally-supported disc galaxies are surprisingly self-similar, in turn hinting at mass-independent self-regulatory mechanisms that have yet to be fully understood (Korsaga et al. 2023, ApJL, 952, L41).
Interestingly, galaxies represent the spatial scale at which the standard cosmological paradigm is facing the most significant observational challenges today, and a number of scaling relation issues have recently been heavily discussed in the literature. I will address these issues in innovative and independent manners, providing powerful novel constraints on galaxy formation and evolution models.
Project 1 To better understand the discrepancy between my recent findings and existing numeri- cal simulations, I will explore the dependence of the HI mass – halo mass scaling relation as a function of stellar mass, redshift, and environment (field, group, and cluster galaxies). To achieve this, I will exploit data from the two large ongoing state-of-the-art surveys exploiting the revolutionary MeerKAT telescope: MIGHTEE-HI (MeerKAT International GigaHertz Tiered Extragalactic Exploration) and LADUMA (Looking At the Distant Universe with the MeerKAT Array; of which I am already a member). MeerKAT is the ideal instrument due to its sensitivity and capacity to cover large volumes (and thus multiple environments and redshifts). The early science MIGHTEE-HI data detected 276 galaxies with spatially-resolved kinematics and optical counterparts up to redshift 0.08 and total baryonic mass ≈ 10^11 M⊙ (Maddox et al. 2021, A&A, 646, A35; Ponomavera et al. 2021, MNRAS, 508, 1195). The full science ob- servations, which have just been completed, will provide one order of magnitude more detections. Combined with the deeper LADUMA survey and existing ancilllary multi-wavelength datasets, these data will allow me to investigate the HI-to-halo mass relation of galaxies up to a redshift of ≈ 0.6 in both the low- and the high-stellar mass regimes (M⋆ ≤ 10^8 M⊙ and M⋆ 10^10 M⊙).
To achieve this, mass models will be constructed that assume an analytic dark mater profile (so- called Dekel-Zhao profile; Freundlich et al. 2020, MNRAS, 491, 4523) characterised by a variable inner slope, a dimensionless concentration parameter, and the halo mass, fitted using an affine- invariant Markov chain Monte Carlo (MCMC) method and the python implementation EMCEE. As simulations of galaxies are very important to understand galaxy formation and evolution pro- cesses, in parallel I will investigate how this HI-to-halo mass relation behaves in state-of-the-art simulations (which suggest that massive disks are significantly dark matter dominated. My experience analysing hydrodynamical simulations (e.g. SIMBA and Illustris- TNG) will ensure I conduct this study efficiently. In cases for which the signal-to-noise ratio is too poor to reliably detect individual galaxies, as is likely with the LADUMA intermediate-redshift galaxies, I will stack the HI data (i.e. co-add the HI spectra of galaxies, detected or not, with known optical redshifts) to derive accurate average properties. My expertise with optical spectroscopic and HI- data analysis is key to effectively carry out this task.
Project 2 The shape of galaxy rotation curves can also be described without dark matter via a modification of the gravitational law at low accelerations (Modifed Newtonian Dynamics, a.k.a. MOND; Milgrom 1983, ApJ, 270, 365). I plan to explore this model and associated scaling relations using different fitting functions interpolating between the Newtonian and MOND regimes (and the mass-to-light ratios of galaxies). This will exploit the aforementioned MIGHTEE-HI and LADUMA surveys, to better understand the mass discrepancy problem and improve the current dynamical models. In addition, the so-called acceleration relation (between the observed acceleration of matter in galaxies and that predicted from the gravity of the luminous matter only) is well-known to depart from the 1:1 relation at low accelerations, thus setting the MOND acceleration scale (a0 parameter). However, this relation has never been explored at very high accelerations. Using the extremely high angular resolution ALMA telescope data from the WISDOM project (mm-Wave Interferometric Survey of Dark Object Masses. I will explore this relation at extremely high accelerations never probed before, and will search for departures from the Newtonian expectation (at the opposite extreme from MOND). Any detection will be a fundamental discovery. This investigation will allow me to compare the results with the aforementioned ones and help address the ongoing debate about the necessity of dark matter to construct the mass models of galaxies.