arXiv subjects

Isabel M. E. Santos-Santos

Publications and source records attributed to Isabel M. E. Santos-Santos.

9 recordsLinked to original sources

arXiv2026

The missing ultra-faint satellites of the Milky Way

We combine the highest resolution N-body simulation of a $\sim10^{12}M_\odot$ $Λ$CDM halo (Aquarius-A) with the semi-analytic model GALFORM to study the full satellite population of Milky Way (MW)-like galaxies. The model assumes that galaxies form only in subhalos whose peak circular velocity exceeds the hydrogen-cooling threshold, and predicts that the total number of satellites ever accreted into the main halo, down to arbitrarily low luminosity, should not exceed a few hundred. The model tracks satellites even after their halos become unresolved ("orphan" galaxies), and includes a novel treatment of dark matter and stellar tidal stripping which takes into account that $Λ$CDM subhalos survive tidal disruption because of their cuspy density profiles. Our results match the massive end of the observed MW satellite mass function and predict that many ultra-faint dwarfs (UFDs) with $M_V>-5$ are missing from the current MW satellite census. The missing UFDs are predicted to avoid the innermost regions of the host, and have properties overlapping those of the recently-discovered ultra-faint compact MW satellites (UFCSs). If the faintest dwarfs are initially as compact as UFCSs, our results suggest that many UFCSs should be dark matter-dominated dwarfs with velocity dispersions $1-3$ km/s which survived disruption because they inhabit the cusp of $Λ$CDM subhalos. UFCSs have predicted densities of $10^{10}$-$10^{11}\,M_\odot/$kpc$^3$, higher than those of more extended UFDs. If confirmed, our results provide strong support for the cuspy nature of $Λ$CDM halos and for the hydrogen-cooling threshold for galaxy formation.

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arXiv2026

Andromeda XXXVI: discovery of a new ultra-faint dwarf galaxy towards M31

We present deep imaging of Andromeda XXXVI (And XXXVI), a dwarf galaxy discovered through visual inspection of the Pan-Andromeda Archaeological Survey, using observations obtained with the OSIRIS+GTC instrument. The colour-magnitude diagram of And XXXVI shows a well-defined red giant branch (RGB). However, constraining a distance is challenging because the tip of the RGB is sparsely populated and no horizontal branch stars are found. The RGB is nevertheless well matched by an old (12.5 Gyr), metal-poor ([Fe/H]= - 2.5) isochrone shifted to the distance of Andromeda (776 kpc). With a projected distance of 119 kpc from M31, And XXXVI is therefore likely a satellite of Andromeda. With MV -5.9 +-0.1, half-light radius rh 64 +30-19 pc, and an ellipticity 0.015 +0.032-0.012, And XXXVI is one of the faintest of ultra-faint M31 dwarfs discovered to date. The discovery of And XXXVI adds to the faint end of M31's satellite luminosity function, suggesting the presence of an even larger population of very faint satellites. Deeper space-based imaging and/or spectroscopic observations are needed to better constrain its position within M31's halo. Combined with a detailed star formation history, such data would help determine whether its old, metal-poor stellar population indicates early quenching, similar to the trends seen in Milky Way satellites, and whether And XXXVI could be considered a reionisation fossil.

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arXiv2024

The diversity of rotation curves of simulated galaxies with cusps and cores

We use $Λ$CDM cosmological hydrodynamical simulations to explore the kinematics of gaseous discs in late-type dwarf galaxies. We create high-resolution 21-cm 'observations' of simulated dwarfs produced in two variations of the EAGLE galaxy formation model: one where supernova-driven gas flows redistribute dark matter and form constant-density central 'cores', and another where the central 'cusps' survive intact. We 'observe' each galaxy along multiple sight lines and derive a rotation curve for each observation using a conventional tilted-ring approach to model the gas kinematics. We find that the modelling process introduces systematic discrepancies between the recovered rotation curve and the actual circular velocity curve driven primarily by (i) non-circular gas orbits within the discs; (ii) the finite thickness of gaseous discs, which leads to overlap of different radii in projection; and (iii) departures from dynamical equilibrium. Dwarfs with dark matter cusps often appear to have a core, whilst the inverse error is less common. These effects naturally reproduce an observed trend which other models struggle to explain: late-type dwarfs with more steeply-rising rotation curves appear to be dark matter-dominated in the inner regions, whereas the opposite seems to hold in galaxies with core-like rotation curves. We conclude that if similar effects affect the rotation curves of observed dwarfs, a late-type dwarf population in which all galaxies have sizeable dark matter cores is most likely incompatible with current measurements.

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arXiv2023

Anisotropies in the spatial distribution and kinematics of dwarf galaxies in the Local Group and beyond

The Local Group (LG) of galaxies is dominated by the M31 and Milky Way (MW) pair, a configuration which suggests that the mass distribution in the LG and its surroundings should be highly anisotropic. We use the APOSTLE cosmological simulations to examine how this anisotropy manifests on the spatial distribution and kinematics of dwarf galaxies out to a distance of 3 Mpc from the MW. The simulations indicate a clear preference for dwarfs to be located close to the axis defined by the MW-M31 direction, even for dwarfs in the LG periphery (LGP; i.e., those at distances 1.25 , varies with angular distance to M31, peaking in the anti-M31 direction and reaching a minimum behind M31. The combined M31-MW mass decelerates the local expansion; the LG "turnaround radius" (where =0) in APOSTLE is located at r ~ 1.25 Mpc from the LG barycentre and the pure Hubble flow (where ~ H_0*d) is not reached out to at least 3 Mpc. The predicted flow is very cold, with a barycentric dispersion of <40 km/s. A comparison of these predicted features with existing observations gives mixed results. There is clear observational evidence for an angular anisotropy in V_{rad} around the LGP, but little evidence for a preferred spatial distribution of LGP dwarfs along the MW-M31 axis. The observed local Hubble flow is also peculiar. Although the coldness of the flow is consistent with the simulations, it is significantly less decelerated: relative to the MW, on average, all galaxies beyond d~1.25 Mpc seem to be on a pure Hubble flow. We argue that these oddities may result from incompleteness and inhomogeneous sky coverage, but a full explanation may need to await the completion of deep all-sky surveys able to fill the gaps in our current inventory of nearby dwarfs.

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arXiv2023

The Tucana dwarf spheroidal: a distant backsplash galaxy of M31?

We use the APOSTLE Local Group (LG) cosmological hydro-simulations to examine the properties of "backsplash" galaxies, i.e, dwarfs which were within the virial boundaries of the Milky Way (MW) or M31 in the past, but are today outside their virial radius ($r_{200}$). More than half of all dwarfs between $1-2\,r_{200}$ of each primary are backsplash. More distant backsplash systems, i.e., those reaching distances well beyond $2\,r_{200}$, are typically close to apocentre of nearly radial orbits, and, therefore, essentially at rest relative to their primary. We use this result to investigate which LG dwarfs beyond $\sim500$ kpc of either primary could be a distant backsplash satellite of MW or M31. Tucana dSph, one of the few known quiescent LG field dwarfs, at $d_{\rm M31}\approx1350$ kpc and $d_{\rm MW}\approx880$ kpc, is a promising candidate. Tucana's radial velocity is consistent with being at rest relative to M31. Further, Tucana is located close to M33's orbital plane around M31, and simple orbit integrations indicate that Tucana may have been ejected during an early pericentric passage of M33 $\sim11$ Gyr ago, a timing which approximately coincides with Tucana's last episode of star formation. We suggest that Tucana may have been an early-infalling satellite of M31 or M33, providing a compelling explanation for its puzzling lack of gas and ongoing star formation despite its isolated nature. In this scenario, M33 should have completed some orbits around M31, a result that may help to explain the relative dearth of M33 satellite-candidates identified so far.

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arXiv2022

Satellite mass functions and the faint end of the galaxy mass-halo mass relation in LCDM

The abundance of the faintest galaxies provides insight into the nature of dark matter and the process of dwarf galaxy formation. In the LCDM scenario, low mass halos are so numerous that the efficiency of dwarf formation must decline sharply with decreasing halo mass in order to accommodate the relative scarcity of observed dwarfs and satellites in the Local Group. The nature of this decline contains important clues to the mechanisms regulating the onset of galaxy formation in the faintest systems. We explore here two possible models for the stellar mass ($M_*$)-halo mass ($M_{200}$) relation at the faint end, motivated by some of the latest LCDM cosmological hydrodynamical simulations. One model includes a sharp mass threshold below which no luminous galaxies form, as expected if galaxy formation proceeds only in systems above the Hydrogen-cooling limit. In the second model, $M_*$ scales as a steep power-law of $M_{200}$ with no explicit cutoff, as suggested by recent semianalytic work. Although both models predict satellite numbers around Milky Way-like galaxies consistent with current observations, they predict vastly different numbers of ultra-faint dwarfs and of satellites around isolated dwarf galaxies. Our results illustrate how the satellite mass function around dwarfs may be used to probe the $M_*$-$M_{200}$ relation at the faint end and to elucidate the mechanisms that determine which low-mass halos "light up" or remain dark in the LCDM scenario.

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arXiv2021

Magellanic satellites in $Λ$CDM cosmological hydrodynamical simulations of the Local Group

We use the APOSTLE $Λ$CDM cosmological hydrodynamical simulations of the Local Group to study the recent accretion of massive satellites into the halo of Milky Way (MW)-sized galaxies. These systems are selected to be close analogues to the Large Magellanic Cloud (LMC), the most massive satellite of the MW. The simulations allow us to address, in a cosmological context, the impact of the Clouds on the MW, including the contribution of Magellanic satellites to the MW satellite population, and the constraints placed on the Galactic potential by the motion of the LMC. We show that LMC-like satellites are twice more common around Local Group-like primaries than around isolated halos of similar mass; these satellites come from large turnaround radii and are on highly eccentric orbits whose velocities at first pericentre are comparable with the primary's escape velocity. This implies $V_{\rm esc}^{\rm MW} (50 $ kpc$)\sim 365$ km/s, a strong constraint on Galactic potential models. LMC analogues contribute about 2 satellites with $M_*>10^5\, M_\odot$, having thus only a mild impact on the luminous satellite population of their hosts. At first pericentre, LMC-associated satellites are close to the LMC in position and velocity, and are distributed along the LMC's orbital plane. Their orbital angular momenta roughly align with the LMC's, but, interestingly, they may appear to "counter-rotate" the MW in some cases. These criteria refine earlier estimates of the LMC association of MW satellites: only the SMC, Hydrus1, Car3, Hor1, Tuc4, Ret2 and Phoenix2 are compatible with all criteria. Carina, Grus2, Hor2 and Fornax are less probable associates given their large LMC relative velocity.

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arXiv2020

Baryonic clues to the puzzling diversity of dwarf galaxy rotation curves

We use a compilation of disc galaxy rotation curves to assess the role of the luminous component ("baryons") in the rotation curve diversity problem. As in earlier work, we find that rotation curve shape correlates with baryonic surface density: high surface density galaxies have rapidly-rising rotation curves consistent with cuspy cold dark matter halos; slowly-rising rotation curves (characteristic of galaxies with inner mass deficits or "cores") occur only in low surface density galaxies. The correlation, however, seems too weak to be the main driver of the diversity. In addition, dwarf galaxies exhibit a clear trend, from "cuspy" systems where baryons are unimportant in the inner mass budget to "cored" galaxies where baryons actually dominate. This trend constrains the various scenarios proposed to explain the diversity, such as (i) baryonic inflows and outflows during galaxy formation; (ii) dark matter self-interactions; (iii) variations in the baryonic mass structure coupled to rotation velocities through the "mass discrepancy-acceleration relation" (MDAR); or (iv) non-circular motions in gaseous discs. Together with analytical modeling and cosmological hydrodynamical simulations, our analysis shows that each of these scenarios has promising features, but none seems to fully account for the observed diversity. The MDAR, in particular, is inconsistent with the observed trend between rotation curve shape and baryonic importance; either the trend is caused by systematic errors in the data or the MDAR does not apply. The origin of the dwarf galaxy rotation curve diversity and its relation to the structure of cold dark matter halos remains an open issue.

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arXiv2017

Diversity of dwarf galaxy IR-submm emission patterns: CLUES from hydrodynamical simulations

The spectral energy distributions (SEDs) of low-mass low-metallicity (dwarf) galaxies are a challenging piece of the puzzle of galaxy formation in the near Universe. These SEDs show some particular features in the submillimeter to far-infrared wavelength range compared to normal, larger, and metal-richer galaxies that cannot be explained by the current models. These can be summarized as: a broadening of the IR peak, which implies a warmer dust component; an excess of emission in the submm ($\sim$500 $μ$m), that causes a flattening of the submm/FIR slope; and a very low intensity of PAH emission features. With the aim of explaining these emission patterns, the SEDs of a sample of 27 simulated dwarf galaxies were calculated using the GRASIL-3D radiation transfer code. This code has the particularity that it separately treats the radiative transfer through dust grains within molecular clouds and within the cirrus, the dense and diffuse components of the gas phase, respectively. The simulated galaxies have stellar masses ranging from 10$^6$-10$^9$ M$_\odot$, and were obtained from a single simulation run within a Local Group environment with initial conditions from the CLUES project. We report a study of the IRAS, Spitzer and Herschel bands luminosities, and of the SFRs, dust, and gas (HI and H$_2$) mass contents. We find a satisfactory agreement with observational data, with GRASIL-3D naturally reproducing the specific spectral features mentioned above. We conclude that the GRASIL-3D two-component dust model gives a physical interpretation of the emission of dwarf galaxies: molecular clouds and cirrus represent the warm and cold dust components, respectively, needed to reproduce observational data.

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