Gas cooling in simulations of the formation of the galaxy population

We compare two techniques for following the cooling of gas and its condensation into galaxies within high resolution simulations of cosmologically representative regions. Both techniques treat the dark matter using N-body methods. One follows the gas using smoothed particle hydrodynamics (SPH) while the other uses simplified recipes from semi-analytic (SA) models. We compare the masses and locations predicted for dense knots of cold gas (the `galaxies') when the two techniques are applied to evolution from the same initial conditions and when the additional complications of star formation and feedback are ignored. We find that above the effective resolution limit of the two techniques, they give very similar results both for global quantities such as the total amount of cooled gas and for the properties of individual `galaxies'. The SA technique has systematic uncertainties arising from the simplified cooling model adopted, while details of the SPH implementation can produce substantial systematic variations in the galaxy masses it predicts. Nevertheless, for the best current SPH methods and the standard assumptions of the SA model, systematic differences between the two techniques are remarkably small. The SA technique gives adequate predictions for the condensation of gas into `galaxies' at less than one percent of the computational cost of obtaining similar results at comparable resolution using SPH.Comment: Revised, Figure 7 added. To appear in MNRA

Dark matter annihilation in the halo of the Milky Way

If the dark matter in the Universe is made of weakly self-interacting particles, they may self-annihilate and emit gamma-rays. We use high resolution numerical simulations to estimate directly the annihilation flux from the central regions of the Milky Way and from dark matter substructures in its halo. Although such estimates remain uncertain because of their strong dependence on the structure of the densest regions, our numerical experiments suggest that less direct calculations have overestimated the emission both from the centre and from halo substructure. We estimate a maximal enhancement of at most a factor of a few with respect to a smooth spherical halo of standard Navarro-Frenk-White (NFW) structure. We discuss detection strategies for the next generation of gamma-ray detectors and find that the annihilation flux may be detectable, regardless of uncertainties about the densest regions, for the annihilation cross-sections predicted by currently popular elementary particle models for the dark matter.Comment: 10 pages, 8 figures, replaced with version accepted by MNRAS (very minor changes

Orthography affects L1 and L2 speech perception but not production in early bilinguals

First published online: 25 August 2021Orthography plays a crucial role in L2 learning, which generally relies on both oral and written input. We examine whether incongruencies between L1 and L2 grapheme-phoneme correspondences influence bilingual speech perception and production, even when both languages have been acquired in early childhood before reading acquisition. Spanish–Basque and Basque–Spanish early bilinguals performed an auditory lexical decision task including Basque pseudowords created by replacing Basque /s̻/ with Spanish /θ/. These distinct phonemes take the same orthographic form, . Participants also completed reading-aloud tasks in Basque and Spanish to test whether speech sounds with the same orthographic form were produced similarly in the two languages. Results for both groups showed orthography had strong effects on speech perception but no effects on speech production. Taken together, these findings suggest that orthography plays a crucial role in the speech system of early bilinguals but does not automatically lead to non-native production.This work was supported by the Basque Government [BERC 2018–2021 program]; the Spanish State Research Agency [BCBL Severo Ochoa excellence accreditation SEV-2015-0490]; the H2020 European Research Council [Marie Skłodowska-Curie grant 843533; ERC Consolidator Grant ERC-2018-COG-819093]; the Spanish Ministry of Economy and Competitiveness [PSI2017-82941-P; Europa-Excelencia ERC2018-092833; RED2018-102615-T]; and the Basque Government [PIBA18-29]

The impact of orthographic forms on speech production and perception: An artificial vowel-learning study

Available online 23 August 2022This study investigates the effect of orthographic forms on phonetic aspects of isolated speech sound production and perception. Three groups of 25 L1-Spanish speakers were exposed to /y/ and /e/ in a multi-session learning study. They heard the same vowels presented with: L1-incongruent orthographic forms, novel orthographic forms, or without orthographic forms. After three exposure sessions, participants were tested on vowel production in an elicited production task and vowel perception in a multiple forced choice task. All groups established new /y/ and / e/ production and perception categories. Incongruent orthographic forms led to less target-like category positions for /y/ but not /e/ in production and perception. Novel orthographic forms only facilitated more target-like perception for /y/. In a fourth session, Auditory-only participants were exposed to incongruent orthography for /y/ and novel orthography for /e/. Sequential exposure to incongruent orthography caused less target-like production and perception category positions, while sequential exposure to novel orthography altered neither. Together these results suggest that orthographic forms affect isolated speech sounds and are encoded at the speech sound level. Incongruent grapheme-to-phoneme mappings from L1 to later-learned languages may critically affect the phonetic characteristics of non-native speech sounds, but learning outcomes depend on specific L1-L2 category contrasts.This work was supported by institutional grants from the Basque Government [BERC 2018–2021 program] and the Spanish State Research Agency [BCBL Severo Ochoa excellence accreditation CEX2020-001010-S] awarded to the BCBL. This project has also received funding from the European Union’s H2020 research and innovation program (Marie Skłodowska-Curie grant agreement No 843533 awarded to AS); the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement No 819093 to CDM); the Spanish Ministry of Economy and Competitiveness [PSI2017-82941-P; Europa-Excelencia ERC2018-092833; RED2018-102615-T (all CDM)]; and the Basque Government [PIBA18-29 (CDM)]

Simulating IGM Reionization

We have studied the IGM reionization process in its full cosmological context including structure evolution and a realistic galaxy population. We have used a combination of high-resolution N-body simulations (to describe the dark matter and diffuse gas component), a semi-analytic model of galaxy formation (to track the evolution of the sources of ionizing radiation) and the Monte Carlo radiative transfer code CRASH (to follow the propagation of ionizing photons into the IGM). The process has been followed in the largest volume ever used for this kind of study, a field region of the universe with a comoving length of L~20/h Mpc, embedded in a much larger cosmological simulation. To assess the effect of environment on the reionization process, the same radiative transfer simulations have been performed on a 10/h Mpc comoving box, centered on a clustered region. We find that, to account for the all ionizing radiation, objects with total masses of M~10^9 Msun must be resolved. In this case, the simulated stellar population produces a volume averaged ionization fraction x_v=0.999 by z~8, consistent with observations without requiring any additional sources of ionization. We also find that environment substantially affects the reionization process. In fact, although the simulated proto-cluster occupies a smaller volume and produces a higher number of ionizing photons, it gets totally ionized later. This is because high density regions, which are more common in the proto-cluster, are difficult to ionize because of their high recombination rates.Comment: 10 pages, 9 figures, minor revisions, in press, MNRAS 343 (2003) 1101-110

The Milky Way's satellite population in a LambdaCDM universe

We compare the structure and kinematics of the 11 known satellites of the Milky Way with high resolution simulations of the formation of its dark halo in a LambdaCDM universe. In contrast to earlier work, we find excellent agreement. The observed kinematics are exactly those predicted for stellar populations with the observed spatial structure orbiting within the most massive ``satellite'' substructures in our simulations. Less massive substructures have weaker potential wells than those hosting the observed satellites. If there is a halo substructure ``problem'', it consists in understanding why halo substructures have been so inefficient in making stars. Suggested modifications of dark matter properties (for example, self-interacting or warm dark matter) may well spoil the good agreement found for standard Cold Dark Matter.Comment: 5 pages, 2 figures, replaced with version accepted by MNRAS (minor changes

A New Approach to the Study of Stellar Populations in Early-Type Galaxies: K-band Spectral Indices and an Application to the Fornax Cluster

New measurements of K-band spectral features are presented for eleven early-type galaxies in the nearby Fornax galaxy cluster. Based on these measurements, the following conclusions have been reached: (1) in galaxies with no signatures of a young stellar component, the K-band Na I index is highly correlated with both the optical metallicity indicator [MgFe]' and central velocity dispersion; (2) in the same galaxies, the K-band Fe features saturate in galaxies with sigma > 150 km/s while Na I (and [MgFe]') continues to increase; (3) [Si/Fe] (and possibly [Na/Fe]) is larger in all observed Fornax galaxies than in Galactic open clusters with near-solar metallicity; (4) in various near-IR diagnostic diagrams, galaxies with signatures of a young stellar component (strong Hbeta, weak [MgFe]') are clearly separated from galaxies with purely old stellar populations; furthermore, this separation is consistent with the presence of an increased number of M-giant stars (most likely to be thermally pulsating AGB stars); (5) the near-IR diagrams discussed here seem as efficient for detecting putatively young stellar components in early-type galaxies as the more commonly used age/metallicity diagnostic plots using optical indices (e.g Hbeta vs. [MgFe]').Comment: 47 pages, 16 figures, ApJ accepte