arXiv subjects

Laura V. Sales

Publications and source records attributed to Laura V. Sales.

105 records · Page 6Linked to original sources

arXiv2014

Satellite abundances around bright isolated galaxies II: radial distribution and environmental effects

We use the SDSS/DR8 galaxy sample to study the radial distribution of satellite galaxies around isolated primaries, comparing to semi-analytic models of galaxy formation based on the Millennium and Millennium-II simulations. SDSS satellites behave differently around high- and low-mass primaries: those orbiting objects with $M_*>10^{11}M_\odot$ are mostly red and are less concentrated towards their host than the inferred dark matter halo, an effect that is very pronounced for the few blue satellites. On the other hand, less massive primaries have steeper satellite profiles that agree quite well with the expected dark matter distribution and are dominated by blue satellites, even in the inner regions where strong environmental effects are expected. In fact, such effects appear to be strong only for primaries with $M_* > 10^{11}M_\odot$. This behaviour is not reproduced by current semi-analytic simulations, where satellite profiles always parallel those of the dark matter and satellite populations are predominantly red for primaries of all masses. The disagreement with SDSS suggests that environmental effects are too efficient in the models. Modifying the treatment of environmental and star formation processes can substantially increase the fraction of blue satellites, but their radial distribution remains significantly shallower than observed. It seems that most satellites of low-mass primaries can continue to form stars even after orbiting within their joint halo for 5 Gyr or more.

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arXiv2014

Stellar feedback by radiation pressure and photoionization

The relative impact of radiation pressure and photoionization feedback from young stars on surrounding gas is studied with hydrodynamic radiative transfer (RT) simulations. The calculations focus on the single-scattering (direct radiation pressure) and optically thick regime, and adopt a moment-based RT-method implemented in the moving-mesh code AREPO. The source luminosity, gas density profile and initial temperature are varied. At typical temperatures and densities of molecular clouds, radiation pressure drives velocities of order ~20 km/s over 1-5 Myr; enough to unbind the smaller clouds. However, these estimates ignore the effects of photoionization that naturally occur concurrently. When radiation pressure and photoionization act together, the latter is substantially more efficient, inducing velocities comparable to the sound speed of the hot ionized medium (10-15 km/s) on timescales far shorter than required for accumulating similar momentum with radiation pressure. This mismatch allows photoionization to dominate the feedback as the heating and expansion of gas lowers the central densities, further diminishing the impact of radiation pressure. Our results indicate that a proper treatment of the impact of young stars on the interstellar medium needs to primarily account for their ionization power whereas direct radiation pressure appears to be a secondary effect. This conclusion may change if extreme boosts of the radiation pressure by photon trapping are assumed.

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arXiv2014

The shape of dark matter subhalos in the Aquarius simulations

We analyze the Aquarius simulations to characterize the shape of dark matter halos with peak circular velocity in the range 8 ~0.75 and ~0.60 at r~1 kpc. Their velocity ellipsoid become strongly tangentially biased in the outskirts as a consequence of tidal stripping.

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arXiv2013

Counterrotating Stars in Simulated Galaxy Disks

Counterrotating stars in disk galaxies are a puzzling dynamical feature whose origin has been ascribed to either satellite accretion events or to disk instabilities triggered by deviations from axisymmetry. We use a cosmological simulation of the formation of a disk galaxy to show that counterrotating stellar disk components may arise naturally in hierarchically-clustering scenarios even in the absence of merging. The simulated disk galaxy consists of two coplanar, overlapping stellar components with opposite spins: an inner counterrotating bar-like structure made up mostly of old stars surrounded by an extended, rotationally-supported disk of younger stars. The opposite-spin components originate from material accreted from two distinct filamentary structures which at turn around, when their net spin is acquired, intersect delineating a "V"-like structure. Each filament torques the other in opposite directions; the filament that first drains into the galaxy forms the inner counterrotating bar, while material accreted from the other filament forms the outer disk. Mergers do not play a substantial role and most stars in the galaxy are formed in situ; only 9% of all stars are contributed by accretion events. The formation scenario we describe here implies a significant age difference between the co- and counterrotating components, which may be used to discriminate between competing scenarios for the origin of counterrotating stars in disk galaxies.

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arXiv2012

Satellites and Haloes of Dwarf Galaxies

We study the abundance of satellite galaxies as a function of primary stellar mass using the SDSS/DR7 spectroscopic catalogue. In contrast with previous studies, which focussed mainly on bright primaries, our central galaxies span a wide range of stellar mass, 10^7.5 < M_*^pri/M_sun < 10^11, from dwarfs to central cluster galaxies. Our analysis confirms that the average number of satellites around bright primaries, when expressed in terms of satellite-to-primary stellar mass ratio (m_*^sat/M_*^pri), is a strong function of M_*^pri. On the other hand, satellite abundance is largely independent of primary mass for dwarf primaries (M_*^pri<10^10 M_sun). These results are consistent with galaxy formation models in the LCDM scenario. We find excellent agreement between SDSS data and semi-analytic mock galaxy catalogues constructed from the Millennium-II Simulation. Satellite galaxies trace dark matter substructure in LCDM, so satellite abundance reflects the dependence on halo mass, M_200, of both substructure and galaxy stellar mass (M_*). Since dark matter substructure is almost scale-free, the dependence of satellite abundance on primary mass results solely from the well-defined characteristic mass in the galaxy mass-halo mass relation. On dwarf galaxy scales, where models predict a power-law scaling, M_* \propto M_200^2.5, similarity is preserved and satellite abundance is independent of primary mass. For primaries brighter than the characteristic mass of the M_*-M_200 relation, satellite abundance increases strongly with primary mass. Our results provide strong support for the steep, approximately power-law dependence of dwarf galaxy mass on halo mass envisioned in LCDM galaxy formation models.

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arXiv2012

The dark matter halos of dwarf galaxies: a challenge for the LCDM paradigm?

The cold dark matter halo mass function is much steeper than the galaxy stellar mass function on galactic and subgalactic scales. This difference is usually reconciled by assuming that the galaxy formation efficiency drops sharply with decreasing halo mass, so that virtually no dwarf galaxies form in halos less massive than ~ 10^10 M_sun. In turn, this implies that, at any given radius, the dark mass enclosed by a galaxy must exceed a certain minimum. We use rotation curves of dwarf galaxies compiled from the literature to explore whether their enclosed mass is consistent with these constraints. We find that almost one half of the dwarfs in our sample with stellar mass between 10^6-10^7 M_sun are at odds with this restriction: either they live in halos with masses substantially below 10^10 M_sun or there is a mechanism capable of reducing the dark mass enclosed by some of the faintest dwarfs. Neither possibility is easily accommodated within the standard LCDM scenario. Extending galaxy formation to halos well below 10^10 M_sun would lead to large numbers of dwarf galaxies in excess of current estimates; at the same time, the extremely low stellar mass of the systems involved makes it unlikely that baryonic effects can reduce their dark matter content. Resolving this challenge seems to require new insights into dwarf galaxy formation, or perhaps a radical revision of the prevailing paradigm.

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arXiv2012

The Origin of Disks and Spheroids in Simulated Galaxies

In the simplest scenario, disk galaxies form predominantly in halos with high angular momentum and quiet recent assembly history, whereas spheroids are the slowly-rotating remnants of repeated merging events. We explore these assumptions using one hundred systems with halo masses similar to that of the Milky Way, identified in a series of cosmological gasdynamical simulations GIMIC. At z=0, the simulated galaxies exhibit a wide variety of morphologies, from dispersion-dominated spheroids to pure disk galaxies. Surprisingly, these morphological features are very poorly correlated with their halo properties: disks form in halos with high and low net spin, and mergers play a negligible role in the formation of spheroid stars, most of which form in-situ. More important to morphology is the coherent alignment of the angular momentum of baryons that accrete over time to form a galaxy. Spheroids tend to form when the spin of newly-accreted gas is misaligned with that of the extant galaxy, leading to the episodic formation of stars with different kinematics that cancel out the net rotation of the system. Disks, on the other hand, form out of gas that flows in with similar angular momentum to that of earlier-accreted material. Gas accretion from a hot corona thus favours disk formation, whereas gas that flows "cold", often along separate, misaligned filaments, favours the formation of spheroids. In this scenario, most spheroids consist of superpositions of stellar components with distinct kinematics, age, and metallicity, an arrangement that might survive to the present day given the paucity of major mergers. Since angular momentum is acquired largely at turnaround, morphology is imprinted early by the interplay of the tidal field and the shape of the material destined to form the galaxy.

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arXiv2011

Clues to the "Magellanic Galaxy" from Cosmological Simulations

We use cosmological simulations from the Aquarius Project to study the orbital history of the Large Magellanic Cloud (LMC) and its potential association with other satellites of the Milky Way (MW). We search for dynamical analogs to the LMC and find a subhalo that matches the LMC position and velocity at either of its two most recent pericentric passages. This suggests that the LMC is not necessarily on its first approach to the MW, provided that the virial mass of the Milky Way is M_200 ~1.8e12 M_sun. The simulation results yield specific predictions for the position and velocity of systems associated with the LMC prior to infall. If on first approach, most should lie close to the LMC because the Galactic tidal field has not yet had enough time to disperse them. If on second approach, the list of potential associates increases substantially. Interestingly, our analysis rules out an LMC association for Draco and Ursa Minor, two of the dwarf spheroidals suggested by Lynden-Bell & Lynden-Bell to form part of the "Magellanic Ghostly Stream". Our results also indicate that the direction of the orbital angular momentum is a powerful test of LMC association. This test, however, requires precise proper motions, which are unavailable for most MW satellites. Of the 4 satellites with published proper motions, only the Small Magellanic Cloud is clearly associated with the LMC. Taken at face value, the proper motions of Carina, Fornax and Sculptor rule them out as potential associates, but this conclusion should be revisited when better data become available. The dearth of satellites clearly associated with the Clouds might be solved by wide-field imaging surveys that target its surroundings, a region that may prove a fertile hunting ground for faint, previously unnoticed MW satellites.

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arXiv2011

The Shape of Dark Matter Haloes in the Aquarius Simulations: Evolution and Memory

We use the high resolution cosmological N-body simulations from the Aquarius project to investigate in detail the mechanisms that determine the shape of Milky Way-type dark matter haloes. We find that, when measured at the instantaneous virial radius, the shape of individual haloes changes with time, evolving from a typically prolate configuration at early stages to a more triaxial/oblate geometry at the present day. This evolution in halo shape correlates well with the distribution of the infalling material: prolate configurations arise when haloes are fed through narrow filaments, which characterizes the early epochs of halo assembly, whereas triaxial/oblate configurations result as the accretion turns more isotropic at later times. Interestingly, at redshift z=0, clear imprints of the past history of each halo are recorded in their shapes at different radii, which also exhibit a variation from prolate in the inner regions to triaxial/oblate in the outskirts. Provided that the Aquarius haloes are fair representatives of Milky Way-like 10^12 Msun objects, we conclude that the shape of such dark matter haloes is a complex, time-dependent property, with each radial shell retaining memory of the conditions at the time of collapse.

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arXiv2011

A Comparison of Galaxy Group Luminosity Functions from Semi-Analytic Models

Semi-analytic models (SAMs) are currently one of the primary tools with which to model statistically significant ensembles of galaxies. The underlying physical prescriptions inherent to each SAM are, in many cases, different from one another. Several SAMs have been applied to the dark matter merger trees extracted from the Millennium Run, including those associated with the well-known Munich and Durham lineages. We compare the predicted luminosity distributions of galaxy groups using four publicly available SAMs (De Lucia et al. 2006; Bower et al. 2006; Bertone et al. 2007; Font et al. 2008), in order to explore a galactic environment in which the models have not been explored to the same degree as they have in the field or in rich clusters. We identify a characteristic "wiggle" in the group galaxy luminosity function generated using the De Lucia et al. (2006) SAM, that is not present in the Durham-based models, consistent to some degree with observations. However, a comparison between conditional luminosity functions of groups between the models and observations of Yang et al. (2007) suggest that neither model is a particularly good match. The luminosity function wiggle is interpreted as the result of the two-mode AGN feedback implementation used in the Munich models, which itself results in flattened magnitude gap distribution. An associated analysis of the magnitude gap distribution between first- and second-ranked group galaxies shows that while the Durham models yield distributions with approximately equal luminosity first- and second-ranked galaxies, in agreement with observations, the De Lucia et al. models favours the scenario in which the second-ranked galaxy is approximately one magnitude fainter than the primary,especially when the dynamic range of the mock data is limited to 3 magnitudes.

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arXiv2010

Feedback and the Structure of Simulated Galaxies at redshift z=2

We study the properties of simulated high-redshift galaxies using cosmological N-body/gasdynamical runs from the OverWhelmingly Large Simulations (OWLS) project. The runs contrast several feedback implementations of varying effectiveness: from no-feedback, to supernova-driven winds to powerful AGN-driven outflows. These different feedback models result in large variations in the abundance and structural properties of bright galaxies at z=2. We find that feedback affects the baryonic mass of a galaxy much more severely than its spin, which is on average roughly half that of its surrounding dark matter halo in our runs. Feedback induces strong correlations between angular momentum content and galaxy mass that leave their imprint on galaxy scaling relations and morphologies. Encouragingly, we find that galaxy disks are common in moderate-feedback runs, making up typically ~50% of all galaxies at the centers of haloes with virial mass exceeding 1e11 M_sun. The size, stellar masses, and circular speeds of simulated galaxies formed in such runs have properties that straddle those of large star-forming disks and of compact early-type galaxies at z=2. Once the detailed abundance and structural properties of these rare objects are well established it may be possible to use them to gauge the overall efficacy of feedback in the formation of high redshift galaxies.

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arXiv2009

The Effect of Tidal Stripping on Composite Stellar Populations in Dwarf Spheroidal Galaxies

We use N-body simulations to study the effects of tides on the kinematical structure of satellite galaxies orbiting a Milky Way-like potential. Our work is motivated by observations of dwarf spheroidal galaxies in the Local Group, for which often a distinction is possible between a cold centrally concentrated metal rich and a hot, extended metal poor population. We find that an important attenuation of the initial differences in the distribution of the two stellar components occurs for orbits with small pericentric radii (r_per < 20 kpc). This is mainly due to: i) the loss of the gravitational support provided by the dark matter component after tidal stripping takes place, which forces a re-configuration of the luminous components, and ii) tides preferentially affect the more extended stellar component, leading to a net decrease in its velocity dispersion as a response for the mass loss, which thus shrinks the kinematical gap. We apply these ideas to the Sculptor and Carina dwarf spheroidals. Differences in their orbits might help to explain, under the assumption of similar initial configurations, why in the former a clear kinematical separation between metal poor and metal rich stars is apparent, while in Carina this segregation is significantly more subtle.

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arXiv2009

Orbital Eccentricity as a probe of Thick Disk Formation Scenarios

We study the orbital properties of stars in four (published) simulations of thick disks formed by: i) accretion from disrupted satellites, ii) heating of a pre-existing thin disk by a minor merger, iii) radial migration and iv) gas rich mergers. We find that the distribution of orbital eccentricities are predicted to be different for each model: a prominent peak at low eccentricity is expected for the heating, migration and gas-rich merging scenarios, while the eccentricity distribution is broader and shifted towards higher values for the accretion model. These differences can be traced back to whether the bulk of the stars in each case is formed 'in-situ' or is 'accreted', and are robust to the peculiarities of each model. A simple test based on the eccentricity distribution of nearby thick disk stars may thus help elucidate the dominant formation mechanism of the Galactic thick disk.

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arXiv2009

The Origin of Extended Disk Galaxies at z=2

Galaxy formation models typically assume that the size and rotation speed of galaxy disks are largely dictated by the mass, concentration, and spin of their surrounding dark matter haloes. Equally important, however, are the fraction of baryons in the halo that collect into the central galaxy, as well as the net angular momentum that they are able to retain during its assembly process. We explore the latter using a set of four large cosmological N-body/gasdynamical simulations drawn from the OWLS (OverWhelmingly Large Simulations) project. These runs differ only in their implementation of feedback from supernovae. We find that, when expressed as fractions of their virial values, galaxy mass and net angular momentum are tightly correlated. Galaxy mass fractions, m_d=M_gal/M_vir, depend strongly on feedback, but only weakly on halo mass or spin over the halo mass range explored here (M_vir>1e11 h^{-1}M_sun). The angular momentum of a galaxy, j_d=J_gal/J_vir, correlates with m_d in a manner that is insensitive to feedback and that deviates strongly from the simple j_d = m_d assumption often adopted in semi-analytic models of galaxy formation. The m_d-j_d correlation implies that, in a given halo, galaxy disk size is maximal when the central galaxy makes up a substantial fraction (~20%-30%) of all baryons within the virial radius. At z=2, such systems may host gaseous disks with radial scale lengths as large as those reported for star-forming disks by the SINS survey, even in moderately massive haloes of average spin. Extended disks at z=2 may thus signal the presence of systems where galaxy formation has been particularly efficient, rather than the existence of haloes with unusually high spin parameter.

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arXiv2009

Disk galaxies at z=2 in OWLS

We use the OWLS (OverWhelmingly Large Simulations) set of cosmological Nbody/gasdynamical simulations to study the properties of simulated galaxies at z=2. We focus on the effect of supernova feedback from evolving stars on the baryonic mass and angular momentum content of galaxies that assemble at the center of 1e11-3e12 h^{-1}M_\odot halos. Our main finding is that the mass and angular momentum of such galaxies are strongly coupled, in a way that is approximately independent of feedback: varying the feedback implementation leads, in a given halo, to large variations in galaxy mass but leaves the galaxy mass-angular momentum correlation largely unaltered. In particular, the ratio between the angular momentum of a galaxy and that of its surrounding halo (j_d=J_gal/J_vir) correlates closely with the galaxy mass (expressed in units of the virial mass of the halo; m_d=M_gal/M_vir). This correlation differs substantially from the m_d=j_d assumption commonly adopted in semianalytic models of galaxy formation. We use these results to infer the sizes of disk galaxies at z=2 expected in the LCDM scenario and to interpret recent observations of extended disks at z~2 by the SINS collaboration

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