Dark satellites and the morphology of dwarf galaxies

One of the strongest predictions of the LambdaCDM cosmological model is the presence of dark satellites orbiting all types of galaxies. We focus here on the dynamical effects of such satellites on disky dwarf galaxies, and demonstrate that these encounters can be dramatic. Although mergers with M_sat > M_d are not very common, because of the lower baryonic content they occur much more frequently on the dwarf scale than for L_*-galaxies. As an example, we present a numerical simulation of a 20% (virial) mass ratio merger between a dark satellite and a disky dwarf (akin to the Fornax dwarf galaxy in luminosity) that shows that the merger remnant has a spheroidal morphology. We conclude that perturbations by dark satellites provide a plausible path for the formation of dSph systems and also could trigger starbursts in gas rich dwarf galaxies. Therefore the transition from disky to the often amorphous, irregular, or spheroidal morphologies of dwarfs could be a natural consequence of the dynamical heating of hitherto unobservable dark satellites.Comment: Submitted to ApJ Letters. 5 pages, 4 figures. A movie showing an encounter between a disky dwarf galaxy and a dark satellite can be found at http://www.astro.rug.nl/~ahelmi/dwarfs-morphologies.htm

Properties of Galaxy Groups in the SDSS: II.- AGN Feedback and Star Formation Truncation

Successfully reproducing the galaxy luminosity function and the bimodality in the galaxy distribution requires a mechanism that can truncate star formation in massive haloes. Current models of galaxy formation consider two such truncation mechanisms: strangulation, which acts on satellite galaxies, and AGN feedback, which predominantly affects central galaxies. The efficiencies of these processes set the blue fraction of galaxies as function of galaxy luminosity and halo mass. In this paper we use a galaxy group catalogue extracted from the Sloan Digital Sky Survey (SDSS) to determine these fractions. To demonstrate the potential power of this data as a benchmark for galaxy formation models, we compare the results to the semi-analytical model for galaxy formation of Croton et al. (2006). Although this model accurately fits the global statistics of the galaxy population, as well as the shape of the conditional luminosity function, there are significant discrepancies when the blue fraction of galaxies as a function of mass and luminosity is compared between the observations and the model. In particular, the model predicts (i) too many faint satellite galaxies in massive haloes, (ii) a blue fraction of satellites that is much too low, and (iii) a blue fraction of centrals that is too high and with an inverted luminosity dependence. In the same order, we argue that these discrepancies owe to (i) the neglect of tidal stripping in the semi-analytical model, (ii) the oversimplified treatment of strangulation, and (iii) improper modeling of dust extinction and/or AGN feedback. The data presented here will prove useful to test and calibrate future models of galaxy formation and in particular to discriminate between various models for AGN feedback and other star formation truncation mechanisms.Comment: 16 pages, 5 figures, submitted to MNRA

On the Hierarchy of Block Deterministic Languages

A regular language is $k$-lookahead deterministic (resp. $k$-block deterministic) if it is specified by a $k$-lookahead deterministic (resp. $k$-block deterministic) regular expression. These two subclasses of regular languages have been respectively introduced by Han and Wood ($k$-lookahead determinism) and by Giammarresi et al. ($k$-block determinism) as a possible extension of one-unambiguous languages defined and characterized by Br\"uggemann-Klein and Wood. In this paper, we study the hierarchy and the inclusion links of these families. We first show that each $k$-block deterministic language is the alphabetic image of some one-unambiguous language. Moreover, we show that the conversion from a minimal DFA of a $k$-block deterministic regular language to a $k$-block deterministic automaton not only requires state elimination, and that the proof given by Han and Wood of a proper hierarchy in $k$-block deterministic languages based on this result is erroneous. Despite these results, we show by giving a parameterized family that there is a proper hierarchy in $k$-block deterministic regular languages. We also prove that there is a proper hierarchy in $k$-lookahead deterministic regular languages by studying particular properties of unary regular expressions. Finally, using our valid results, we confirm that the family of $k$-block deterministic regular languages is strictly included into the one of $k$-lookahead deterministic regular languages by showing that any $k$-block deterministic unary language is one-unambiguous

How research-based theatre is a solution for community engagement and advocacy at regional medical campuses: The Health and Equity through Advocacy, Research, and Theatre (HEART) program

Background: Regional medical campuses are often located in geographic regions that have different populations than the main campus, and are well-positioned to advocate for the health needs of their local community to promote social accountability within the medical school.Methods: At the Niagara Regional Campus of McMaster University, medical students developed a framework which combined research, advocacy, and theatre to advocate for the needs of the local population of the regional campus to which they were assigned. This involved a qualitative study using semi-structured interviews with homeless individuals to explore their experience accessing the healthcare system and using a transformative framework to identify barriers to receiving quality healthcare services. Findings from the qualitative study informed a play script that presented the experiences of homeless individuals in the local health system, which was presented to health sciences learners and practicing health professionals. Participants completed two instruments to examine the utility of this framework.Results: Research-based theatre was a useful intervention to educate current and future health professionals about the challenges faced by homeless individuals in the region. Participants from both shows felt the framework of research-based theatre was an effective strategy to promote change and advocate for marginalized populations.Conclusion: Research-based theatre is an innovative approach which can be utilized to promote social accountability at regional medical campuses, advocating for the health needs of the communities in which they are located, with the added bonus of educating current and future health professionals

On DDO154 and Cold Dark Matter halo profiles

We investigate the claim by Burkert and Silk (1997) that the observed rotation curve of the dwarf irregular galaxy DDO154 cannot be reconciled with the universal CDM halo profile of Navarro, Frenk & White (1996,1997) even when allowance is made for the effect of violent gas outflow events on the structure of the galaxy. By means of N-body simulations we show that under certain conditions it is possible to obtain a reasonable fit to the observed rotation curve without invoking Burkert & Silk's proposed spheroidal MACHO component. We are able to best reproduce the observed decline in the rotation curve by postulating additional hidden disc mass, in an amount that is compatible with disc stability requirements. In the process we improve upon the results of Navarro, Eke & Frenk (1996) on the formation of halo cores by mass loss by using actual haloes from Cold Dark Matter simulations instead of Hernquist (1990) distributions.Comment: LaTeX (mn.sty), 8 pages, 6 figures included; updated to match final version to appear in MNRA

Feedback from galactic stellar bulges and hot gaseous haloes of galaxies

We demonstrate that the feedback from stellar bulges can play an essential role in shaping the halo gas of galaxies with substantial bulge components by conducting 1-D hydrodynamical simulations. The feedback model we consider consists of two distinct phases: 1) an early starburst during the bulge formation and 2) a subsequent long-lasting mass and energy injection from stellar winds of low-mass stars and Type Ia SNe. An energetic outward blastwave is initiated by the starburst and is maintained and enhanced by the long-lasting stellar feedback. For a MW-like galactic bulge, this blastwave sweeps up the halo gas in the proto-galaxy and heats up the surrounding medium to a scale much beyond the virial radius of the halo, thus the accretion of the halo hot gas can be completely stopped. In addition, the long-lasting feedback in the later phase powers a galactic bulge wind that is reverse-shocked at a large radius in the presence of surrounding intergalactic medium and hence maintains a hot gaseous halo. As the mass and energy injection decreases with time, the feedback evolves to a subsonic and quasi-stable outflow, which is enough to prevent halo gas from cooling. The two phases of the feedback thus re-enforce each-other's impact on the gas dynamics. The simulation results demonstrate that the stellar bulge feedback may provide a plausible solution to the long-standing problems in understanding the MW type galaxies, such as the "missing stellar feedback" problem and the "over-cooling" problem. The simulations also show that the properties of the hot gas in the subsonic outflow state depend sensitively on the environment and the formation history of the bulge. This dependence and variance may explain the large dispersion in the X-ray to B-band luminosity ratio of the low $L_X/L_B$ Es.Comment: v2, discussions added, accepted for publication in MNRA

The environmental dependence of clustering in hierarchical models

In hierarchical models, density fluctuations on different scales are correlated. This induces correlations between dark halo masses, their formation histories, and their larger-scale environments. In turn, this produces a correlation between galaxy properties and environment. This correlation is entirely statistical in nature. We show how the observed clustering of galaxies can be used to quantify the importance of this statistical correlation relative to other physical effects which may also give rise to correlations between the properties of galaxies and their surroundings. We also develop a halo model description of this environmental dependence of clustering.Comment: 11 pages, 6 figures, MNRAS in pres

An analytical model for the accretion of dark matter subhalos

An analytical model is developed for the mass function of cold dark matter subhalos at the time of accretion and for the distribution of their accretion times. Our model is based on the model of Zhao et al. (2009) for the median assembly histories of dark matter halos, combined with a simple log-normal distribution to describe the scatter in the main-branch mass at a given time for halos of the same final mass. Our model is simple, and can be used to predict the un-evolved subhalo mass function, the mass function of subhalos accreted at a given time, the accretion-time distribution of subhalos of a given initial mass, and the frequency of major mergers as a function of time. We test our model using high-resolution cosmological $N$-body simulations, and find that our model predictions match the simulation results remarkably well. Finally, we discuss the implications of our model for the evolution of subhalos in their hosts and for the construction of a self-consistent model to link galaxies and dark matter halos at different cosmic times.Comment: 14 pages, 10 figures (caption for figure 10 fixed). Accepted for publication in Ap

Internal properties and environments of dark matter halos

We use seven high-resolution $N$-body simulations to study the correlations among different halo properties (assembly time, spin, shape and substructure), and how these halo properties are correlated with the large-scale environment in which halos reside. The large-scale tidal field estimated from halos above a mass threshold is used as our primary quantity to characterize the large-scale environment, while other parameters, such as the local overdensity and the morphology of large-scale structure, are used for comparison. For halos at a fixed mass, all the halo properties depend significantly on environment, particularly the tidal field. The environmental dependence of halo assembly time is primarily driven by local tidal field. The mass of the unbound fraction in substructure is boosted in strong tidal force region, while the bound fraction is suppressed. Halos have a tendency to spin faster in stronger tidal field and the trend is stronger for more massive halos. The spin vectors show significant alignment with the intermediate axis of the tidal field, as expected from the tidal torque theory. Both the major and minor axes of halos are strongly aligned with the corresponding principal axes of the tidal field. In general, a halo that can accrete more material after the formation of its main halo on average is younger, is more elongated, spins faster, and contains a larger amount of substructure. Higher density environments not only provide more material for halo to accrete, but also are places of stronger tidal field that tends to suppress halo accretion. The environmental dependencies are the results of these two competing effects. The tidal field based on halos can be estimated from observation, and we discuss the implications of our results for the environmental dependence of galaxy properties.Comment: Accepted for publication in MNRA