Simulation codes for galaxy formation and evolution take on board as many physical processes as possible beyond the standard gravitational and hydrodynamical physics. Most of this extra physics takes place below the resolution level of the simulation
s and is added in a sub-grid fashion. However, these sub-grid processes affect the macroscopic hydrodynamical properties of the gas and thus couple to the on-grid physics that is explicitly integrated during the simulation. In this paper, we focus on the link between partial ionization and the hydrodynamical equations. We show that the energy stored in ions and free electrons constitutes a potential energy term which breaks the linear dependence of the internal energy on temperature. Correctly taking into account ionization hence requires modifying both the equation of state and the energy-temperature relation. We implemented these changes in the cosmological simulation code Gadget2. As an example of the effects of these changes, we study the propagation of Sedov-Taylor shock waves through an ionizing medium. This serves as a proxy for the absorption of supernova feedback energy by the interstellar medium. Depending on the density and temperature of the surrounding gas, we find that up to 50% of the feedback energy is spent ionizing the gas rather than heating it. Thus, it can be expected that properly taking into account ionization effects in galaxy evolution simulations will drastically reduce the effects of thermal feedback. To the best of our knowledge, this potential energy term is not used in current simulations of galaxy formation and evolution.
In this paper we investigate whether Smoothed Particle Hydrodynamics (SPH), equipped with artificial conductivity, is able to capture the physics of density/energy discontinuities in the case of the so-called shearing layers test, a test for examinin
g Kelvin-Helmholtz (KH) instabilities. We can trace back each failure of SPH to show KH rolls to two causes: i) shock waves travelling in the simulation box and ii) particle clumping, or more generally, particle noise. The probable cause of shock waves is the Local Mixing Instability (LMI), previously identified in the literature. Particle noise on the other hand is a problem because it introduces a large error in the SPH momentum equation. We also investigate the role of artificial conductivity (AC). Including AC is necessary for the long-term behavior of the simulation (e.g. to get $lambda=1/2, 1$ KH rolls). In sensitive hydrodynamical simulations great care is however needed in selecting the AC signal velocity, with the default formulation leading to too much energy diffusion. We present new signal velocities that lead to less diffusion. The effects of the shock waves and of particle disorder become less important as the time-scale of the physical problem (for the shearing layers problem: lower density contrast and higher Mach numbers) decreases. At the resolution of current galaxy formation simulations mixing is probably not important. However, mixing could become crucial for next-generation simulations.
We present optical VLT spectroscopy of 16 dwarf elliptical galaxies (or dEs) comparable in mass to NGC 205, and belonging to the Fornax cluster and to nearby groups of galaxies. Using ULySS and STECKMAP, we derive radial profiles of the SSP-equivalen
t ages, metallicities and star-formation histories. The old stellar population of the dEs, which dominates their mass, is likely coeval with that of massive ellipticals or bulges, but the star formation efficiency is lower. Important intermediate age (1-5 Gyr) populations, and frequently tails of star formation until recent times are detected. These histories are reminiscent of their lower mass dSph counterparts of the Local Group. Most galaxies (10/16) show significant metallicity gradients, with metallicity declining by 0.5 dex over one half-light radius on average. These gradients are already present in the old population. The flattened (or discy), rotating objects (6/16) have flat metallicity profiles. This may be consistent with a distinct origin for these galaxies or it may be due to their geometry. The central SSP-equivalent age varies between 1 and 6 Gyr, with the age slowly increasing with radius in the vast majority of objects. The group and cluster galaxies have similar radial gradients and star-formation histories. The strong and old metallicity gradients place important constraints on the possible formation scenarios of dEs. Numerical simulations of the formation of spherical low-mass galaxies reproduce these gradients, but they require a longer time for them to build up. A gentle depletion of the gas, by ram-pressure stripping or starvation, could drive the gas-rich, star-forming progenitors to the present dEs.
We present high spatial resolution 21cm HI observations of EA01A and EA01B, a pair of interacting post-starburst, or E+A, galaxies at z = 0.0746. Based on optical HST/WFPC2 images, both galaxies are known to display disturbed morphologies. They also
appear to be linked by a bridge of stars. Previous HI observations Chang et al. (2001) had already uncovered sizable quantities of neutral gas in or near these galaxies but they lacked the spatial resolution to locate the gas with any precision within this galactic binary system. We have analysed deep, high resolution archival VLA observations of the couple. We find evidence for three gaseous tidal tails; one connected to EA01A and two emanating from EA01B. These findings confirm, independently from the optical imaging, that (i) EA01A and EA01B are actively interacting, and that, as a consequence, the starbursts that occurred in these galaxies were most likely triggered by this interaction, and that (ii) 6.6+-0.9 10^9 Msun of neutral gas are still present in the immediate vicinity of the optical bodies of both galaxies. The HI column density is lowest at the optical positions of the galaxies, suggesting that most of the neutral gas that is visible in our maps is associated with the tidal arms and not with the galaxies themselves. This might provide an explanation for the apparent lack of ongoing star formation in these galaxies.
The presence of black holes (BHs) at the centers of dwarf elliptical galaxies (dEs) has been argued both theoretically and observationally. Using archival HST/WFPC2 data, we found the Virgo cluster dwarf elliptical galaxy VCC128 to harbor a binary nu
cleus, a feature that is usually interpreted as the observable signature of a stellar disk orbiting a central massive black hole. Debattista et al. 2006 estimated its mass M sim 6 10^6 - 5 10^7 Msun. One of the most robust means of verifying the existence of a BH is radio continuum and/or X-ray emission, however because of the deficiency of gas in dEs, radio continuum emission forms the best option here. We have tried to detect the X-band radio emission coming from the putative black hole in VCC128 when it accretes gas from the surrounding ISM. While we made a positive 4 sigma detection of a point source 4.63 south-west of the binary nucleus, no statistically significant evidence for emission associated with the nuclei themselves was detected. This implies either that VCC128 has no massive central black hole, which makes the nature of the binary nucleus hard to explain, or, if it has a central black hole, that the physical conditions of the ISM (predominantly its density and temperature) and/or of the surrounding accretion disk do not allow for efficient gas accretion onto the black hole, making the quiescent black hole very hard to detect at radio wavelengths.
