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Register a new userFormation of Andromeda II via a gas-rich major merger and an interaction with M31
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Andromeda II (And II) has been known for a few decades but only recently observations have unveiled new properties of this dwarf spheroidal galaxy. The presence of two stellar populations, the bimodal star formation history (SFH) and an unusual rotation velocity of And II put strong constrains on its formation and evolution. Following Lokas et al. (2014), we propose a detailed model to explain the main properties of And II involving (1) a gas-rich major merger between two dwarf galaxies at high redshift in the field and (2) a close interaction with M31 about 5 Gyr ago. The model is based on N-body/hydrodynamical simulations including gas dynamics, star formation and feedback. One simulation is designed to reproduce the gas-rich major merger explaining the origin of stellar populations and the SFH. Other simulations are used to study the effects of tidal forces and the ram pressure stripping during the interaction between And II and M31. The model successfully reproduces the SFH of And II including the properties of stellar populations, its morphology, kinematics and the lack of gas. Further improvements to the model are possible via joint modelling of all processes and better treatment of baryonic physics.
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It is widely accepted that within the framework of LCDM a significant fraction of giant-disk galaxies has recently experienced a violent galactic merger. We present numerical simulations of such major mergers of gas-rich pure disk galaxies, and focus on the innermost stellar component (bulge) of the disk remnants. The simulations have high spatial and mass resolutions, and resolve regions deep enough to allow bulge classification according to standard kinematical and structural characteristics. In agreement with recent studies we find that these bulges are dominated by stars formed in the final coalescence process. In contrast to the common interpretation of such components as classical bulges (i.e. similar to intermediate luminosity ellipticals), we find they are supported by highly coherent rotations and have Sersic indices n<2, a result leading to their classification as pseudo-bulges. Pseudo-bulge formation by gas rich major mergers of pure disks is a novel mode of pseudo-bulge formation; It complements pseudo-bulge growth by secular evolution, and it could help explain the high fractions of classically bulge-less giant disk galaxies, and pseudo-bulges found in giant Sc galaxies.
We present the first comparison between the lifetime star formation histories (SFHs) of M31 and Milky Way (MW) satellites. Using the Advanced Camera for Surveys aboard the Hubble Space Telescope, we obtained deep optical imaging of Andromeda II (M$_{V} = -$12.0; log(M$_{star}$/M$_{odot}$) $sim$ 6.7) and Andromeda XVI (M$_{V} = -$7.5; log(M$_{star}$/M$_{odot}$) $sim$ 4.9) yielding color-magnitude diagrams that extend at least 1 magnitude below the oldest main sequence turnoff, and are similar in quality to those available for the MW companions. And II and And XVI show strikingly similar SFHs: both formed 50-70% of their total stellar mass between 12.5 and 5 Gyr ago (z$sim$5-0.5) and both were abruptly quenched $sim$ 5 Gyr ago (z$sim$0.5). The predominance of intermediate age populations in And XVI makes it qualitatively different from faint companions of the MW and clearly not a pre-reionization fossil. Neither And II nor And XVI appears to have a clear analog among MW companions, and the degree of similarity in the SFHs of And II and And XVI is not seen among comparably faint-luminous pairs of MW satellites. These findings provide hints that satellite galaxy evolution may vary substantially among hosts of similar stellar mass. Although comparably deep observations of more M31 satellites are needed to further explore this hypothesis, our results underline the need for caution when interpreting satellite galaxies of an individual system in a broader cosmological context.
The numerous streams in the M31 halo are currently assumed to be due to multiple minor mergers. Here we use the GADGET2 simulation code to test whether M31 could have experienced a major merger in its past history. It results that a 3+/-0.5:1 gaseous rich merger with r(per)=25+/-5 kpc and a polar orbit can explain many properties of M31 and of its halo. The interaction and the fusion may have begun 8.75+/-0.35 Gyr and 5.5 +/-0.5 Gyr ago, respectively. With an almost quiescent star formation history before the fusion we retrieve fractions of bulge, thin and thick disks as well as relative fractions of intermediate age and old stars in both the thick disk and the Giant Stream. The Giant Stream is caused by returning stars from a tidal tail previously stripped from the satellite prior to the fusion. These returning stars are trapped into elliptical orbits or loops for almost a Hubble time period. Large loops are also predicted and they scale rather well with the recently discovered features in the M31 outskirts. We demonstrate that a single merger could explain first-order (intensity and size), morphological and kinematical properties of the disk, thick disk, bulge and streams in the halo of M31, as well as the distribution of stellar ages, and perhaps metallicities. It challenges scenarios assuming one minor merger per feature in the disk (10 kpc ring) or at the outskirts (numerous streams & thick disk). Further constraints will help to properly evaluate the impact of such a major event to the Local Group.
Using N-body simulations we study the origin of prolate rotation recently detected in the kinematic data for And II, a dSph satellite of M31. We propose an evolutionary model for the origin of And II involving a merger between two disky dwarf galaxies whose structural parameters differ only in their disk scale lengths. The dwarfs are placed on a radial orbit towards each other with their angular momenta inclined by 45 deg to the orbital plane and by 90 deg with respect to each other. After 5 Gyr of evolution the merger remnant forms a stable triaxial galaxy with rotation only around the longest axis. The origin of this rotation is naturally explained as due to the symmetry of the initial configuration which leads to the conservation of angular momentum components along the direction of the merger. The stars originating from the two dwarfs show significantly different surface density profiles while having very similar kinematics in agreement with the properties of separate stellar populations in And II. We also study an alternative scenario for the formation of And II, via tidal stirring of a disky dwarf galaxy. While intrinsic rotation occurs naturally in this model as a remnant of the initial rotation of the disk, it is mostly around the shortest axis of the stellar component. The rotation around the longest axis is induced only occasionally and remains much smaller that the systems velocity dispersion. We conclude that although tidal origin of the velocity distribution in And II cannot be excluded, it is much more naturally explained within the scenario involving a past merger event. Thus, in principle, the presence of prolate rotation in dSph galaxies of the Local Group and beyond may be used as an indicator of major mergers in their history or even as a way to distinguish between the two scenarios of their formation.
We present the result of radio and optical observations of the S0 galaxy IC 4200. We find that the galaxy hosts 8.5 billion solar masses of HI rotating on a ~90 deg warped disk extended out to 60 kpc from the centre of the galaxy. Optical spectroscopy reveals a simple-stellar-population-equivalent age of 1.5 Gyr in the centre of the galaxy and V- and R-band images show stellar shells. Ionised gas is observed within the stellar body and is kinematically decoupled from the stars and characterised by LINER-like line ratios.We interpret these observational results as evidence for a major merger origin of IC 4200, and date the merger back to 1-3 Gyr ago.
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