Ancient Merger Research Articles

The vast thin plane of M31 corotating dwarfs: an additional fossil signature of the M31 merger and of its considerable impact in the whole Local Group

The recent discovery by Ibata et al. of a vast thin disc of satellites (VTDS) around M31 offers a new challenge for the understanding of the Local Group properties. This comes in addition to the unexpected proximity of the Magellanic Clouds (MCs) to the Milky Way (MW), and to another vast polar structure (VPOS), which is almost perpendicular to our Galaxy disc. We find that the VTDS plane is coinciding with several stellar, tidally induced streams in the outskirts of M31, and, that its velocity distribution is consistent with that of the giant stream (GS). This is suggestive of a common physical mechanism, likely linked to merger tidal interactions, knowing that a similar argument may apply to the VPOS at the MW location. Furthermore, the VTDS is pointing towards the MW, being almost perpendicular to the MW disc, as the VPOS is. We compare these properties to the modelling of M31 as an ancient, gas-rich major merger, which has been successfully used to predict the M31 substructures and the GS origin. We find that without fine tuning, the induced tidal tails are lying in the VTDS plane, providing a single and common origin for many stellar streams and for the vast stellar structures surrounding both the MW and M31. The model also reproduces quite accurately positions and velocities of the VTDS spheroidal dwarfs. Our conjecture leads to a novel interpretation of the Local Group past history, as a gigantic tidal tail due to the M31 ancient merger is expected to send material towards the MW, including the MCs. Such a link between M31 and the MW is expected to be quite exceptional, though it may be in qualitative agreement with the reported rareness of MW–MCs systems in nearby galaxies.

Chemical analysis of ancient relicts in the Milky Way disk

We present detailed analysis of two groups of F- and G- type stars originally found to have similarities in their orbital parameters. The distinct kinematic properties suggest that they might originate from ancient accretion events in the Milky Way. From high resolution spectra taken with the spectrograph FIES at the Nordic Optical Telescope, La Palma, we determined abundances of oxygen, alpha- and r-process elements. Our results indicate that the sample of investigated stars is chemically homogeneous and the abundances of oxygen, alpha and r-process elements are overabundant in comparison with Galactic disk dwarfs. This provides the additional evidence that those stellar groups had the common formation and possible origin from disrupted satellites. 1. KINEMATIC SUBSTRUCTURES IN THE SOLAR NEIGHBORHOOD Observations and theoretical simulations have made much progress and provided us with tools to search for past accretion events in the Milky Way and beyond. The current events are the Sagittarius and Canis Major dwarf spheroidal galaxies, merging into the Galaxy. Traces of such mergers are visible in stellar trails revealed in the Galactic halo by large surveys (e.g. (9)). Can we also find such traces of ancient merger events in the solar neighborhood? Such events are much harder to discern among the stars of the disk. Combining kinematic and chemical analyses we could answer this question. Helmi et al. (2006), using Nordstrom et al. (2004) catalog, from correlations between the apocentre (A), pericentre (P) and z-angular momentum (Lz) - the so called APL space of orbital parameters - identified three new coherent groups of stars and suggested that those might correspond to remains of disrupted satellites. Stars in those groups cluster around regions of roughly constant eccentricity, they have the distinct distribution of kinematics, metallicity (Fe/H) and age, providing evidences of their extragalactic origin. In our high resolution spectroscopic study we aim to investigate the Milky Way disc. F - and G type dwarf stars are very useful in studing the ancient history of our Galaxy since they are long-lived, and their atmospheres reflect their initial chemical composition. In this study, we measured abundances of oxygen, - and r-process elements and here we discuss our results. The method of detailed chemical analysis, that we used in this study, can be found in our previous paper (8).

Origin of rotational kinematics in the globular cluster system of M31: a new clue to the bulge formation

Abstract We propose that the rotational kinematics of the globular cluster system (GCS) in M31 can result from a past major merger event that could have formed its bulge component. We numerically investigate kinematical properties of globular clusters (GCs) in remnants of galaxy mergers between two discs with GCs in both their disc and halo components. We find that the GCS formed during major merging can show strongly rotational kinematics with the maximum rotational velocities of ∼140–170 km s−1 for a certain range of orbital parameters of merging. We also find that a rotating stellar bar, which can be morphologically identified as a boxy bulge if seen edge-on, can be formed in models for which the GCSs show strongly rotational kinematics. We thus suggest that the observed rotational kinematics of GCs with different metallicities in M31 can be closely associated with the ancient major merger event. We discuss whether the formation of the rotating bulge/bar in M31 can be due to the ancient merger.

Chemodynamical analysis of bulge stars for simulated disc galaxies

We analyse the kinematics and chemistry of the bulge stars of two simulated disc galaxies using our chemodynamical galaxy evolution code GCD+. First we compare stars that are born inside the galaxy with those that are born outside the galaxy and are accreted into the centre of the galaxy. Stars that originate outside of the bulge are accreted into it early in its formation within 3 Gyrs so that these stars have high [alpha/Fe] as well as having a high total energy reflecting their accretion to the centre of the galaxy. Therefore, higher total energy is a good indicator for finding accreted stars. The bulges of the simulated galaxies formed through multiple mergers separated by about a Gyr. Since [alpha/Fe] is sensitive to the first few Gyrs of star formation history, stars that formed during mergers at different epochs show different [alpha/Fe]. We show that the [Mg/Fe] against star formation time relation can be very useful to identify a multiple merger bulge formation scenario, provided there is sufficiently good age information available. Our simulations also show that stars formed during one of the merger events retain a systematically prograde rotation at the final time. This demonstrates that the orbit of the ancient merger that helped to form the bulge could still remain in the kinematics of bulge stars.

Formation of the Small Magellanic Cloud: An Ancient Major Merger as a Solution to the Kinematical Differences between Old Stars and H i Gas

Recent observations of the Small Magellanic Cloud (SMC) have revealed that the H I gas shows a significant amount of rotation (Vc ~ 60 km s−1), while no or little rotation is evident for the old stellar populations. We suggest that this unique kinematical difference between these components in the SMC can be caused by a major merger event which occurred in the early stage of the SMC formation. Our simulations show that dissipative dwarf-dwarf merging can transform two gas-rich dwarf irregulars into a new dwarf, which consists of a spheroidal stellar component and a rotating extended H I disk. The remnant of this dwarf-dwarf merging shows significantly different kinematics between stars and gas, in the sense that a gas disk rotates rapidly while a stellar component shows little rotation. We thus suggest that the simulated dwarf having a dynamically hot spheroid and an extended gas disk finally evolves into the present SMC after efficient stripping of the outer gas via tidal fields of the Galaxy and the Large Magellanic Cloud. We also suggest that spatial distributions and kinematics of RGB and AGB stars with different ages in the possible spheroidal component of the SMC can provide valuable information on whether and when a past major merger event really occurred in the SMC.

AnXMM-Newtonobservation of the dynamically active binary cluster A1750

We present results from the XMM-Newton observation of the binary cluster A1750 at z = 0.086. We have performed a detailed study of the surface brightness, temperature and entropy distribution and confirm that the two main clusters of the system (A1750 N and A1750 C) have just started to interact. From the temperature distribution, we calculate that they are likely to merge sometime in the next 1 Gyr. The more massive cluster, A1750 C, displays a more complicated temperature structure than expected. We detect a hot region associated with a density jump ~450 kpc east of the cluster centre, which appears to be a shock wave. This shock is not connected with the binary merger, but it is intrinsic to A1750 C itself. From simple physical arguments and comparison with numerical simulations, we argue that this shock is related to a merging event that A1750 C has suffered in the past 1-2 Gyr. The larger scale structure around A1750 suggests that the system belongs to a rich supercluster, which would presumably increase the likelihood of merger events. These new XMM-Newton data thus show that A1750 is a complex system, where two clusters are starting to interact before having re-established equilibrium after a previous merger. This merger within a merger indicates that the present day morphology of clusters may depend not only on on-going interactions or the last major merging event, but also on the more ancient merger history, especially in dense environments.

Correlations between UBV colors and fine structure in E and S0 galaxies - A first attempt at dating ancient merger events

A study of 69 E and S0 galaxies located mostly in the field and in groups reveals that the UBV colors become systematically bluer at any given luminosity as the amount of merger-induced fine structure increases. To quantify such fine structure, we define an index Σ that measures ripples, jets of luminous matter, X structures, and boxy isophotes; it ranges between 0 and 7.6 for the above galaxies. The correlations between UBV colors and this index Σ closely resemble the correlations found earlier between CN, Mg 2 , and Hβ line strengths and the same Σ in 36 ellipticals [Schweizer et al. ApJ, 364, L33 (1990)]