The Formation of Large Galactic Disks Through the Hierarchical Scenario: Further Consequences

Abstract

Using the deepest and most complete observations of distant galaxies, we investigate the progenitors of present-day large spirals. Observations include spatially-resolved kinematics, detailed morphologies and photometry from UV to mid-IR. Six billions years ago, half of the present-day spirals were starbursts experiencing major mergers, evidence for this is provided by their anomalous kinematics and morphologies. They are consequently modeled using hydrodynamics models of mergers and it perfectly matches with merger rate predictions by state-of-the-art-ΛCDM semi-empirical models. Furthermore imprints in the halo of local galaxies such as M31 or NGC5907 are likely caused by major merger relics. This suggests that the hierarchical scenario has played a major role in shaping the massive galaxies of the Hubble sequence. Linking galaxy properties at different epochs is the best way to fully understand galaxy formation processes and we have tested such a link through generated series of simulations of gas-rich mergers. Mergers have expelled material in galactic haloes and beyond, possibly explaining 60% of the missing baryons in Milky-Way (MW) mass galaxies. A past major merger in M31 might drastically affect our understanding of Local Group galaxies, including MW dwarfs. We also propose future directions to observationally constrain the necessary ingredients in galaxy simulations.

Galaxy Mass Assembly with VLT & HST and lessons for E-ELT/MOSAIC

“…In fact, the merger model of M31 of Hammer et al (2010) predicted the disk of satellites discovered by Ibata et al (2013). Spatial locations of these M31 satellites are drawing two loop systems (see Figure 3d and Hammer et al 2013) similar to those found around NGC5907, i.e., typical signatures of ancient major mergers.…”

“…The later provides a robust explanation of the stellar Giant Stream, which could be made of tidal tail stars captured by the galaxy gravitational potential of the stellar loops, 5.3 Gyrs after a 3:1 merger (first passage, 6.8 Gyrs ago, f initial gas =60%, f initial baryons =17%, see Wang et al 2012). (c): Stellar particles associated to an ancient 3:1 merger at M31 location (GADGET2, 8M particles see Hammer et al 2010Hammer et al , 2013Fouquet et al 2012), and forming a plane seen almost edge-on, coinciding with the observed satellite plane (almost seen edge-on, in blue color) found by Ibata et al (2013). (d): Same as (c) but rotated by 90 degrees to reveal the loops system in the loop plane.…”

“…Perhaps the numerous bulges with low Sersic indices (often called pseudo-bulges, see Kormendy 2013 and references therein) found in local galaxies are at odd with a major merger history. Forming pseudo bulge galaxies is however expected through ancient gas-rich mergers (Keselman & Nusser 2012;Hammer et al 2012), though further investigations are necessary. The scatter of the M stellar − SF R relation for emission line galaxies could be too narrow (0.3 dex, Noeske et al 2007) for accommodating major mergers, which are expected to show strong and short living bursts during the fusion phase.…”

“…Both MW and M31 possess fossil satellite systems, which might be linked together because the Ibata et al (2013) disk of satellites is pointing to the MW. Not surprisingly it is also a prediction of the modeling of M31 as an ancient merger, and the main associated tidal tail may have reached the MW (Fouquet et al 2012;Hammer et al 2013).…”

“…Examination of nearby galaxy outskirts reveals fossil imprints of such ancient merger events, under the form of well organized stellar streams. Perhaps our neighbor, M31, is the best illustration of an ancient merger, which modeling in 2010 leads to predict the gigantic plane of satellites discovered by Ibata et al (2013). There are still a lot of discoveries to be done until the ELT era, which will open an avenue for detailed and accurate 3D-spectroscopy of galaxies from the earliest epochs to the present.…”