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25 June 2012

Galaxy disks do not need to survive in the L-CDM paradigm

Full title: Galaxy disks do not need to survive in the L-CDM paradigm: the galaxy merger rate out to z~1.5 from morpho-kinematic data

Authors: Puech, M., et al
Paper: here

The main result of the paper is the derivation of the time evolution of the galaxy merger rate using a sample of intermediate mass (M_stellar=10^10-10^11 M_sun) emission-line galaxies for which they have both kinematic (IMAGES survey) and morphological (from HST) data. These galaxies are chosen because they are the most likely progenitors of local spirals (see justifications in the paper). A large fraction of these galaxies are not relaxed morphologically and/or kinematically and the authors demonstrate that the disturbances are most likely triggered by major mergers.

The authors then match as many as possible of these observed, disturbed galaxies to simulated major mergers by comparing the real and simulated velocity fields and morphology etc. This allows them to associate a time since the merger began with each observation and they group them into “pre-fusion”, “post-fusion” (the main starburst period) and “relaxation” phases. They plot the observed SFR (normalised by gas mass) versus the time since the start of merger to illustrate that the IMAGES galaxies sample all the phases of an average merger well. They can then derive a merger rate from: rate = ngal in merger phase/time period for this phase, and a corresponding redshift for this rate: z= average start time of mergers in this phase. This gives them merger rates of 5.5, 10.1, 11.1 %/Gyr at z=0.72, 1.12, 1.55 respectively. They show that these merger rate values are in agreement with those from the Hopkins et al. 2009 semi-empirical model to within a factor of 2-3.

The authors draw some additional conclusions, including:
· Given the high gas fractions they infer for the major mergers occurring at z>0.6, a significant disc should be able to reform by z=0.
· The merger rate from the semi-empirical model is not in conflict with the fraction of bulgeless galaxies observed locally.
· Since cold streams have been shown to be suppressed below z~1.5, gas-rich major mergers could potentially take over as a channel for massive, thin disk formation at this redshift.
· Overall: there is no disc survival problem, but a period in which discs could/should be rebuilt at z<1.5.

1 comment:

  1. The method of determining the phase of the merger by matching with simulations is nice, but it seems there are many caveats on the final merger rate derived. For example, a post-fusion phase timescale is not determined and so is set to the pre-fusion phase value (based on Fig. 4, even though some of the post fusion points lie at large t values). It seems unlikely the main starburst (“post-fusion” phase) will endure for as long as the presence of kinematically/morphological disturbances that mark the pre-fusion phase, as these can be triggered long before the galaxies come together for the final time. It seems strange that the relaxation phase also has a very similar duration.

    The evidence that the mergers occurring around 0.6<z<1.5 should be gas-rich was not totally convincing in the form in which it is presented here and this is crucial if the authors want to argue that the discs are rebuilt (although we note that it may be discussed thoroughly in one of the other papers by the same authors that is cited). The claims that “gas fractions at a level of 80-100% are probably close to average conditions” at z~1.5 seems surprising (even though they are derived from observations) and the initial gas fractions inferred from the simulations could be unreliable, due to the usual issues of what feedback recipes are involved etc.