The Hubble sequence: just a vestige of merger events?

Abstract

We investigate whether the Hubble sequence can be reproduced by the relics of merger events. We verify that, at z median = 0.65, the abundant population of anomalous starbursts -i.e. with peculiar morphologies and abnormal kinematics -is mainly linked to the local spirals. Their morphologies are dominated by young stars and are intimately related to their ionised-gas kinematics. We show that both morphologies and kinematics can be reproduced by using gas modelling from Barnes ' (2002, MNRAS, 333, 481) study of major mergers. Their gas content may be indirectly evaluated by assuming that distant starbursts follow the Kennicutt-Schmidt relation: the median gas fraction is found to be 31%. Using our modelling to estimate the gas-to-star transformation during a merger, we identify the gas fraction in the progenitors to be generally above 50%. All distant and massive starbursts can be distributed along a temporal sequence from the first passage to the nuclei fusion and then to the disk rebuilding phase. This later phase has been recently illustrated for J033245.11-274724.0, a distant compact galaxy dominated by a red, dust-enshrouded disk. This active production of rebuilt disks is in excellent agreement with model predictions for gaseous rich encounters. It confirms that the rebuilding spiral disk scenarioa strong and recent reprocessing of most disks by major mergers -is possibly an important channel for the formation of presentday disks in grand-design spirals. Because half of the present-day spirals had peculiar morphologies and anomalous kinematics at z median = 0.65, they could indeed have been in major merger phases 6 Gyr ago, and almost all at z ∼ 1. It is time now to study in detail the formation of spiral disks and of their substructures, including bulge, disks, arms, bars and rings that may mainly originate from instabilities created during the last major merger. Many galaxies also show a helicoidal structure, which is probably due to a central torque, and seems to play an important role in regulating the angular momentum of the newly-formed disks.

How was the Hubble sequence 6 Gyr ago?

“…This is not mainly caused by the effects described above. This result slightly affect the conclusions of Neichel et al (2008) and Hammer et al (2005Hammer et al ( , 2009, who assumed that besides E/S0, all quiescent galaxies have to be spirals. This simply reduces further the fraction of regular, rotating disks in the past Hubble diagram whose fraction is 2.3 times lower than in the present-day galaxies.…”

“…In the following we assume that the actual limit in linking past to present Hubble sequence is realized when a galaxy is simply suppressed by being accreted by a more massive galaxy. Hammer et al (2009), assuming that all objects could be modeled as a merger or a merger remnant, have derived the corresponding merger mass ratio (see their Fig. 6, bottom).…”

“…A.2). Hammer et al (2009) find that distant starbursts have a gas fraction averaging 31%. This is based on assuming the Kennicutt law that links surface density of star formation rate to that of gas mass.…”

The Dawes Review 1: Kinematic Studies of Star-Forming Galaxies Across Cosmic Time

“…However, in CDM models it has long been supposed that such ellipticals could continue to accrete gas (either pristine or expelled during the merger) and form new discs of young stars; they would then transform back in to a spiral albeit one with a large bulge-to-disc ratio (Barnes 2002;Springel & Hernquist 2005). Hammer et al (2009) posit a ' sprial rebuilding scenario' in which half of today's major spirals were in an active major merger phase 6 Gyr ago, and all have had a merger since z = 1, (with the Milky Way being exceptional) and the disc is then rebuilt by re-accreting the original gas. This may have more angular momentum than cosmological accretion as it retains that of the original disc.…”

Loops formed by tidal tails as fossil records of a major merger

“…Simulations with low gas fractions (≤10%) also show that a significant fraction of the gas can survive a major merger and form a new disc (Barnes & Hernquist 1996;Barnes 2002). By observing and analysing the progenitors of local spirals at intermediate redshift, Hammer et al (2005Hammer et al ( , 2007Hammer et al ( , 2009 suggest that many present-day discs could have been rebuilt after a major merger. Recent theoretical advances also support this scenario.…”