How do galaxies die? Contradictions between two papers

[Novanglus]

Hi all!
My name is Nova and I have recently started a research project on the life cycle of planets, stars, galaxies, etc. I posted this question on reddit, but no one seemed particularly interested. Someone suggested I consult the ultimate physics community on the web, Physics Forums, so here I am.

These two papers (one published in February, the other, a few days ago) seem to be contradicting each other. One says they will die from "strangulation" because their source of fuel eventually runs out (they measured the average change of metallicity in thousands of galaxies to determine this). The other says they die because their source of fuel is actually stripped away. Am I not understanding something? Can they both be right?
My hunch says both *can* happen, but the latter is much less likely and more research needs to be done.

A bit about myself. I am an astronomy major and a novice shroomer (mushroom farmer, non-psychadelic).
Thanks so much
Nova

1.) May: http://phys.org/news/2015-05-galactic-death-strangulation.html

2.) February: http://phys.org/news/2015-02-fast-die-young-galaxies-gas.html#nRlv

P.S. Let me know if you need me to link to the research papers themselves.

 

[marcus]

Welcome! I'm glad to see you--this is an interesting question and it seems as yet unresolved. You could give us the arXiv links to the professional grade papers. I'm not an expert but I like to see what the authors actually said.

One is here:
http://arxiv.org/abs/1501.07653
Misalignment between cold gas and stellar components in early-type galaxies
O. Ivy Wong, K. Schawinski, G.I.G. Józsa, C.M. Urry, C.J. Lintott, B.D. Simmons, S. Kaviraj, K.L. Masters
(Submitted on 30 Jan 2015)
Recent work suggests blue ellipticals form in mergers and migrate quickly from the blue cloud of star-forming galaxies to the red sequence of passively evolving galaxies, perhaps as a result of black hole feedback. Such rapid reddening of stellar populations implies that large gas reservoirs in the pre-merger star-forming pair must be depleted on short time scales. Here we present pilot observations of atomic hydrogen gas in four blue early-type galaxies that reveal increasing spatial offsets between the gas reservoirs and the stellar components of the galaxies, with advancing post-starburst age. Emission line spectra show associated nuclear activity in two of the merged galaxies, and in one case radio lobes aligned with the displaced gas reservoir. These early results suggest that a kinetic process (possibly feedback from black hole activity) is driving the quick truncation of star formation in these systems, rather than a simple exhaustion of gas supply.
Comments: 12 pages, 9 figures, accepted for publication in MNRAS
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I don't see any necessary contradiction. Say MOST die from strangulation, that is the primary cause.
But there is a small class of galaxies that "die young". Inexplicably stop forming new stars, and turn from blue to red.
The other is here:
http://arxiv.org/abs/1505.03143
Strangulation as the primary mechanism for shutting down star formation in galaxies
Yingjie Peng, Roberto Maiolino, Rachel Cochrane
(Submitted on 12 May 2015)
Local galaxies are broadly divided into two main classes, star-forming (gas-rich) and quiescent (passive and gas-poor). The primary mechanism responsible for quenching star formation in galaxies and transforming them into quiescent and passive systems is still unclear. Sudden removal of gas through outflows or stripping is one of the mechanisms often proposed. An alternative mechanism is so-called "strangulation", in which the supply of cold gas to the galaxy is halted. Here we report that the difference between quiescent and star forming galaxies in terms of stellar metallicity (i.e. the fraction of metals heavier than helium in stellar atmospheres) can be used to discriminate efficiently between the two mechanisms. The analysis of the stellar metallicity in local galaxies, from 26,000 spectra, clearly reveals that strangulation is the primary mechanism responsible for quenching star formation, with a typical timescale of 4 billion years, at least for local galaxies with a stellar mass less than 10^11 solar masses. This result is further supported independently by the stellar age difference between quiescent and star-forming galaxies, which indicates that quiescent galaxies of less than 10^11 solar masses are on average observed four billion years after quenching due to strangulation.
Published in Nature on 14 May 2015 ( http://dx.doi.org/10.1038/nature14439 )
11 pages, 9 figures (4 in mainarticle 5 more in the extra material.)