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I heard Matthew O'Dowd mention on PBS Space-Time that dead galaxies cannot start re-forming stars even if they collect gas and/or dust from elsewhere, or even merge with another galaxy....

Why? And what, exactly, causes galaxies to stop forming new stars to begin with? A plain lack of material, or the wrong temperature, or the wrong 'starter' material? (Carbon monoxide, e.g.?)

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    $\begingroup$ Could you provide the exact reference/quote? It could be useful to understand the context and what he means by "dead galaxies" $\endgroup$
    – Prallax
    Oct 2 at 6:19
  • $\begingroup$ @Prallax "dead" in the context of galaxies conventionally means "not forming stars". $\endgroup$ Oct 2 at 9:44
  • $\begingroup$ @PeterErwin I was asking because in general it is not true that a galaxy that presently is not forming stars can't begin forming again (as detailed in pela's answer). I thought that maybe the claim in the OP was referring to something more specific. $\endgroup$
    – Prallax
    Oct 2 at 12:19
  • $\begingroup$ Hi Kurt Hikes, did I manage to answer your question? If so, I'd be happy if you could accept it, otherwise feel free to ask for clarification :) $\endgroup$
    – pela
    Oct 9 at 8:44
  • $\begingroup$ Kurt...? $\endgroup$
    – pela
    Oct 15 at 21:40
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It is not true that "dead", or quenched galaxies cannot start forming stars again. It is called rejuvenation and is observed occasionally (e.g. Chauke et al. 2019), although not commonly (Akhshik et al. 2021). It is also seen in simulations, where some quiescent galaxies return to the star-forming main sequence (Behroozi et al. 2019).

Quenching

What causes galaxies to cease forming stars is not entirely settled, but is generally thought to be a lack of cold gas. In fact, this was convincingly confirmed only last week by Whitaker et al. (2021) who, using Hubble+ALMA observations and aided by gravitational lensing, showed that six quiescent galaxies in the early Universe lacked virtually all cold gas.

How quenched galaxies have lost their cold gas is another open question. Major mergers was thought to be an efficient way to strip galaxies of their gas, but is probably not the most significant mechanism (Gabor et al. 2010; Weigel et al. 2017). So probably feedback from star formation and/or active galactic nuclei is (at least part of) the reason, either by mechanically blowing out the gas, or by heating it (e.g. Cheung et al. 2016). In principle, the gas should cool within the order of 1 giga-year, but at least for a while it can be kept hot through energy injection from central supermassive black holes (Whitaker et al. 2013) or perhaps gravitational heating (Johansson et al. 2009).

Rejuvenation

So, rejuvenating through merging is not impossible, although it might just result in a larger quiescent galaxy, as least in the case of "dry" merging, i.e. with another gas-poor galaxy. Minor merging has been suggested as a mechanism to rejuvenate quenched galaxies (Manelli et al. 2014), as has more smooth gas re-fueling (Marino et al. 2011; Mancini et al. 2019).

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    $\begingroup$ Wouldn't mergers cause the gas to heat up? $\endgroup$ Oct 2 at 16:58
  • $\begingroup$ @RBarryYoung Some gas will heat up, but shocks between gas also initiate star formation. This star formation, and also the increased AGN activity will also heat up the gas. Gas-rich ("wet") merging ~always results in starbursts, whereas gas-poor ("dry") merging typically doesn't. In this case, however, we would have "mixed" merging, i.e. one gas-rich and one gas-poor galaxy. I'm not sure there's a consensus on the result (see e.g. Lin+ 10). It also depends on the galaxies' relative angular momentum… $\endgroup$
    – pela
    Oct 2 at 21:54

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