# Why is SPT0418-47 (“the most distant Milky Way look-alike”) expected to evolve into an elliptical galaxy?

Phys.org's ALMA sees most distant Milky Way look-alike describes the image reconstruction of a strongly and very nicely lensed z = 4.2 galaxy by a by a foreground galaxy at z = 0.263 and says:

"What we found was quite puzzling; despite forming stars at a high rate, and therefore being the site of highly energetic processes, SPT0418-47 is the most well-ordered galaxy disc ever observed in the early Universe," stated co-author Simona Vegetti, also from the Max Planck Institute for Astrophysics. "This result is quite unexpected and has important implications for how we think galaxies evolve." The astronomers note, however, that even though SPT0418-47 has a disc and other features similar to those of spiral galaxies we see today, they expect it to evolve into a galaxy very different from the Milky Way, and join the class of elliptical galaxies, another type of galaxies that, alongside the spirals, inhabit the Universe today.

and links to Rizzo et al. (2020) in Nature: A dynamically cold disk galaxy in the early Universe. Also see (YouTube and ESO)

Question: Why is SPT0418-47 ("the most distant Milky Way look-alike") expected to evolve into an elliptical galaxy? Is there something about this particular observation that indicates that, or is that just what galaxies "back then" did, even if they had a disk-like phase?

The full paper can be downloaded from the ESO site. This contains the following:

Dusty starburst galaxies are believed to be the progenitors of early-type galaxies (ETGs), which are the most massive galaxies observed today, dominated by old stellar populations. The most popular evolutionary track for this transformation predicts that the dusty-starburst phase is followed by a quenching phase, during which AGN feedback leads to gas consumption and heating with the consequent formation of a population of compact quiescent galaxies at $$z \approx 2$$. In the final phase, dry minor mergers are expected to be responsible for a growth in galaxy size and the transformation into present-day ETGs.

(AGN = active galactic nucleus)

They then compare the stellar/baryonic mass with the likely influence of dry mergers:

The comparison between the ETGs and the stellar/baryonic quantities for SPT0418–47 in the size–mass plane (Fig. 4a) indicates that this starburst galaxy should increase its stellar mass by a factor of 6 (3 for the red diamond) and its effective radius by a factor of 11 (3 for the red diamond), in order to evolve into an average ETG (yellow cross). This is in agreement with a simple toy model for mergers, in which a single dry major merger event would be responsible for an increase in both the size and stellar mass of SPT0418–47 by a factor of 3.

They also compare the distribution of dark matter with local ETGs:

Finally, we derived the fraction of dark-matter mass within the effective radius and found that, with a value of $$f_{\rm DM} (< R_{\rm e}) = 0.095^{+0.004}_{-0.004}$$ (red diamond), the central regions of SPT0418–47 are dominated by baryons. As shown in Fig. 4c, such a low fraction of dark matter is compatible with observations of local ETGs, implying that the physical mechanisms responsible for the mass and size growth of this galaxy with cosmic time should preserve the dark-matter contribution within the innermost ∼1 kpc.

Note that the term "early-type galaxy" refers to elliptical and lenticular galaxies, which are near the start of the Hubble sequence. It does not refer to evolutionary stage, as noted in Hubble (1927) "The classification of spiral nebulae":

The nomenclature, it is emphasized, refers to position in the sequence, and temporal connotations are made at one's peril. The entire classification is purely empirical and without prejudice to theories of evolution—comparison with theories will be the more significant for this very reason.

The standard cold-dark-matter model ("Lambda CDM") says that galaxy formation is seeded by initial density fluctuations in the dark matter and gas. The earliest galaxy formation will happen in the densest such fluctuations (denser = stronger gravity = faster collapse), which will be local overdensities within an initial, larger-scale overdensity. Such regions will almost certainly have other local overdensities, which will also form early proto-galaxies. Since these are near to the first, they're more likely to merge early in the universe's history.

So if you see a massive galaxy at a high redshift (early in the universe's history), it mostly likely means that it's in a very dense region of the early universe -- e.g., the core of what will become a cluster of galaxies -- which means there will be other massive galaxies forming (or soon to form) nearby, which will probably merge with this galaxy fairly rapidly and turn it into an elliptical. (Plus, since this is happening in a dense region, the gravity of that region will draw in other galaxies not immediately nearby, leading to more mergers over time.)

• Thank you for the clear and simple explanation! – uhoh Aug 22 '20 at 12:10