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Justin Read
@ReadDark
, 13 tweets, 7 min read Read on Twitter
The "ultra-diffuse" dwarf NGC 1052-DF2 is back in the news with two new papers better-measuring the kinematics of its stars and GCs:

arxiv.org/pdf/1901.03711…
adsabs.harvard.edu/abs/2018arXiv1…

I couldn't help but notice how similar DF2 is to the nearby Sagittarius dwarf

[A thread; 1/N]
The Sagittarius dwarf (Sag) is a nearby galaxy, first discovered by Ibata et al. in 1995, that we know is currently being torn apart by tidal forces from the Milky Way.

adsabs.harvard.edu/abs/1995MNRAS.…
adsabs.harvard.edu/abs/1997AJ....…

[2/N]
Similarly to Sag, DF2 is ~84 kpc in projection from a massive companion galaxy, NGC1052 (that is 5-10x more massive than the MW). DF2, like Sag, also has no HI gas, indicative of past interactions:

arxiv.org/pdf/1806.10141…
adsabs.harvard.edu/abs/2018MNRAS.…
adsabs.harvard.edu/abs/2019MNRAS.…

[3/N]
Comparing Sag & DF2 is complicated by the uncertain distance to DF2 that lies in the range 13-20Mpc. This affects its size, stellar mass, dynamical mass & distance to NGC1052:

adsabs.harvard.edu/abs/2018ApJ...…
adsabs.harvard.edu/abs/2018RNAAS.…
adsabs.harvard.edu/abs/2018arXiv1…
adsabs.harvard.edu/abs/2018arXiv1…

[4/N]
Nonetheless, the main properties of Sag & DF2 are remarkably similar. Only the stellar mass differs, by a factor 2-5 depending on the assumed distance to DF2.

[5/N]
Both Sag & DF2 also have similar globular cluster populations. Sag has >8 GCs compared with >10 in DF2. However, half of Sag's GCs lie beyond Rhalf. This could indicate that Sag is more strongly tidally disrupted than DF2.

adsabs.harvard.edu/abs/2003AJ....…
adsabs.harvard.edu/abs/2018ApJ...…

[6/N]
Sag is amazing because we can use its observed tidal debris to reconstruct its stellar and dynamical mass *before* it fell in to the MW. This gives an original stellar and halo mass of M*~1.2e8 Msun & M200 > 6e10 Msun:

adsabs.harvard.edu/abs/2012MNRAS.…
adsabs.harvard.edu/abs/2017MNRAS.…

[7/N]
The reconstructed stellar mass of Sag is similar to that of DF2 today! That could suggest that DF2 is similar to Sag but less tidally disrupted, or that DF2 started out a bit more massive than Sag. Either way, tides are very important for Sag and likely also to be for DF2.

[8/N]
Indeed, a couple of recent papers have modelled dwarf galaxies being tidally stripped and shocked as they orbit in larger host galaxies, naturally forming ultra-diffuse galaxies just like Sag and DF2:

adsabs.harvard.edu/abs/2018MNRAS.…
adsabs.harvard.edu/abs/2018arXiv1…

[9/N]
Interestingly, both of the above works require the pre-infall galaxies to have central dark matter cores, consistent with observations of isolated gas-rich dwarfs:

e.g. adsabs.harvard.edu/abs/2017MNRAS.…

[10/N]
Of course, we shouldn't expect Sag and DF2 to be identical. They likely have slightly different progenitor mass, different orbits, different host galaxy potentials etc. But their similarity suggests that tides play a key role in the formation of ultra-diffuse dwarfs.

[11/N]
If the above is correct, a number of predictions follow:

1. Galaxies like Sag and DF2 should be relatively common [O(1) around MW-mass galaxies]

2. We should see tidal features around DF2 if we look deep enough

3. We won't find a tidally isolated DF2-analogue

[12/N]
Indeed, there may be a hint of tidal features around DF2 in the current data. It would be great to follow up on this!

arxiv.org/pdf/1806.10141…
adsabs.harvard.edu/abs/2018arXiv1…

[End]
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More from @ReadDark see all

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Justin Read
@ReadDark
There is a growing consensus in the astro simulation community that, once we begin to resolve the interstellar medium (ISM), gas cooling and heating causes gravitational potential fluctuations that kinematically "heat up" dark matter [a thread 1/N]
If we can get the gravitational potential to fluctuate "significantly" on a timescale comparable to the local dynamical time, then dark matter (DM) is "heated up":

adsabs.harvard.edu/abs/1996MNRAS.…
adsabs.harvard.edu/abs/2005MNRAS.…
adsabs.harvard.edu/abs/2012MNRAS.…
adsabs.harvard.edu/abs/2014Natur.…

[2/N]
A "significant" fluctuation here can actually be quite small. Fluctuating just ~25% of the total enclosed mass can be sufficient if repeated enough times:

adsabs.harvard.edu/abs/2016MNRAS.…

DM heating can occur even when stars and gas do not dominate the enclosed mass at any time.

[3/N]
Read 13 tweets
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Justin Read
@ReadDark
We have found observational evidence that dark matter is “heated up” at the centres of dwarf galaxies:

arxiv.org/abs/1808.06634…

This solves the long-standing “cusp-core” problem in LCDM, and suggests that dark matter is a cold, collisionless, fluid.

[A thread]
I gave a ~20 minutes talk at KITP about this paper which you can watch here:

online.kitp.ucsb.edu/online/cdm-c18…

For a Twitter summary of the results, please read on ... [1/N]
On large scales, the standard cosmological model (LCDM) gives an excellent description of the cosmic microwave background radiation and the growth of structure. However, on small scales, in our cosmic backyard known as the Local Group, there have been long-standing puzzles. [2/N]
Read 18 tweets
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Justin Read
@ReadDark
This recent press release on some of the latest galaxy formation simulations has reignited an old debate about dark matter versus modified theories of gravity. Sadly, many commentators seem to be missing the point. (A thread, 1/N.)

quantamagazine.org/coder-physicis…
In mod. grav. theories, all the force comes from the visible stars & gas. Given good observations, we can accurately predict the gravitational force field without knowing the first thing about how galaxies form. Such theories are easy to test in disc galaxies & work well. (2/N.)
By contrast, dark matter theories exist in the context of a cosmological model that explains the formation and evolution of all structure in the Universe. For these theories to predict the force field in a given galaxy, we are required to understand how galaxies form. (3/N.)
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@DudeDarkmatter
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