Object Of The Week November 19, 2023 - Two dwarfs with barely any dark matter

[wvreeven]

NGC1052-DF2 (LEDA 3097693), NGC1052-DF4

Ultra Diffuse Galaxies

Cetus

RA: 02 41 46.80 (DF2), 02 39 15.11 (DF4)
DEC: -08 24 09.3 (DF2), -08 06 58.6 (DF4)

Mag: 16.2 (DF2), 16.5 (DF4)
Diam: 0.707' x 0.707' (DF2), 0.550' x 0.550' (DF4)

Spectacular discoveries can still be made occasionally using amateur size equipment, say an array of Canon 400 mm tele-lenses. The Dragonfly array in New Mexico of the University of Toronto [1] is such an array. Currently consisting of 2 arrays of 24 lenses each, with an expansion to 120 lenses planned, it aims to map the entire area of the Sloan Digital Sky Survey. Currently it has completed 70% of it. Among its many discoveries is NGC1052-DF2 [2].

STScI-01EVT0RHXR1CY4HGXMHKC0HJRA.jpg
Credit: NASA, ESA, and P. van Dokkum (Yale University).

NGC1052-DF2 map.jpg
Map of NGC1052-DF2 within the NGC 1052 group. Note that NGC1052-DF4 also is faintly visible to the right of NGC 1035 and just below the number 5 in the label.

A 2018 paper by Van Dokkum et al. [3] indicates that the galaxy lies at some 20 Mpc and it part of the NGC 1052 group, based on spectroscopic observations of 10 luminous compact objects. Based on their compact morphologies (just-resolved in HST images) and colors, Van Dokkum identifies them as globular clusters. Based on the remarkably small spread in velocity between these globular clusters, an estimate of the total mass of the galaxy was made. This dynamical mass was found to be consistent with the stellar mass, leaving little room for dark matter.

This is where the discussion about the distance to the galaxy started. For instance, a group of the Instituto de Astrofísica de Canarias determined the distance to be merely 13 Mpc [4], making this a perfectly normal dwarf galaxy. However, a more recent study [5] places the galaxy even further away, at 22 Mpc. Pieter van Dokkum has set up a website about NGC1052-DF2 [6], arguing why the lower value for the distance of 13 Mpc, in his opinion, is unlikely to be valid.

In 2018 Dragonfly discovered another low-dark matter galaxy near NGC 1052, NGC1052-DF4 was discovered [7], deepening the mystery.

NGC1052-DF4.jpg
Image of NGC1052-DF4 taken from [7]

As for observing these galaxies visually, I really don't know if it's possible or not. Both have visual magnitudes within the range of 16" telescopes or larger. But both of them being Ultra Diffuse Galaxies may put them well out of range of even Jimi's telescope. I'm looking forward to knowing if any of you have attempted any of these galaxies and whether you succeeded or not. I am currently away from my 20" telescope and don't know when I can use it again. If Cetus still is favorable in the sky when I do, I'll try myself.


As always,

"Give it a go and let us know!
Good luck and great viewing!"

[1] https://www.dragonflytelescope.org/
[2] https://simbad.u-strasbg.fr/simbad/s...t=LEDA+3097693
[3] https://arxiv.org/abs/1803.10237
[4] https://www.iaa.csic.es/en/news/new-...ut-dark-matter
[5] https://iopscience.iop.org/article/1...41-8213/ac0335
[6] https://www.pietervandokkum.com/ngc1052-df2
[7] https://iopscience.iop.org/article/1...38-4357/aae7c8

[Uwe Glahn]

I knew already that I heard about these designation and voila, I found an entry for NGC 1052 DF2.

I noted with my 27" under 7mag skies and 172x-293x: glow already suspected in the searching eyepiece; with 172x and 293x barely but steadily visible with averted vision; very diffuse and large round glow; position and size clear

[wvreeven]

Thanks for posting your positive observation here, Uwe! That gives me courage for giving it a try with my 20".