5 Attachment(s)
Object of the Week October 29th, 2023 – NGC 253, the Sculptor Galaxy, and its companions
NGC 253 = UGCA 13 = PGC 2789 = Sculptor Galaxy
Galaxy in Sculptor
RA: 00 47 33
Dec: -25 17 20
Mag: 8.0
Size: 28.8' x 5.9'
Wait, what!? How has this galaxy never been featured as an OOTW yet? Sometimes we overlook the showpieces because they are well-known. However, in writing this OOTW, I've learned so much more about this galaxy and ferreted out several advanced targets that I'm excited to share.
NGC 253 is perhaps the most prized galaxy of the southern skies after the Magellanic clouds, certainly the brightest. It's known by many names: Sculptor Galaxy, Sculptor Filament, Silver Coin, Silver Dollar. I've somewhere seen the German name "das Rheingold", but Steinicke/Jakiel [7] lists that to be a different galaxy. An easy binocular target for even average skies, surely many here have observed it multiple times. Heck, there are even several reports of it being visible to the unaided eye under excellent conditions!
The galaxy was discovered on September 23rd 1783 by Caroline Herschel [1]. It is described in the NGC as "very remarkable; very, very bright; very, very large; very much elongated; gradually brighter in the middle" [2]. The galaxy is highly inclined from us, estimated at about 78° [2]. Morphologically, it would be described as a flocculent spiral [3], type "Sc" [2]. Studies however indicate a bar, perhaps 7kpc in size [4], that's not visible in optical images due to the high inclination of the galaxy [5]. Modern data [6] says that the galaxy lies 3.70 Mpc (about 12 Mly) away, which puts its visible diameter at about 30 kpc, typical of a spiral galaxy. The Sculptor galaxy is the brightest member of the Sculptor Galaxy Group (including NGC 55, NGC 247, NGC 300 and NGC 7793) [6,7] and along with several other galaxies, forms a filamentary structure extending from the local group to about a distance of 7 Mpc [6].
SSCs
Due to gas being channeled along its bar to the nucleus, NGC 253 is undergoing a nuclear starburst, leading to the formation of Super Star Clusters (SSCs) in its nucleus [5,8]. In 1996, A. M. Watson and collaborators [9] discovered a number of star clusters in the nuclear region, one of which has been confirmed to be an SSC [9,8]. Leroy et. al. [8] also discovered 13 other super star clusters using radio imaging, but they are highly obscured by dust to be visible even in near-infrared images. The least obscured SSC in question has J2000 coordinates 00:47:32.99 -25:17:19.74 (#5 in [8] and "bright blob" in [9], note coordinate issue in [9]), and is detected in V-band imaging from the HST [9] provoking hopes of a visual observation, although the daunting V mag estimate of 22.2 screams otherwise.
Even if we cannot resolve the SSC in question, the knot from the entire starburst complex seems tractable to visual observation and I have therefore left the coordinates for the SSC in the table.
Attachment 5265
HST V-band (F555W) image of the region around the "bright blob" [9]
Attachment 5266
ESO Optical view of NGC 253, annotated with an arrow pointing to the above HST field
Globular Clusters
I found several studies finding candidate globular clusters associated with NGC 253 [10,11,12]. It seems it is not trivial to identify whether an object is a globular cluster, a Milky Way star or a background galaxy, and a combination of spectroscopic and photometric studies give the best confirmation [10]. It appears that the study by Cantiello et. al. [10] is the latest available catalog, in which the brightest (g band) bona-fide globular cluster is designated [LA83] NGC 253 35, shining at about 18.4 mag. This is likely tractable with large amateur telescopes (like Jimi's 48"). I have listed a few more "tractable" globulars in the table.
Attachment 5267
DSS2 Color overlay of globular cluster [LA83] NGC 253 35
Dwarf Galaxies
Many extremely low surface brightness dwarf companions of NGC 253 have been discovered [13,14,15,16,17,6], some thanks to the contributions of amateur astrophotographers! Looking at their DSS2 fields, though, shows virtually nothing, except for Donatiello III (01:09:24.55 ?27:20:49.6) [16]. Even with Do III, the likelihood of successful visual observation seems poor, but I have listed it in the table since I retain a sliver of hope. There have been previous discussions here and on CN referencing NGC 253-dw2, but no positive observation as anticipated.
However, Karachentsev and others [6] identify two other dwarf galaxies, WOC2017-07 (GALEXASC J005501.01-231008.9, coordinates in discovery paper [18] seem displaced from optical center) and PGC 704814, as associated with NGC 253. They list eight galaxies with radial velocity measurements that also lie in the gravitational sphere of influence of NGC 253, to create a candidate set of galaxies associated with NGC 253. Of these, two are well-known spirals (NGC 247 and NGC 7793), and I have listed the remaining six obscure dwarfs in the table. I imagine some of these dwarfs are amenable to visual observation in larger aperture scopes under excellent skies.
Attachment 5268
HST image of WOC2017-07 from reference [6]
Other interlopers
Steve Gottlieb lists 4 nearby galaxies on a CloudyNights post, and 2 of them have redshifts in SIMBAD indicating that they are far background galaxies. This is probably the case with the other two as well. I added Steve's listing of 4 field galaxies to the table.
HII regions
I have not been able to dig into the HII regions, although it looks like there are some bright, isolated HII/SFRs in this galaxy (e.g. at 00h:47:33 -25:15:30, [U2000] 15). Perhaps someone else can tabulate more in the comments, and I will update the table!
Table of advanced observing targets
I have chosen here some potential advanced observing targets based on the above discussion
| Identifier(s) |
Type |
RA (J2000) |
Dec (J2000) |
Mag (band) |
References and Notes |
| "Bright Blob" |
Super Star Cluster |
00:47:32.99 |
-25:17:19.74 |
22.2 (V) |
[9,4,8] Should be able to see the knot of the entire complex, maybe not resolve SSC |
| [LA83] NGC 253 35 |
Globular Cluster |
00:46:15.3 |
-25:20:06 |
18.4 (g) |
[11,10] Brightest bona-fide globular acc. [10] |
| [LA83] NGC 253 24 |
Globular Cluster |
00:47:33.2 |
-25:23:32 |
19.4 (B) |
[10] Bright star close by |
| [LA83] NGC 253 40 |
Globular Cluster |
00:47:17.7 |
-25:07:03 |
19.3 (g) |
[10] |
| [U2000] 15 |
HII region |
00:47:33.4 |
-25:15:34 |
|
[19] |
| ESO 540-31 = DDO 6 = PGC 2902 |
Dwarf Galaxy |
00:49:49.3 |
-21:00:58 |
15.4 (V) |
[6] |
| ESO 540-32 = PGC 2933 |
Dwarf Galaxy |
00:50:24.5 |
-19:54:23 |
16.0 (V) |
[6] Associated with NGC 247 |
| KDG 2 = ESO 540-30 = PGC 2881 |
Dwarf Galaxy |
00:49:21 |
-18:04:31 |
16.4 (B) |
[6] |
| PGC 704814 |
Possible Dwarf Galaxy |
23:58:40.70 |
-31:28:03 |
16.7 (B) |
[6] Perhaps easiest. Poor velocity measurement, yet [6] says it's a plaus. companion of NGC 7793 |
| SDIG = ESO 349-31 = PGC 621 |
Dwarf Galaxy |
00:08:13.4 |
-34:34:41 |
15.5 (B) |
[6] Sculptor Dwarf Irregular Galaxy, assoc. with NGC 7793 |
| WOC2017-07 = GALEXASC J005501.01-231008.9 |
Dwarf Galaxy |
00:55:01.9 |
-23:10:08 |
|
[6] extremely LSB |
| Donatiello III |
Dwarf Galaxy |
01:09:24.55 |
?27:20:49.6 |
18.75 (V) |
[16] extremely LSB |
| 2MASX J00472496-2521253 |
Likely Background Galaxy |
00:47:24.9 |
-25:21:25 |
17.5 (V) |
CN thread |
| 2MASS J00465933-2527218 = MAC 0046-2527 |
Likely Background Galaxy |
00:46:59.3 |
-25:27:20 |
16.6 (J) |
CN post, S. Gottlieb |
| LEDA 198196 = MAC 0047-2526 |
Background Galaxy |
00:47:29.0 |
-25:26:26 |
16.3 (B) |
CN post, S. Gottlieb |
| LEDA 198197 = MAC 0048-2507 |
Background Galaxy |
00:48:21.8 |
-25:07:36 |
16.3 (B) |
CN post, S. Gottlieb |
Visual Observation
Like most others, I have only observed the main galaxy NGC 253. My first observation was in December 2005 using a 10x50 binoculars. "I was able to see a long, bright patch of light". Subsequently, I looked at it through an 8" scope, describing it as "Majestic. Some bright stars are also in the field."
My earliest good observation was from Coorg District, Karnataka, India on 20th December 2009, when I made a sketch of the galaxy from a 17.5" f/5 Discovery dob.
Attachment 5269
The most striking feature was the dark lane marked. I think I meant the 31mm Nagler and not 13mm.
This object was fresh on my mind because of a much more recent observation, with my 25x100 binoculars. The night after the recent October 2023 annular eclipse, Steve and I were observing from the Bortle 2 skies of Bluff, UT. I wrote down "Bright, hard to miss. Elongated. Even in this small aperture [and magnification], it shows structure, with a brighter core region and perhaps a nucleus.". NGC 247, NGC 288, NGC 300, NGC 55 were also picked out nearby. This was part of my warm-up exercise before tackling an unrelated local-group member, the daunting Sculptor Dwarf.
I do not know of any observations of the other objects in the table, except for the 4 background galaxies listed by Steve, although I hope to see some in the comments. There are reports of observations of the background galaxies on the CN thread, apertures ranging from an 18" to Jimi's 48".
This forum is all about encouraging and sharing visual observation, and you don't need a monster dob to enjoy NGC 253 itself. And then for the veterans using very large aperture, this field still has a lot more to offer. Therefore,
GIVE IT A GO AND LET US KNOW!
References
[1] Courtney Seligman's website on NGC 253
[2] Photographs of Two Great Southern Galaxies, J. C. Duncan (1946)
[3] Cosmic Pinwheels, R. J. Buta, World Scientific (2021)
[4] A Young Super Star Cluster in the Nuclear Region of NGC 253, K. A. Kornei and N. McCrady (2009)
[5] Distribution and Kinematics of Molecular Gas in Barred Spiral Galaxies. II. NGC 253, K. Sorai et. al. (2000)
[6] Distance and Mass of the NGC 253 Galaxy Group, I. D. Karachentsev et. al. (2021)
[7] Galaxies and How To Observe Them, W. Steinicke and R. Jakiel, Springer (2006)
[8] Forming Super Star Clusters in the Central Starburst of NGC 253, A. K. Leroy et. al. (2018)
[9] The Discovery of Young, Luminous, Compact Stellar Clusters in the Starburst Galaxy NGC 253, A. M. Watson et. al. (1996)
[10] A VST and VISTA study of globular clusters in NGC 253, M. Cantiello et. al. (2018)
[11] Spectroscopy of globular cluster candidates in the Sculptor group galaxies NGC 253 and 55, M. A. Beasley and R. M. Sharples (2000)
[12] Search for the globular clusters in nearby galaxies I. NGC 253, A. Blecha (1985)
[13] Discovery of a New Faint Dwarf Galaxy Associated With NGC 253, D. J. Sand et. al. (2014)
[14] Satellite accretion in action: a tidally disrupting dwarf spheroidal around the nearby spiral galaxy NGC 253, A. J. Romanowsky et. al. (2016)
[15] A tidally disrupting dwarf galaxy in the halo of NGC 253, E. Toloba et. al. (2016)
[16] Tracing satellite planes in the Sculptor group I. Discovery of three faint dwarf galaxies around NGC 253, D. Martinez-Delgado et. al. (2021)
[17] Hubble Space Telescope Observations of NGC 253 Dwarf Satellites: Three Ultra-faint Dwarf Galaxies, B. Mutlu-Pakdil et. al. (2022)
[18] A deep Parkes HI survey of the Sculptor group and filament: HI mass function and environment, T. Westmeier et. al. (2017)
[19] Circumnuclear Supernova Remnants and H II Regions in NGC 253, J. S. Ulvestad (2000)
