Object of the Week October 29th, 2023 – NGC 253, the Sculptor Galaxy, and its companions

[akarsh]

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 (https://en.wikipedia.org/wiki/Sculptor_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.
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HST V-band (F555W) image of the region around the "bright blob" [9]

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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 (http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/611/A21), in which the brightest (g band) bona-fide globular cluster is designated [LA83] NGC 253 35 (http://simbad.cds.unistra.fr/simbad/sim-id?Ident=%5BLA83%5D%20NGC%20253%2035), 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.

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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 (https://www.deepskyforum.com/showthread.php?820-NGC-253-dw2-anyone) and on CN (https://www.cloudynights.com/topic/789409-ngc-253-dwarf-galaxy/) referencing NGC 253-dw2 (https://simbad.u-strasbg.fr/simbad/sim-basic?Ident=NGC%20253-dw2), but no positive observation as anticipated.

However, Karachentsev and others [6] identify two other dwarf galaxies, WOC2017-07 (GALEXASC J005501.01-231008.9 (https://simbad.u-strasbg.fr/simbad/sim-id?Ident=GALEXASC%20J005501.01-231008.9), 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.

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HST image of WOC2017-07 from reference [6]

Other interlopers
Steve Gottlieb lists 4 nearby galaxies on a CloudyNights post (https://www.cloudynights.com/topic/789409-ngc-253-dwarf-galaxy/?p=11371380), 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 (https://simbad.cds.unistra.fr/simbad/sim-id?Ident=%401259283&Name=%5bU2000%5d%2015&submit=submit)). 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)TypeRA (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 (https://en.wikipedia.org/wiki/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 (https://www.cloudynights.com/topic/789409-ngc-253-dwarf-galaxy/?p=11371380)



2MASS J00465933-2527218 = MAC 0046-2527
Likely Background Galaxy
00:46:59.3
-25:27:20
16.6 (J)
CN post, S. Gottlieb (https://www.cloudynights.com/topic/789409-ngc-253-dwarf-galaxy/?p=11371380)



LEDA 198196 = MAC 0047-2526
Background Galaxy
00:47:29.0
-25:26:26
16.3 (B)
CN post, S. Gottlieb (https://www.cloudynights.com/topic/789409-ngc-253-dwarf-galaxy/?p=11371380)



LEDA 198197 = MAC 0048-2507
Background Galaxy
00:48:21.8
-25:07:36
16.3 (B)
CN post, S. Gottlieb (https://www.cloudynights.com/topic/789409-ngc-253-dwarf-galaxy/?p=11371380)







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.
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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 (https://www.cloudynights.com/topic/789409-ngc-253-dwarf-galaxy/), 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 (https://cseligman.com/text/atlas/ngc2a.htm#253)
[2] Photographs of Two Great Southern Galaxies, J. C. Duncan (1946) (https://adsabs.harvard.edu/pdf/1946PASP...58..233D)
[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) (https://iopscience.iop.org/article/10.1088/0004-637X/697/2/1180/pdf)
[5] Distribution and Kinematics of Molecular Gas in Barred Spiral Galaxies. II. NGC 253, K. Sorai et. al. (2000) (https://academic.oup.com/pasj/article/52/5/785/1505201)
[6] Distance and Mass of the NGC 253 Galaxy Group, I. D. Karachentsev et. al. (2021) (https://iopscience.iop.org/article/10.3847/1538-3881/abe8d1)
[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) (https://iopscience.iop.org/article/10.3847/1538-4357/aaecd1/pdf)
[9] The Discovery of Young, Luminous, Compact Stellar Clusters in the Starburst Galaxy NGC 253, A. M. Watson et. al. (1996) (https://authors.library.caltech.edu/53645/1/1996AJ____112__534W.pdf)
[10] A VST and VISTA study of globular clusters in NGC 253, M. Cantiello et. al. (2018) (https://www.aanda.org/articles/aa/pdf/2018/03/aa31325-17.pdf)
[11] Spectroscopy of globular cluster candidates in the Sculptor group galaxies NGC 253 and 55, M. A. Beasley and R. M. Sharples (2000) (https://academic.oup.com/mnras/article/311/4/673/1746403)
[12] Search for the globular clusters in nearby galaxies I. NGC 253, A. Blecha (1985) (https://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1986A%26A...154..321B&defaultprint=YES&filetype=.pdf)
[13] Discovery of a New Faint Dwarf Galaxy Associated With NGC 253, D. J. Sand et. al. (2014) (https://iopscience.iop.org/article/10.1088/2041-8205/793/1/L7/pdf)
[14] Satellite accretion in action: a tidally disrupting dwarf spheroidal around the nearby spiral galaxy NGC 253, A. J. Romanowsky et. al. (2016) (https://academic.oup.com/mnrasl/article/457/1/L103/2589712)
[15] A tidally disrupting dwarf galaxy in the halo of NGC 253, E. Toloba et. al. (2016) (https://iopscience.iop.org/article/10.3847/2041-8205/816/1/L5/meta)
[16] Tracing satellite planes in the Sculptor group I. Discovery of three faint dwarf galaxies around NGC 253, D. Martinez-Delgado et. al. (2021) (https://www.aanda.org/articles/aa/pdf/2021/08/aa41242-21.pdf)
[17] Hubble Space Telescope Observations of NGC 253 Dwarf Satellites: Three Ultra-faint Dwarf Galaxies, B. Mutlu-Pakdil et. al. (2022) (https://iopscience.iop.org/article/10.3847/1538-4357/ac4418)
[18] A deep Parkes HI survey of the Sculptor group and filament: HI mass function and environment, T. Westmeier et. al. (2017) (https://academic.oup.com/mnras/article/472/4/4832/4107119)
[19] Circumnuclear Supernova Remnants and H II Regions in NGC 253, J. S. Ulvestad (2000) (https://iopscience.iop.org/article/10.1086/301428/pdf)

[Raul Leon]

Hi, here's my observation from 10/19/2017: Ngc 253 is a galaxy in Sculptor: magnitude:6.99 ; size: 26.8' x 4.6' ; aka the Silver Coin galaxy; large, elongated and bright this galaxy shows a wealth of detail in the eyepiece! I used a 21mm Ethos at 115x with my 14.5 StarStructure f/4.35271

[ScottH]

Good research there, Akarsh. I've seen NGC 253 naked-eye and hopefully, am not the only one on this forum to have done so! I've tried for the globular cluster [LA83] NGC 253 35 (+17.58b) once with my friend's 36-inch and failed.

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However, I did end up seeing some sort of star-forming region and/or OB association with that same telescope on the same night without distinctly looking for it in the northern half of the galaxy.

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Hubble ASC image

In the image below (https://esahubble.org/images/opo9510a/), I've marked where the "bright blob" (a comical choice since it's truly fainter than everything right around it!) from my reading of the Watson et al (1996) (https://ui.adsabs.harvard.edu/abs/1996AJ....112..534W/abstract)paper. So, please tell me you didn't put the red cross-hairs on the image you included the caption "HST V-band (F555W) image of the region around the "bright blob" [9]"!

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Scott H.

[akarsh]

Raul, that's a fantastic sketch, thanks for sharing!

Scott, there's a wealth of info in your response. Very cool stuff.


Good research there, Akarsh. I've seen NGC 253 naked-eye and hopefully, am not the only one on this forum to have done so!



Oh, excellent. I had no idea it might be possible from the northern hemisphere. What latitude was this from and what sky conditions were you under?


I've tried for the globular cluster [LA83] NGC 253 35 (+17.58b) once with my friend's 36-inch and failed.

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I'm surprised to learn that someone has even attempted this globular so far. I mean, clearly this is the sort of thing you go after, I should've expected it. The magnitude makes me believe it is tractable in Jimi's 48", after all we have seen a cluster I believe of a similar magnitude in NGC 4449. But there's a big difference here -- the altitude at culmination is a lot lower.




However, I did end up seeing some sort of star-forming region and/or OB association with that same telescope on the same night without distinctly looking for it in the northern half of the galaxy.

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So reading off coordinates from your image, it looks like 0:47:42.50 -25:15:37 is the SFR you saw. I'm curious was it not possible to see the SFR/HII I mentioned in the OOTW post that appears as a pink blob in e.g. the image you shared, at 00:47:33 -25:15:30? I see that you haven't marked it on your sketch, perhaps it is visually faint or perhaps you were not looking for it?

I wonder if you already have any designations for these. I found the HII that I am referring to as [U2000] 15 (https://simbad.cds.unistra.fr/simbad/sim-id?Ident=%25401259283&Name=%5BU2000%5D%2015) in SIMBAD, listed as an HII region. The only hit I got around the coordinates of the SFR you saw in SIMBAD is a Near-IR source 2MASS J00473350-2515337 (https://simbad.u-strasbg.fr/simbad/sim-id?Ident=2MASS%20J00474305-2515403).

It looks like the "[U2000]" designation comes from this paper (https://iopscience.iop.org/article/10.1086/301428/pdf) whic




In the image below (https://esahubble.org/images/opo9510a/), I've marked where the "bright blob" (a comical choice since it's truly fainter than everything right around it!) from my reading of the Watson et al (1996) (https://ui.adsabs.harvard.edu/abs/1996AJ....112..534W/abstract)paper. So, please tell me you didn't put the red cross-hairs on the image you included the caption "HST V-band (F555W) image of the region around the "bright blob" [9]"!

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You're right, I forgot to point out that the red cross-hairs are on the center of the HST field which is not the center of the bright blob. Is the bright blob not brighter than its immediate surroundings? Or am I misreading the position of it on the image you shared?

Clear Skies
Akarsh

[ScottH]

Oh, excellent. I had no idea it might be possible from the northern hemisphere. What latitude was this from and what sky conditions were you under?

From my home at latitude 36.1*N. I didn't have a sky quality meter at the time, but it was a truly special evening. I saw several things that night with the naked-eye and 7x35 binoculars that I know it was performing as a Bortle 2 site that day.


I'm surprised to learn that someone has even attempted this globular so far. I mean, clearly this is the sort of thing you go after, I should've expected it. The magnitude makes me believe it is tractable in Jimi's 48", after all we have seen a cluster I believe of a similar magnitude in NGC 4449. But there's a big difference here -- the altitude at culmination is a lot lower.

Hey, it was worth a shot! But if I'm ever able to pay Mr.48" in West Texas a visit in autumn, it's high on my list to attempt again. As to NGC 4449, I've thoroughly researched and observed that galaxy with the 36-inch. So, I'm pretty sure I know which cluster you're talking...though I don't think I saw it.


So reading off coordinates from your image, it looks like 0:47:42.50 -25:15:37 is the SFR you saw. I'm curious was it not possible to see the SFR/HII I mentioned in the OOTW post that appears as a pink blob in e.g. the image you shared, at 00:47:33 -25:15:30? I see that you haven't marked it on your sketch, perhaps it is visually faint or perhaps you were not looking for it?

I wonder if you already have any designations for these. I found the HII that I am referring to as [U2000] 15 (https://simbad.cds.unistra.fr/simbad/sim-id?Ident=%25401259283&Name=%5BU2000%5D%2015) in SIMBAD, listed as an HII region. The only hit I got around the coordinates of the SFR you saw in SIMBAD is a Near-IR source 2MASS J00473350-2515337 (https://simbad.u-strasbg.fr/simbad/sim-id?Ident=2MASS%20J00474305-2515403). It looks like the "[U2000]" designation comes from this paper (https://iopscience.iop.org/article/10.1086/301428/pdf)

Good questions. If my memory is correct, I did not study nor use an image of the galaxy before I visually studied it and drew it. So, I wasn't looking for anything, and afterwards regretted not specifically looking for that H II ([U2000] 15) on the galaxy's west side. Meaning I can't say it wasn't visible since I didn't focus on it. And no, I was never able to come up with a catalog designation in SIMBAD for what the object I saw.



You're right, I forgot to point out that the red cross-hairs are on the center of the HST field which is not the center of the bright blob. Is the bright blob not brighter than its immediate surroundings? Or am I misreading the position of it on the image you shared?

Well, in the HST image I posted, the "bright blob" isn't brighter than the real blob just barely east of it. And you yourself said its about +22.2 Vmag...

Scott H.

[Uwe Glahn]

Wow, next fantastic write-up Akarsh, thanks for the very informative compilation.

NGC 253 is often reviled from Mid-European observers because of its low position in the sky. But of course its a shining object from far more south.

I started with a binocular sketch, made 21 years ago from a light polluted Bortle 4 sky and 50°N. Within a 3° circle it shows as a pair together with NGC 288. With 20x the galaxy started to show some mottling, but no clear detail.

sketch: 20x125, NELM 6m0+
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home (http://www.deepsky-visuell.de/Zeichnungen/NGC253_NGC288.htm)

Later from Namibia I tried some GC's after 1983ApJ...265..166L. With 24" and 400x I could steadily see A with averted vision and B as a popping object.

From the Alps I tried the Dwarf after the deepskyforum.com thread that you refer as a galaxy 2MASX J00472496-2521253. With the 27" it was steadily visible with averted vision. Three co-observers did confirm the observation.

[akarsh]

Thanks for your report, Uwe!



Later from Namibia I tried some GC's after 1983ApJ...265..166L. With 24" and 400x I could steadily see A with averted vision and B as a popping object.


Ah, so that's where the [LA83] designations in SIMBAD come from! I looked up [LA83] NGC 253 A (https://simbad.u-strasbg.fr/simbad/sim-basic?Ident=[LA83]%20NGC%20253%20A) and [LA83] NGC 253 B (https://simbad.u-strasbg.fr/simbad/sim-basic?Ident=[LA83]%20NGC%20253%20B) in SIMBAD and both are listed as galaxies. This would not ordinarily mean much because SIMBAD is plagued with errors, but however SIMBAD lists a redshift from this source (https://academic.oup.com/mnras/article/311/4/673/1746403) (I did not verify looking at the source). It looks like "B" is a galaxy at a redshift of 0.1+, over 1.3 billion year light travel time, and "A" also has a large redshift of about 0.05+. Unfortunately neither seem to be globulars, but it isn't every day that you see an individual galaxy over a billion ly away in a 24".

[ScottH]

So, while doing research on a different paper, I learned of a 2018 paper by Rodríguez, Baume, and Feinstein titled
Identification and Analysis of the Young Population in the Starburst Galaxy NGC 253 (https://ui.adsabs.harvard.edu/abs/2018MNRAS.479..961R/abstract). It's a catalog of young stellar groups (YSGs) in NGC 253 that has yet to be ingested into SIMBAD. Using the VizieR table overlayed on Aladin, I was able to figure out that the OB association I saw is either their [RBF2018] NGC 253 AS043, AS068, AS177 or a combination of them. To boot, you can see in the image below (which was in the paper) that they have the OB association (or YSG, you choose) circled even though it lies to the upperleft and just off the chart edge.


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