NGC 7090, a Southern Edge-on Galaxy

Photo in this article´s headline by ESA/Hubble & NASA                                                                                                     

A Good Example of This Type of Galaxy 

to Observe From the Southern Hemisphere

Figure 1. DSS image of NGC 7090. North is up.

August is a good month to aim your telescope at a not-very prominent constellation -Indus- and observe a 30 million light years away fine edge-on galaxy there, NGC 7090. Of course, you must stay in the Southern Hemisphere to see it high in the sky, around 60° by local midnight anywhere around latitude -33°, in early August. This galaxy reaches roughly 68° by the first half of that month. If you try to find this galaxy from a latitude of +32° for example, it only reaches about 3.7° during its transit. So, if you travel to the Southern Hemisphere, I recommend this galaxy to be included in your observing program. 

John Frederick William Herschel (1792 - 1871) discovered this good example of an edge-on galaxy, in 1834. It is also catalogued in the Arp-Madore Catalogue of Southern Peculiar Galaxies as an example of Category 15 "Galaxies with Tails Loops of Material or Debris". The authors of this catalog state "This is an important category because, if the galaxies really are single, it implies internal activity is responsible for that tails or loops. According to Wolfgang Steinicke's Revised NGC and IC Catalog, and Simbad Database, among others,  this is an SBc galaxy. However, Volker Heesen et al. in their paper "Advective and diffusive cosmic ray transport in galactic haloes" (February 2016) state that the morphological type of this galaxy is Scd. A historical identification by Dreyer is, NGC 7090 (= GC 4679 = JH 3872, 1860 RA 21 26 36, NPD 145 10.8) "pretty bright, pretty large, very much extended 127° ry gradually then pretty suddenly a little brighter middle".

Figure 2

The DSS image above shows the field you will be able to see through any telescope that gives you a 1° field of view. I observed this galaxy in early August some years ago at around 4am UT (1am local time) with an 8-inch Schmidt-Cassegrain telescope.

At 63x, this galaxy looks elongated northwest-southeast and is surrounded by some stars of 9 and 10 magnitude. The field is interesting. Observing with averted vision, a very faint star is visible superimposed on the galaxy, in its southeast edge, indicated by a small arrow in Figure 2 to the left. I could not find this star in some of the planetarium software like Skymap Pro 6 and Stellarium. The picture on the bottom panel of Figure 2 is from Simbad Database. Seemingly, there is not a stellar label for this star viewed against de galaxy. The small red circles on the picture depict three stars visible in the 5 arc minutes field of view, with UCA 178-229232 and UCA 178-229229 as high proper motion stars of magnitudes 12.3 and 14.7 respectively. On the other hand, TYC 8811-1272-1 is a 13.1 star. The Simbad Database states that a high mass x-ray binary1 -CXO2 J213633.8-543402- is situated in that position with coordinates 21 36 33.85  Dec. -54 34 02.7 (J2000.0), so that is surely the object I could see through the telescope. You can read about this kind of object in the paper "X-ray emission from star-forming galaxies– I. High-mass X-ray binaries" by S. Mineo et al. (September 2011). NGC7090 was previously known to host two highly variable/transient ULXs (Walton et al. 2011b; Earnshawetal.2019b; Liu et al. 2019; Song et al. 2020). D. J. Walton et al. in the paper "A new transient ultraluminous X-ray source in NGC7090" (November 2020) claim the discovery of a third ultraluminous X-ray source, and explore the possibility that ULX3 is a new member of the ULX pulsar population. For illustration purposes, I plot it on the lower panel section of the picture here.

The galaxy is barely visible so averted vision improves its detection, appearing faint with the central region appearing smooth in brightness.

At 118x the galaxy looks clearly elongated. Averted vision is necessary if you try to glimpse any detail of this edge-on object. The star is more clearly visible, and the region labeled with a black line square in Figure 2 is the brightest part, the core of NGC 7090. The faint extensions of this object are not easy to see through an 8-inch telescope.

Higher magnification (160x) starts to show a "granular" structure or surface in the brightest region of the galaxy when observed using averted vision. A small region of the galaxy can be barely glimpsed immediately to the southeast of  CXO J213633.8-543402 (indicated with a small red circle in the upper panel of Figure 2) appearing narrower than the central region.
 
A faint view of the region enclosed by the square in Figure 2 was possible at 222x, appearing "granular" like at 160x.

Figure 3

According to Volker Heesen et. al. in their paper " Advective and diffusive cosmic ray transport in galactic haloes" (February 2016), NGC 7090 has a thin and a thick radio disc featuring a prominent polarized radio halo, particularly at λ22 cm.

This image from NASA/ESA Hubble Space Telescope zooms in to get a view of the brighter central portion of NGC 7090. A pattern of pinkish red regions, the location of ongoing star formation, and a large group of cool stars packed in a compact, spheroidal region could explain why a sort of grainy area is visible in an 8-inch telescope or similar at high magnification.

How to Find the Galaxy?

21h 36m 29s   -54° 33' 26" (J2000.0)

Click on the image to enlarge

How to find it,
About 3°08' to the west of the 4.4 magnitude star Delta (δ) Indi.

Stars in map up to visual magnitude 6.

______________________________________________________________________________________________________________________________________________________________________

1_ A high-mass X-ray binary (HMXB) is a binary star system that is strong in X rays, and in which the normal stellar component is a massive star: usually an O or B star, a Be star, or a blue supergiant. The compact, X-ray-emitting, component is a neutron star or black hole.^[7] A fraction of the stellar wind of the massive normal star is captured by the compact object, and produces X-rays as it falls onto the compact object.

In a high-mass X-ray binary, the massive star dominates the emission of optical light, while the compact object is the dominant source of X-rays.

2_ Standard designation for sources detected in observations made with the Chandra X-ray Observatory.

There are 3 standard designations for sources detected in observations made with the Chandra X-ray Observatory.

CXO is reserved for use by the Chandra X-ray Observatory Center (CXC) for designating sources from projects of an institutional nature.

ESO 510-20 Galaxy System

 

Is This Object Within Reach of Big Amateur Telescopes, 

or is it Just a Candidate for Astrophotography?

We are at the end of the summer here in the Southern Hemisphere. Under the usual cool (rather cold) nights at this time of the year (April) in any observing site that offers dark skies, like in the Andes mountains, you can see constellations Centaurus and Hydra, a good reservoir of galaxies and groups of galaxies. The easternmost part of the long constellation Hydra reaches an altitude of 60 degrees around local midnight in late March-early April. The area is high enough to aim a big telescope, like a 24-inch one, and look for a very peculiar and unusual interacting system nicknamed the Dentist´s Chair because of its shape resembling that chair nobody wants to seat on. Other designations for this object are ESO 510-20, PGC 49560, and AM 1353-272 in the Arp-Madore Catalogue of Southern Galaxies and Associations.

Figure 1. The Hydra-Centaurus Region. North is left, East down

Figure 1 to the right plots the distribution of galaxies brighter than magnitude 12. As you can notice, the easternmost end of the long constellation Hydra, at the border with the constellation Centaurus, contains a good sample of galaxies (center of the image).

Figure 1 in the paper "Large Velocity Gradients in the Tidal Tails of the Interacting Galaxy AM 1353 −272 (“The Dentist’s Chair”)" by Peter M. Weilbacher et al. 2002, included in this article as Figure 2 with their permission, shows the galaxy system in detail. According to A. Monreal-Ibero et al. in their paper "Towards DIB mapping in galaxies beyond 100 Mpc A radial profile of the λ5780.5 diffuse interstellar band in AM 1353-272 B⋆". (2015), the system consists of two components. The main galaxy (A) presents two prominent ∼40 kpc long tidal tails. The companion (B) is a low-luminosity disk-like galaxy of disturbed morphology undergoing a strong starburst and with high extinction. They state that component ‘C’ is located 38 Mpc behind the interacting pair. " The long tidal tails of “A” host a number of blue knots. Given their colors, these knots could be physically linked with "A" (paper "On the Formation and Evolution of Dwarf Galaxies in Tidal Tails" by Peter M. Weilbacher 2002). 

Now, is this object within the reach of big amateur telescopes, or is it just a candidate for astrophotography? 

First Observing Attempt

Figure 2

Remember, the results of this observation are based on one single night with the conditions at that moment mentioned below. My plan is to go to another observing site to make a new observation to compare results.

The sky on the night of the observation was pretty steady with a decent seeing. However, I can tell that the night before, during the observing weekend, was darker. The observing site, according to some sources, has a Bortle 2 scale (average dark sky). In my humble opinion, the site might be close to the border with Bortle 3, because although the sky toward the North and West is truly dark, some light pollution from the big city of Mendoza and the much closer but smaller town of Uspallata interfere at low altitude, near the southern and eastern horizons. I guess we can take a visual magnitude of 7 like the average limiting magnitude of the area since it is right in the limit of Bortle 2 and 3. I consider the area good enough to carry out challenging observing projects. For sure you can find even darker skies if you drive some miles northward, closer to the "Pampa el Leoncito", where the biggest observatory in Argentina is situated (CASLEO). Pampa El Leoncito is still a very dark area from where I was able to glimpse, with two observer partners, the Gegenschein a few years ago, giving us the idea of a very good sky. Back to the observing site and the night of the observation of ESO 510-20, I can mention that at the moment of the observation, a slight wind interfered with the analysis. Earlier that day, the wind caused some dust to remain high in the atmosphere, making the sky brighter than the night before (I didn´t measure the limiting magnitude at the observing site).

Photo by the author showing the position of the galaxy system in the sky

Once I aimed the telescope at the region where the target is situated, indicated in the picture above (roughly 9 degrees from the 2-magnitude star Menkent), it was necessary to recognize the surrounding field in detail. Using low magnification to have a field of view around 1 degree wide, some stars are identified, like HIP 68099, a red giant situated about 437 light-years appearing yellow through the telescope. HIP 68099 is the brightest star in the field with a visual magnitude of 6.8. Also, HD 121469, a 9.2 magnitude star, and TYC 6724-613-1 with a magnitude of 10.5, two of the brightest stars in the field close to the galaxy system, were visible.

Figure 3. DSS STSci image

At 76x, you can find the brightest star in the field, HIP 68099, a 6.8 magnitude red giant branch star, the brightest one in the field (see Figure 3). Beyond the mentioned stars, the group of stars encircled by a red ellipse is very useful in a  field of view of 15 arc minutes. That asterism, to the right of the bright star HiP 68099, composed of stars with visual magnitudes between 13 and 15, was useful not only for identifying the field of view, among other stars but also to determine the precise position of the galaxy system. It looks like an arrow pointing to ESO 510-20. At this magnification, of course, the galaxy system is not visible. ESO 510-20 seems to lie roughly equidistant from the stars labeled with blue circles in Figure 3 that can be identified at this magnification. Observing carefully and applying averted vision, a sort of  "star" seems to appear in the area of the galaxy system.

Using 144x, the view of the stars immediately surrounding the position of ESO 510-20 is more clear. However, nothing is visible where the target is. Once again, averted vision makes it possible to momentarily glimpse, very faintly, a ghostly and small dot there. Higher power is a must to try to catch this obscure object of the southern skies.

Even if higher magnification shows a very detailed field regarding the faint stars surrounding the precise area of the galaxy system, it is not evident and averted vision doesn´t improve the very dim ghostly brightness that was also visible at lower magnifications.

After observing for a long time, the target's position in the sky was about 84 degrees, late in the night, so it had a perfect altitude to observe a challenging object. At even higher magnification (360x) the result is the same, hard to glimpse a very small and faint dot using averted vision.

Figure 4. North is up, East to the left

Now, is that extremely faint dot visible with all the magnifications a star situated there, or can be maybe the brighter core of the largest component of the galaxy system? Making a photo of the galaxy system dimmer (Figure 4), simulating in some way what I could see through an eyepiece, you can notice that component A (the brightest one) and the round feature to the northeast (component C in Figure 2) remain visible, so maybe that is what I could see because seemingly no stars brighter than the system´s core lie there according to the DSS image.

Conclusion

So, is ESO 510-20 a visual object? Well, you can not expect at all to see the entire shape of this galaxy system, unless you take a deep photo. However, is possible to try to glimpse the brighter central part of component A or even more, component C. Now, based on the single observing night I dedicated to this object I should say it is not an object for visual astronomy. This interactive system seems to be reserved for astrophotographers or observers with instruments bigger than 24-inch. However, I think that it is convenient to make a new observation of this object to determine if a challenging object like this one is visible or not. I think it is always convenient and fruitful to observe the same target again, from the same place or even from a site with better observing conditions (i.e. darkness, transparency, seeing, general weather conditions, etc.) and compare results. 

I didn´t find enough information regarding the visual magnitude of ESO 510-20. The magnitude in B-band is displayed in several sources, like SIMBAD Database, as 15.9. Just by way of example to know the importance of a dark sky let's imagine a galaxy with a visual magnitude of 16 and a size of 0.5 x 0,5 arc minutes (the apparent size of the component A of ESO 510-20  is smaller). Under a dark sky with a limiting magnitude of 7, the hypothetic object should be visible through a 24-inch telescope, with 140x as the optimum detection magnification. The situation changes if the sky is slightly "brighter" (limiting magnitude drops to 6.7 for example). Under this last situation, the target should not be visible, at least in theory. Unfortunately, I did not measure the limiting magnitude that night, but as was aforementioned the night before was definitely darker and for sure with an even higher limiting magnitude. The observation of challenging objects like ESO 510-20 implies a sky as dark as possible, so limiting magnitude is an important variable we need to have into account when organizing this kind of observing session.

The fact of not observing this object through a 24-inch telescope doesn´t mean it is not possible to detect it visually. Besides the mentioned conditions (seeing, weather, limiting magnitude, etc.) an appropriate condition of the observer and the equipment is undoubtedly a must as well (i.e. eye performance, a very well-collimated telescope, appropriate dark adaptation, good use of averted vision, etc.). Two observers under the same sky can achieve different results, depending on their own capabilities and personal conditions at the moment of the observation. I think the question of this article is open. For sure the "Dentist´s Chair" system is a good target for long-exposure photography, but it is, for sure, a challenging target for those observers who like to take their eyes and telescopes to the limit of their performances.

Although the nickname of this deep-sky object can sound "intimidating", it is worth the effort to try to glimpse at least some features of this intriguing interacting system during an observing session on any autumn night in the southern hemisphere. If you can, include it in your observing list!

Lindsay-Shapley Ring

A Peculiar Galaxy, an interesting Target for Big Telescopes, Resides in the Far Southern Skies

As a visual observer, galaxy morphology has always been a very exciting topic for me. To observe different kinds of galaxies, from the classic elliptical and spiral ones to those in groups and clusters showing peculiar shapes is something very interesting, especially if you have a chance to use a big telescope that allows you to detect the fainter and finer details of them. Among the peculiar members in the world of the galaxy morphology, we have the so-called annular galaxies or ring galaxies. As James L. Higdon et al. state in their paper "Why Are Ring Galaxies Interesting?" (2010), these objects are formed by the near central passage of a companion through a spiral along the rotation axis. The brief additional gravitational force induces epicyclic motions throughout the disk, which act to form radially propagating orbit-crowded rings of gas and stars. On other of their papers, James L. Higdon et al. argue that ring galaxies are striking examples of the ability of collisions to transform both the morphology and star-forming activity of a spiral galaxy. Numerical studies since the mid-1970s argue persuasively that the optically prominent rings are in fact outwardly propagating zones of strong orbit crowding within the disk of a spiral induced by the near central passage of a companion galaxy ("Wheels of Fire. IV. Star Formation and the Neutral Interestellar Medium in the Ring Galaxy AM¹ 0644-741" 2011).

The 24-inch telescope at the "Altos Limpios" Nature Reserve. 

Photo by the author.

If you are observing in the Southern Hemisphere during the summer season take a look, for a moment, to the Large Magellanic Cloud. It is a must-to-see object because of its size, brightness, and remarkable structure. Well, now aim your eyes about 8 degrees southeast of that galaxy. In that region of the sky, more specifically in constellation Volans, an interesting annular galaxy resides, 300 million light-years away. I´m talking about PGC 19481, also known as Graham A, the Southern Ellipse, AM 0644-7411 (Arp & Madore 1987), or Lindsay-Shapley Ring.

A way to find this peculiar galaxy is to look for the 5.1 magnitude star alpha Mensae, one of the stars that are part of the inconspicuous constellation Mensa. The galaxy lies about 2.1 degrees east-northeast of it. The J2000 coordinates of the nucleus of this galaxy are R.A. 06h43m06.s18   Dec. −74◦14 10".7.

The Lindsay-Shapley ring galaxy (AM 0644-741) appears in Category 6 of the Arp Madore catalog of Southern Peculiar Galaxies and Associations, a category that includes any galaxy with an apparent luminous ring around it, and attempts to exclude objects which appear to be spiral galaxies which have had their arms tightly wound into nearly circular appearance.

Below, there is a short video by Chandra Observatory about this enigmatic object in the Southern Sky,

Figure 1. Field of view of the Lindsay-Shapley Ring galaxy. 

"In AM 0644 741, we see a strong ring of bright blue stars in the Cerro Tololo Inter-American Observatory (CTIO) and Hubble Space Telescope (HST ) images, with associated HII regions. Unlike the Cartwheel, which shows spokes and is presumed to be a second ring-formation event (Struck-Marcell & Higdon 1993), AM 0644-741 is likely to be the first ring formed" (see the paper "Dynamical Parameters for AM 0644 741" Alex Antunes and John Wallin (2007).

As can be read on the paper "Morphology and enhanced star formation in a Cartwheel-like ring galaxy", F. Renaud et al. 2017), the Lindsay-Shapley Ring galaxy also displays an enhanced star formation activity in the furthest quadrant of its ring to the nuclei (Higdon et al. 2011).

The picture at the top of this article shows the 24-inch (0.61 meters) used to observe this galaxy. It was set up in a good field in the ranger station at the nature reserve named "Altos Limpios". You need to drive for almost 2 hours northeast from Mendoza City, in Argentina, to reach it. It is important to say that I saw this galaxy when it was about 32 degrees high in the sky (it reaches 48 degrees at its highest) and seeing was not the best that night, so maybe better results regarding the structures seen in this galaxy can be achieved with better-seeing conditions and with the galaxy at higher altitude.

The galaxy was searched, at the beginning of the observation, using 98x. After several minutes scanning the area the target was found, appearing, at this magnification, as a subtle nebulosity close to a quasi-stellar hazy spot when observing with averted vision, which is, actually, the core of the galaxy. The ring diameter, in arcsec, is 85 x 49. An asterism formed by 4 stars of magnitudes between 12 and 12.9 for those labeled in Figure 1, and magnitude 14.5 for the fourth one in the asterism labeled with a blue circle, was useful to confirm that the area was the correct (see Figure 1). A star indicated with a black arrow in Figure 1, is visible close to the galaxy´s nucleus.

Figure 2. DSS image of the Lindsay-Shapley Ring galaxy and its 

companions.

The Lindsay-Shapley Ring galaxy seems to be a member, according to the SIMBAD Database, of a small group of galaxies. The two nearest galaxies (at least in projection on the sky) were also detected through the 24-inch telescope. I´m talking about the galaxies LEDA² 19455, situated about 1.2 arcmin southeast of the target galaxy, and LEDA 19454, 1.05 arcmin to the north- northeast (see Figure 2). The first one, a distorted elliptical, was visible using averted vision as a small hazy patch. Both Graham (1974) and Few, Madore, and Arp noted an extended and low surface brightness light distribution between the ring galaxy and LEDA 19455. Partly on the basis of this emission, these researchers concluded that LEDA 19455 was the intruder galaxy. The other companion, fainter, was hardly visible using also averted vision.

Applying a little higher magnification (i.e. 122x), the view was similar to that one at lower magnification. However, the nucleus of the galaxy seems to be a little easier to see, like a faint defocused star. The outer part of the galaxy, including of course the ring structure, shows up when averted vision is applied, appearing as a slightly elongated nebulosity. It is not easy to see, and the beautiful structure seen in those detailed and colorful pictures provided by ground-based telescopes and the Hubble Space Telescope is not detected. The view is rather blurry and faint. The nearby galaxies are again visible, looking faint. LEDA 19455 appears similar to the nucleus of Lindsay-Shapley Ring galaxy, both in size and brightness.

203x is a good magnification to observe the group of galaxies. LEDA 19455 looks now brighter and its presence there is undoubtful, showing a round shape. The LEDA 19454 galaxy is fainter, as said before, but it can be detected. The central part of Lindsay-Shapley Ring galaxy looks bigger and its brightness seems to be smooth. When the galaxy is observed with averted vision, an arc-shaped structure of very faint nebulosity is visible just south of the nucleus

Figure 3. Picture credit of James L. Higdon and John F. 

Wallin, appearing as Fig 2 in their paper "Wheels of 

Fire. III. Massive Star Formation in the "Double-Ringed"

ring galaxy AM 0644-741".  

The view of the galaxy is interesting at 244x. The nucleus is easily visible through this instrument. The whole galaxy looks like a rather elongated nebulosity, a ghostly nebulosity among the surrounding stars in the field of view of the telescope. At this power and observing more carefully, some of the structure, very faint, and indicated with an arrow in Figure 2, seems to be detected for moments. AM 0644-741 possesses twin interlocked rings, each with slightly different centers, position angles, and ellipticity. The double-ring structure makes AM 0644-741 unique among ring galaxies, much like the Cartwheel and its network of spokes. A total of 54 H II complexes were identified in the rings using Figure 2a of the paper "Wheels of Fire. III. Massive Star Formation in the "Double-Ringed" Ring Galaxy AM 0644-741" by James L. Higdon & John F. Wallin (1997). I think the slightly brighter portion visible there, barely visible even with averted vision, are some of the HII regions present in the galaxy. The authors of the mentioned paper state that the most luminous H II complexes tend to be found near the intersection of the A-ring and B-ring (see Figure 3 here) which occurs in an area of the galaxy coincident with the one I observed, indicated with the arrow in Figure 2.

In the SIMBAD Database web page, two x-ray sources are indicated as present in that area also (i.e. [WFM2018] AM0644x1, and [WFM2018] AM0644x2.).

A long time ago, something amazing happened in this distant galaxy, and you can be a witness of that when observing it from a dark sky site.

1 _ Catalogue of Southern Peculiar Galaxies and Associations (Arp, Madore): 6,445 peculiar and interacting galaxies found on IIIa-J southern sky survey plates, types, positions, and characteristic sizes, cross-identifications and selected photographs. 

2 _ (Lyon-Meudon Extragalactic Database). Extension of PGC catalog in the Lyon-Meudon Extragalactic Database 1989: Catalogue of principal galaxies, 1ST edition: N=73197.
The first designation was PGC, but PGC and LEDA are now equivalent.