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!