NGC 7090, a Southern Edge-on Galaxy

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.

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

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.