NGC 1365

[AusGuy]

NGC 1365 has appeared a few times in the Deep Sky forum but, so far as I can determine, it has not yet made an appearance here in this EAA forum. Popularly known as The Great Barred Spiral, it is a beautiful galaxy and well worth observing but a bit of a challenge for visual observers unless they have a fairly large telescope. But for EAA it's a cinch. The image below is a screenshot from an observing session with my 8 inch Newtonian and I think that I can confidently say that it displays most of the galaxy's visual components. It's shown at 100% resolution. With my imaging system, when the camera's full frame is displayed the galaxy appears quite small although it's then possible to frame it together with some of its adjacent galaxies. The live view is not a patch on long exposure images, of course, but that's not the point. Being able to see this level of detail quite painlessly is a reward in itself. Each exposure was only 2 minutes at 400 gain but it took quite a few additions to the stack to bring out the fainter parts of the spiral arms.

Located in the constellation Fornax, NGC 1365 is part of the Fornax Cluster of galaxies. Its distance is generally quoted as being 60 million light years (NASA, ESA, ESO) and its diameter as being 200,000 light years (but see below). There are many articles on the internet describing the nature of this galaxy and its active central black hole and they are easily found. So I won't bore you with the same details here. Rather I'll confine this to what can be seen or reasonably determined from the EAA view.

Generally referred to as a double-barred galaxy, the fainter main bar runs pretty much horizontally across the centre in this image. I measure its angular size to be 2' 40” making it 43,550 ly long. The brighter inner core is set obliquely across it. At first sight that inner core looks to have a rounded “Z” shape but closer inspection shows that it is in fact a 21,600 ly long ovoid with obscuring dust lanes coming in from top left and bottom right. “Proper” AP images (such as https://apod.nasa.gov/apod/ap210108.html ) show that the two dust lanes are simply the left and right sides of an almost continuous dust lane that runs right across the front of the main bar. But here the overexposed core of the galaxy washes out the dust lane's middle part.

The dust lanes themselves show some structure with wisps stretching downwards at the left and upwards at the right. They are much better seen in the APOD image and seem to relate to the ends of various spiral arms that are starting there.

The two brightest opposing spiral arms show evidence of many star forming regions strung out along them. Our view only shows hints of the red colour of the emission nebulae that are evident in the APOD image. We can see other arms as well all following the galaxy's general spiral shape. But that brings up what seems to be an obvious question – why do the outer arms seem to curve back towards the centre of the galaxy? It seems as if the whole galaxy has been warped and the arms are being bent away from the plane of a straight-forward flattened disk. Or is it just an optical illusion? So far I have not found the answer to that conundrum. But maybe you know the answer?

As to distance and size, most sources seem to follow each other in quoting figures of 60 mly and 200 kly respectively but these are suspiciously round and it seems probable that the actual values (as best as can be determined) are somewhat different. Is that important here? Probably not. The Fornax Cluster is located well beyond our local group and the fact that NGC 1365 still displays the size and detail that it does must mean that it is extremely large (as galaxies go). But some internet sources are annoyingly inconsistent in their description of its distance and size. For example, Wikipedia quotes a distance of 56.2 mly, a size of 205 kly and an angular size of 11.2 arc minutes. But simple trigonometry shows that at that distance 11.2' subtends only 183 kly. However, there is one source that goes into the matter is some detail. It can be found at https://ned.ipac.caltech.edu/level5/Lindblad/Lind2.html where values are shown as distance 18.6 mpc (60,707,000 ly), angular size 11.2' and long axis 61 kpc (199,092 ly). So it seems that NGC 1365 really is twice as large as our Milky Way galaxy's disk.

[Mike JW]

Hi David,

Excellent read and so informative, highlighting just what can be observed using the EAA technique. I fancy the answer to why do the arms curve back to the centre could be due to the angle of view?

Mike

[Martin Meredith]

Really interesting to read about this southern galaxy.

I see that LEDA provides an estimate of 12.0' whereas Simbad gives 10.72'. Perhaps different sources uses different criteria for deciding where the arms end? Whichever it is, it would fit very nicely on my sensor!

In principle I could observe this at 11 degrees above the horizon but it is in the direction where I suffer most light pollution. I have observed a few galaxies in Fornax and it is one of those constellations (Grus being another one) that I'm aware of having some great DSOs having pored through the Night Sky Observer's Guide. One day...

Martin

[AusGuy]

Thanks for your feedback Mike and Martin. As regards size, in the original post I had initially thought to expand on the uncertainty of quoted values for angular measurement and linear size but decided against it in the interests of brevity. But since you mention the LEDA and Simbad values Martin I'll give it a go now.

In the image attached below I've used a screen display measuring tool that I have to evaluate the EAA image. It would have been nice to have been able to use the APOD image for this purpose since it is much clearer but it is copyrighted so the EAA image will have to suffice. I have marked various stars with letters. For those marked A and B I have determined their angular separation by using the measuring tools in SkyTools3 and Cartes du Ciel. They both agree on a value of 9'54”. The measuring tool has a base (or calibration) circle which I've coloured red, placed it over the image so that it exactly spans stars A and B and set its angular value to 9'54”. The tool has a secondary measuring circle which I've coloured yellow. When I set its size value to 10.72' or even 11.2' I found that it did not fully encompass the galaxy's outer arms. So I've enlarged it so that it does. You can check this by referring to stars marked C and D and finding them on the APOD image. The resulting value for the enlarged measuring circle is 12'22”. But with an accepted distance of 60,700,000 ly that angular measurement results in a computed size for the galaxy of 218,382 ly which is significantly larger than the usually quoted 200,000 light years. Now it probably seems brave to the point of being foolhardy to challenge the linear size value quoted by so many eminent sources but that's the way the cards seem to fall. Could the measuring tool be inaccurate? Possibly, but I doubt it. It uses the simple arithmetical logic :

Calculated measurement angle = Entered calibration angle x M/B

where :

M = number of screen pixels across the horizontal diameter of the measuring circle

B = number of screen pixels across the horizontal diameter of the calibration circle

And, as I mentioned, both SkyTools3 and Cartes du Ciel agree on the value of the calibration angle.

David

[AusGuy]

It just goes to show that one should not write in haste in the wee hours after midnight. In preparing the second post above the wrong star was marked D causing an error in the measurement. Oops.

The better representation is given below. When I set the measuring circle's size to 10.72' it does not fully enclose the galaxy's outer arms. But enlarging it to 11.2' (11'12”) gives a much better fit although arguably just a little too small. Thus it seems that the size values given in the second source reference quoted above are the more correct. So unfortunately for the galaxy it has had to revert back to its usual size!

David