M33 in narrow-band HST


Chris Heapy

Macclesfield, UK

Telescope was my TeleVue NP127is refractor mounted on a Losmandy G11 equatorial mount, guided using a TeleVue Pronto guidescope plus SX Lodestar guidecamera. The main CCD camera was an Atik490EX operating at 1×1 binning (equivalent to approximately 1.2 arcseconds per pixel).
Image processing was carried out mainly using StarTools and PixInsight, with some minor colour balancing done using Photoshop.

This image of the famous Triangulum Spiral Galaxy M33 is unusual in that it was acquired using Astrodon Ultra-Narrow bandwidth (3nM) nebula filters. My objective was an attempt to reveal the active star forming areas in this 3 million light year-distance galaxy which I thought should be visible as areas of blue/red/yellow when rendered using the standard Hubble Space Telescope (HST) palette. In this colour scheme the active HII regions appear as Greens (for ionised Hydrogen-alpha), Reds (for Sulphur-II) and Blue (for triple-ionised oxygen, OIII), and where these areas overlap, as H-a and SII frequently do, the final colour appears as yellows and browns which is the natural combination of the two. By using these narrow band filters the appearance of stars is suppressed, and whilst these would not be resolved anyway using amateur equipment the usual misty appearance of star-filled spiral arms is missing. This image encompasses most of the visible galaxy and what remains is a beautiful scattering nebulae of all sizes, distributed principally along the spiral arms – or rather where the spiral arms would be if they were visible!
Some of these discrete nebulae are huge and actually have their own catalog designation, the largest (top-right) is ngc604 and is one of the largest HII regions known with a diameter of approx 1500 light years and a spectrum similar to our familiar Great Orion Nebula. All these areas represent active star forming regions, clusters of young energetic stars forming at the centre of vast clouds of hydrogen gas and dust which are actively hollowing-out the cloud, stellar winds and intense UV radiation driving the gas and dust outward and ionising radiation causing it emit its own light, and by this process we can see it using the narrow band nebula filters. The typical structure of such areas is revealed in this image; a blue (OIII) centre region which would be closest to the star cluster with the red/yellow ionised gas driven out to the periphery. There are many parallels to be seen much closer in our own Milky Way, ngc281 (the Pacman Nebula) for example exhibits an identical structure, and being a lot close is much easier to study and understand. Also in the image are some features which I find more difficult to interpret, large areas of OIII-bright (blue) nebula with no associated H-alpha region.
This image consists of 10 x 20min sub-frames for each channel, so a total of 10 hours exposure. This was acquired over several nights in early October 2014 from my back-garden observing site in a suburban area of Macclesfield, Cheshire.