M31 the Andromeda Galaxy and its Nuclear Spiral

Higher resolutions: M31 RGB narrowband 2000x2000 (50%), 4000x4000 (100%)
  M31 broadband continuum subtracted Ha 2000x2000 (50%), 4000x4000 (100%)

About this Image

Main Subject M31 the Andromeda Galaxy
ObjectsM31 (NGC 224), M32 (NGC 221), M110 (NGC 205), NGC206
Data NED database entry, Atlas of the Andromeda Galaxy
Description At a distance of 2.5 million light years M31 is the closest major galaxy to the Milky Way, and conversly the most distant object observable with the naked eye - albeit as a small murky patch on the very edge of visibility. It's only the nucleus that is bright enough to register, the disk of the galaxy itself extends invisibly over an area of sky more than three times the diameter of the full moon. In astrophotographs though its true scale and grandure can be fully appreciated.

Although the main image of Andromeda above looks reasonably natural it's actually a false colour narrowband image. Red indicates the relative concentration of ionized hydrogen gas, green the oxygen, and blue sulphur. This imaging technique is particularly effective at increasing contrast and detail of the nebulae embedded within the dust lanes. In the full size image you can see many prodigous bubbles of nebulosity that have been heated, blown up and will ultimately be carved out by the ultraviolet light and stellar winds of the young stars birthed within their cores.

Moving your mouse pointer on and off the image above will alternate between the colour narrowband image of M31 and a continuum subtracted Ha version. The continuum subtracted image shows even better contrast and reveals hitherto invisible structures within M31 - in particular the enigmatic "Nuclear Spiral" at its heart. This structure was only discovered in the early 80's by Jacoby et al. It is hard to determine if the nuclear spiral is oriented on a different plane from the rest of the galaxy, or is actually more of a spherical knot.

The image of the of the nuclear spiral below has been cropped and reduced from the full sized continuum subtracted image, and processed to better emphasise its structure.

M31's nuclear spiral

The continuum subtraction technique takes narrowband imaging one step further by reducing contamination of pure emission line sources. The contamination originates from black body radiation emitters, i.e. stars, emitting some of their radiation output into the narrowband emission lines. This reduces contrast and the detection limit for emission line objects such as nebulae and supernova remnants. The variant of the technique used for the M31 image uses a broadband red filtered image to sample the continuum radiation and subtracts this from the narrowband filtered image - leaving behind just emission line only objects. Note how the foreground stars virtually disappear, as does the bulk of M31 and its satellite galaxies. See the notes section at the bottom of this page for technical details.

M31 - nebulae image crops

M31 nebulae M31 nebulae M31 nebulae

M31 - our odd sister galaxy

Andromeda has various strange morphological features compared to many spiral galaxies. The disk is warped, the arms are more circular than spiral, larger than typical stellar associations, and the nuclear spiral at the core of the galaxy, to name a few. It is theorized that the small companion galaxy M32 ploughed head on into M31 a couple of hundred million years ago. Simulations show that such a collision could have disrupted the classic spiral arrangement into the strange almost concentric rings that we see in Andromeda today. M32 is the dwarf eliptical galaxy seemingly hovering just above the disk mid-left in the main image. It has been speculated that it actually started out as a small spiral galaxy, but lost its disk in the collision with Andromeda. What we see today may simply be what remains of its central bulge. In addition a much earlier close pass by the local spiral galaxy M33 may also have influenced the structure we see today, such as the warped disk. It has also been proposed that many of the Andromeda's characteristics can be explained by its creation from the collision of two spiral galaxies many billions of years ago.

The image below is a geometric stretch of Andromeda that tries to simulate an overhead view. Though this causes some obvious gross distorions it does provide an easier visualization of the overall structure of the dust lanes.

Geometric Stretch of M31

The interactions with M32 or M33 may also be the root cause of the massive star cloud known as NGC 206. It's one of the largest such regions in our local galaxy group and no comparably sized stellar association exists within the Milky Way itself. NGC 206 is the bright concentration of stars just in front of the odd intersection of two dust lanes towards the left of M31's disk, and shown below as a 100% resolution crop. It's possible that some individual supergiant stars have been resolved within the image, but it is difficult to be sure of distinguishing these from tight clusters of stars that are undoubtably also constituents of NGC 206.

NGC 206

Andromeda is one of the few intergalactic objects that exhibit a blue shift and is approaching the Milky Way at around 100km/s. The effects of satellite galaxy mergers will be nothing compared to the eventual cataclysmic collision and merger of Andromeda and the Milky Way. Don't hold your breath for this event though as it's not expected for another 4.5 billion years or so - double the present age of our solar system (...more than enough time to get around to those DIY jobs you have been putting off forever. Err, moving swifly on...).

Technical Details

Location Pumpkin Patch Observatory, Bourn, Cambridge, UK
Acquisition Date 2009 10 11 - 2010 01 06
Exposure 20x20m Ha, 21x20m OIII, 19x20m SII. Total 20hrs narrowband (Another 20+ hrs of exposure was rejected due to insufficient quality). 13x5m Red used for broadband continuum subraction.
Optics Astro Optik 400mm Cassegrain @ prime focus (f=1200mm)
Filters Astrodon Ha, OIII, SII 6nm narrowband; Red E-Series (60nm bandwidth); YR Cyclops filter wheel
Mount Paramount ME
Guiding Borg 101ED/SBIG ST402ME
Camera FLI Microline 16803
Software Maxim/DL 5, CCDStack, Photoshop CS
NotesR=Ha, G=O3, B=S2. This false colour mapping coupled with a colour balance targetting white stars & central core, along with a grey background, rendered a relatively natural "broadband" look to the galaxy.

The broadband continuum subtraction processing was based on the work of Filipe Alves and Mikael Svalgaard. Thanks for sharing guys, and in particular to Filipe for bringing it to my attention. The technique was described earlier in a paper by William H. Waller.

CCDStack was used for deconvolution-based FWHM matching of the red continuum image to the Ha, and then scaling and subtraction of this from the Ha master.

Broadband filter based continuum subtraction can perhaps be viewed as a poor man's method of continuum subtraction. The superior technique is to use specialist off-band narrowband continuum filters. These filters have a similar bandwidth to the narrowband filters, but are slightly offset from the emission line. This avoids the continuum sample including the emission line itself, as well as other lines, and also provides a closer match of star FWHMs between the continuum and narrowband images. This results in improved isolation of the pure emission line objects, and cleaner removal of stars. Broadband continuum subtraction does however require less exposure time to aquire the necessary equivalent signal level compared to narrowband continuum filters and it doesn't require the purchase of an expensive specialist filter.

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