I am very pleased to present what I believe is the first image to reveal the Hydrogen alpha emission surrounding Centaurus A.  

Centaurus A is likely the most imaged and studied southern galaxy, so it is pretty exciting to be able to show a new feature in this patch of sky.  Due to its close proximity, Centaurus A has the largest angular extent of any radio galaxy, and it is well known for having a narrow radio jet emanating from its central black hole which results in an ionised hydrogen filament to the North of the galaxy. In addition to the narrow radiojets, there are large radio emitting lobes to the North and South of the galaxy, which span a massive 8 degrees across the sky.

While we are all familiar with the stellar component of galaxies, they are surrounded by huge halos of gas.  These halos extend out to the galaxies virial radius, and this gas is called the circumgalactic medium (CGM). Gas flows into the CGM from the intergalactic medium, and eventually falls into the galaxy feeding ongoing star formation.  Some of the gas in the CGM may also be uplifted from the galaxy by active galactic nucleus (AGN) outflows or may be left over from previous tidal merger events. The CGM is predicted to contain as many baryons (effectively matter containing protons and neutrons) and heavy elements as the galaxy itself, and without the CGM, star formation within the galaxy would eventually cease. The CGM is difficult to detect as it is very large, diffuse and faint.   As a result, most studies into the CGM have relied on detecting the CGM through absorption spectrum from quasars that happen to be located behind the halo in our line of sight. 

Prior to starting this project, I came across a few papers where new Ha clouds were directly imaged around M51 and M82. These Ha clouds were thought to possibly be components of the circumgalactic medium of the galaxies, and so I decided to embark on this project with the aim of attempting to image the Ha halo/CGM of Centaurus A.   In most galaxies, the CGM is thought to extend out ~200kpc from the galaxy, which in the case of Centaurus A would be an angular radius of ~1.6 degrees, so a wide field was definitely required!

For this image, I have processed the Ha image by doing a continuum subtraction using the broadband R filter for the continuum and thus revealing the isolated Ha emission.  This is a very useful technique to remove the continuum that our narrowband filters do not completely block.  @Vicent Peris  from PixInsight has published an excellent writeup on continuum subtraction using broadband filters (https://pixinsight.com/examples/M31-Ha/) and William Waller wrote an article about the broadband technique in 1990 (https://adsabs.harvard.edu/full/1990PASP..102.1217W) for those interested.

It was exciting to see distinct Ha clouds and streams present on the Ha continuum subtraction image, which almost appear to be spiralling into the galaxy. There is a large convex cloud to the North of Centaurus A  ~1.59 degrees (~95 kpc)  from the galaxy centre, and there are further circumferential clouds around Centaurus A with a varying radius of 30 - 50 arc minutes (30 - 50 kpc).  The Ha emission is largely distinct from the galactic cirrus in the region, and so I do not think it represents extended red emission from the cirrus.  The Ha is extremely faint and I have added the central RGB galaxy in the main image via a mask.  As a bit of a sanity check, I compared my image against the SHASSA Ha image of the Centaurus A region.  SHASSA is a wide angle Ha sky survey using a 50mm lens at a resolution of ~47 “/pixel.  After heavily stretching the SHASSA continuum subtracted image, you can see that there is faint Ha emission in the same location and morphology confirming my findings.  


© Mathew Ludgate & the Southern H-Alpha Sky Survey Atlas (SHASSA)

It would be very interesting to further characterise the Ha clouds detected in my image.  Spectroscopy of the clouds will require a very large scope given their low brightness but could give useful information  such as the cloud velocity, metallicity and estimates of its density. This information could help determine the origin of the clouds -are they related to Centaurus A or are they part of the Milky Way in our line of sight?