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 M81. 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 to ~200kpc from the galaxy, which in the case of Centaurus A would be an angular radius of 3.3 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.
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.
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?
That's a great image and a really good contribution to the science behind what we do. As an ignorant amateur, how can you be sure that the hydrogen is associated with Centaurus A and not foreground hydrogen in our galaxy?
That's a great image and a really good contribution to the science behind what we do. As an ignorant amateur, how can you be sure that the hydrogen is associated with Centaurus A and not foreground hydrogen in our galaxy?
Kevin
Thanks Kevin! Great question, without further data I can’t be sure as to the origin of the Ha. It is quite likely that some or all of the Ha in my image is associated with the Milky Way but would be amazing if some of it is part of Centaurus A. There aren’t any known Milky Way high velocity clouds in this part of the sky afaik, so that’s a plus to them being part of Centaurus A. Also the shape of the upper convex cloud is in about the same location as the large radio-lobe from the galaxy so maybe that suggests it could be associated to the outflow from the AGN. Ultimately, spectroscopy would definitely help determine the origin but I suspect would need a very very very large scope 😂
Wonderful image and I thoroughly enjoyed reading the technical points! It really does demonstrate how complex and interconnected some of these deep sky objects are. Congrats on another fantastic project.
A magnificent image and great contribution. Well done.
I would conjecture the Ha is in the Milky Way as average galaxy images don't show surrounding Ha clouds and the distances involved would be mind boggling. The clouds would be many hundreds of times larger than Cent A itself. Whilst possible it just doesn't seem likely.
The Milky Way on the other hand is chock full of Ha clouds as a wide angle shot reveals.
Also Ha surrounding a distant galaxy, where would the ionizing radiation come from?
Love that technique. Just when you think you have seen it all in this game someone posts an image like yours!
Wonderful image and I thoroughly enjoyed reading the technical points! It really does demonstrate how complex and interconnected some of these deep sky objects are. Congrats on another fantastic project.
Thanks David! It is pretty amazing how everything is connected by the cosmic web. Cheers, Mat
A magnificent image and great contribution. Well done.
I would conjecture the Ha is in the Milky Way as average galaxy images don't show surrounding Ha clouds and the distances involved would be mind boggling. The clouds would be many hundreds of times larger than Cent A itself. Whilst possible it just doesn't seem likely.
The Milky Way on the other hand is chock full of Ha clouds as a wide angle shot reveals.
Also Ha surrounding a distant galaxy, where would the ionizing radiation come from?
Love that technique. Just when you think you have seen it all in this game someone posts an image like yours!
Greg.
Thanks Greg! You raise some very good points 👍 I agree it is likely some/all of the Ha is related to the Milky Way but the gas halos around galaxies are amazingly large. For example, the Ha cloud that was discovered near M82 is 44kpc from the galaxy, which is a similar distance to the inner ring of clouds in my image 😳
As to ionisation sources, the Ha could be photo-ionised from local or background UV radiation, from starburst activity from CenA, from AGN outflows (likely with CenA), or from shock ionisation from infalling gas. It is also possible the ionisation is a light echo from a previous more active starburst period or AGN outflows. Getting some spectroscopy done would be great and would give a few clues about the ionisation mechanism 😀
Sub zero cool Matthew. I once saw a mono shot of the Andromeda galaxy with some foreground Ha nebs. Always very cool to see in a widefield. And yours is in color.
Sub zero cool Matthew. I once saw a mono shot of the Andromeda galaxy with some foreground Ha nebs.
Always very cool to see in a widefield. And yours is in color.
<quote>how can you be sure that the hydrogen is associated with Centaurus A and not foreground hydrogen in our galaxy?<quote>
That was my first question, but then I noticed a filament of ionized hydrogen extending from Cen a itself, then noticed that the immediate area around CentA appears to be clear, in fact it looks like CentA is surrounded by a bubble of Halpha, and the image is showing the densest parts visible through the sides of the bubble, whilst the foreground and background of the bubble is too faint to image.
It's a really great image that does show just how much hydrogen gas there is just floating out there. Like a few others, I'd agree that some of the Ha emission surrounding the Cent A galaxy (most specifically the Ha jets) are indeed a part of Cent A.
The vast majority of the Ha emission in this image would have to be hydrogen gas within our galaxy. Where the idea does start to get interesting is that a good portion of the baryonic mass (possibly 70%) within some galaxy clusters is hydrogen gas not inside galaxies at all. This is typically towards the centres of galaxy clusters which are long "dead" elliptical galaxies which certainly doesn't describe Cent A.
The two papers here and here go into a bit of detail around the surrounding mechanisms.
In short, Ha directly surround a galaxy like Ha may not be detectable due to its temperature (too hot). It's estimated that some 40-50% of the baryonic matter in the universe is in the hot intergalactic medium but it's difficult to confirm as it's so difficult to detect given it's temperature and lack of emission. This further points to most of the Ha emission in the image being within our galaxy
Oh I love your posts, Mat You're always doing something unique and I love the science behind everything.
That said, I'd also be hesitant to call these emissions around Centaurus. I mean, they're incredibly bright if so. I know this is a massive amount of integration, but thinking of the Inverse-square law (the Intensity of the radiation is inversely proportional to the square of the distance) that hydrogen would have to be almost half as bright as a black hole's accretion disk to be picked up at this distance, and what would cause it to be that bright?
Sub zero cool Matthew. I once saw a mono shot of the Andromeda galaxy with some foreground Ha nebs. Always very cool to see in a widefield. And yours is in color.
Thanks Marc! As Allan has mentioned RBA published his fantastic image of the Ha clouds around M31, and Vicet Peris from Pixinsight also did a great monochrome M31 recently 👍
<quote>how can you be sure that the hydrogen is associated with Centaurus A and not foreground hydrogen in our galaxy?<quote>
That was my first question, but then I noticed a filament of ionized hydrogen extending from Cen a itself, then noticed that the immediate area around CentA appears to be clear, in fact it looks like CentA is surrounded by a bubble of Halpha, and the image is showing the densest parts visible through the sides of the bubble, whilst the foreground and background of the bubble is too faint to image.
Thanks William! The morphology of the Ha certainly helps look like it could be associated with CenA -a large inner bubble and then spiral arms. I've attached an image from Hummels et al, ApJ 2019 showing a hydrodynamical model of the CGM in a 180kpc x 180kpc area (which would be a 3 x 3 angular degree area for Cen A) -this shows fairly similar streams and an inner ring 🤔:
Interesting some of the absorption spectrum studies of the CGM have shown the amount of ionised gas increases as you move further away from the galaxy 😳
The h-alpha glow is without doubt within the Milky Way (there are professional surveys that show this)
Still not convinced?
Think about the brightness of the diffuse h-alpha flux assuming it was emitted at the distance of Cent-A (~13 Million light years).....
well..."brightness" is perhaps the wrong term....it should be incredibly dim.
Occam's razor
...but if not, then write up the paper and I'd expect you'll be in Stockholm soon enough .
Physics aside, a *really* interesting image and brings something new to the table
Really well done
Many thanks Peter! I agree Milky Way gas is most likely, and needs to be excluded before we accept it being part of CenA. The M82 Ha cloud I mentioned earlier was determined to most likely be associated with M82 rather than the Milky Way based on the information from the spectroscopy etc. The image of the M82 Ha cloud was taken with a pathfinder instrument for the Dragonfly array, which uses 3 x Canon 400mm f2/8 lens so very similar to me. For their image they detected the M82 Ha cloud on 2 images -one with a single lens integration of 95 hours and the other image with a single lens integration of 25 hours. M82 and CenA are both 12 million light years way from us, so it is at least possible that I have picked up some Ha associated with CenA with my 51.5 hours of Ha. All I need is 4-5 hours of time on a 8-10m scope and I'll let know for sure 😂
It's a really great image that does show just how much hydrogen gas there is just floating out there. Like a few others, I'd agree that some of the Ha emission surrounding the Cent A galaxy (most specifically the Ha jets) are indeed a part of Cent A.
The vast majority of the Ha emission in this image would have to be hydrogen gas within our galaxy. Where the idea does start to get interesting is that a good portion of the baryonic mass (possibly 70%) within some galaxy clusters is hydrogen gas not inside galaxies at all. This is typically towards the centres of galaxy clusters which are long "dead" elliptical galaxies which certainly doesn't describe Cent A.
The two papers here and here go into a bit of detail around the surrounding mechanisms.
In short, Ha directly surround a galaxy like Ha may not be detectable due to its temperature (too hot). It's estimated that some 40-50% of the baryonic matter in the universe is in the hot intergalactic medium but it's difficult to confirm as it's so difficult to detect given it's temperature and lack of emission. This further points to most of the Ha emission in the image being within our galaxy
Thanks Colin! Thanks for the papers, I'll check those out 👍 The Intergalactic Medium (IGM) and CGM are pretty amazing, and how they are all linked through a filamentary gaseous cosmic web is mind blowing. As you mention, the gas accounts for a very large fraction of the matter in the universe and we know fairly little about it.
A beautiful and fascinating image Mathew! Excellent!
As to whether the emissions are associated with Cent. A, I very much doubt it. HII emission requires a relatively nearby ionising energy source like from type O or B stars. The energy density that far from a galaxy would be very low. However, some of the literature describes isolated dots of intergalactic HII regions, but surmise they are ionised by one or more nearby and equally isolated energetic stars.
Last edited by marc4darkskies; 19-07-2022 at 08:34 AM.
I once saw a mono shot of the Andromeda galaxy with some foreground Ha nebs. Always very cool to see in a widefield. And yours is in color. 
