madbadgalaxyman
20-05-2011, 10:51 AM
Dear fellow Galaxies Friends,
This is the second instalment in a series about unusual southern galaxies as revealed by the ultraviolet eyes of the satellite called GALEX
The interesting Barred Spiral galaxy NGC 986 in Fornax is less familiar to most of us than famous and spectacular Fornax galaxies such as NGC 1365 and NGC 1097. True, NGC 986 is actually significantly smaller than NGC 1097, with the bright part of NGC 986 extending some 2.5 arcminutes. However, in many ways NGC 986 has a unique structure and appearance.
Here is a blue plus red composite of NGC 986 from the Digital Sky Survey ; this composite image was created by the Aladin website, which is one of the best versions of DSS on the internet.
94363
Despite the fact that DSS is, these days, about equivalent to a very modest amateur image, the main features of NGC 986 are plain to see in this DSS image; so this object is not too hard.
FEATURES SEEN ON THE DSS IMAGE
The “bulge”, or central brightening, is obvious, but I have put bulge in inverted commas because this galaxy is a case in which the central brightening may not actually be a spheroidal
object (spheroidal is approximately “elliptical” or "oval" or "spherical" in common language).
In many barred spirals, for example NGC 986, there is a pseudo-bulge instead of a bulge.
A pseudo-bulge is a flat (not round) object formed by the successive bursts of star formation, as gas is funnelled to the centre of a galaxy..... by moving inwards along the bar. A flattened pseudo-bulge may eventually, with time, gradually build up into a true spheroidal ("spherical") bulge!
The bar structure that extends on either side of the bulge is very long and prominent, and it is surrounded by a rather rough and patchy ring of low contrast. It is an absolute rule of galaxy morphology that virtually every bar structure that is found in the universe is surrounded by a ring (or broken ring) structure of some sort!!
Ring structures that surround the large-scale bright bar found within a galaxy are known in the trade as “inner rings”.
Now....... surrounding this inner ring is a faint trace of a broken outer ring.
Madbadgalaxyman's Question: If this outermost ring is telling us the true orientation of NGC 986, that is, that this galaxy is relatively close to face-on, than what does this tell us about the true (intrinsic) shape of the inner ring?
A. The inner ring has to be an oval, in true three-dimensional space. Indeed, it was a great achievement by Ron Buta (Univ. of Alabama) and his colleagues to prove that many structures within spiral galaxies are NOT intrinsically round, but instead oval or elliptical in real space.
NGC 986 - THE ULTRAVIOLET VIEW FROM GALEX
Here is the view of NGC 986 from the GALEX satellite, with
the displayed blue colour coding for Far-ultraviolet light (FUV) and yellow coding for Near-ultraviolet (NUV) light. (This image was obtained with the GalexView virtual telescope).
94364
Remember, yellow (NUV) comes from older stars, and blue light in this image (actually FUV) comes from very young and massive and hot stars.....that is, the blue channel shows the location of current star formation activity.
Note how the bulge is modestly visible and the bar is not visible, in ultraviolet. The stellar population of the bulge and bar should be old, as old red stars don't put out much ultraviolet light.
However, when we look at the data from the GALEX satellite, and I display only the Far-ultraviolet image taken by the satellite, we see that there is significant FUV light coming from the centre of NGC 986:
94365
The above version I have made of the FUV image shows only the brightest FUV features in this galaxy. It reveals a strong central concentration of FUV light, which is due to a powerful burst of Massive Stars formation at the very centre of this galaxy. It also shows that enhancement of star-formation (traced by FUV) at the two ends of the bar that is typically found in barred spiral galaxies.
The following version of the same FUV data is at a logarithmic scale, and it plainly shows, in addition to the central FUV concentration, the inner ring that surrounds the bar. (but not the bar itself!). This ring is obviously a region of strong star formation:
94366
Furthermore, this stretched version of the FUV image plainly shows the outermost broken ring that surrounds the inner bar-straddling ring. These broken rings are sometimes called pseudo-rings, in the opaque jargon of professional astronomy.
NGC 986 is therefore a “two-ringed galaxy”.
Amazingly, some galaxies, such as NGC 1433 and NGC 1291 , are “three-ringed galaxies”.....they have an additional Very Small tight ring that immediately surrounds the very centre of the galaxy. (sounds something like a three ring circus!)
OTHER IMAGES - RED, H-alpha and HUBBLE
Now I will display an R-band (red) image of NGC 986 from the online "De Vaucouleurs Atlas of Galaxies". This image is of decent, though not high, angular resolution, and it could be a fair approximation of what average-to-good amateur imagers can achieve without too much trouble :
94368
Note that this displayed image shows the bar and the patchy inner ring that surrounds the bar, but it does not show the faint outermost broken ring.
H-alpha imaging of NGC 986:
I have not included the Hydrogen alpha image of this galaxy, as this post is already too long. Might post it later. Suffice it to say that the bar is unusually rich in interstellar gas (prominent in CO submillimetre emission) and the bar is very prominent in H-alpha images.
Lastly, here is a spectacular Hubble Space Telescope image:
94367
Cheers,
madbadgalaxyman
____________________
This is the second instalment in a series about unusual southern galaxies as revealed by the ultraviolet eyes of the satellite called GALEX
The interesting Barred Spiral galaxy NGC 986 in Fornax is less familiar to most of us than famous and spectacular Fornax galaxies such as NGC 1365 and NGC 1097. True, NGC 986 is actually significantly smaller than NGC 1097, with the bright part of NGC 986 extending some 2.5 arcminutes. However, in many ways NGC 986 has a unique structure and appearance.
Here is a blue plus red composite of NGC 986 from the Digital Sky Survey ; this composite image was created by the Aladin website, which is one of the best versions of DSS on the internet.
94363
Despite the fact that DSS is, these days, about equivalent to a very modest amateur image, the main features of NGC 986 are plain to see in this DSS image; so this object is not too hard.
FEATURES SEEN ON THE DSS IMAGE
The “bulge”, or central brightening, is obvious, but I have put bulge in inverted commas because this galaxy is a case in which the central brightening may not actually be a spheroidal
object (spheroidal is approximately “elliptical” or "oval" or "spherical" in common language).
In many barred spirals, for example NGC 986, there is a pseudo-bulge instead of a bulge.
A pseudo-bulge is a flat (not round) object formed by the successive bursts of star formation, as gas is funnelled to the centre of a galaxy..... by moving inwards along the bar. A flattened pseudo-bulge may eventually, with time, gradually build up into a true spheroidal ("spherical") bulge!
The bar structure that extends on either side of the bulge is very long and prominent, and it is surrounded by a rather rough and patchy ring of low contrast. It is an absolute rule of galaxy morphology that virtually every bar structure that is found in the universe is surrounded by a ring (or broken ring) structure of some sort!!
Ring structures that surround the large-scale bright bar found within a galaxy are known in the trade as “inner rings”.
Now....... surrounding this inner ring is a faint trace of a broken outer ring.
Madbadgalaxyman's Question: If this outermost ring is telling us the true orientation of NGC 986, that is, that this galaxy is relatively close to face-on, than what does this tell us about the true (intrinsic) shape of the inner ring?
A. The inner ring has to be an oval, in true three-dimensional space. Indeed, it was a great achievement by Ron Buta (Univ. of Alabama) and his colleagues to prove that many structures within spiral galaxies are NOT intrinsically round, but instead oval or elliptical in real space.
NGC 986 - THE ULTRAVIOLET VIEW FROM GALEX
Here is the view of NGC 986 from the GALEX satellite, with
the displayed blue colour coding for Far-ultraviolet light (FUV) and yellow coding for Near-ultraviolet (NUV) light. (This image was obtained with the GalexView virtual telescope).
94364
Remember, yellow (NUV) comes from older stars, and blue light in this image (actually FUV) comes from very young and massive and hot stars.....that is, the blue channel shows the location of current star formation activity.
Note how the bulge is modestly visible and the bar is not visible, in ultraviolet. The stellar population of the bulge and bar should be old, as old red stars don't put out much ultraviolet light.
However, when we look at the data from the GALEX satellite, and I display only the Far-ultraviolet image taken by the satellite, we see that there is significant FUV light coming from the centre of NGC 986:
94365
The above version I have made of the FUV image shows only the brightest FUV features in this galaxy. It reveals a strong central concentration of FUV light, which is due to a powerful burst of Massive Stars formation at the very centre of this galaxy. It also shows that enhancement of star-formation (traced by FUV) at the two ends of the bar that is typically found in barred spiral galaxies.
The following version of the same FUV data is at a logarithmic scale, and it plainly shows, in addition to the central FUV concentration, the inner ring that surrounds the bar. (but not the bar itself!). This ring is obviously a region of strong star formation:
94366
Furthermore, this stretched version of the FUV image plainly shows the outermost broken ring that surrounds the inner bar-straddling ring. These broken rings are sometimes called pseudo-rings, in the opaque jargon of professional astronomy.
NGC 986 is therefore a “two-ringed galaxy”.
Amazingly, some galaxies, such as NGC 1433 and NGC 1291 , are “three-ringed galaxies”.....they have an additional Very Small tight ring that immediately surrounds the very centre of the galaxy. (sounds something like a three ring circus!)
OTHER IMAGES - RED, H-alpha and HUBBLE
Now I will display an R-band (red) image of NGC 986 from the online "De Vaucouleurs Atlas of Galaxies". This image is of decent, though not high, angular resolution, and it could be a fair approximation of what average-to-good amateur imagers can achieve without too much trouble :
94368
Note that this displayed image shows the bar and the patchy inner ring that surrounds the bar, but it does not show the faint outermost broken ring.
H-alpha imaging of NGC 986:
I have not included the Hydrogen alpha image of this galaxy, as this post is already too long. Might post it later. Suffice it to say that the bar is unusually rich in interstellar gas (prominent in CO submillimetre emission) and the bar is very prominent in H-alpha images.
Lastly, here is a spectacular Hubble Space Telescope image:
94367
Cheers,
madbadgalaxyman
____________________