Quote:
Originally Posted by Weltevreden SA
SN 2013aa seems quite remote into the halo of 5643, 2x beyond the last
visible traces of spiral arms and roughly 4x the virial radius. Even though the spectral data recorded thus far and dist modulus indicate the
SN is part of the galaxy, it's the first time I recall a 1a occurring so seemingly remote from its host galaxy.
......has occurred far out in the galactic boonies where there's very low gas and dust density, not much in the way of magnetohydodynamic turbulence, and no visible bar torque or recent interactivity with another high-mass object. Could 2013aa be in a faint dwarf orbiting 5643?
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A very interesting comment, and it is very good to hear from someone in South Africa; a large proportion of our IIS members seem to come from Australia, and we don't get too many posts from SA. This is odd, as we share exactly the same sky!!
There has been a lot of talk in the recent literature about XUV ("extended ultraviolet") disks in spiral galaxies, as can be detected using the Far-Ultraviolet imaging band of the GALEX satellite;
It has been proven, from Galex FUV detections of star-forming knots found in the very outermost regions of spiral galaxies (the FUV bandpass is inordinately sensitive to current star formation), that star formation can occur
a long way outside of the optically-bright disk of a spiral galaxy.
A good nearby example of this phenomenon is M81, in which the low-density intergalactic medium has nonetheless somehow managed to form knots of OB stars!!
As the wavelength-range and sensitivity of astronomical observations have progressively increased, there has been a tendency for astronomers to find ever more inter-galactic gas between the galaxies and ever more gas outside of the bright optically-evident bodies of galaxies. So astronomers now know of several sources of gas that can provide the raw materials for forming stars in the outermost parts of galaxies.
For instance:
- it has turned out to be the case that many elliptical galaxies are NOT gas-poor, as was the common so-called “knowledge”(actually a prejudice!) found in the textbooks about galaxies. There is often a massive interstellar medium of very hot X-ray emitting gas that surrounds an elliptical galaxy, and this gas could potentially cool enough to form new stars. There is also a colossal reservoir of very hot gas associated with many clusters of galaxies, often with more mass existing in the form of hot gas than is found in all of the stars of all of the galaxies.
- low column-density HI (neutral atomic Hydrogen gas) has been found in the outermost regions of many galaxies, and the total amount of this gas can be comparable to the amount of gas actually found within the prominent central regions of a galaxy. This gas was undetectable until very recently, due to its low density!
- Further evidence for the existence of large reservoirs of cold (and therefore, potentially star forming) gas existing outside of the obviously visible disk components of spiral galaxies can be found from considering the star-formation history of our own Galaxy; the Milky Way Galaxy has been forming stars steadily for all of its history, and the ongoing star-formation would have exhausted its total gas supply several times during its history, were there not some mechanism by which cold gas falls into the disk from above it and below it. The interstellar gas in spiral galaxies seems to be recharged by gas which is falling in from the halo of the spiral galaxy, or even perhaps falling in from intergalactic space.
- the intergalactic gas surrounding a galaxy, or the Low column-density gas that is actually associated with the outermost regions of a galaxy, can be caused to form new stars by the energetic effects of plasma beams originating in a central Active Galactic Nucleus, or by the energy of powerful shock waves that exit the disk of a spiral galaxy due to the effects of multiple supernovae.
For instance, in NGC 5128, the plasma beam from the central AGN has set off some modest star-formation in the outermost regions of this galaxy.
- Supershells (powerful shock waves, in a gaseous medium, coming from multiple supernovae) can expand outwards, reaching outwards to points which are a long way from the obvious bright disk component of a spiral galaxy, thereby providing adequate energy for the inception of star formation in clouds of cold gas existing in the outermost regions of galaxies.
One further point is that a galaxy does not have to look the same now as it did three billion years ago;
- there is evidence that a massive interstellar medium can be stripped from the outer parts of a disk galaxy by encounters with other galaxies or by the effects of "ram pressure stripping" of a galaxy's Interstellar Medium caused by high-velocity passages of the galaxy through a low density intergalactic medium. Thus, outermost stars and other outer features that are observed in a galaxy could be a residual of previous structures that no longer exist.
- Another process is also likely to exist, which affects a galaxy in the opposite way to the aforementioned exhaustion or stripping of gas from the outer regions of some galaxies;
the outer disks of spiral galaxies, and even the outer disks of elliptical galaxies(!!!) , can be gradually built up from the infall of gas!!
This seems to be happening in the S0/E galaxy NGC 1316, as was strongly indicated in a recent paper by Prof. Kenneth C. Freeman (of ANU) and colleagues.
Best regards,
madbadgalaxyman
Madbadgalaxyman's comment of the day;
"Yes, Virginia, elliptical galaxies
do have disk components. Disks in elliptical galaxies just happen to be very faint, or very small in radius."