Quote:
Originally Posted by Shiraz
Most hi res images of NGC253 show indistinct darker filaments/columns rising from around the central zone, as in the attached image. The paper you referenced indicated that such features are well known in other galaxies and at other wavelengths, so could they be consistent with a cold gas outflow in NGC253 - they even seem to be in about the right place.
Regards Ray
|
You have been observant, Ray, in noting the existence of the dust columns/plumes/chimneys . There is also, plausibly, some 'breakout' of the dust from the plane of NGC 253, at other points within the disk component of this galaxy which are not so close to its central region.
The connection between the vertical dust features and the outflow of cold molecular hydrogen gas is plausible on astrophysical grounds.
There have been various observations made at many wavelengths in NGC 253 and in other spirals, of material that seems to be exiting "vertically" from the plane of a spiral galaxy....... but nothing like a consistent picture has emerged of what is going on.
Agreed, Ray, NGC 253 is a very good example of a galaxy with dust columns/filaments. I have been calling this sort of feature seen in edge-on spiral galaxies "vertical plumes and chimneys". (a spectacular example of possible dust ejection from the centre of a galaxy is NGC 1808 , in which the narrow radial dust lanes somewhat resemble a fountain!)
The "vertical" (at right angles to the plane of a galaxy) dust features in NGC 253 were studied in a paper by Sofue & Wakamatsu & Malin (1994, AJ,
108, 2102) using an unsharp masked AAT plate that David Malin had made of this galaxy. (Their image of dust in NGC 253 is still well worth looking at, if you can find the paper version instead of a poor quality scan). Their paper called it a "boiling, steaming" interstellar medium, if I remember correctly. Their results were very tentative, sort of saying"we observed this" but with minimal hard facts to support their interpretation. However, this was indeed a forward-looking paper. Nonetheless, studies of the "disk-halo interface" in spiral galaxies remained a backwater for many years after this paper was written!
You are right that the presence of dust columns or chimneys coming from the central zone, as observed in the optical regime, is highly consistent with the outflow of cold molecular gas from NGC 253.....because there is a very close relationship between dust and molecular gas in the interstellar medium of galaxies;
As a rough and ready rule, one can say that there is virtually a one-to-one correspondence between the distribution of dark nebulae in a galaxy and the distribution of cold molecular hydrogen gas [as traced for instance by the CO(1-0) line ] ; the reason for this is that the cold hydrogen gas within a galaxy (which starts in neutral atomic form) can only form itself into diatomic hydrogen molecules if it is shielded from starlight by interstellar dust.
(Because ultraviolet photons act to photodissociate diatomic molecular hydrogen, molecular hydrogen gas is only found in regions where starlight is subject to strong extinction from significant amounts of interstellar dust)
(( there are some good lecture notes around about the role of dust and molecules in the interstellar medium. These two sets of slides are pretty good:
http://www.astro.umd.edu/~richard/AS.../Lecture_9.pdf
http://www2.astro.psu.edu/users/alex/astro497_7.pdf
))
So most of the dark nebulae that we see in the band of the Milky Way actually correspond to clouds of molecular hydrogen gas!
There are two recent threads in the Imaging Forum which may be relevant to the idea of outflow of gas at right angles to the principal plane of a spiral galaxy:
(1) "The Wolf Nebula" by gregbradley
(on about page 2 or 3 of the imaging forum)
(2) "The Milky Way around 11 hours" by gregbradley
(about page 3 or 4 of the imaging forum)
In the Wolf Nebula (RCW 113) thread, I discuss the possible utility of amateur H-alpha imaging surveys in finding very faint H-alpha emitting gaseous shells and gas outflows that may be causing gas to travel in a direction away from the plane of the Milky Way.
In "The Milky Way around 11 hours" thread, I discuss the possibility of detecting dust outflows from the Milky Way, by making ultra-deep optical images of the band of the Milky Way.
In the following image, I have displayed (at low resolution) part of an 360 degree Milky Way image by Nick Risinger , but with increased contrast and brightness. Some of the dust features rising above and below the plane of the Milky Way may be analogous to those seen in NGC 253 :
