madbadgalaxyman
30-09-2013, 10:41 PM
by Robert Anthony Lang
Today's astronomers understand the interstellar gas within spiral & irregular galaxies very approximately, and they have little better than a "back of the envelope sketch" of what is going on; it is a very difficult problem to model the extremely large range of pressures, temperatures, gas phases, and spatial scales, that characterize the distribution of the interstellar gas within galaxies. Add to this even more layers of complexity..... because various novel processes are currently being elucidated that have the effect of either removing gas from the plane of a spiral (or irregular) galaxy or contributing fresh gas to it. It is because of this remarkable complexity that astronomers have no single widely-accepted model of the interstellar medium that accurately reproduces the ever-changing (with time) space distribution of the density & energy & temperature & composition & velocities of the interstellar gas within galaxies. Links to papers & books that include descriptions of some of the competing models of the ISM, can be found within these threads:
http://www.iceinspace.com.au/forum/showthread.php?t=104637
http://www.iceinspace.com.au/forum/showthread.php?t=93498
http://www.iceinspace.com.au/forum/showthread.php?t=91492
An important unsolved scientific problem is to accurately assess the effects on the interstellar medium of the enormous energies deposited into the interstellar gas of a spiral galaxy by OB stars and supernovae, and to figure out the workings of several recently discovered processes that serve to remove gas from a galaxy (perhaps even permanently) or contribute fresh gas to a galaxy from its halo and from intergalactic space.
For instance, there is fairly good evidence that the "high velocity clouds" of HI (= cold neutral Atomic Hydrogen gas) are falling into the disks (planes) of spiral galaxies, with the gas originating from the halo of a galaxy and/or originating from intergalactic space. In fact, astronomers are searching for the elusive and Largely Undiscovered ultra-low-density intergalactic medium , as this gas is expected to "refuel" galaxies with fresh gas. This new gaseous material, on reaching the plane of a spiral galaxy, provides fresh raw material to fuel the formation of new generations of stars, even if a galaxy has previously exhausted all of its interstellar gas (via star formation & gas removal). For example, possible clouds of intergalactic gas between M33 and M31 were discovered recently.....see this preprint about this discovery; arxiv.1305.1631 .
A good & easy review of observational evidence for the accretion of cold gas by galaxies can be found at: http://www.astron.nl/~oosterlo/accretionReview/
The below graphic shows evidence for probable infall of cold gas into several galaxies:
148754
(Top panel shows HI gas clouds that surround our Galaxy ; bottom right panel shows HI clouds surrounding NGC 891 ; bottom left panel is HI clouds surrounding M31)
Indirect evidence for the arrival of fresh gas into spiral galaxies is the fact that the typical large spiral (for instance, our own Galaxy) is thought to have continually formed 1 to 2 solar masses per annum of stars at a relatively constant rate, over the last 5 to 10 billion years, and therefore it could not sustain this Star Formation Rate without fresh supplies of gas being delivered to the disk component.
The gas within a galaxy is not a closed system, and it can easily be removed from a galaxy; one major process that can remove interstellar gas from galaxies located within clusters of galaxies is called "ram pressure stripping", in which the high-velocity passage of a galaxy through its host Cluster of Galaxies causes the gas contained in a galaxy to be stripped away from it; the very-low-density and Very Hot (> five million degrees) inter-galaxy gas that exists within rich clusters of galaxies can push out the gas that is contained in a galaxy. This gas stripping has been observed in some Virgo Cluster spiral galaxies, but it has been difficult to prove that ram pressure stripping is actually happening.
One well understood process of gas removal from galaxies is when regions of very active Star Formation which recently created many OB stars & supernovae (e.g. those in M82 & NGC 253) eject a lot of Dust and Ionized Gas (H-alpha & X-ray emitting gas) from a spiral galaxy. Also, supernovae create large (200 to 2000 parsecs across) bubble-like "shells" and "supershells" of expanding HI (atomic hydrogen gas) that can result in gas being pushed a long way up or down from the plane of a spiral or irregular galaxy. Astronomers also envisage "galactic fountains" of gas exiting into the halo of a spiral galaxy, and then later falling back into the more central parts of the galaxy.
Much of this knowledge about the processes that add or subtract gas from a galaxy is very new in astronomers' minds, therefore the processes of "gas addition and gas subtraction" are poorly understood. There is an ongoing paradigm shift away from regarding the gas content of a spiral galaxy as being fixed and towards the idea that gas is exiting and entering galaxies all the time.
DISCOVERY OF A HIGH MASS MOLECULAR OUTFLOW IN THE MODERATE STARBURST GALAXY NGC 253:
Gas expulsion and gas accretion in galaxies is a very fresh field of research, so major surprises are still occurring; one of them is the recent discovery of a massive wind of cold and molecular gas exiting from NGC 253.
Well known to astronomers are the relatively low-mass outflowing winds of ionized gas.....gas expulsions that exit at high speed from parts of actively star forming galaxies that contain enormous numbers of young & massive & luminous stars (O & B stars) plus the consequent supernovae (for instance in M82 & NGC 1482). Astronomers are also well aware of dust chimneys and dust plumes that exit a disk galaxy in a direction "vertically" at 90 degrees to the plane of an actively star-forming spiral galaxy (e.g. these are prominent in NGC 891). [[for a discussion of vertical dust chimneys in spiral galaxies, see madbadgalaxyman's posts in Greg Bradley's imaging forum thread entitled "The Milky Way around 11 hours"]].
In contrast, high mass outflows of molecular gas are much less known and have therefore often been left out of astronomers' models of gas outflows from galaxies (!), even though there was a 2002 observation by Walter & Weiss & Scoville that detected huge amounts of molecular gas (> 3 x 10E8 solar masses) flowing away from M82. Thus, the recent discovery of large masses of molecular gas outflowing from the plane of NGC 253 is highly significant; and here is a quick overview of this work (which used ALMA telescope observations):
http://www.almaobservatory.org/en/press-room/press-releases/621-starburst-to-star-bust-alma-sheds-light-on-mystery-of-missing-massive-galaxies
This work was published in "Nature" on 25th July 2013.
Or find it here: http://www.eso.org/public/archives/releases/sciencepapers/eso1334/eso1334a.pdf
The authors derive a very high outflow rate of over three solar masses per annum for the molecular gas exiting from the centre of NGC 253, but possibly as high as 9 solar masses of material per annum, for the molecular gas which is being expelled from the powerful Centrally Located concentration of OB stars (= the circum-nuclear starburst) in this galaxy. This rapid gas outflow, with gas exiting away from the plane of NGC 253, is compared (by the authors) to the usage of the interstellar gas at an estimated rate of three solar masses per annum to form new stars in this centralmost nuclear (circumnuclear) region of NGC 253 which is only a few hundred parsecs across.
( Note: At least half, or even the majority, of the current star formation within NGC 253 is taking place in a small starburst region that is located near to its centre!)
CONTINUED IN NEXT POST !!!
Today's astronomers understand the interstellar gas within spiral & irregular galaxies very approximately, and they have little better than a "back of the envelope sketch" of what is going on; it is a very difficult problem to model the extremely large range of pressures, temperatures, gas phases, and spatial scales, that characterize the distribution of the interstellar gas within galaxies. Add to this even more layers of complexity..... because various novel processes are currently being elucidated that have the effect of either removing gas from the plane of a spiral (or irregular) galaxy or contributing fresh gas to it. It is because of this remarkable complexity that astronomers have no single widely-accepted model of the interstellar medium that accurately reproduces the ever-changing (with time) space distribution of the density & energy & temperature & composition & velocities of the interstellar gas within galaxies. Links to papers & books that include descriptions of some of the competing models of the ISM, can be found within these threads:
http://www.iceinspace.com.au/forum/showthread.php?t=104637
http://www.iceinspace.com.au/forum/showthread.php?t=93498
http://www.iceinspace.com.au/forum/showthread.php?t=91492
An important unsolved scientific problem is to accurately assess the effects on the interstellar medium of the enormous energies deposited into the interstellar gas of a spiral galaxy by OB stars and supernovae, and to figure out the workings of several recently discovered processes that serve to remove gas from a galaxy (perhaps even permanently) or contribute fresh gas to a galaxy from its halo and from intergalactic space.
For instance, there is fairly good evidence that the "high velocity clouds" of HI (= cold neutral Atomic Hydrogen gas) are falling into the disks (planes) of spiral galaxies, with the gas originating from the halo of a galaxy and/or originating from intergalactic space. In fact, astronomers are searching for the elusive and Largely Undiscovered ultra-low-density intergalactic medium , as this gas is expected to "refuel" galaxies with fresh gas. This new gaseous material, on reaching the plane of a spiral galaxy, provides fresh raw material to fuel the formation of new generations of stars, even if a galaxy has previously exhausted all of its interstellar gas (via star formation & gas removal). For example, possible clouds of intergalactic gas between M33 and M31 were discovered recently.....see this preprint about this discovery; arxiv.1305.1631 .
A good & easy review of observational evidence for the accretion of cold gas by galaxies can be found at: http://www.astron.nl/~oosterlo/accretionReview/
The below graphic shows evidence for probable infall of cold gas into several galaxies:
148754
(Top panel shows HI gas clouds that surround our Galaxy ; bottom right panel shows HI clouds surrounding NGC 891 ; bottom left panel is HI clouds surrounding M31)
Indirect evidence for the arrival of fresh gas into spiral galaxies is the fact that the typical large spiral (for instance, our own Galaxy) is thought to have continually formed 1 to 2 solar masses per annum of stars at a relatively constant rate, over the last 5 to 10 billion years, and therefore it could not sustain this Star Formation Rate without fresh supplies of gas being delivered to the disk component.
The gas within a galaxy is not a closed system, and it can easily be removed from a galaxy; one major process that can remove interstellar gas from galaxies located within clusters of galaxies is called "ram pressure stripping", in which the high-velocity passage of a galaxy through its host Cluster of Galaxies causes the gas contained in a galaxy to be stripped away from it; the very-low-density and Very Hot (> five million degrees) inter-galaxy gas that exists within rich clusters of galaxies can push out the gas that is contained in a galaxy. This gas stripping has been observed in some Virgo Cluster spiral galaxies, but it has been difficult to prove that ram pressure stripping is actually happening.
One well understood process of gas removal from galaxies is when regions of very active Star Formation which recently created many OB stars & supernovae (e.g. those in M82 & NGC 253) eject a lot of Dust and Ionized Gas (H-alpha & X-ray emitting gas) from a spiral galaxy. Also, supernovae create large (200 to 2000 parsecs across) bubble-like "shells" and "supershells" of expanding HI (atomic hydrogen gas) that can result in gas being pushed a long way up or down from the plane of a spiral or irregular galaxy. Astronomers also envisage "galactic fountains" of gas exiting into the halo of a spiral galaxy, and then later falling back into the more central parts of the galaxy.
Much of this knowledge about the processes that add or subtract gas from a galaxy is very new in astronomers' minds, therefore the processes of "gas addition and gas subtraction" are poorly understood. There is an ongoing paradigm shift away from regarding the gas content of a spiral galaxy as being fixed and towards the idea that gas is exiting and entering galaxies all the time.
DISCOVERY OF A HIGH MASS MOLECULAR OUTFLOW IN THE MODERATE STARBURST GALAXY NGC 253:
Gas expulsion and gas accretion in galaxies is a very fresh field of research, so major surprises are still occurring; one of them is the recent discovery of a massive wind of cold and molecular gas exiting from NGC 253.
Well known to astronomers are the relatively low-mass outflowing winds of ionized gas.....gas expulsions that exit at high speed from parts of actively star forming galaxies that contain enormous numbers of young & massive & luminous stars (O & B stars) plus the consequent supernovae (for instance in M82 & NGC 1482). Astronomers are also well aware of dust chimneys and dust plumes that exit a disk galaxy in a direction "vertically" at 90 degrees to the plane of an actively star-forming spiral galaxy (e.g. these are prominent in NGC 891). [[for a discussion of vertical dust chimneys in spiral galaxies, see madbadgalaxyman's posts in Greg Bradley's imaging forum thread entitled "The Milky Way around 11 hours"]].
In contrast, high mass outflows of molecular gas are much less known and have therefore often been left out of astronomers' models of gas outflows from galaxies (!), even though there was a 2002 observation by Walter & Weiss & Scoville that detected huge amounts of molecular gas (> 3 x 10E8 solar masses) flowing away from M82. Thus, the recent discovery of large masses of molecular gas outflowing from the plane of NGC 253 is highly significant; and here is a quick overview of this work (which used ALMA telescope observations):
http://www.almaobservatory.org/en/press-room/press-releases/621-starburst-to-star-bust-alma-sheds-light-on-mystery-of-missing-massive-galaxies
This work was published in "Nature" on 25th July 2013.
Or find it here: http://www.eso.org/public/archives/releases/sciencepapers/eso1334/eso1334a.pdf
The authors derive a very high outflow rate of over three solar masses per annum for the molecular gas exiting from the centre of NGC 253, but possibly as high as 9 solar masses of material per annum, for the molecular gas which is being expelled from the powerful Centrally Located concentration of OB stars (= the circum-nuclear starburst) in this galaxy. This rapid gas outflow, with gas exiting away from the plane of NGC 253, is compared (by the authors) to the usage of the interstellar gas at an estimated rate of three solar masses per annum to form new stars in this centralmost nuclear (circumnuclear) region of NGC 253 which is only a few hundred parsecs across.
( Note: At least half, or even the majority, of the current star formation within NGC 253 is taking place in a small starburst region that is located near to its centre!)
CONTINUED IN NEXT POST !!!