This thread continues from the other one by Robert Anthony Lang about the massive Molecular Outflow in NGC 253
Here is a model of gas outflowing from the central region of NGC 253, from a 2009 paper.
Do you notice something missing here? This model shows X-ray emitting gas and
H-alpha emitting gas being ejected at right angles to the plane of NGC 253 by energy originating from OB stars and supernovae, but it
does not show any ejection of cold molecular gas, as was discovered in the recent work.
Oops!!
The molecular gas outflows were unaccountably missing from the minds of astrnonomers (!), until the the advent of sensitive & high-resolution millimeter & submillimeter imaging (e.g. using the new ALMA radio telescope) made molecular outflows an observational fact.
The reader will appreciate that if more gas exits the central region of NGC 253 than is used up in forming stars, in a given period of time, then
these vigorous processes of gas consumption and gas expulsion could greatly affect the future amount of remaining interstellar gas belonging to NGC 253. These numbers indicate the possibility that vigorous removal of Interstellar Gas from this galaxy, together with rapid consumption of the interstellar gas by means of the formation of new stars, could (
plausibly) eventually result in the cessation of the formation of massive & luminous stars of the sort that line spiral arms. It is, in fact, very likely that some galaxies that start as Hubble type Sb or Sc do eventually turn into Hubble type Sa or S0 galaxies, simply because these galaxies either exhaust or expel all of their interstellar gas, thus leaving no remaining interstellar gas to make future generations of stars.
One of the authors Alberto Bolatto explains the importance of this NGC 253 result for understanding the formation of galaxies in the early history of the universe: "the amount of (outflowing) gas we measure gives us very good evidence that some growing galaxies spew out more gas than they take in. We may be seeing a present-day example of a very common occurrence in the early universe." Bolatto is here referring to that early era (>= 9 billion years ago) in our universe when the majority of the stars in the large (non-dwarf) galaxies were formed in super-powerful bursts of star formation of the order of 100-200 solar masses per annum per each galaxy. It will be challenging to explain how the galaxies that we see in today's universe formed, if super-powerful
outflows of very large masses of gas accompanied the gas
inflows and
gravitational collapses of gas into stars that were necessary to build up the majority of the stellar mass that we now observe in galaxies.
In related work, Aleks Diamond-Stanic (Univ. of Wisconsin-Madison) has found observational evidence for remarkably fast (up to 2500 km/s) gas outflows from some distant dwarf galaxies that contain intense concentrations of very luminous stars; gas outflows which are powerful enough to permanently shut down all of the star formation in a galaxy. If you think a tornado or a hurricane is a fast wind, just stand in the way of a 1000 kilometers per second wind!
An interesting set of slides by Diamond-Stanic about this process can be found here:
http://www.cge.uci.edu/2012_Workshop...ond-Stanic.pdf and the relevant paper is: (2012), Astrophysical Journal Letters,
755, L26
Many of my main points are well summarized in the first section of the aforementioned paper in ApJL:
" The central regions of elliptical galaxies are thought to form in compact starbursts (Kormendy et al.
2009; Hopkins et al.
2009). Feedback associated with such starbursts can produce outflows driven by thermal energy from supernova explosions (Chevalier & Clegg
1985), stellar winds (Leitherer et al.
1992), and momentum input from both supernova ram pressure and radiation pressure on dust grains (Murray et al.
2005). It has been argued that such feedback imposes a limit on the maximum star formation rate (SFR) surface density for starbursts (Lehnert & Heckman
1996; Meurer et al.
1997; Murray et al.
2005; Thompson et al.
2005) and the maximum stellar surface density for elliptical galaxies and star clusters (Hopkins et al.
2010).
Galactic winds are ubiquitous in star-forming galaxies at all redshifts and generally exhibit outflow velocities in the 100-500 km s–1 range, which can be attributed to the stellar processes described above (Heckman et al.
2000; Shapley et al.
2003; Martin
2005; Rupke et al.
2005; Weiner et al.
2009; Rubin et al.
2010). Outflows with significantly higher velocities (V > 1000 km/s ) were discovered by Tremonti et al. (
2007) in a sample of massive (
M* equals 10E11
M☉) post-starburst galaxies at
z equals 0.6, and it was suggested that a more energetic source such as feedback from an accreting supermassive black hole (Silk & Rees
1998; Di Matteo et al.
2005) may be responsible for launching the winds (see Fabian
2012, for a recent review).
However, it is also plausible that feedback from a compact starburst could expel gas with such large velocities. Indeed, there is evidence for a positive correlation between outflow velocity and starburst luminosity (Martin
2005; Rupke et al.
2005; Tremonti et al.
2007), albeit with significant scatter. Furthermore, Heckman et al. (
2011) recently found outflows with maximum velocities reaching 1500 km/s in a sample of local starbursts with compact nuclei "
