Molecular outflow in NGC 253 (continued) - implications for galaxy evolution

[madbadgalaxyman]

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!!


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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/talks/ADiamond-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 (http://www.iceinspace.com.au/forum/apjl_755_2_26.text.html#apjl439476r 17); Hopkins et al. 2009 (http://www.iceinspace.com.au/forum/apjl_755_2_26.text.html#apjl439476r 14)). Feedback associated with such starbursts can produce outflows driven by thermal energy from supernova explosions (Chevalier & Clegg 1985 (http://www.iceinspace.com.au/forum/apjl_755_2_26.text.html#apjl439476r 2)), stellar winds (Leitherer et al. 1992 (http://www.iceinspace.com.au/forum/apjl_755_2_26.text.html#apjl439476r 19)), and momentum input from both supernova ram pressure and radiation pressure on dust grains (Murray et al. 2005 (http://www.iceinspace.com.au/forum/apjl_755_2_26.text.html#apjl439476r 27)). It has been argued that such feedback imposes a limit on the maximum star formation rate (SFR) surface density for starbursts (Lehnert & Heckman 1996 (http://www.iceinspace.com.au/forum/apjl_755_2_26.text.html#apjl439476r 18); Meurer et al. 1997 (http://www.iceinspace.com.au/forum/apjl_755_2_26.text.html#apjl439476r 23); Murray et al. 2005 (http://www.iceinspace.com.au/forum/apjl_755_2_26.text.html#apjl439476r 27); Thompson et al. 2005 (http://www.iceinspace.com.au/forum/apjl_755_2_26.text.html#apjl439476r 42)) and the maximum stellar surface density for elliptical galaxies and star clusters (Hopkins et al. 2010 (http://www.iceinspace.com.au/forum/apjl_755_2_26.text.html#apjl439476r 15)).
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 (http://www.iceinspace.com.au/forum/apjl_755_2_26.text.html#apjl439476r 12); Shapley et al. 2003 (http://www.iceinspace.com.au/forum/apjl_755_2_26.text.html#apjl439476r 37); Martin 2005 (http://www.iceinspace.com.au/forum/apjl_755_2_26.text.html#apjl439476r 22); Rupke et al. 2005 (http://www.iceinspace.com.au/forum/apjl_755_2_26.text.html#apjl439476r 36); Weiner et al. 2009 (http://www.iceinspace.com.au/forum/apjl_755_2_26.text.html#apjl439476r 48); Rubin et al. 2010 (http://www.iceinspace.com.au/forum/apjl_755_2_26.text.html#apjl439476r 33)). Outflows with significantly higher velocities (V > 1000 km/s ) were discovered by Tremonti et al. (2007 (http://www.iceinspace.com.au/forum/apjl_755_2_26.text.html#apjl439476r 43)) 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 (http://www.iceinspace.com.au/forum/apjl_755_2_26.text.html#apjl439476r 39); Di Matteo et al. 2005 (http://www.iceinspace.com.au/forum/apjl_755_2_26.text.html#apjl439476r 5)) may be responsible for launching the winds (see Fabian 2012 (http://www.iceinspace.com.au/forum/apjl_755_2_26.text.html#apjl439476r 7), 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 (http://www.iceinspace.com.au/forum/apjl_755_2_26.text.html#apjl439476r 22); Rupke et al. 2005 (http://www.iceinspace.com.au/forum/apjl_755_2_26.text.html#apjl439476r 36); Tremonti et al. 2007 (http://www.iceinspace.com.au/forum/apjl_755_2_26.text.html#apjl439476r 43)), albeit with significant scatter. Furthermore, Heckman et al. (2011 (http://www.iceinspace.com.au/forum/apjl_755_2_26.text.html#apjl439476r 11)) recently found outflows with maximum velocities reaching 1500 km/s in a sample of local starbursts with compact nuclei "

[Shiraz]

very interesting Robert.

Most hi res images of NGC253 show indistinct darker filaments/columns rising from around the central zone, as in the attached image. This is just broadband luminance (I did not take any Ha at the time), but I think the filaments are still reasonably easy to see. 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. What do you think?

Regards Ray

[madbadgalaxyman]

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/ASTRO421/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 :

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