Wednesday, December 05, 2007

Winds, winds everywhere

Blogging on Peer-Reviewed Research
With the NSF and Suzaku proposal deadlines safely behind me I've been spending this week catching up on the astrophysical literature on galactic winds that has appears in the last few months.

There is actually quite a bit of it, which I thought I would share with you as evidence that galactic winds are both quite common, potentially very close to home, and also potentially quite important in understanding the nature of galaxies and galaxy formation (blogging is also a useful way of making notes for myself, of course).

The following is a list of galaxies for which observational evidence of galactic-scale winds has recently been obtained:

Leon et al (2007, A&A, 473, 747) find evidence for outflow from NGC 6764, [NED link]. This galaxy is a LINER (often classified as Seyfert 2) and known Wolf-Rayet galaxy (a class of starburst galaxy). A limb-brightened outflow cone or elongated bubble is visible in the optical. The galaxy itself is pretty faint (IRAS f60 flux 6.6 Jy, D ~ 34 Mpc) so its not surprising that no Chandra or XMM-Newton data exist on this object.

NGC 4460 [NED link] in the Canes Venatici I cloud of galaxies (which also hosts classic starbursts such as NGC 4449 and NGC 4631) is an edge-on spiral (lenticular) that shows a classic limb-brightened nuclear outflow cone, see Kaisin & Karachentsev 2007 (astro-ph/0710.0711). They note "compact emission disk in its core, from which diffuse emission protruberances originated along the minor axis." Distance ~ 9.6 Mpc. I was not aware of this object before now, but their continuum-subtracted H-alpha image shows a classic superwind-like morphology. IR warm, f60/f100 ~ 0.48, indicating a genuine starburst. But f60 is only 3.2 Jy, so it is faint because it has a low absolute SF rate.

Jimenez-Vicente et al (2007, MNRAS, 382, L16) find spectroscopic evidence for a low velocity outflow (about 100 km/s) from the center of Messier 100, a beautiful spiral galaxy at a distance of D ~ 16 Mpc (I discussed this result before here, but as its now been published its worth mentioning again). Globally M100 is not quite a starburst galaxy, but it is possible that a weaker form of outflow might occur from its central regions.

Good candidates for poorly-collimated, kiloparsec or larger, AGN-driven winds (as opposed to large-scale jet activity or nuclear-scale warm-absorbers) are much rarer than for the "classic" starburst-driven type of galactic wind.

IC 5063 [NED link] has kiloparsec-scale ~600 km/s outflow of neutral hydrogen and ionized gas, see Morganti et al 2007 (astro-ph/0710.1189). IC 5063 is a bulge-dominated SA galaxy with a prominent dust lane (possibly a minor merger remnant) with Seyfert 2 activity at a distance of D~47 Mpc. [Added 08 Oct 2007]

Theoretical work on galactic winds is rarer than observational work, but some significant new papers on related to winds (not necessarily starburst-driven) have also appeared.

Jackie Cooper and colleagues (Cooper et al 2007, ApJ, in press, see astro-ph/0710.5437) present some of the first(*) 3-D simulations of a starburst-driven galactic wind (or superwind). Their main innovation is to model the initial ambient ISM the supernova explode into as a multi-phase medium with a tenuous inter-cloud component and dense clouds drawn from a Kolmogorov density spectrum (as you would expect to be created by turbulence). Despite this major difference from the previous generation of superwind models that were forced to assume a homogeneous ISM (e.g Suchkov et al 1994, Strickland & Stevens 2000), Cooper confirm our (SS2000) finding that the soft X-ray emission from superwinds arises in low-volume regions where the SN-driven hot wind interacts with dense cool clumps and clouds. An mpg movie of one of their simulations is available here.

The main limitation of their models are the small scales they can simulate at high resolution in 3-D - only a cube 1 kpc on a side over a time of only 1 Myr, compared to the 10+ kpc, 10+ Myr scales of real winds.

The idea that our own galaxy, the Milky Way, has some form of weak galactic wind has been kicking around for a while (e.g. Sofue 1989; Bland-Hawthorn & Cohen 2003; Keeney et al 2006). However, although star formation is more vigorous in the center of our Galaxy than it is on average through the disk, the Milky Way is by no means a starburst galaxy so we would not expect there to be enough mechanical energy released by the stellar winds and supernova to drive a starburst-driven wind. Furthermore the scale of the supposed MW outflow varies from study to study, from a few 100 pc through 1-kpc-scale out to 10 to 100 kpc-scale winds, so I'm normally very skeptical of claims that Milky Way has a galactic wind.

However, a paper worth mentioning by Everett et al (astro-ph/0710.3712) argues for a 1-kpc-scale outflow driven by a mix of cosmic ray and thermal pressure. Cosmic rays typically carry between only 10 and 30% of the kinetic energy released by supernovae, which is why they're often ignored in the theoretical picture of starburst-driven superwinds -- in starburst galaxies we have more than enough energy to drive the observed winds. However, as stated above, the extra energy in the cosmic rays might be important for a small outflow from the center of the Milky Way.

Theoretical models of purely cosmic-ray-driven galactic winds are not new (e.g. Breitschwerdt et al 1991), but it is nice to see mixed thermal plus CR-driven wind models being developed and applied.

The Everett model appears to generate the same form of wind solution as the pure CR-driven winds - an initially low velocity (v less than 200 km/s) flow that slowly accelerates to higher velocity as the height above the plane of the Galaxy increases to several kpc. This behavior is unlike the velocity of the warm neutral and ionized gas observed in classic supernova-driven winds, where the measured velocities reach (typically) 200 - 600 km/s within a few hundred parsecs of the starburst region and then appear roughly constant.

Although interesting from a theoretical standpoint I think it is necessary that more observational evidence accumulate, specifically kinematic evidence of outflow from both absorption and emission-line studies, before we can be confident that the Milky Way galaxy does host some form of galactic wind.

The effect of supernovae and stellar winds from massive stars on the interstellar gas that they themselves formed out of is an example of an astrophysical feedback loop. Feedback from massive stars, and/or AGN, is thought to be important in regulating galaxy formation and evolution, but the exact mechanisms by which this proceeds and the significance of the effects of feedback are by no means clear as yet.

At one point SN feedback was believed to be very powerful, such at it might actually destroy low mass galaxies by blowing all the gas out of them and preventing further star formation, or even by unbinding them and their dark matter halos completely
(Dekel & Silk 1986).

More recently the pendulum of opinion has swung in the other direction, as more detailed theoretical work demonstrated that supernova-driven winds could not efficiently blow all the gas out of even the lowest mass dwarf galaxies. Supernova-driven winds could eject metals from galaxies (elements heavier than hydrogen and helium that are created in stellar nucleosynthesis and ejected in supernova), but the majority of the interstellar gas would remain behind in the galaxy (e.g. Mac Low & Ferrara 1999).

Now, in a result that is sure to widespread attention, Maschenko et al (astro-ph/0711.4803) claim that SN feedback nevertheless can affect the shape of the dark matter halos of dwarf galaxies. The shape of DM halos has been a problem for some time, with theory predicting a different central shape (cuspy) to that inferred from observations of gas and stellar motions (constant density core).

For galaxies that are just forming most of the mass is either gas (rather than stars) or dark matter. Although overall there is thought to be more mass in dark matter than in "normal" baryonic matter, in the centers of these protogalaxies the condensed baryonic gas is a significant contributor to the gravity. Mashchenko et al claim that their simulations show that the first SN explosions perturb and stir the gas around, which then moves the dark matter around purely by gravity, thus smearing out the cuspy DM predicted by normal theory and turning into the smoother cores observed in real galaxies.

If true this would be a very significant result. While I work on feedback and think it very important for understanding the nature of the galaxies we observe in the Universe today, I am somewhat skeptical of this result. This work relies on accurately modeling star formation, and the hydrodynamics of the interstellar gas, over a wide range of physical and temporal scales. Ultimately it comes down to whether you believe their numerical method (smoothed particle hydrodynamics), and their implementation of SN feedback within the code, is accurate. Are these results result, or merely numerical artifacts? Time will tell.

(*) 3-D simulations of multiple SN explosions in proto-dwarf galaxies, single star clusters, or small regions of a starburst have been done before now. All of these situations are more simplified than simulating a wind in a modern, more massive, galaxy, so the Cooper simulations can be considered some of the first published 3-D sims of galactic winds. Annoyingly I've been sitting on a set of completed 3-D sims of winds covering larger physical scales than the Cooper models for two years now without publishing them.... argh!

[Update 6:39pm: Replace paragraphs lost after blogger decided to delete random paragraphs.]

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