Saturday, December 22, 2007

Asteroid impacts might be beneficial (in the long term)

The image on the left is an artists impression (from of the first moments of the K-T impact, the asteroid impact that probably caused the mass extinction event at the end of the Cretaceous period about 65 million years ago.

Although mass extinctions wipe out many of the species that are present on Earth at the time of the extinction event, the number and diversity of animal and plant species ultimately increases after mass extinctions. Thus, although mass extinctions are in the short term (several million years) destructive, on the longer terms of hundreds of millions of years they may actually beneficial. Carl Zimmer discusses this, more specifically asteroid-related extinctions, in an article at Wired (NB, only a few of the documented mass extinctions have been plausibly shown to be due to asteroid impact).

The ambiguous and poorly-understood long-term effect of destructive astronomical events (such as asteroid impacts, supernovae, gamma-ray bursts, AGN, etc) on biological life is something that renders assessing the size of the galactic habitable zone difficult, if not impossible, at the present and with our current understanding of these astronomical events and the fossil record.

In order to arrive at a conception of a very small Galactic Habitable Zone (visible in the images from the 2001 Gonzalez et al Scientific American article) , i.e. that the Solar system and the Earth was very unusual (a "privileged planet" that was then evidence of divine favor [or in code: Intelligent Design]), Guillermo Gonzalez assumed that anything that increases the chances of asteroid impacts, or nearby supernova, or high UV or cosmic ray fluxes, was negative and harmful to life.

But those assumptions are by no means robustly justified by existing data, nor are they unique (as the opposite effect, as discussed in Zimmer's article, could plausibly be true), as other astrobiologists pointed out the him at the time. Indeed, it could be that external events are ultimately responsible for driving greater evolutionary diversity and hence for increasing the chances of complex multi-cellular life evolving.

While on the subject of asteroid impacts, has two asteroid-related articles.

The first article, by Charles Choi, discusses new simulations by researchers at Sandia National Labs that suggest that the 1908 Tunguska explosion could have been caused by meteorite only 20 meters in diameter, smaller that previously thought. As there the number of asteroids of a given size is a strongly decreasing function of the their size., this implies that Tunguska-level events might be more common than previously thought. However, the actual destruction caused by the Tunguska explosion is also probably less than previously estimated, so rest easy!

The second article, by Alicia Chang, discusses an asteroid that has a 1 in 75 chance of hitting Mars (not us) this coming January 30th. This asteroid, 2007 WD5, would also cause a Tunguska-level explosion, equivalent to an explosion of about 15 Megatons of TNT.

Norman Levitt Deconstructs Steve Fuller’s Postmodernist Critique of Evolution

Steve Fuller, a self-proclaimed leftist sociologist at the University of Warwick (just south of Birmingham, which is where I did my undergraduate and graduate degrees) was a witness for the defense in the Dover School board trial (i.e for the pro-ID school board). He has a new book out ("Science v. Religion? Intelligent Design and the Problem of Evolution") defending himself and critiquing evolutionary science.

Skeptic magazine has a review of the book by Norman Levitt, a mathematician and author with Paul Gross of several books on the academic culture wars (e.g. "The Flight from Science and Reason", "Higher Superstition" and "Prometheus Bedeviled"), which is worth a read (more for the insight into the unlikely alliance between the leftist and rightist anti-science groups than for a detailed critique of Fuller's book).

Tuesday, December 18, 2007

The Death Star Galaxy

Being in galaxy in the path of a jet emanating from a supermassive black hole in a neighboring galaxy might be quite catastrophic:

A powerful jet from a super massive black hole is blasting a nearby galaxy, according to new findings from NASA observatories. This never-before witnessed galactic violence may have a profound effect on planets in the jet's path and trigger a burst of star formation in its destructive wake

Known as 3C321, the system contains two galaxies in orbit around each other. Data from NASA's Chandra X-ray Observatory show both galaxies contain super massive black holes at their centers, but the larger galaxy has a jet emanating from the vicinity of its black hole. The smaller galaxy apparently has swung into the path of this jet.

This NASA press release is getting quite a bit of press (e.g. BBC, Yahoo news), and some of the quotes in the press articles certainly spin up the catastrophic angle.

From the BBC article:
The combined effects of this radiation and particles travelling at almost the speed of light could have disastrous consequences for the atmospheres of any Earth-like planets lying in the path of the jet.

For example, protective layers of ozone in the planet's upper atmosphere could be destroyed, which could result in the mass extinction of any life that had evolved on the planet.
From the Yahoo news article:
The larger galaxy has a multi-digit name but is called the "death star galaxy" by one of the researchers who discovered the galactic bullying, Daniel Evans of the Harvard-Smithsonian Center for Astrophysics.

Tens of millions of stars, including those with orbiting planets, are likely in the path of the deadly jet, said study co-author Martin Hardcastle of the University of Hertfordshire in the United Kingdom.

If Earth were in the way — and it's not — the high-energy particles and radiation of the jet would in a matter of months strip away the planet's protective ozone layer and compress the protective magnetosphere, said Evans. That would then allow the sun and the jet itself to bombard the planet with high-energy particles.

And what would that do life on the planet?

"Decompose it," Tyson said.

"Sterilize it," Evans piped in.
However, the actual scientific paper (by Evans et al, still undergoing peer review but available here: astro-ph/0712.2669) wisely ignores the stuff that gets the press excited. None of the speculation regarding zapping planets or star formation in the press stories is in the scientific paper at all. Indeed, its rather sad that the straight facts are not considered exciting enough for the press stories, and instead the speculative stuff gets pushed to the top.

Indeed I have to wonder, given the lack of any discussion in the preprint, if the quotes regarding the effect on planetary atmospheres are based on actual calculations of the estimated particle flux in the target galaxy? Or are they purely speculation based a the hard-to-resist gut feeling that "blacks holes are powerful, so they must be capable of anything"?

Speaking in general (as I don't know the answer to the questions above) I feel this is a real problem with press releases and articles that deviate from the actual scientific paper - the public is potentially being fed speculation rather than the actual peer-reviewed science.

Now it is possible that the quotes in the press articles are based on quantitative comparisons to other peer-reviewed literature on the effect astrophysical events may have on planetary atmospheres or biological activity - the trouble is we don't and can't easily know whether this is the case.

There have been calculations of the effect of nearby supernovae, Gamma Ray bursts on the atmosphere of Earth-like planets (e.g. Smith et al, 2004, Icarus, 171, 229; Scalo & Wheeler, 2002, ApJ, 566, 723; Hunt, 1978, Nature, 271, 430), and also of the effects of neutron star mergers (Dar et al, 1998, Phys Rev Letters, 80, 5813), but I am not aware of equivalent publications regarding jets from AGN.

The image shown comes from the press release, and is described in the accompanying caption:

This composite image shows the jet from a black hole at the center of a galaxy striking the edge of another galaxy, the first time such an interaction has been found. In the image, data from several wavelengths have been combined. X-rays from Chandra (colored purple), optical and ultraviolet (UV) data from Hubble (red and orange), and radio emission from the Very Large Array (VLA) and MERLIN (blue) show how the jet from the main galaxy on the lower left is striking its companion galaxy to the upper right. The jet impacts the companion galaxy at its edge and is then disrupted and deflected, much like how a stream of water from a hose will splay out after hitting a wall at an angle.

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.]

How not to get tenure

Speaking of the Templeton Foundation, it is claimed that the TF(*) was one of the few sources of funding that Guillermo Gonzalez managed to obtain. Indeed, it seems he only managed to bring in $170,000 spread over 5 years, compared to the average of $1,300,000 other ISU physics and astronomy faculty had brought in while they were on tenure track!

GG's failure to bring in normal levels of funding, in addition to a dramatic drop in publication rate and lack of grad student/postdocs, were major factors in him being denied tenure. These are, along with undergraduate teaching, major aspects of being a professional scientist in tenure-track although tenure is not based on a fixed set of rules.

Many good scientists do not get tenure (e.g. Rob Knob of the Galactic Interactions blog), and many good scientists never even get onto tenure-track in the first place. Given that GG appears to have failed to satisfy the requirements in many ways it is totally unsurprising that he was denied tenure at Iowa State University.

That he espoused an unscientific astronomical version of Intelligent Design and had close links to the Discovery Institute was also, quite rightly and quite fairly, another aspect of concern for the the faculty in the Physics department he was attempting to get tenure from. The Discovery Institutes's anti-secular and anti-scientific agenda, coupled with its manifest dishonesty is no secret. The recently revealed emails clearly show that GG's DI/ID links were known and were (entirely fairly) viewed negatively but were not used as a litmus test to "discriminate" against him. Of course, this will not stop Gonzalez and the DI is hijacking a routine and just decision in order to play politics.

After all, the faculty must have been aware of his views when they offered him tenure-track in the first place - they were no secret in the astronomical community - yet he did get a tenure track position. I have no doubt that while the faculty may have viewed GG's views with distaste they would have given him tenure had he satisfied the standard requirements of all tenure track faculty: bring in funding, mentor students and postdocs, be scientifically productive. Guillermo Gonzalez has only himself to blame for his current position.

Gonzalez could have used those tenure track years to engage in peer-reviewed research to develop the concept of galactic habitability and turn it from a poorly-constrained hypothesis into a robust theory. He could easily have applied for grants to pay for several grad students and postdocs to to work with him to expand our knowledge of the role galaxies play in habitable planet formation and evolution.

But Guillermo Gonzalez didn't attempt to further science. Instead he decided to present his speculative and religiously-distorted views of Galactic Habitability to the unsuspecting public as scientific fact through his book "The Privileged Planet", bypassing peer review altogether.

In the mean time real science, done by real scientists, went ahead and left Guillermo Gonzalez behind. The ADS abstract service records 440 astronomy-related abstracts with the word habitable in 2006-2007 alone. If we repeat the search requiring the surname Gonzalez be one of the authors we get 1 abstract, and its a M. Gonzalez, not Guillermo. I have to expand the search to 2000-2007 before the Guillermo Gonzalez(**) appears, and then only in three abstracts, two of which are reviews rather than new work. Again, by way of comparison 107 abstracts contained "Galactic" and "Habitable" in the abstract between 2000 and 2007 (1353 with the word Habitable alone).

If Guillermo Gonzalez really believed that astrophysics did indeed provide convincing evidence of a Designer (specifically a Christian God) why would he have abandoned actual research?

(*) I am informed that the TF fund many good studies the interaction between science and religion, and that the TF is opposed to the "culture war" spin presented by fundamentalists such as the DI. Some of the people they occasionally fund are less rational though.

(**) There is also another Guillermo Gonzalez in professional astronomy.

Anyway, all this politics is tiring. In my next post we'll be back to discussing really interesting stuff. Yup, more on galactic winds!

Monday, December 03, 2007

Bob Park on Paul Davies and the Templeton Prize

Bob Park discusses Paul Davies's infamous Op-Ed (previously mentioned here) and the Templeton Prize in his November 30th "What's New" column.

Protecting NASA research funding

Space News Business Report (at discusses Alan Stern's (NASA associate administrator for sciences) changes to restore and protect research funding.