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Galactic Habitability and Sgr A*

Yesterday I looked at evidence for oxygen in a galaxy so distant that we are seeing it as it was a mere 500 million years after the Big Bang. It’s an intriguing find, because that means there was an even earlier generation of stars that lived and died, seeding the cosmos with elements heavier than hydrogen and helium. It’s hard to imagine the vast tracts of time since populated with stars and, inevitably, planets without speculating on where and when life developed.

But as we continue to speculate, we should also look at the factors that could shape emerging life in galaxies like our own. Tying in neatly with yesterday’s post comes a paper from Amedeo Balbi (Università degli Studi di Roma “Tor Vergata”), working with colleague Francesco Tombesi. The authors are interested in questions of habitability not in terms of habitable zones in stellar systems but rather habitable zones in entire galaxies. For we know that at the center of our Milky Way lurks the supermassive black hole Sgr A*, whose effects must be considered.

Such black holes are known to produce vast amounts of ionizing radiation in the highly visible form of quasars or active galactic nuclei (AGN). The atmospheric loss and biological damage inflicted on a rocky planet as it is exposed to intense X-ray and extreme ultraviolet radiation can be extreme, and such conditions would have marked our own galaxy’s AGN phase.

The concern here is to examine how ionizing radiation can impact habitability by exposing planetary surfaces to high-energy fluxes, while also degrading planetary atmospheres. We’ve looked at these issues now and again on this site, with particular regard to red dwarf stars, the most common class of star in the galaxy but also a type prone to flare activity particularly when young. We now consider whether there are regions in our galaxy that would be less likely to be habitable because of the effects of Sgr A*, which was not always as quiet as it is now.

Image: Centaurus A is one of the active galactic nuclei closest to Earth. It emits strong radio emission and produces a relativistic jet. Credit: ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimetre); NASA/CXC/CfA/R.Kraft et al. (X-ray).

AGNs represent a class of galaxies that appear in a wide range of shapes and spectral features. Some of that spectral variation may, according to one theory, involve our viewing angle and the obscuring effects of dust. The peak of Sgr A*’s active phase is thought to have occurred less than 8 billion years ago and to have lasted between 107 and 109 years. The paper examines how this activity would affect the habitability of the Milky Way.

Image: An artist impression of the quasar ULAS J1120+0641. Credit: ESO/M. Kornmesser.

The researchers compared the XUV flux at various distances from galactic center to the dosage that would prove lethal for organisms on Earth, producing ‘critical fluxes’ for complex life as well as for prokaryotes (some radiation-resistant terrestrial prokaryotes can survive high radiation doses). How far from Sgr A* would a planet have to be to be exposed to a critical flux?

The issue is complicated by the possible response of local organisms, which might evolve to cope with increased radiation under varying environmental conditions, and it is also true that radiation doses lower than lethality could spur biological mutations. But given what they assume to be plausible values for a lethal absorbed dose of ionizing radiation, the authors believe that complex life would have been in jeopardy during Sgr A*’s peak active phase at distances as large as 10 kpc [32,600 light years] from galactic center. By comparison, the Sun lies about 8 kpc from the center of the Milky Way, having formed well after the Sgr A* peak.

From the paper:

This may not have prevented the appearance of life per se, since prokaryotes could have survived to higher fluxes. However, we point out that the biological effect of the ionizing radiation from Sgr A* would have been in addition to that of any other source of ionizing radiation, for example from the host star, and would add to the loss of a large fraction of the atmosphere. Even if some organisms might have survived by developing radiation-resistance or finding protected niches, the global effect on the biosphere would have been significant.

Let me circle back around to the question of atmosphere loss, also critical in assessing life’s chances in the era of Sgr A* peak activity. Assuming that the torus of the AGN would have been co-aligned with the galactic plane, the authors produce the chart below, showing the total amount of atmospheric mass lost at the end of the AGN phase of Sgr A* by a planet with the same density as Earth, as a function of the distance from the galactic center.

Image: This is Figure 1 from the paper. Caption: The total mass lost at the end of the AGN phase of Sgr A* by a terrestrial planet at distance D from the galactic center, in units of the atmosphere mass of present day Earth. Each curve was computed assuming a value for the efficiency of hydrodynamic escape of either ε = 0.1 or ε = 0.6. An optical depth τ = 1 corresponds to locations close to the galactic plane (maximum attenuation by the AGN torus) while τ = 0 corresponds to high galactic latitudes (no attenuation). Credit: Amedeo and Tombesi.

The conclusion is stark: Rocky planets in the galactic bulge would have been exposed to enough XUV radiation during peak Sgr A* activity to lose a significant fraction of their atmosphere. The mass loss for distances from the galactic plane of 0.5 kpc or less could be comparable to the atmosphere of Earth today. It would take significant volcanism and outgassing to regenerate an atmosphere sufficiently to repair such a loss.

…our results imply that the inner region of the Milky Way might have remained uninhabitable until the end of the AGN phase of its central black hole, and possibly thereafter. This has important consequences in assessing the likelihood of ancient life in the Galaxy, and should be taken into account in future studies of the Galactic habitable zone. It also suggests further investigations on the relation between supermassive black holes in galactic cores and planetary habitability.

The paper is Balbi and Tombesi, “The habitability of the Milky Way during the active phase of its central supermassive black hole,” Scientific Reports 7, article #: 16626 (2017). Full text.


Comments on this entry are closed.

  • Alex Tolley May 31, 2018, 14:08

    So the conclusion is that ancient ET (“old ones”) if they exist, most likely appeared at the edge of the galaxy, and nowhere near the center. The rate of stable, habitable conditions and life formation would be reduced for the galaxy as a whole during this active period.

    I’m not convinced about the problems for protist life in the deep oceans and even the lithosphere. X and UV radiation don’t penetrate well, so unless the atmosphere is removed and the oceans evaporate, it seems to me that bacteria should be safe enough.

    Terrestrial life is remarkably adaptive to what we once thought extreme conditions. Some organisms are extremely radiation resistant. Others remain alive exposed to high UV in the upper atmosphere. Even mammals seem to live around Chernobyl, apparently doing well as there is no human encroachment.

    Of perhaps more interest is whether we can broadly indicate whether there remains a zone in the galaxy that is relatively inimical to surface living multi-cellular life. How far does it extend and does it impact our ideas of where other biological civs may reside. For machine civs, maybe this is their safe zone as it precludes biological civs invading.

    • ljk May 31, 2018, 16:31

      That is why Robert Bradbury and others advocated looking for Dyson Shell/Swarms/Matrioshka and Jupiter Brains not near Galactic Center but way out at the galactic edge in the cold, dark regions away from most stars. That is where conditions would be cold enough to handle these “superbrains”.


      They might also use globular star clusters for resources, so that is another place to check. We will obviously have better luck at this stage of the SETI game to find the “big boys” who conduct astroengineering projects and are not worried about reactions from the ant colonies.

  • Geoffrey Hillend May 31, 2018, 21:59

    Most of the radiation is blocked by the dust at 8 kp. Active galactic nucleus x rays go long distance from the polar regions, but the dust blocks the horizontal radiation across the galactic plane or disk, so it does not go very far. It depends on how much dust there is in the galaxy. When a galaxy crosses in front of the poles it can be over a million light years away but still get hit by the high energy x rays which still have a maximum width which might take a long time a solar system in the receiving galaxy to cross the width of the x rays.

  • Robin Datta June 1, 2018, 0:09

    Such scientifically formed concepts as these could guide SETI in where to look and what to expect, getting the most bang for the buck. Hoping for a “big bang” :-)

  • Douglas Muir June 1, 2018, 3:27

    Clicked through to read the article. I note that the “less than 8 billion years ago” figure is not well supported, and that’s a weak spot in the paper. (Three cites are given for it; all of them are talking about black hole evolution in the general sense, without particular reference to Sgr A.)

    Doug M.

  • Douglas Muir June 1, 2018, 4:04

    Meanwhile, a point about cosmologically ancient life.

    1) Metallicity in the universe generally, and in our galaxy particularly, tends to increase over time. Makes sense, right? You have cycle after cycle of supernovae forging heavy elements and throwing them out into space, and it’s (mostly) a one-way process.

    2) There’s probably a minimum metallicity required for a biosphere to support life. Earth biology makes heavy use of a whole cocktail of elements, including several that are only created in supernovae. It may be possible for an alien biology to only use light elements, but it’s a big question mark.

    3) Putting 1 and 2 together we can conclude that there’s probably a minimum age for any galaxy before which life is unlikely or impossible, because there just aren’t enough heavy elements around to support a complex biology. There may be Ancient Elder Races out there, but there’s a hard limit to just how ancient they can be.

    A couple of other twists. Metallicity in stellar populations tends to increase with time; within a galaxy, it also tends to /decrease/ with distance from the galactic core. Greater stellar density -> more supernovae throwing heavy elements into your prestellar gas clouds -> faster processing of atoms through nucleosynthesis. But out in the thinly populated regions of the arms, things happen more slowly. So, the stars 3 megaparsecs out from the core are on average a lot less metallic than the stars 1 megaparsec out, and this has been true for a long time. This in turn means that (a) there is a galactic zone where average stellar metallicity is sufficient for life, and (b) this zone appeared first in the galactic core and then gradually migrated outwards over astronomical time.

    — you’ll notice I said “average”. Of course there’s a distribution function. We actually have a pretty good understanding of the distribution function of stellar age, location, and metallicity. One interesting point: the Sun (and by implication the Solar System) is unusually metal-rich. It’s around the 90th percentile for a star of its cohort and galactic location. (This is about the only thing about the Sun that’s really unusual — it’s otherwise a really typical solitary G dwarf.)

    Anyway: if the linked article is correct, it doesn’t bode well for the Elder Races. The galactic regions where you’d expect cosmologically ancient life to first appear are the bulge and the inner part of the arms, and these are exactly the ones that would have been zapped by an active galactic nucleus. Sometimes the early worm gets the bird.

    Doug M.

    • Alex Tolley June 1, 2018, 12:46

      Megaparsec? Did you mean kiloparsec?

      • Douglas Muir June 1, 2018, 17:03

        Yes, I did. Talking about metallicity within the Milky Way here. Metallicity on a cosmic scale is its own deep topic.

        Doug M.

  • Marshall Eubanks June 1, 2018, 9:35

    I am not at all convinced. The era they propose for the active AGN period of our galaxy is less than 8 billion years ago* – about the time the spiral arms formed, and 4+ billion years after the first high metallicity planetary systems formed in the bulge. I think that civilizations even a little more advanced than us could deal with an active AGN in the galactic center, or even may have caused it for industrial purposes. It would not have hurt the galactic arms much, as at that period they were just forming and wouldn’t have had many established planetary systems.

  • Andrew Wright June 1, 2018, 15:12

    Can’t we hope that some rocky planets in the galactic bulge started with very thick atmospheres, so that losing some of it made them more habitable, rather than less?

  • Geoffrey Hillend June 2, 2018, 21:41

    If they started with a thick atmosphere, then most likely they are not habitable worlds from the start. Being close to the central black hole is probably not a good place for life to start. Only the future and better technology will allow us to know for sure.

  • ljk June 5, 2018, 10:19

    Globular clusters not as old as thought?

    By Deborah Byrd in Space | June 5, 2018

    New research suggests that globular clusters – once thought to be nearly as old as the universe itself – aren’t that old, after all. They might be only around 9 billion years old.


    If GSC are younger than once thought, that means they formed when heavier elements were available, therefore making them even more appealing targets for advanced ETI to utilize and for us to SETI and METI-ize.

  • ljk June 8, 2018, 11:45

    More mystery objects near Milky Way’s giant black hole

    By Deborah Byrd in Space | June 7, 2018

    Astronomers call them G-objects. The 2 previously known ones came incredibly close to the Milky Way’s central black hole, yet survived. Now astronomers report 3 more of these mystery G-objects near the heart of our galaxy.


    Astronomers said on June 6, 2018, that they analyzed 12 years of data gathered at the W. M. Keck Observatory in Hawaii to discover several more of the bizarre objects known as G-objects. Only two examples were previously known of these strange galactic inhabitants, which are located behind a shroud of galactic dust, near Sagittarius A* (pronounced Sagittarius A-star), the supermassive black hole at our Milky Way galaxy’s heart. Astronomers discovered the first G-object – G1 – in 2004 and the second – G2 – in 2012. Both were thought to be gas clouds until they made their closest approach to the black hole. Both G1 and G2 somehow managed to survive the hole’s gravitational pull, which wouldn’t have happened if they were gas clouds; a 4-million-solar-mass black hole like Sagittarius A* can shred gas clouds apart. Now these same astronomers report three more of the strange G-objects – which they’ve labeled G3, G4 and G5 – near the galaxy’s heart. The astronomers said they:

    … look like gas clouds, but behave like stars.

    Astronomer Anna Ciurlo – a member of the Galactic Center Orbits Initiative at UCLA – led a team that reached this conclusion. She announced the team’s result at the American Astronomical Society meeting going on this week in Denver, Colorado. Ciurlo said in a statement:

    These compact dusty stellar objects move extremely fast and close to our galaxy’s supermassive black hole. It is fascinating to watch them move from year to year. How did they get there? And what will they become? They must have an interesting story to tell.

  • ljk June 12, 2018, 10:00
  • ljk June 12, 2018, 13:15

    Could an exoplanet survive orbiting a black hole? How about one million alien worlds…