Astronomy Without A Telescope – Not So Ordinary

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Sorry – a bit of southern sky bias in this one. But it does seem that our favourite down under naked eye objects are even more unique than we might have thought. The two dwarf galaxies, the Large and Small Magellanic Clouds, orbit the Milky Way and have bright star forming regions. It would seem that most satellite galaxies, in orbit around other big galaxies, don’t. And, taking this finding a step further, our galaxy may be one of a declining minority of galaxies still dining on gas-filled dwarf galaxies to maintain a bright and youthful appearance.

We used to think that the Sun was an ordinary, unremarkable star – but these days we should acknowledge that it’s out of statistical mid-range, since the most common stars in the visible universe are red dwarfs. Also, most stars are in binary or larger groups – unlike our apparently solitary one.

The Sun is also fortunately positioned in the Milky Way’s habitable zone – not too close-in to be constantly blasted with gamma rays, but close-in enough for there to be plenty of new star formation to seed the interstellar medium with heavy elements. And the Milky Way itself is starting to look a bit out of the ordinary. It’s quite large as spiral galaxies go, bright with active star formation – and it’s got bright satellites.

The Lambda Cold Dark Matter (CDM) model of large scale structure and galaxy formation has it that galaxy formation is a bottom-up process, with the big galaxies we see today having formed from the accretion of smaller structures – including dwarf galaxies – which themselves may have first formed upon some kind of dark matter scaffolding.

Through this building-up process, spinning spiral galaxies with bright star forming regions should become common place – only dimming if they run out of new gas and dust to feast on, only losing their structure if they collide with another big galaxy – first becoming a ‘train wreck’ irregular galaxy and then probably evolving into an elliptical galaxy.

The  Lambda CDM model suggests that other bright spiral galaxies should also be surrounded by lots of gas-filled satellite galaxies, being slowly draw in to feed their host. Otherwise how is it that these spiral galaxies get so big and bright? But, at least for the moment, that’s not what we are finding – and the Milky Way doesn’t seem to be a ‘typical’ example of what’s out there.

The relative lack of satellites observed around other galaxies could mean the era of rapidly accreting and growing galaxies is coming to a close – a point emphasised by the knowledge that we observe distant galaxies at various stages of their past lives anyway. So the Milky Way may already be a relic of a bygone era – one of the last of the galaxies still growing from the accretion of smaller dwarf galaxies.

Supernova 1987a, which exploded near the Tarantula Nebula of the Large Magellanic Cloud. Credit: Anglo-Australian Observatory.

On the other hand – maybe we just have some very unusual satellites. To a distant observer, the Large MC would have nearly a tenth of the luminosity of the Milky Way and the Small MC nearly a fortieth– we don’t find anything like this around most other galaxies. The Clouds may even represent a binary pair which is also fairly unprecedented in any current sky survey data.

They are thought to have passed close together around 2.5 billion years ago – and it’s possible that this event may have set off an extended period of new star formation. So maybe other galaxies do have lots of satellites – it’s just that they are dim and difficult to observe as they are not engaged in new star formation.

Either way, using our galaxy as a basis for modelling how other galaxies work might not be a good idea – apparently it’s not so ordinary.

Further reading: James, P. A. And Ivory C.F. On the scarcity of Magellanic Cloud-like satellites.

10 Replies to “Astronomy Without A Telescope – Not So Ordinary”

  1. apparently it’s not so ordinary.

    Cool, and the preceding article is also touching on something that could be distinguished. But one can notice that these types of claims have a tendency to come and go.

    As for the Sun I would argue that it is ordinary for stars having planets.

    For binaries or multiples, click on the Binary filter in exoplanet database, not many of those are reported with planets so far.

    For the galactic habitable zone, it is wide and migrating outwards as the stars gets old enough. At most maybe life here is young as life in the universe goes if star formation is prolonged. Incidentally that would a priori argue that the habitable zone is larger in other galaxies! (So now I’m curious about such estimates in general. Q’s feeding Q’s, as always.)

  2. ScienceToday 29 sep 2007 wrote:
    Earlier this year, CfA astronomers reported measuring the 3-d velocities of the Magellanic Clouds through space with greater accuracy than ever before. The velocities were anomalously high.
    Two explanations were proposed: 1. the Milky Way is more massive than previously thought, or 2. the Magellanic Clouds are not gravitationally bound to the Milky Way.
    Further analysis by Besla and her colleagues verified the second explanation. The parabolic orbit they calculated for the Clouds, based on the observed velocities, shows that both are on their first pass by the Milky Way.
    Is this false science today?

  3. 29 Sep 2007 Science wrote:
    Earlier this year, CfA astronomers reported measuring the 3-d velocities of the Magellanic Clouds through space with greater accuracy than ever before. The velocities were anomalously high.
    Two explanations were proposed: 1. the Milky Way is more massive than previously thought, or 2. the Magellanic Clouds are not gravitationally bound to the Milky Way.
    Further analysis by Besla and her colleagues verified the second explanation. The parabolic orbit they calculated for the Clouds, based on the observed velocities, shows that both are on their first pass by the Milky Way.
    Is this false science now?

  4. Emmanuel, that is an interesting observation.

    The paper seems to assume that the MCs are bound to MW (at least I can find no reference), but implies they are by way of a simulation result which is referenced but inconclusively described. (I.e. you can’t tell if the merger was an assumption or a result without hunting the reference down.)

    As of 6 May -09, Universe Today (by way of Fraser Cain) wasn’t bound to the bound scenario:

    “If you’re lucky enough to live in the Earth‘s southern hemisphere, you’re familiar with two fuzzy patches in the night sky. These are the Large and Small Magellanic Clouds, which may or may not be companion galaxies to our own Milky Way. … Astronomers used to think that the Magellanic Clouds were companion galaxies to the Milky Way, but recent velocity measurements have calculated that they’re not gravitationally bound to the Milky Way. Instead, they’re just passing nearby us before they travel away again.”

    I don’t know if that is the latest result however.

    In the paper the criteria for satellites is a distance, and it’s larger than the MC distance, so it doesn’t affect their statistics. Which btw it is, now that I’m looking at it, it’s a seemingly nice distribution with brighter “satellites” and more and more weaker ones. Some galaxies have more MC analogs than MW has. (One had 3 SMC analogs, for example.)

    So even if MW isn’t “typical” it isn’t unique either. (Except for that the MCs constitute a binary.)

  5. @ emanueldewitt

    Not at all. Not sure if there is consensus on Besla’s first pass theory – but it ranks as top shelf science. Here’s a different take on the story: http://www.newscientist.com/article/dn12652-milky-way-keeps-a-light-grip-on-speedy-neighbours.html.
    I read this as the LMC being almost certainly caught and the SMC possibly. I also note a recent lecture where she definitely refers to the Clouds being in orbit:
    http://www.cita.utoronto.ca/index.php/Events-Calendar/2010/The-Formation-of-the-Magellanic-Stream
    Anyhow – agree they could be very new satellites, galactic time-wise.

  6. My previous comment was caught in some web SMBH, but the gist was:

    Interesting note from EdW, but it won’t affect the paper since they use distance (rather further than the MCs) as the association criteria.

    Also on the topic of being outside statistical midrange, the paper show a distribution with objects brighter than the MCs to many more dimmer. Confirming that, one galaxy had more MC type objects (3 SMC equivalents). So if the MW is an unsound basis for modeling the average galaxy, it isn’t outside the distribution’s typical range either.

    It is the binary nature of the MCs that looks to be rare so far.

  7. Everybody knows that smaller satellite galaxies like the SMC and LMC of the milky way, will ultimately collide merge with their parent galaxy along dark matter tidal streaming stripped stars lost from the smaller galaxies. The gravity that pulls together the merging galaxies comes not from the gases stars and dust each galaxy contains gains or loses, but rather from their black holes, that are proven to collide in space by gravity. The unbound gravitational condition of the LMC with the milky way is observational proof of this. The biggest black hole collision in our visible region of the universe was perhaps two super duper clusters black hole collisions, causing what I believe is called the big-bang baby universe standard model, a mere 14.6 BY ago, and not even a trillion number like the national debt.

  8. A recent paper on the Magellanic Stream( http://arxiv.org/PS_cache/arxiv/pdf/1009/1009.0001v1.pdf ) does look at the implications of the 2007 work by Besla (Sec 5) and notes in the abstract “We estimate that the age of the ~140 degree-long MS is ~2.5 Gyr which coincides with bursts of star formation in the Magellanic Clouds and a possible close encounter of these two galaxies with each other that could have triggered the formation of the MS”

    It seems that the MS extends at least 200 degrees and most likely more. This paper finds this to be consistent with supernovae-induced shocks excavating material out of the MCs as a possible mechanism for the formation of the MS, in lieu of tidal forces or ram-pressure stripping. They also mention a known gap in the ages of globular clusters, between 3 and 13Gyr, in the LMC (but not the SMC) that is also consistent with the ‘interacting cloud’ hypothesis.

  9. @JimHenson

    Crikey. ‘The gravity that pulls together the merging galaxies comes not from the gases stars and dust each galaxy contains gains or loses, but rather from their black holes, that are proven to collide in space by gravity.’

    Gravity doesn’t select which component of mass it chooses to act upon. Black holes represent a huge amount of density within a small volume – but Sagittarius A* is just 4 million solar masses. The Milky Way contains maybe 4 billion stars and a cumulative mass that is about 5 orders of magnitude higher than Sag A*

  10. supermassiveblack holes have high relativistic light speed velocities. Galactic black holes might be moving away or towards one another at light speed, which is much faster then the orbital velocities and redshifts of its stars, dust and gases. when this occurs, the outer stars can be lost streamed to the larger parent galaxy. this is not because of stellar galactic matter gravity pulling them away from the smaller galaxy, but is caused by a black hole gravitational effect. In fact there was a blog here on how to smash collide stars together. Even with black holes pulling them towards the center of the galaxy, the best excuse the gravity experts can come up with on why stars seldom have been observed to collide unless they are tight fit binaries in long time decaying orbits, is that the stars are so far apart compared to their sizes. Consider if the stars were rocks like earth or a marble. Swirl it around a black hole the way the stars do, and tell me if the damn thing wouldn’t make a lot of noise and bang your eardrums? Stars just observationally do not collide and bump together the way solid gravity objects would that are not around magnetic fields and black holes, such as inside our solar system.

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