UCSC: Andromeda's Disk Indicates Violent History

[bystander]

Study of Andromeda's Disk Indicates More Violent History than Milky Way
University of California, Santa Cruz | 2015 Jan 08

Survey data reveal a more disordered stellar population in our galactic neighbor than in our own galaxy, suggesting more recent bombardment of Andromeda by smaller galaxies

[c][attachment=0]studyofandro[1].jpg[/attachment][/c]

A detailed study of the motions of different stellar populations in the disk of the Andromeda galaxy has found striking differences from our own Milky Way, suggesting a more violent history of mergers with smaller galaxies in Andromeda's recent past.

The structure and internal motions of the stellar disk of a spiral galaxy hold important keys to understanding the galaxy's formation history. ...

The new study, led by UC Santa Cruz graduate student Claire Dorman and Guhathakurta, combined data from two large surveys of stars in Andromeda, one conducted at the W. M. Keck Observatory in Hawaii and the other using the Hubble Space Telescope. The Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo (SPLASH) survey has used the Keck/DEIMOS multi-object spectrograph to measure radial velocities of more than 10,000 individual bright stars in Andromeda. The recently completed Panchromatic Hubble Andromeda Treasury (PHAT) survey provides high-resolution imaging at six different wavelengths for more than half of these stars. ...

Dorman's analysis revealed a clear trend related to stellar age, with the youngest stars showing relatively ordered rotational motion around the center of the Andromeda galaxy, while older stars displayed much more disordered motion. Stars in a "well ordered" population are all moving coherently, with nearly the same velocity, whereas stars in a disordered population have a wider range of velocities, implying a greater spatial dispersion. ...

[Ann]

I read the full article referred to in bystander's post, and came to this passage:

The comparison to the Milky Way revealed substantial differences suggesting that Andromeda has had a more violent accretion history in the recent past. "Even the most well ordered Andromeda stars are not as well ordered as the stars in the Milky Way's disk," Dorman said.

In the currently favored "Lambda Cold Dark Matter" paradigm of structure formation in the universe, large galaxies such as Andromeda and the Milky Way are thought to have grown by cannibalizing smaller satellite galaxies and accreting their stars and gas. Cosmologists predict that 70 percent of disks the size of Andromeda's and the Milky Way's should have interacted with at least one sizable satellite in the past 8 billion years. The Milky Way's disk is much too orderly for that to have happened, whereas Andromeda's disk fits the prediction much better.

"In this context, the motion of the stars in Andromeda's disk is more normal, and the Milky Way may simply be an outlier with an unusually quiescent accretion history," Guhathakurta said.

Wow. As some of you know, I have this sneaking suspicion that the Earth is not "typical" (if only because I suspect that its rich and varied biosphere is not typical or common at all), and I think that the Sun is not "typical" (I suspect that it is rather quiescent and unusually life-friendly for its class, which is in itself not the most typical kind of star in the universe - but yes, I do realize that the Sun is not truly "atypical") - and now it appears that the Milky Way itself is rather "atypical", and it may possibly be unusually life-friendly because of its well-ordered, quiescent nature!

Wow! I get a kick out of thinking that we here on the Earth may have won the most amazing cosmic lottery. I know that many of you, probably most of you, don't agree with me and definitely don't wish me to be right (because that would make it harder to find life on other planets). And of course I may be completely wrong.

But to me - well, how do I put it? I often hear people saying that we Earthlings are so insignificant. We mean nothing to the universe. But to me it is the other way round. If we are truly rare, as I believe we are (and yes, I may be wrong ), then we are not insignificant at all. We could be amazing. Perhaps the universe outdid itself when it created us, thanks to a series of happy coincidences.

To me that is fantastic to think about!

Ann

[geckzilla]

There are ways in which humanity is significant and ways it is insignificant given our current information. One statement about how insignificant humans are need not be in conflict with one about how significant they are.

[rstevenson]

...Wow. As some of you know, I have this sneaking suspicion that the Earth is not "typical" (if only because I suspect that its rich and varied biosphere is not typical or common at all), and I think that the Sun is not "typical" (I suspect that it is rather quiescent and unusually life-friendly for its class, which is in itself not the most typical kind of star in the universe - but yes, I do realize that the Sun is not truly "atypical") - and now it appears that the Milky Way itself is rather "atypical", and it may possibly be unusually life-friendly because of its well-ordered, quiescent nature!

Wow! I get a kick out of thinking that we here on the Earth may have won the most amazing cosmic lottery. I know that many of you, probably most of you, don't agree with me and definitely don't wish me to be right (because that would make it harder to find life on other planets). And of course I may be completely wrong.

But to me - well, how do I put it? I often hear people saying that we Earthlings are so insignificant. We mean nothing to the universe. But to me it is the other way round. If we are truly rare, as I believe we are (and yes, I may be wrong ), then we are not insignificant at all. We could be amazing. Perhaps the universe outdid itself when it created us, thanks to a series of happy coincidences.

Given the vastness of the universe, I think it's perfectly reasonable to say that humanity is the result of amazing coincidences, and that intelligent life forms may be numerous throughout the universe. In other words, we (and every other sentient race) could be described as both amazing and non-unique.

Rob