In search of Tidal Tails…

newtidal.gifOkay, at this point it is old news, but, just to state the old news one more time for anyone who missed it – our galaxy has tidal tails.

The first press release on tidal tails that I know of occurred at the 2001 San Diego meeting of the AAS, and was related to work announced by Heather Morrison (Case Western Reserve University) describing the stream created when the Sagittarius Dwarf Galaxy fell into and was shredded by the Milky Way.

Since then, there have been memorable releases at AAS Seattle in 2003, AAS DC in 2005, AAS Seattle in 2007 and at last week’s AAS in Honolulu. There may have been other releases, but those I’ve forgotten.

That’s a lot of releases for one small topic, and while I have started to giggle a bit when I see a new tidal stream in my packet of press releases, I don’t think the press releases have been without merit. Understanding these sprawling families of stars helps us understand the history of our Milky Way. The Milky Way was formed through the merger of dwarf galaxies, and these mergers left (and continue to leave) beind streams of stars. In theory, as galaxies merge together, they are elongated as they are shredded within gravitational potential wells. Observing tidal streams and tidal tails provides checks on our theories (and in a wispy, mist-in-a-breeze kind of way, they are also beautiful).

And the way they find them is just good science.

Using the Sloan Digital Sky Survey, astronomers look for families of stars that have colors and brightnesses characteristic of a co-evolving system at a set distance. This means they are looking for stars that formed from the same cloud of dust and gas that are evolving together at the same distance. Their colors and magnitudes are what you see in a Hertzsprung-Russell diagram. In such a system, any differences between stars are related to differences in mass. Open clusters and globular clusters are examples of co-evolving systems.

By taking a synthetic population (for instance, an imaginary population that includes every possible mass of star for a system the same age and with the same ingredient list as M13), and stepping out in distance, it is possible to look for three dimensional structures. Off to the west, the stars may trace out a small nearby oval, but as distance is increased, the stream my arc east, forming larger and thinner structures. Once a structure is found, it can become a game of hide-and-go-seek to discover how it warps around the galaxy, and at this time, the structures can only be found using the best known pattern recognition algorithm available – the human mind.

So, PhD Astronomers stare at monitors and step through the halo of the galaxy looking for the ghosts of now destroyed galaxies. It is an expensive enterprise, consuming time (which really means salary) and yielding only a few tails here and there. But . . . It is solid science.

These tails, like Hansel and Gretel’s bread crumbs, trace out the paths of galaxies. These paths are defined by luminous and dark matter, and by understanding the orbits of dieing dwarf galaxies we can understand the dynamics of the galactic interactions that builds galaxies like our own.

Violence begets galaxies. Galaxies beget life. And life (at least some of the life in the field of astronomy) tests its theories on the remnants of that violence.


  1. Vijayaraghavan. E June 7, 2007 at 9:42 am #

    Those bright spots in the trails, I suppose, are the cores of the byegone halaxies. Do they show any trace of black holes?

  2. Mayonaze June 7, 2007 at 2:44 pm #

    Given the age of the universe, an average density of galaxies, and a speed/direction for the Milky Way, would it be possible to predict the number of galaxies that are incorporated into the Milky Way?

  3. pamela June 7, 2007 at 3:03 pm #

    Hi Vijayaraghaven,

    To search for black holes you need to be able to measure the velocities of the stars in the bright knots – in the remainders of galaxy cores. Unfortunately, these stars are generally too far away for us to get sufficient measurements of enough of them for anything to be said for certain. This is definately an interesting and open question!

    Hi Mayonaze,

    This is a hard question to answer because galaxies come in a range of sizes and have undergone different amounts of mass loss. Think of it this way – If you know the mass and age of a person, can you determin how many meals/calories they’ve consumed? An educated guess is possible, and you can say “If this, then this…” but an accurate answer requires information that can’t be gleaned from a modern snap shot.


  4. Lorne Ipsum June 8, 2007 at 12:13 pm #

    Just a quick question here…

    When the discovery of the remains of a dwarf galaxy is announced, how do the discoverers draw the line between dwarf galaxy remnants and open clusters? Is it a function of morphology (tail = dwarf), or the critter’s orbit, or is it a stellar chemistry thing?

  5. pamela June 8, 2007 at 2:37 pm #

    Open clusters are the stars born in a star forming region (a collapsing, fragmenting, molecular cloud) that all have the same chemistry and same age, and they are young. By themselves, they are not rich in dark matter. Dwarf galaxies can have open clusters. Dwarf galaxies are small galaxies that have Mass to Luminous Mass ratios of 50-500 (globular clusters have ratios of <5). The dwarfs in the halo are all old, and are not forming stars. In tidal streams, how the stars move defines if they are shredded globular clusters of shredded dwarf galaxies. It all becomes a question of dynamics.

  6. Lorne Ipsum June 15, 2007 at 7:21 pm #

    Gotcha — thanks!

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