GREAT GALAXY: The Milky Way maintains a fleet of some two dozen satellite galaxies whose motions help reveal its mass.
Image: NASA/ESA/Hubble Heritage Team
Although scientists know the masses of the sun and Earth, it's a different story for the galaxy. Mass estimates range widely: At the low end, some studies find that the galaxy is several hundred billion times as massive as the sun whereas the largest values exceed two trillion solar masses. Astronomers would have an easier task if the galaxy consisted solely of stars. But a huge halo of dark matter engulfs its starry disk and vastly outweighs it. Now remarkable observations of a small galaxy orbiting our own have led to a new number.
In studies of the Milky Way's mass one little galaxy plays an outsize role: Leo I. "The value of Leo I is twofold," says Michael Boylan-Kolchin of the University of California, Irvine. "It's both very distant and moving quite quickly." Discovered in 1950 and located 850,000 light-years from the Milky Way's center, Leo I is a dwarf spheroidal galaxy and the farthest of the many galaxies that are thought to orbit our own. Most of the Milky Way's dark matter halo should fit inside Leo I's orbit—that is, if the dwarf galaxy is actually in orbit and not just passing by.
Astronomers know from Leo I's Doppler shift that it is racing away from us. If the Milky Way has enough mass, its gravity will hold it in orbit. Moreover, astronomers would be able to observe the motion of Leo I and use it to deduce the Milky Way's total mass—including its dark matter halo—out to the dwarf galaxy's great distance. But if the Milky Way does not have enough mass, Leo I will fly away, its high speed revealing little of consequence.
To deduce Leo I's path through space, astronomers have to determine the small galaxy's precise motion. The Doppler shift reveals Leo I's velocity along our line of sight, but no one knew how fast the little galaxy was moving across it. Determining that requires measuring its proper motion—the change in the galaxy's position from one year to the next. Proper motion is easy to gauge for a nearby star but difficult to measure for another galaxy, because far-off objects have tiny proper motions.
Sangmo Tony Sohn of the Space Telescope Science Institute and his colleagues therefore used the Hubble Space Telescope to compare Leo I's position in 2006 and 2011 with more than a hundred background galaxies. In work submitted to The Astrophysical Journal, Sohn's team reports success: the first proper motion measurement of Leo I.
"It's a powerful piece of work," says Timothy Beers, the director of Kitt Peak National Observatory, who was not affiliated with the research. "It strikes me as utterly amazing that we have instruments that can measure proper motions that far away." Scott Tremaine, an astronomer at the Institute for Advanced Study, agrees: "The measurement of the proper motion really is a tour de force."
Combined with the Doppler shift, the proper motion reveals that Leo I orbits the Milky Way at 200 kilometers per second. By comparison, that's nearly as fast as the sun orbits the Milky Way's center, even though the dwarf galaxy is much farther away. Says Boylan-Kolchin, "To sustain a similar velocity at that far a distance requires a lot of extra mass."
How much mass? In a companion study Boylan-Kolchin and his colleagues simulate how giant galaxies such as the Milky Way grow by swallowing lesser galaxies, finding that dwarf galaxies moving as fast as Leo I are almost always bound to the giants, which means Leo I is a true satellite. His team then derives a mass for the Milky Way of 1.6 trillion suns. "That's on the high side," Beers comments. "But it's not outrageous."
See what we're tweeting about
10 Comments
Add CommentExcellent article discussing in quite analytical manner issue estimating the mass of MW galaxy. However, article does not specifies the elements of MW apart from stars which constitute the mass of MW. Apart from stars, there are also Black Holes, MACHOs etc in MY galaxy. There is no census of BHs ( both micro and supermassive) in MW galaxy.
Reply | Report Abuse | Link to thisFurther article states that orbit of Leo I cuts across dark matter halo enveloping and that Leo I is about 8,50,000 light years away from us i.e. from Sun. There are no correct estimates for the size of dark matter halo enveloping MW. May be dark matter halo of MW might be extending much further than 8,50,000 light years away from Leo I and this dwarf galaxy might also be having its own dark matter halo. Apart from MW galaxy, Leo I could also be under gravitational influence of other dwarf galaxies in its neighborhood. Due to close proximity of Leo I with other dwarf galaxies in local group, its motion may be influenced more by these local galaxies rather than MW galaxy. Further, all those dwarf galaxies in local group may also have their dark matter halo
In view of above, estimation of mass of MW galaxy solely from the motion of Leo I appears to follow a quite simplistic model
Its going to be the same size as our national debt
Reply | Report Abuse | Link to thisInteresting paper on a better approximation to our galaxy's info.
Reply | Report Abuse | Link to thisOf course it still depends on IF Leo I actually orbits the MW or not. Authors have considered this, so it seems all is in its right perspective - a slightly better guess at how big the MW is.
Nice, interesting one, SA.
My enthusiasm about this topic outweighs my ignorance, so please excuse my ignorance. Would someone be able to answer this question:
Reply | Report Abuse | Link to thisWhy can't it be that there's something with more mass pulling the dwarf galaxy away from us, black hole maybe? Undetected galaxy?
Recall our method of determining distance is derived from
Reply | Report Abuse | Link to thisvariables such as brightness of distant objects. So any
estimates are just that. Though learning about space is important, if we don't pay more attention to conditions here on Earth, the possibility exists we may never find out.
i do not know what area leo 1 is in , but , is andromeada only 150,000 ly away from it , as it is 1,000,000 ly from us , and it has aproximate mass/size maybe it is influential ?
Reply | Report Abuse | Link to this0.06 Mly = radius of Milky Way
Reply | Report Abuse | Link to this0.35 Mly = radius of MW dark matter halo
0.85 Mly = distance from MW to Leo I
2.54 Mly = distance from MW to Andromeda
3.20 Mly = distance from Andromeda to Leo I
Thanks for putting it into perspective.
Reply | Report Abuse | Link to thisInteresting indeed! While the proper motions of Leo I and M31 have been precisely determined of the past several years, Leo I's full orbit has been implied by very complex evaluations of models (including large numbers of assumed conditions) representing potential orbits extrapolated from the very small sample of measured proper motions.
Reply | Report Abuse | Link to thisSeveral commentators have raised interesting points about relative locations. Cramer supplies some relative distances to relevant objects from some source. A very interesting chart is included in
http://en.wikipedia.org/wiki/Local_group#Component_galaxies
That chart is based on current positions of objects relative to the Milky Way, not considering any past or future relative locations. It is difficult to see, but the reference planar chart axis includes both the MW and Andromeda (M31); other objects are not found on that plane (the position of each is indicated by a small white dot).
Th collective center of mass for the Local Group currently lies somewhere in between the MW and Andromeda. It must be considered that Leo I may not be orbiting the MW but, like most Local Group members, may be moving generally about its center of mass, severely perturbed by its larger members. At these distances, Local Group members cannot be confined to orderly Keplerian orbits. Reading the referenced research report, I did not find any determinant evaluation of this possibility, despite Acoyauh2's confidence that they have done so.
While Cramer points out the current relative distance from the MW to Leo I and to Andromeda, the research identifies potential orbital paths of Leo I around the MW. If I interpret Figures 10-12 in the research report correctly, the projected orbits of Leo I should take it to locations nearer the Andromeda galaxy than the MW! Based on the article's quote of an estimate of the Andromeda galaxy's mass as being twice the MW's it's difficult to understand how a stable orbit around the MW could possibly be maintained by Leo I!
The very difficult analysis seems highly unorthodox to me, as I understand it. First, the orbit of Leo I is derived based on a low mass model of the MW. Then the MW mass was evaluated for mid and maximum mass values. It was eventually determined by the researchers "that the observed kinematics of Leo I are more consistent
with high-mass than with low-mass MW models." That is to say, they find that the larger of three parametric evaluations of MW mass seems to best fit the orbit they derived for Leo I.
(continued)
Reply | Report Abuse | Link to thisA most interesting example of extraordinary assumptions made in their analyses is that, as the presumed mass of the MW was incremented, the mass of M31 was commensurately decremented!
Of course, the principal assumption made is that the MW (and presumedly all other galaxies) are enveloped by a dark matter halo that (in the case of the low mass MW scenario) contains more than 90% of total galactic mass.
Interestingly, the researchers also evaluated a Keplerian model, correctly explaining:
"The assumption of a Keplerian potential for the MW is not as unreasonable as it may seem at first. The large Galactocentric distance of Leo I, ... 260.6 kpc, combined with its significant tangential velocity, implies that much of the MW's mass is inside the Leo I orbit at all times." It must be added, however, that another prerequisite condition for Keplerian compliance is that the orbital's motion not be perturbed by external masses.
The Keplerian model of the galaxy and found it to be inadequate, as "a Keplerian model is too concentrated, and therefore overestimates the acceleration as Leo I approaches the MW." Of course, these researcher's evaluations presumed an enormously massive dark matter halo, which would produce overestimates of acceleration if the dark matter halo did not actually exist...
Other researchers have noted that, when evaluated without presuming any dark matter halo, hundreds of nearer halo objects comply with Keplerian rotation curves, for the reasons given above. However, if a dark matter halo does exist for the MW, there is no explanation for why those nearer halo objects would comply with Keplerian rotation curves while galactic disk objects do not. For references, please see: "Inappropriate Application of Kepler's Empirical Laws of Planetary Motion to Spiral Galaxies Created the Perceived Galaxy Rotation Problem - Thereby Establishing a Galactic Presence for the Elusive, Inferred Dark Matter",
http://fqxi.org/data/essay-contest-files/Dwyer_FQXi_2012__Questionin_1.pdf
- especially the "Supplemental Information" section.
While these researchers precise determination of the proper motions of Leo I and M31 are very important, I find nothing has been definitively determined about the orbital motions of galaxies in the Local Group, whether Leo I is in stable orbit around the Milky Way or the actual mass of the Milky Way.