The galaxy's ancient brown dwarf population revealed

Nov 20, 2013 by Robert Massey
A brown dwarf from the thick-disk or halo is shown. Although astronomers observe these objects as they pass near to the solar system, they spend much of their time away from the busiest part of the Galaxy, and the Milky Way's disk can be seen in the background. Credit: John Pinfield

(Phys.org) —A team of astronomers led by Dr David Pinfield at the University of Hertfordshire have discovered two of the oldest brown dwarfs in the Galaxy. These ancient objects are moving at speeds of 100-200 kilometres per second, much faster than normal stars and other brown dwarfs and are thought to have formed when the Galaxy was very young, more than 10 billion years ago. Intriguingly the scientists believe they could be part of a vast and previously unseen population of objects. The researchers publish their results in the Oxford University Press journal Monthly Notices of the Royal Astronomical Society.

Brown dwarfs are star-like objects but are much less massive (with less than 7% of the Sun's mass), and do not generate internal heat through nuclear fusion like . Because of this simply cool and fade with time and very old brown dwarfs become very cool indeed - the new discoveries have temperatures of 250-600 degrees Celsius, much cooler than stars (in comparison the Sun has a surface temperature of 5600 degrees Celsius).

Pinfield's team identified the new objects in the survey made by the Wide-field Infrared Survey Explorer (WISE), a NASA observatory that scanned the mid-infrared sky from orbit in 2010 and 2011. The names are WISE 0013+0634 and WISE 0833+0052, and they lie in the Pisces and Hydra constellations respectively. Additional measurements confirming the nature of the objects came from large ground-based telescopes (Magellan, Gemini, VISTA and UKIRT). The infrared sky is full of faint red sources, including reddened stars, faint background (large distances from our own Milky Way) and nebulous gas and dust. Identifying cool brown dwarfs in amongst this messy mixture is akin to finding needles in a haystack. But Pinfield's team developed a new method that takes advantage of the way in which WISE scans the sky multiple times. This allowed them to identify cool brown dwarfs that were fainter than other searches had revealed.

The team of scientists then studied the infrared light emitted from these objects, which are unusual compared to typical slower moving brown dwarfs. The spectral signatures of their light reflects their ancient atmospheres, which are almost entirely made up of hydrogen rather than having the more abundant heavier elements seen in younger stars. Pinfield comments on their venerable ages and high speeds, "Unlike in other walks of life, the Galaxy's oldest members move much faster than its younger population".

Stars near to the Sun (in the so-called local volume) are made up of 3 overlapping populations - the thin disk, the thick disk and the halo. The thick disk is much older than the thin disk, and its stars move up and down at a higher velocity. Both these disk components sit within the halo that contains the remnants of the first stars that formed in the Galaxy.

Thin disk objects dominate the local volume, with thick disk and halo objects being much rarer. About 97% of local stars are thin disk members, while just 3% are from the thick-disk or halo. Brown dwarfs population numbers probably follow those of stars, which explains why these fast-moving thick-disk/halo objects are only now being discovered.

There are thought to be as many as 70 billion brown dwarfs in the Galaxy's thin disk, and the thick disk and halo occupy much larger Galactic volumes. So even a small (3%) local population signifies a huge number of ancient brown dwarfs in the Galaxy. "These two brown dwarfs may be the tip of an iceberg and are an intriguing piece of astronomical archaeology", said Pinfield. "We have only been able to find these objects by searching for the faintest and coolest things possible with WISE. And by finding more of them we will gain insight into the earliest epoch of the history of the Galaxy."

Explore further: Image: NGC 6872 in the constellation of Pavo

More information: "A deep WISE search for very late type objects and the discovery of two halo/thick-disk T dwarfs: WISE 0013+0634 and WISE 0833+0052", D. J. Pinfield et al, Monthly Notices of the Royal Astronomical Society, in press.
A pre-publication version of the paper is available on arXiv: arxiv.org/abs/1308.0495

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philw1776
1.7 / 5 (10) Nov 20, 2013
Why only 70 billion expected? With 70% of "real" stars being M dwarfs, why wouldn't the assumed # of these still less massive objects be even higher? It seems that nature makes far more less massive objects be they planets, moons, asteroids or stars. What mechanism intervenes to stop this propensity with low mass "failed" stars?
GSwift7
not rated yet Nov 20, 2013
What mechanism intervenes to stop this propensity with low mass "failed" stars?


The truth is that we really don't know. That 70 billion figure is just a guess.

However, I think the reasoning is that it takes an unusual situation to end up with a brown dwarf. You would need a star to start forming, which requires a certain minimum amount and density of cool gas. Then something has to happen that stops the collapse of that gas before it reaches full size, such as another star forming nearby which then heats the gas up and blows it away before it can collapse. Without some kind of intervention, I think the theory is that the minimum amount and density of gas that will lead to collapse ends up giving you red dwarves or larger bodies.
Mimath224
1.3 / 5 (10) Nov 20, 2013
Will such numbers of BD's have an effect on the DM figure? Perhaps not not as the article suggests the halo contains 'rarer' numbers.
Lurker2358
1.5 / 5 (15) Nov 21, 2013
Galaxy modeling is wrong because the N-body equation for a galaxy cannot be solved by computers.

You can model this with a simple, 4 object linear model, representing the mass of objects at various distances from the center,a nd the gravity they have on one another.

Object A is in the center.
Object B is 1 full unit away.
Object C is between A and B at a half-unit.
Object D is opposite of B, at a half unit, or 1.5 units away from B.

If you treat A, C and D as one object with mass at the center, you get 3 units of Force upon object B.

If you instead solve their force equations individually and add them, which is the correct method, you get 5.44 units of Force upon object B.

Viola, a huge portion of so-called "Dark Matter" is immediately explained away.
GSwift7
2.3 / 5 (3) Nov 21, 2013
Mimath:

Will such numbers of BD's have an effect on the DM figure?


No, the DM estimates already include normal objects that are just too small or dim for us to see. We can get an idea of how much normal matter is hiding in the darkness from the percentage of light that is blocked when it passes through from more distant sources. So, we may not know how that normal matter is divided between brown dwarfs, dust motes, planets, etc, but we have a pretty good ballpark estimate of the total for 'normal' matter. If we were to discover that brown dwarves are MUCH more numerous than we thought, then that just has to be subtracted from some other category, such as red dwarves or dust clouds.
antialias_physorg
5 / 5 (4) Nov 21, 2013
Will such numbers of BD's have an effect on the DM figure?

Dark matter is estimated to be more than 4 times as much as regular matter. If their estimate is accurate then we're talking about 3% as many brown dwarfs as other suns with less than 7% of the mass of our sun per object. That doesn't amount to much as a percentage of total mass.

a huge portion of so-called "Dark Matter" is immediately explained away.

..if scientists used such naive methods. But they don't. They use finite element modeling which already takes care of this effect (No one does direct modeling beyond a two body problem).
Zephir_fan
Nov 21, 2013
This comment has been removed by a moderator.
AstroRudy2
1.3 / 5 (10) Nov 21, 2013
According to data on this sub-stellar population collected by studying microlensing of a distant quasar by a foreground galaxy, the mass spectrum of this population increases STRONGLY as you go to lower masses. In other words, this population of observed free-floating planets in the foreground galaxy increases a factor of 1000 for every drop in mass by factor 10. So if these guys observe 2 objects of 10 Jupiter mass, there are 2000 of Jupiter mass and so on. This has been measured down to lunar mass. If correct, this discovery makes free roaming planets, also called rogue planets, the baryonic dark matter (aka. the missing baryons). See arXiv 1212.2962
Q-Star
not rated yet Nov 21, 2013
If correct, this discovery makes free roaming planets, also called rogue planets, the baryonic dark matter (aka. the missing baryons). See arXiv 1212.2962


Are ya really Rudy? Wow, Ewan and Rudy in a two day period. Welcome aboard guys. We need more working astrophysicists.

Please clarify for me (and others). Ya aren't suggesting that "this missing baryonic dark matter" accounts for all the "dark matter" necessary to the LamdaCDM model are ya? I know your work in this area very well, I know that's not what ya are saying, so my question was rhetorical. But the people on this site are very prone to taking that remark to mean "all dark matter is now accounted for",,, they will take the slightest misunderstanding and proclaim that it is over and done.

Anyhoo, we sure do need guys like ya and Ewan here, I really hope ya both will post often. Any chance ya can get Avi to take part too?
Mimath224
1 / 5 (9) Nov 21, 2013
GSwift7,antialias_physorg; Yes appreciate your points but '...We can get an idea of how much normal matter is hiding in the darkness from the percentage of light that is blocked when it passes through from more distant sources...'
How sure are we about the amount of light at source so that we can estimate the % blocked?

'...They use finite element modeling which already takes care of this effect (No one does direct modeling beyond a two body problem)...'
(Think Zephir_fan got his vectors mixed.) What form of PDEs (num. sol) would best used for this brown dwarf scenario?

Zephir_fan
Nov 21, 2013
This comment has been removed by a moderator.
Mimath224
1.4 / 5 (9) Nov 22, 2013
Zephir_fan my apologies! I meant to comment on Lurker2358, as you indicate. Can I have my bk points removed?
AstroRudy2
1.6 / 5 (7) Nov 22, 2013
This is a reply to Q-star. The effect of these primordial planets on the transmission of the universe was described in astro-ph:0512.236 . When the anomalous supernova brightness was seen by Adam Riess, they looked for the reddening signature of dust and concluded it was not there and also ASSUMED that the dark matter could not cause the problem. But the achromatic signature of refraction by all these planets was not considered until astro-ph:0512.236. This refraction would have evolved in a known way with the expansion and cooling of the universe and can exactly explain the supernova brightness defect. Hence no need for dark energy.
The LambdaCDMtheory is parametter fitted to cosmic structure on cosmic scales of 1 Mpc to 1 Gpc, but when you look outside this range, it fails completely. Studies of the dwarf spheroidal galaxies in the MW Halo show that the Lambda CDM predictions completely fail as follows:
1. Lambda-CDM predicts the inbound satellite galaxies enter on randomly.
AstroRudy2
1.5 / 5 (8) Nov 22, 2013
Regarding other dark matter: CERN and the underground detectors show it cannot exist, except as good old neutrinos. It is likely that neutrinos importantly guided structure formation until baryons took over at z = 1000 when the plasma neutralized and the viscosity dropped 13 orders of magnitude, allowing all the hydrogen to collapse into primordial clusters containing a fog of hydrogen condensations with the for droplet size planetary mass. This is described in solid Kolmogorov hydrodynamics guided by gravity. The concentric heirarchical structuring of planets in clumps of globular cluster mass defeats all the usual assumptions of a uniform (Gaussian) distribution of matter.
Q-Star
not rated yet Nov 22, 2013
This is a reply to Q-star.,,,, blah, blah, blah,,,,Lambda CDM predictions completely fail as follows


Well then,, ya can tickle me with a rusty nail. Ya sure have changed your tune. When did this great epiphany strike ya?
Q-Star
not rated yet Nov 22, 2013
Regarding other dark matter: CERN and the underground detectors show it cannot exist, except as good old neutrinos.


Holy spherical cow jumping over the moon. Rudy have ya been enjoying the drink this fine morning? Ya aren't very good at this. It only took ya two posts to over play your hand. (Actually ya pushed a tad to hard with putting so much "gravitas" into your profile. A real person wouldn't have done that, they would know how temping that would be to cranks and crackpots.)

Send me an email and I'll make a public apology. Psst, I'll give ya hint, even though the real Rudy would know that my profile contains just enough information that anyone who knows me in the real world would immediately know who I am. Think Megan and Jeffrey.
GSwift7
5 / 5 (2) Nov 22, 2013
lol, yeah, I call BS too.

There's no evidence to support vast numbers of free-floating planetary mass objects. There was a numerical/statistical analysis done by Stanford that used the assumed total mass of the Milky Way and subtracted the mass of all the known, visible objects and then computed how many planetary mass objects it would take to make up the balance. That gave them an upper bound limit of around 100,000 planets for each visible star, but actual observations of microlensing events caused by free floating planets set the actual number closer to 2 per star.

Anything as massive as a globular cluster composed solely of planetary mass objects seems like a statistical improbability, since the same rules that apply elsewhere would suggest that stars would form. Also, I think it is unlikely that we wouldn't be able to see the lensing effects in our current surveys. And finally, the mass distribution required for something like that is just crazy.