Weekly Space Hangout – June 12, 2015: Astronomy in Chile Educator Ambassadors Program

Host: Fraser Cain (@fcain)

Special Guest: This week we welcome Astronomy in Chile Educator Ambassadors Program Participants:
Michael Prokosch (Seeing Stars Blog, [email protected])
Tim Spuck ([email protected])
Brian Koberlein (@briankoberlein / briankoberlein.com)
Vivian White ([email protected]).

Guests:
Jolene Creighton (@jolene723 / fromquarkstoquasars.com)
Brian Koberlein (@briankoberlein / briankoberlein.com)
Morgan Rehnberg (cosmicchatter.org / @MorganRehnberg )
Alessondra Springmann (@sondy)
Continue reading “Weekly Space Hangout – June 12, 2015: Astronomy in Chile Educator Ambassadors Program”

Dwarf Galaxies That Dance? Andromeda Observations Reveal A Larger Cosmic Mystery

Astrophoto: Andromeda Galaxy by Fabio Bortoli

What is up with these dwarf galaxies? A survey of thousands of galaxies using the Sloan Digital Sky Survey reveals something interesting, which was first revealed by looking at the massive Andromeda Galaxy nearby Earth: dwarf galaxies orbiting larger ones are often in disc-shaped orbits and not distributed randomly, as astronomers expected.

The finding follows on from research in 2013 that showed that 50% of Andromeda’s dwarf galaxies are in a single plane about a million light-years in diameter, but only 300,000 light-years thick. Now with the larger discovery, scientists suspect that perhaps there is a yet-to-be found process that is controlling gas flow in the cosmos.

“We were surprised to find that a large proportion of pairs of satellite galaxies have oppositely directed velocities if they are situated on opposite sides of their giant galaxy hosts,” stated lead author Neil Ibata of Lycée International in France.

“Everywhere we looked, we saw this strangely coherent coordinated motion of dwarf galaxies,” added Geraint Lewis, a University of Sydney physicist. “From this we can extrapolate that these circular planes of dancing dwarfs are universal, seen in about 50 percent of galaxies. This is a big problem that contradicts our standard cosmological models. It challenges our understanding of how the universe works, including the nature of dark matter.”

The astronomers also speculated this could show something unexpected in the laws of physics, such as motion and gravity, but added it would take far more investigation to figure that out.

The findings were published in the journal Nature.

Source: University of Sydney

Gas Cloud Survives Collision With Milky Way

A high-velocity cloud hurtling toward the Milky Way should have disintegrated long ago when it first collided with and passed through our Galaxy. The fact that it’s still intact suggests it’s encased in a shell of dark matter, like a Hobbit wrapped in a mithril coat.

Mapping dark matter — the unseen stuff that makes up more than 80 percent of cosmic matter — near our Galaxy is crucial to fully understanding how the Milky Way assembled over cosmic time.

This firstly requires detailed observations of nearby dwarf galaxies — galaxies each totaling a mass less than 10% of the Milky Way’s 200 to 400 billion stars — because they’re enshrouded in dark matter. More recently, it has been suggested that nearby high velocity clouds of hydrogen gas are encased in dark matter as well. But the effects of their dark matter halos remain unknown.

So Matthew Nichols from the Sauverny Observatory in Switzerland and colleagues set out to observe the Smith Cloud — a high-velocity cloud of hydrogen gas located 8,000 lightyears away in the constellation Aquila — in order to better constrain its dark matter halo. They used the Green Bank Telescope (GBT) in west Virginia in order to detect the faint radio emission of neutral hydrogen.

“The Smith Cloud is really one of a kind. It’s fast, quite extensive, and close enough to study in detail,” said Nichols in a press release.  At its distance the cloud (9,800 lightyears long and 3,300 lightyears wide) covers almost as much sky as the constellation Orion.

“It’s also a bit of a mystery; an object like this simply shouldn’t survive a trip through the Milky Way, but all the evidence points to the fact that it did,” said Nichols. Previous studies of the Smith Cloud revealed that it first passed through our Galaxy many millions of years ago. By reexamining and carefully modeling the cloud, Nichols’ team now believes that it’s actually wrapped in a substantial halo of dark matter.

“Based on the currently predicted orbit, we show that a dark matter free cloud would be unlikely to survive this disk crossing,” said coauthor Jay Lockman from the National Radio Astronomy Observatory. “While a cloud with dark matter easily survives the passage and produces an object that looks like the Smith Cloud today.”

Not only does this study help astronomers start to characterize the dark matter enshrouding these seemingly harmless clouds, but it helps strengthen the case that the Smith Cloud isn’t purely a cloud of hydrogen gas, but a failed dwarf galaxy, originating from farther away in space. The presence of dark matter, however, will have to be further confirmed.

The paper has been submitted to the Monthly Notices of the Royal Astronomical Society and is available for download here.

The Hidden Galaxy in the Zone of Avoidance

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There are some places astronomers dare not tread. One of the prime places is beyond the disk of our own galaxy where the numerous stars and clouds of dust along the line of sight make observations messy to say the least. This obscured portion of the sky is known as the Zone of Avoidance. But despite the challenges, one team of astronomers has searched through it and found a previously undiscovered galaxy lurking not too far from our own.

To discover this galaxy, the team, lead by graduate student Travis McIntyre at the University of New Mexico, used the gigantic Arecibo radio telescope. This telescope is adept at finding emission at the 21 centimeter wavelength emitted by cool, atomic hydrogen. This long wavelength is relatively immune to the diminishing effects of gas and dust within our galaxy.

After the initial discovery, the team followed up with further observation using the Expanded Very Large Array, which also operates in the radio, as well as the 0.9 meter Southeastern Association for Research in Astronomy telescope, which is an optical telescope, in hopes of peering through some of the muck.

While the galaxy was easily recovered in the second radio search, and the optical images showed a faint clump, the centers of the two did not appear to line up. The visual and radio components seemed not to overlap almost at all. A portion of the reason for this is that the team was unable to image the faint galaxy out to its full extent before the contamination from our own galaxy overwhelmed the signal. As such, the two likely overlap more than is indicated by the study, but this would still indicate that the distribution of hydrogen gas within it is severely lopsided.

Another possibility is that the object detected isn’t really a galaxy at all and is a coincidence of an alignment between a high velocity cloud and an independent cluster of stars. However, such clouds of gas tend to travel in packs and no others are known in the area, making this possibility unlikely.

If the object is a galaxy, it is likely a blue dwarf galaxy with some 10 million solar masses. The team expects that, while the galaxy is relatively nearby, this galaxy is not likely to bea member of the local group because, were it that close, it would be unprecedentedly small. As such, they applied Hubble’s Law to give a rough distance of 22 million light years but caution that at such distances, there is a large velocity dispersion and this estimate may be unreliable.

Searching for galaxies like this one in the Zone of Avoidance are important to astronomers because the mass of such undiscovered galaxies may help to resolve the unexpected “discrepancy between the cosmic microwave background dipole and what is expected from gravitational acceleration imparted on the Local Group by matter in the local universe.”

The Hercules Satellite – A Galactic Transitional Fossil

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On Friday, I wrote about the population of the thick disk and how surveys are revealing that this portion of our galaxy is largely made of stars stolen from cannibalized dwarf galaxies. This fits in well with many other pieces of evidence to build up the general picture of galactic formation that suggests galaxies form through the combination of many small additions as opposed to a single, gigantic collapse. While many streams of what is, presumably, tidally shredded galaxies span the outskirts of the Milky Way, and other objects exist that are still fully formed galaxies, few objects have yet been identified as a satellite that is undergoing the process of tidal disruption.

A new study, to be published in the October issue of the Astrophysical Journal suggests that the Hercules satellite galaxy may be one of the first of this intermediary forms discovered.

In the past decade, numerous minor stellar systems have been discovered in the halo of our Milky Way galaxy. The properties of these systems have suggested to astronomers that they are faint galaxies in their own right. Although many have elongated and elliptical shapes (averaging an ellipticity of 0.47; 0.15 higher than that of brighter dwarf galaxies that orbit further out), simulations have suggested that even these stretched dwarfs are still able to remain largely cohesive. In general, the galaxy will remain intact until it is stretched to an ellipticity of 0.7.  At this point, a minor galaxy will lose ~90% of its member stars and dissolve into a stellar stream.

In 2008, Munoz et al. reported the first Milky Way satellite that was clearly over this limit. The Ursa Major I satellite was shown to have an ellipticity of 0.8. Munoz suggested that this, as well as the Hercules and Ursa Major II dwarfs were undergoing tidal break up.

The new paper, by Nicolas Martin and Shoko Jin, further analyzes this proposition for the Hercules satellite by going further and examining the orbital characteristics to ensure that their passage would continue to distort the galaxy sufficiently. The system already contains an ellipticity of 0.68, which puts it just under the theoretical limit.

The team looked to see just how closely the satellite would pass to our own galactic center. The closer it passed, the more disruption it would feel. By projecting the orbit, they estimated the galaxy would come within ~6 kiloparsecs of the galactic center which is about 40% of the radius of the galaxy overall. While this may not seem especially close Martin and Jin report that they cannot conclude that it will be insufficient. They state that disruption would be dependent on “the properties of the stellar system at that time of its journey in the Milky Way potential and, as such, out of reach to the current observer.”

However, there were some telling signs that the dwarf may already be shedding stars. Along the major axis of the galaxy, deep imaging has revealed a smaller number of stars that does not appear to be bound to the galaxy itself. Photometry of these stars has shown that their distribution on a color-magnitude diagram is strikingly similar to that of the Hercules galaxy itself.

At this point, we cannot fully determine if the Hercules galaxy is doomed to become another stellar stream around the Milky Way, but if it is not truly in the process of breaking up, it seems to be on the very edge.

Milky Way Has a “Squashed Beachball”-Shaped Dark Matter Halo

This illustration shows the visible Milky Way galaxy surrounded by a "squashed beachball"-shaped dark matter halo. Source: UCLA

This illustration shows the visible Milky Way galaxy surrounded by a “squashed beachball”-shaped dark matter halo. Source: UCLA

Our galaxy is shaped like a flat spiral right? Not if you’re talking about dark matter. Astronomers announced today that the Milky Way’s dark matter halo, which represents about 70% of the galaxy’s mass, is actually shaped like a squashed beachball.

Dark matter is completely invisible, but it still obeys the law of gravity, so the existence of dark matter haloes, and their shape, can be inferred by monitoring the orbits of dwarf galaxies orbiting the much larger Milky Way.

Unfortunately, to determine the orbit of an object, you have to measure its position at several points in that orbit, and dwarf galaxies take about a billion years to go around the Milky Way. Astronomers just haven’t been around long enough to watch even a fraction of a complete orbit. Luckily, they don’t have to.

Dwarf galaxies, just like their full-sized counterparts, and made of billions of stars. When the tidal forces from a big galaxy like the Milky Way act on a dwarf galaxy, the result is a streamer of stars that trace out the dwarf galaxy’s orbit. By using data from huge all-sky surveys, a group of astronomers led by David Law at UCLA were able to reconstruct the orbit of the Sagittarius Dwarf Galaxy. There was just one problem: different parts of the dwarf galaxy had different orbits, which led to wildly different dark matter halo shapes.

Law and his colleagues Steven Majewski (University of Virginia) and Kathryn Johnston (Columbia University) solved this problem by allowing models of the dark matter halo to be “triaxial” – in other words, have different lengths in all three dimensions. The best model solution results in a halo shaped like a beach ball that has been squashed sideways.

“We expected some amount of flattening based on the predictions of the best dark-matter theories,” said Law, “but the extent, and particularly the orientation, of the flattening was quite unexpected. We’re pretty excited about this, because it begs the question of how our galaxy formed in its present orientation.”

Sagittarius is not the only dwarf galaxy orbiting the Milky Way, and Law and his colleagues plan to study the orbits of other dwarf galaxies to refine their model. “It will be important to see if these results hold up as precise orbits are measured for more of these galaxies. In the meantime, such a squashed dark-matter halo is one of the best explanations for the observed data.”

This illustration shows the visible Milky Way galaxy (blue spiral) and the streams of stars represent the tidally shredded Sagittarius dwarf galaxy. Click the image for a flyaround view. Source: UCLA

This illustration shows the visible Milky Way galaxy (blue spiral) and the streams of stars represent the tidally shredded Sagittarius dwarf galaxy. Source: UCLA