Megan's Blog

In the news this month: a roundup of stories from the 217th AAS meeting

In the news this month we roundup of some highlights from the 217th meeting of the American Astronomical Society held in Seattle during January. The annual meetings of the American Astronomical Society are the largest gatherings of astronomers on the planet, and the presentations cover topics across the whole field of astronomy and astrophysics, including observational results, theoretical studies and simulations. Here are some of the highlights from this year's meeting.

Starting big, astronomers working on the Sloan Digital Sky Survey released the largest colour map of the sky ever made. It's freely available, but be warned - it's big! Covering a third of the sky and created from millions of 2.8 megapixel images obtained by a dedicated 2.5-m telescope over the last decade, the full image is more than a terapixel in size - that's more than one trillion pixels. But it's not just a pretty picture. The full data release, the eighth from the SDSS project, contains a catalogue of objects as well as spectra allowing astronomers anywhere in the world to use the data as the basis for a diverse range of investigations into questions of galaxy evolution, dark matter and dark energy, the distribution and motion of stars in our own Galaxy, and much, much more.
Press release
SDSS-III collaboration (2011). The Eighth Data Release of the Sloan Digital Sky Survey: First Data from SDSS-III Astrophysical Journal Supplements arXiv: 1101.1559v1

One use for sky surveys like the SDSS is searching for distant galaxies which can tell us about star and galaxy formation in the early universe. Because they are so far away, these first galaxies appear very faint by the time their light reaches us here on Earth. But there is a way around this. Gravitational lensing is the effect whereby the matter in a foreground galaxy can bend the light of a background object, making it appear distorted and magnified. This can be a helpful effect, allowing astronomers to see objects more distant than would otherwise be possible, but in surveys where the aim is to discover the size and brightness distributions of early galaxies, this effect can confuse the results. At the AAS meeting, a team of astronomers led by Stuart Wyithe at the University of Melbourne have estimated that as many as 20 per cent of the most distant galaxies currently detected appear brighter than they actually are, because of this lensing effect. With deeper surveys planned in order to probe the early universe, this lensing effect means that the best place to look for these primitive galaxies is probably near larger foreground galaxies, but understanding the lensing effects will be crucial to determining accurate statistics.
Press release
Wyithe JS, Yan H, Windhorst RA, & Mao S (2011). A distortion of very-high-redshift galaxy number counts by gravitational lensing. Nature, 469 (7329), 181-4 PMID: 21228870

Closer to home, spiral galaxies like the Milky Way often have numerous satellite galaxies orbiting around them. Over time, these galaxies slowly spiral inwards and are eventually disrupted, becoming streams of stars that are often only detectable in large surveys. Others are just too dim to see. But Sukanya Chakrabarti, a researcher at the University of California, has developed a new method of detecting such galactic companions. These dwarf galaxies may be too small and dim to be seen directly, but their mass affects the surrounding regions of their parent galaxies, causing ripples in the clouds of hydrogen within the spiral arms. Chakrabarti's method uses these ripples to infer the mass and location of otherwise invisible dwarf galaxies and has already been used to infer the existence of an undiscovered dwarf on the opposite side of the Milky Way to the Earth. The technique has also been tested on spiral galaxies in the nearby universe where high resolution radio observations can map the hydrogen gas in detail, correctly predicting the location of the companion to the Whirlpool galaxy, M51.
Press release
Sukanya Chakrabarti, Frank Bigiel, Philip Chang, & Leo Blitz (2011). Finding Dark Galaxies From Their Tidal Imprints Astrophysical Journal arXiv: 1101.0815v1

Many galaxies are spirals, like our own Milky Way, containing large reservoirs of gas from which stars are currently being formed, while other so-called early-type galaxies are largely devoid of gas and no longer producing new stars. One of the current problems with our understanding of galaxy evolution is just how galaxies move from the spiral star-forming phase to the gas-poor "red and dead" phase of ellipticals. In a poster presented at the AAS meeting, a team have discovered that one particular elliptical galaxy is rapidly shedding molecular gas from its core. The galaxy, NGC1266, located in the constellation of Eridanus, is pumping out some 13 solar masses worth of molecular gas each year at speeds of up to 400 kilometres per second. Such a strong outflow could completely strip the galaxy of molecular gas required to form stars in just 100 million years, about 1 per cent of the age of the Milky Way. Many galactic outflows are driven by powerful starformation activity, but in this case there is little starformation occuring and the more likely culprit is a central black hole.
Press release

The question of which came first, galaxies, or the supermassive black holes at their cores, is an ongoing debate in astrophysics. There is a direct relation between the mass of a spiral galaxy's central bulge of stars and that of its supermassive black hole, suggesting that black holes and bulges affected each others' growth. Previous studies have found galaxies in the early universe where the black holes were more massive than this relationship would suggest, implying that black holes came first. Now, astronomers have discovered a dwarf galaxy with a central supermassive black hole but no central bulge of stars, which they say strengthens the case that black holes did come first. This dwarf galaxy has an irregular shape, and strong radio and X-ray emission characteristic of outflows from the region around a black hole, and is likely to be similar to the first galaxies which formed in the early universe.
Press release

Black holes are not all supermassive. GRS1915+105 is a binary system in the Milky Way with a black hole just 14 times the mass of the Sun, feeding on material from a companion star. As material from the companion spirals towards the black hole, it forms an X-ray emitting disk with material at its inner edge travelling at speeds of up to 50 per cent of the speed of light. Observations of the system at certain times show short pulses of X-rays being emitted every 50 seconds. Now, using a combination of observations from the Chandra X-ray Observatory and the Rossi X-ray Timing Explorer, a team think they know what's going on. In this phase, the inner region of the disk emits enough radiation to push material away from the black hole. Eventually the disk gets so bright and so hot that it disintegrates and falls towards the black hole, before the cycle begins again. Between pulses, the inner part of the disk refills from material further away from the black hole, while the radiation emitted heats up the outer disk and drives material away from the system, eventually limiting the amount of matter which the black hole can consume, and pushing the system into one of its other known states.
Press release

More well-known periodic objects are pulsars, compact remnants left over when stars larger than eight times the mass of the Sun end their lives as supernovae. One of the brightest and well-observed pulsars lies in the heart of the Crab nebula in Taurus, a pulsar which has long been thought of as one of the steadiest high energy sources in the sky. So steady, in fact, that X-ray telescopes use it as a calibrator, and the brightness of other sources are often quoted in units of "millicrabs". But now, observations made with several high energy instruments have revealed that the Crab pulsar is far less steady than has been assumed. Observations with the Gamma-ray Burst Monitor on the Fermi satellite suggested that the Crab pulsar was dimming, but to prove it was a real effect rather than an instrumental problem affecting the observations, the team made further observations with several other high energy instruments, confirming that the pulsar has dimmed by seven per cent over two years. The result has implications for the in-flight calibration of X-ray instruments, as well as possible effects on previous results where the Crab pulsar was used to calibrate the observations.
Press release

It's not just pulsars which vary. A class of stars known as Cepheid variables have a direct relationship between their maximum brightness and their period of variability. If you can measure the period, then you can calculate how bright the star would be at a given distance. By comparing this to how bright the star actually appears, you can calculate how far away it is. This relationship has long been used as a rung on the so-called cosmic distance ladder, allowing the distances of objects throughout the universe to be determined. Since Cepheids are the first rung on this ladder, and each rung on the ladder relies on the accuracy of the previous one, it is vital to much of cosmology that the calibration of Cepheid variability is accurate. But in a study carried out with the Spitzer space telescope, astronomers have discovered that the first star in the class, delta Cephei, is losing mass in a stellar wind at a rate which alters its mass and creates a surrounding nebula which affects the stars' apparent brightness. Further observations showed that as many as 25 per cent of Cepheids are also losing mass at a significant rate, with implications for distance measurements that underpin much of modern cosmology.
Press release

Even the smallest of stars turn out to be not so constant. A study of more than 215,000 red dwarf stars has found that even these old stars produce flares strong enough to disrupt the atmosphere of any orbiting planets. Originally observed in a survey to search for dimming due to transiting planets, the data were later searched for evidence of flaring and produced several interesting results. The average flare duration was 15 minutes, and some flares increased the brightness of the star by 10 per cent, making them brighter than flares on our own, much larger, Sun. The astronomers also found that variable red dwarfs were about one thousand times more likely to flare than non-variable red dwarfs, possibly due to their strong magnetic fields.
Press release

Phew!

---

This blog post is a news story from the Jodcast, aired in the February 2011 edition.