Archive for November 14, 2017

Merging Galaxies in the Early Universe

Posted in The Universe and Stuff with tags , , , , on November 14, 2017 by telescoper

I just saw this little movie circulated by the European Space Agency.

The  source displayed in the video was first identified by European Space Agency’s now-defunct Herschel Space Observatory, and later imaged with much higher resolution using the ground-based Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. It’s a significant discovery because it shows two large galaxies at quite high redshift (z=5.655) undergoing a major merger. According to the standard cosmological model this event occurred about a billion years after the Big Bang. The first galaxies are thought to have formed after a few hundred million years, but these objects are expected to have been be much smaller than present-day galaxies like the Milky Way. Major mergers of the type seen apparently seen here are needed if structures are to grow sufficiently rapidly, through hierarchical clustering, to produce what we see around us now, about 13.7 Gyrs after the Big Bang.

The ESA press release can be found here and for more expert readers the refereed paper (by Riechers et al.) can be found here (if you have a subscription to the Astrophysical Journal or for free on the arXiv here.

The abstract (which contains a lot of technical detail about the infra-red/millimetre/submillimetre observations involved in the study) reads:

We report the detection of ADFS-27, a dusty, starbursting major merger at a redshift of z=5.655, using the Atacama Large Millimeter/submillimeter Array (ALMA). ADFS-27 was selected from Herschel/SPIRE and APEX/LABOCA data as an extremely red “870 micron riser” (i.e., S_250<S_350<S_500<S_870), demonstrating the utility of this technique to identify some of the highest-redshift dusty galaxies. A scan of the 3mm atmospheric window with ALMA yields detections of CO(5-4) and CO(6-5) emission, and a tentative detection of H2O(211-202) emission, which provides an unambiguous redshift measurement. The strength of the CO lines implies a large molecular gas reservoir with a mass of M_gas=2.5×10^11(alpha_CO/0.8)(0.39/r_51) Msun, sufficient to maintain its ~2400 Msun/yr starburst for at least ~100 Myr. The 870 micron dust continuum emission is resolved into two components, 1.8 and 2.1 kpc in diameter, separated by 9.0 kpc, with comparable dust luminosities, suggesting an ongoing major merger. The infrared luminosity of L_IR~=2.4×10^13Lsun implies that this system represents a binary hyper-luminous infrared galaxy, the most distant of its kind presently known. This also implies star formation rate surface densities of Sigma_SFR=730 and 750Msun/yr/kpc2, consistent with a binary “maximum starburst”. The discovery of this rare system is consistent with a significantly higher space density than previously thought for the most luminous dusty starbursts within the first billion years of cosmic time, easing tensions regarding the space densities of z~6 quasars and massive quiescent galaxies at z>~3.

The word `riser’ refers to the fact that the measured flux increases with wavelength from the range of wavelengths measured by Herschel/Spire (250 to 500 microns) and up 870 microns. The follow-up observations with higher spectral resolution are based on identifications of carbon monoxide (CO) and water (H20) in the the spectra, which imply the existence of large quantities of gas capable of fuelling an extended period of star formation.

Clearly a lot was going on in this system, a long time ago and a long way away!



Landing on the Nose

Posted in Biographical with tags , , , on November 14, 2017 by telescoper

The other day I noticed a story about a plane carrying 53 passengers that did an emergency landing at Belfast airport. Here is a picture from that link:

The story caught my attention for a couple of reasons. One is that the airline and type of plane (a Bombardier Dash 8 Q400) were exactly the same as the one I took from Cardiff to Dublin a few weeks ago. The other is that something very similar happened to me many years ago, on a trip to America. That event may even have involved the same type of plane, as it was a twin turbo-prop, but in my memory the one I was on was a bit smaller than the one shown above. I don’t remember the name of the airline either (though it might have been the now-defunct US Airways) but it was taking me to State College (Pennsylvania, USA), from either Pittsburgh or Philadelphia, for a meeting at Penn State University. I’m not sure of the year, either, but it must have been around 1990.. I’m not sure of the year, either, but it must have been around 1990. From all that you can infer that my memory isn’t all that good, but I do remember the details of the emergency landing extremely well!

We were approaching State College when I noticed that the plane started circling around the airport, which was visible below. Circling prior to landing owing to air traffic restrictions is not an unfamiliar experience for anyone coming into land at, e.g., London Heathrow, but State College airport is in a fairly remote location in rural Pennsylvania so a problem with the air traffic seemed unlikely. Eventually the pilot came onto the intercom and explained that there was a problem with the undercarriage under the nose of the aircraft and we would have to make an emergency landing. The circling was an attempt to use up fuel to reduce the risk of fire on landing. The dozen or so people on the plane seemed quite scared as the pilot explained the procedure, including the brace position to be assumed when was making its landing.

As it happened, I was seated in a window seat next to the emergency door on the port side near the front of the aircraft so it would be my job to open it and get out quickly to let everyone else get out. As we came into land I studied the instructions over and over again. I am not a particularly courageous individual, and I think having that to concentrate on is the best explanation for why I actually didn’t feel all that scared. I was too busy concentrating on the task at hand to let anything else into my head.

Soon we were coming into land. I could see fire engines from their lights flashing either side of the runway as we came down. The pilot shouted “BRACE! BRACE! BRACE!” as the plane touched down on the wheels under the wings, and was a sharp deceleration as the braking systems were deployed. When we were moving sufficiently slowly the pilot dropped the nose, the plane dipped forward and there was a scraping sound as the plane veered to port. It tilted again which I (correctly) assumed was because it had left the runway and was on the grass verge.

The shout came `OPEN THE DOORS’ and I followed the instructions to the letter, turning the handle, pulling the door towards me so it detached and then flinging it out of the aircraft. It worked like clockwork. I felt like a hero, but that sense of pride soon vanished. Forgetting that the plane had tipped forward, I misjudged the step onto the emergency chute that had deployed and, instead of proceeding in an orderly fashion, I tripped on the way out and fell flat on my face. Fortunately, it was not far down to the ground from the door and it had been raining so I fell onto wet grass rather than concrete. I picked myself up and followed the instructions of the firemen to get the hell away from the plane. I’m sure they were laughing as I ran past them to safety.

The plane was safely evacuated and nobody was hurt. Nobody else got covered in mud like I did either…

Later on I got to know a guy who worked on safety training for cabin crew at British Airways. He told me that one of the most important things about an emergency situation on an aircraft is to give the passengers something to do to keep them occupied. That is the best way to prevent panic. It certainly worked with me!