Previously available here, below is a description of the January 21st episode of this radio show, where I try to describe the current thinking (and many questions) on how galaxies formed:
- Galaxy Formation: Pretty much every galaxy is believed to have a super-massive black hole (SMBH) at its center, but is unclear if the galaxy formed before the SMBH or vice versa. Recent radio observations of distant galaxies suggest that the SMBH formed first, and then the galaxy around it (link), though recent optical observations of other distant galaxies suggest the opposite. As matter falls into the SMBH, it heats up and radiates a lot of energy, which is absorbed by the surrounding gas, causing it to heat up and possible decreasing the amount of future gas will fall into the SMBH. This process was originally believed to be very sporadic and violent, but recent evidence suggests this process may be gentler in some galaxies (link). Most galaxies are either spiral galaxies currently forming stars (like the Milky Way, referred to as "blue spirals" because young, massive stars cause the galaxy to appear bluish) or elliptical galaxies consisting almost entirely of old stars (referred to as "red ellipticals" or "red, dead galaxies" which old, low mass stars give the galaxy a red color). A population of red spirals do exist, and are either cases where the spiral galaxy ran out of gas and dust to form new stars, or have so much gas and dust that the blue light produced by young, massive stars is absorbed and the galaxy appears red (link). Galaxies where a lot of material appears to be falling into (a process astronomer call "accretion") the SMBH are often referred to as AGN, or "Active Galactic Nuclei." Discoveries of AGN with the Swift telescope have found differences in the types of galaxies that host nearby and distant AGN (link). It is currently believed that the massive (Milky Way-sized) galaxies observed today or the result of smaller, proto-galaxies merging together in earlier times. This process predicts that the relationship between different current properties of a galaxy (e.g., its size, mass, total light output, gas content) is complicated because they are effected differently by the merging process. However, a recent study found that all of these parameters just depend on the mass of the galaxy - which is very puzzling indeed. Even if this is true, there is a wide diversity in the properties of nearby galaxies, and to study this the Hubble Space Telescope has been measuring the properties of individual stars in these galaxies to determine the history of how quickly they formed stars, etc. (link). One feature of spiral galaxies is that some of these appear to have "bars" in the center (like the Milky Way does, actually), while others don't. A recent survey of galaxies done as part of the COSMOS project has found that the fraction of spirals with bars has tripled over the last 7 billion years (link) . It is currently believed that the merger of two galaxies together will lead to a short-lived but intense increase in the rate at which the resultant galaxy forms stars - called a starburst galaxy. If this correct, then astronomers expect that all startburst galaxies should either show morphological evidence for a recent merger, or be in a crowded regions of galaxies where mergers are likely to occur. An exception to this was NGC 1569, but recent Hubble observations allowed a new measurement of the distance to the galaxy, placing it in the midst of 10 other galaxies (link). Starbursts galaxies are a very active area of research since they play an important role in understanding the history of star formation in the universe, and observations of distant galaxies suggests that star formation in the universe peaked ~2 billion or so years after the Big Bang (about 12 billions years ago; link). The rate of star formation in these galaxies is huge, star formation rates as high as 4000 new stars a year (the Milky Way current makes about 10) has been estimated in some galaxies (link).
- News: Op-ed article in the November 24, 2008 New York Times on on-going problems at NASA with astronomy programs and others going over budget. Also in the New York Times, Dr. Aaron Hirsh write a guest column for their "The Wild Side" arguing for "citizen science" - stepping up a widespread data taking and analysis network. Astronomy has some similar networks, e.g. amateur astronomers searching for supernovae and other transient events, and projects like Galaxy Zoo.
- Wednesday Morning Astronomer: In this article, Gregg Easterbrook speculates that the excess of radio emission from deep outer space is the sound of interstellar war (No. Radio waves are actually light, not sound waves), and discusses recent observations of the Sun's motion around the center of the Milky Way that suggests the inner part of the Milky Way has 50% more mass than previously thought. One reader writes in that this was discovered using a simple method - which is very true, but very hard to do precisely.
- Calendar of upcoming Astronomy/science events in the greater Poughkeepsie/New York City area. Galaxy Formation (continued): As I mentioned earlier, the merger of galaxies is believed to be the most important process in producing the galaxies we see today. Not surprisingly, what happens when two galaxies merge is very complicated, and much of our current understanding comes from studying the Antennae Galaxies - the nearest example of a galaxy merger. Recent observations of this pair of interacting galaxies change the distance from 65 million light-years to 45 million light-years, important in measuring the properties of these galaxies (link). A recent survey of interacting galaxies suggest that all galaxies have undergone a "major merger" in the last 6 billion years, and the peak in the merger rate of galaxies corresponds with the peak of the fraction of starburst galaxies in the universe - suggesting a link between the two. Evidence for a recent merger in a very distant galaxy has been uncovered thanks to gravitational lensing - which makes this galaxy appear much brighter than it ordinarily would (link). New observations suggests that SMBH were common inside galaxies 12 billion years ago, based on observations of two colliding galaxies at this time (link). The merging of two galaxies is also believed to drive gas towards the center of the galaxy, where it falls into the SMBH - creating a Seyfert galaxy. If so, the distribution of hydrogen gas inside such galaxies show evidence for being disrupted by such a merger - and they do (link). I said before that the merger/collision of two galaxies can lead to a period of intense star formation. Well, there is evidence that in some cases, this actually can cause star formation to stop (link). Additionally, the energy radiated by gas falling into the SMBH triggered by a galaxy merger/collision can also stop star formation in the outer part of the gas (link). Not just galaxies can merge / collide, this happens to galaxy clusters as well, and one example of a colliding galaxy cluster is surrounding by a diffuse haze of very low frequency radio emission (link). These collisions may also explain the existence of the magnetic field which exists in the void between galaxies inside galaxy clusters. (article)
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