Merry Christmas!

Many years ago on this day, an amazing child was born. This child forever changed the way we understood the world, and his ideas opened up an unprecedented era of enlightenment and discovery.

Happy birthday, Isaac Newton! 

* Thank you, Sam Collopy, for pointing this out. *

 

 

Merging Black Holes Pt. 2: Ripples in Spacetime

The above Youtube video shows a merging event generating gravitational waves. Now, this video shows two merging white dwarfs, but two merging black holes will do the same thing - only on a much larger scale. It seems crazy to think that this sort of event happens in the universe, but is currently invisible to us. However, in the coming years, that should change.

Gravitational waves are a hot topic in modern astrophysics. They are analogous to most waves, such as water waves or electromagnetic waves (light), except that they are ripples on the background curvature of spacetime itself.

So, what's the big deal with gravitational waves, aside from the fact that they're a cool physics buzzword?

Well, the most exciting prospect of gravitational waves is their potential in the study of different astrophysical phenomena. For most of astronomy, electromagnetic radiation has been the sole source of probing the universe (or, photons for short... it's amazing what we can learn from light rays). Electromagnetic radiation comes from the superposition of (mostly) randomly oriented photons being emitted or absorbed from electrons, atoms, or molecules. Gravitational waves, on the other hand, are generated most strongly from large bulk motions of mass. And, even more incredibly, gravitational waves are not dampened by intervening matter. As a result, the detection of gravitational waves should open an entirely new area of astrophysics previously unseen.  Perhaps, like the advent of infrared and radio astronomy, gravitational wave astronomy will usher in a new era of unprecedented scientific discovery and advancement.

Interferometers like the Laser Interferometer Space Antenna (LISA) and the Laser Interferometer Gravitational-Wave Observatory (LIGO) are currently the most promising ways to detect gravitational waves. Simply put, like most interferometers, a laser is shot at a beamsplitter, which splits the laser beam into two directions (usually perpendicular to each other). These beams hit mirrors, and are then reflected back to a detector, where they are recombined. *Note: I have simplified the process quite dramatically - I have never been very good with instrumentation, so I'm not going to try to go into any specifics.*

(Is it bad that I can't ever say laser without thinking of Dr. Evil's "laser")?

Very simple interferometer design, except for gravitational wave detectors, the movable mirror would not be moved by anything other than gravitational waves. The screen has a great example of what fringe patterns look like.

Since gravitational waves are ripples in the fabric of spacetime itself, a gravitational wave passing through a point in space causes the gravitational field to increase or decrease in magnitude. As a result, if a gravitational wave hits one of the mirrors, the oscillation in gravitational strength should shift the mirror back and forth. This causes the path length for one of the mirrors to deviate from the original value, so that when the beams are later recombined at the detector, the observed fringe patterns should change in accordance with the mirror oscillations. These fringe patterns can then be used to deduce the strength of the passing gravitational wave.

Gravitational wave schematic. The top portion shows the three phases of coalescence: the quasi-circular inspiral phase, the plunge and merger, and the ringdown. The bottom portion shows the amplitude of the emitted gravitational waves that correspond to the above merger process. (Image credit: Baumgarte and Shapiro 2011).

Black hole mergers, incidentally, are especially interesting in the study of gravitational waves. This is because gravitational waves are theorized to carry away most of the angular momentum and energy from these systems, which causes the merging black holes to coalesce. The above figure (that I scanned, sorry for the poor quality, ha ha!) shows a schematic of the phases of a black hole merger, and how those phases relate to gravitational wave strength. The top portion shows a cartoon of the merger, including the early quasi-circular inspiral phase, the plunge and merger phase, and the final ringdown stage. The bottom part shows the wave amplitude (or, strength) versus time. For two black holes approaching each other in the early inspiral phase, the emission of gravitational radiation causes the orbits to circularize and decay. Gravitational waves at this point will be at relatively low levels. But during the late inspiral phase, when the binary is in tight, circular orbits, and during the actual merger, gravitational waves should be at their strongest. This is where LIGO and LISA will be able to detect gravitational waves.

Unfortunately, even though the very early and late stages of coalescence has been simulated fairly easily, the actual late inspiral and merging phases are not as simple. For that part of the merger, post-Newtonian and perturbation methods break down. Therefore, providing templates for the signatures of merging black holes requires advancements in numerical relativity, which will push computational techniques to their limits. 

For more info, please see:
Abramovici, A. et. al 1992, Science, Vol. 256, pp. 325-333
Baumgarte, T. W. & Shapiro, S. L. 2011, Physics Today, Vol. 64 No. 10, pp. 32-3
Berti, E., Cardoso, V. & Will, C. M. 2006, Phys. Rev., 73, 6

Merging Black Holes Pt. 1: An Introduction

There are monsters in the night sky. If you were unfortunate enough to pass by one, there would be nothing to stop the inexorable pull toward destruction. But for such behemoths, you wouldn't be able to see them. You could see the effects of their hunger (if they happened to be hungry) in bright, energetic X-rays. But that's all. That's all you could see of one of the mysteries of astrophysics today - supermassive black holes.

However, funny enough, these monsters may see and remember you. According to Stephen Hawking, black holes can dissipate, because the huge gravitational field may occasionally pop out particles (it's all about E=mc², where energy is converted to mass). Over time, as more and more particles are ejected from the black hole, the mass that originally fed the black hole will once again return to the universe, making the black hole no more.

Artist rendition of a black hole. All images stolen shamlessly from the internet.

Massive black holes are everywhere in the universe, yet their formation is still largely unknown. Cosmological models suggest that massive black holes form from the mergers of smaller seed black holes, but can we reproduce this behaviour in simulations? Do these simulations generate surprises about relativity we previously didn't know about? In the next 5 posts, I'll discuss recent simulations of black hole mergers, some of the main techniques used for handling computational relativity, and possible observational signatures of these merging monster systems. Oh, and spoiler alert - there will be a surprise from these simulations, which suggests that there are some things about General Relativity that can continue to surprise us.

Feeding black hole.

Black holes are relatively simple objects compared to most physical systems. They are described by analytic solutions to Einstein's equations of General Relativity, and depend on only three parameters: charge, mass, and spin. For most astrophysical black holes, the description is further simplified because the charge is usually set to zero. This happens because in the accretion disk, the charged, ionized material rearranges itself to neutralize the black hole charge at large scales. More specifically, the gas is treated as having infinite conductivity and is therefore able to support infinite currents. These currents are generated from charge imbalances and move charges in such a way as to cancel charge imbalances, thus causing the whole system to neutralize.

Black holes also span a ridiculously large range of masses, from the predicted tiny holes from string theory to supermassive holes as large as some small galaxies. However, it is still unclear how supermassive black holes are formed. Population III stars are thought to be one of the most likely sources of the first seed black holes. These stars are the theorized first generation of stars, and consequently form from the primordial gas of pure hydrogen. A consequence of this is that the gas is unable to fragment during the formation process, meaning that these stars are extremely massive and live fairly short lives. It is predicted that the stars with masses of 25 and 140 times the mass of our sun formed the first seed black holes.

However, there is a problem with this scenario. This problem arises in the growth of these smaller seed black holes to the supermassive black holes seen today. If black hole growth proceeds purely by accretion, a liberal estimate of a growth timescale can be computed if the black hole accretes at the Eddington rate. If it is assumed that Population III stars form 100 solar mass seed black holes, then it would take roughly 0.8 Gyrs (or 800 million years) of continuous, uninterrupted accretion to form a black hole of 1 billion times the mass of our sun. But, according to observations, quasars at a redshift of z=6 have been observed. This means that massive black holes must have been in place when the Universe was only about 1 Gyr (1 billion years) old. This means that given even the best estimates for black hole growth, black holes would have had to start accretion in the universe's infancy. Therefore, there must be another mechanism that increases the black hole mass rather quickly.

The current cosmological understanding is that large black holes must then grow bottom up, where small seed black holes merge to form successively larger black holes. In this way, the formation of supermassive black holes can proceed at the quick pace observations seem to imply. But how common are black hole mergers? And if they did merge, do we know if they'll act the way we think they'll act? 

Rendition of a quasar.

In addition to the growth rate of black hole formation, supermassive black hole formation scenarios must also provide explanations for their role in galactic evolution. It is undeniable that large black hole formation is a common occurrence, since they reside in the vast majority of galaxies (in particular bulge galaxies). They are also known to be the source of quasars and active galactic nuclei. To date, though, the interaction between black hole formation and their environments are only just being explored.

But, even though this field is in it's infancy, it seems like a promising avenue to study how the universe works in conditions wholly alien to us - to push the boundaries of the physics we know into the physics of the unknown. 

* Note * This is just a compilation of knowledge I have read from people who actually do this kind of research. I haven't included references in the proper style for fear that someone may use this as a paper. But, I don't feel right about not acknowledging the people who provided us with this knowledge. If you are interested in this subject, please please please look at these sources!

Baumgarte, T. W. & Shapiro, S. L. 2011, Physics Today, Vol. 64 No. 10, pp. 32-37

Carroll, B. W. & Ostlie, D. A. 2007, An Introduction to Modern Astrophysics: 2nd ed., Pearson Education Inc.

Fan, X. et. al 2001, AJ, 121, 54-65

Madau, P. & Rees, M. J. 2001, ApJ, 551, L27-L30

Madau, P. & Quataert, E. 2004, ApJ, 606, L17-L20

Volonteri, M. 2010, Astron. Astrophys. Rev., 18, 279-315
This is a really good review paper on black holes, by the way!

Grad School Lesson #1

Well, I feel like a jerk. I wrote a post about how much schoolwork we grad students get, but then we get a week off of homework. Dang.

So, now I've decided to keep a running, improvised set of lessons from grad school. Perhaps these tidbits will be useful to you. Or... perhaps not. But, for the sake of knowledge, my first lesson is:

Lunchables are made for children, and do not make a good meal substitute.

I was hungry the entire day, and the sugar from the dang thing put me in a very silly mood. Case in point: this picture made me laugh for way longer than it should have....

Hmmm... I really should have wolfed down some caffeine, just to see the awesomeness that would have ensued. But, alas, I didn't, and I spent the rest of the day giggling in my office. I wonder what my office mates thought about that...

Also, do not touch other people's erasers.

Don't even do this by accident. Some grad students are very territorial about this.  Even if it's not their erasers.

Until next time, happy computing. I'll be kicking my computer in anticipation of the next post. :)

Advisor Meeting Analog

I'm pretty sure this is how most advisor meetings go.Thanks to Ryan Shea for finding this. Hopefully he won't find out that I shamelessly stole this from him on facebook.

The World Is Beautiful

This past week was a bit stressful. I felt like I was running around doing things, but not really accomplishing anything. Things happened that were out of my control. 

Sometimes it's hard to let these things go.

Sometimes it's hard to keep going through grad school, because I realize how little I know. Sometimes I get caught up with trying to be the best, when the "best" is some ficticious concept we all made up.

I remember when, as an adolescent, I picked up a telescope and stared at the wonders of the sky. I felt pretty good about myself. Then I went through high school and college and got into a great grad school. University of Arizona is the $#&%. Everybody is intelligent. Everybody is passionate. Compared to these people, I felt at best average. Suddenly, being good wasn't... well... good enough. I wanted to be best. It wasn't enough to pick up a telescope anymore and have fun.

Ah...  the shackles I imprison myself in!

So, this morning (while procrastinating on starting on some homework), I was surfing through some youtube videos. I stumbled on one made by the amazing photographer Terje Sorgjerd. I don't know if everyone has already seen this video, but I think it's breathtakingly beautiful. It reminds me that the world is beautiful and that it's more than made-up definitions of success. But enough of me, I'll let you decide for yourself (only, I highly recommend you go full screen on this one).

AGN Activity Increased by Galaxy Mergers?

Sometimes I forget the amazing things we take as given. Some months ago, a friend of mine asked if scientists had ever figured out if black holes exist. I replied by saying:

"Well, DUH! Of course they exist! They exist at the centers of every known galaxy. That's something everybody knows, right?".

If you really think about it, though, black holes, and their existence, are astounding. They are so astounding, in fact, that the man who predicted their existence was convinced they were too weird to exist in nature. That man was Albert Einstein.

Today, it's pretty conclusive that black holes are ubiquitous throughout the universe. They range in size from about the mass of our sun to gargantuan masses of billions of times as large (although there seems to be a curious deficiency in intermediate mass black holes - which would connect the smaller, stellar mass ones with the supermassive ones at galactic centers).

Establishing that black holes do exist, and are quite common, I'm going to go further and say that astronomers have seen two main types - those that are actively accreting gas, and those that lie dormant. Our Milky Way's black hole is a dormant type, and the ones currently feeding are called AGNs (Active Galactic Nuclei).The pretty picture below (stolen shamelessly from somewhere on the internet), shows a representation of an AGN.

Though there are different types of AGNs, they all are thought to stem from one physical model (more or less, there are still fights about specifics). In short, the rotating supermassive black hole is surrounded by a heated accretion disk. The continuum radiation from the infall of gas into the black hole photoionizes the surrounding gas, which creates emission lines we can see on Earth. In addition, a large disk of clumpy, optically thick clouds (or, clouds too thick to see through in some wavelengths, mainly optical in this case) surround the accretion disk, which can obscure some AGNs if the AGN is oriented in such a way that the clouds are between us and the black hole. Some AGNs also host twin collimated jets. These outflows emerge in opposite directions from the central black hole, and travel at velocities close to the speed of light. Heated bubbles from these jets can reach massive sizes, and how these jets impact galactic evolution (for it seems very likely they must have some effect), is not well known.

This all brings me to a paper I presented at Science Coffee last Thursday: The Impact of Galaxy Interactions on AGN Activity in zCOSMOS by Silverman et al. This study looked at pairs of interacting galaxies using Chandra X-ray data and the spectroscopic redshifts from zCOSMOS. The main point of this study was to see if galaxy mergers could increase the chances of AGN activity. The main thought is that if a gas-rich galaxy began the process of merging, the gas transferred from that galaxy could provide the fuel to feed the supermassive black hole.

Previous studies have been inconclusive about the effect of galaxy mergers on the probability of AGN activity. These studies (well, the ones mentioned) used Hubble Space Telescope (HST) observations to detect AGN activity, and they tended to conclude that they saw no significant increase in AGNs with merging galaxies. However, Hubble observes in optical wavelengths. Because of this, the thick clouds around the accretion disk could hide AGN activity, depending on the orientation of the galaxy. In an effort to get a more complete sample of AGNs, this new study uses X-rays from Chandra to detect AGNs, which are able to pass through the clouds.

To determine if galaxies in their survey sample were interacting, the authors looked at interacting pairs of galaxies between 0.25<z<1.05, distances of less than 75 kpc apart, and velocity differences of less than 500 km/s. This, of course, does not include galaxies that have already merged. The figure below shows some of their interacting pairs. The left panels show Chandra X-ray data (AGN detections), and the right panels show HST images. The red crosses denote AGNs, and the yellow crosses denote centers of normal galaxies. (Sorry this is so small. This is the largest size I can get on the blog).

They found that AGN activity was more common for interacting systems by about a factor of 2 over AGN activity in galaxies not interacting. In addition, for just the interacting pairs, they found a higher fraction of AGNs for lower physical separations and lower velocity differences. This seems to support the idea that more closely interacting galaxies were more likely to host an AGN. They even found a slight increase in AGN probability for interacting galaxies that were more comparable in size (though their sample was not statistically large enough to make that a very strong conclusion).

I think this study does make a point that galaxy mergers may have an effect on the probability of a galaxy hosting an AGN. How big is that effect? I don't know. Like the authors, I do agree that galaxy mergers can't be a dominant factor in deciding if a galaxy is active or not (about 80% of the AGN sample were in galaxies not interacting). However, they do seem more likely to spur AGN activity, and because of that, shouldn't be discounted entirely.