Galactic Center Plasma

The Milky Way Galaxy’s central regions are a dynamic and fascinating laboratory. Besides the presence of the supermassive black hole, the galactic center also plays host to a disproportionate amount of massive stars, gas at high densities and high temperatures, and the presence of a strong galactic wind. All this and more combines to create a dynamic process that we do not entirely understand yet.

Remarkably, the evidence we have to know and understand the galactic center comes from observations taken at wavelengths outside of the visible spectrum. Our Milky Way is a flat spiral disk galaxy, and our sun and solar system are very nearly embedded right in the middle of the plane of the disk of the Milky Way. We’re about 8 kpc (or about 26000 light years) away from the center of the Milky Way, yet we are positioned just 30 pc (or about 100 ly) above the plane of the Milky Way’s disk. This means that when we look towards the Milky Way’s center, we have to observe through the entire intervening Milky Way plane located between us and the center, and its containing gas and dust.

This gas and dust along the way tends to cause extinction for shorter wavelengths, and allows longer wavelengths to pass through1. This means, for galactic center observations, wavelengths beyond 1 µm (which is in the infrared portion of the spectrum) can be used. The dust and gas along the way also becomes transparent at more energetic portions of the electromagnetic spectrum, for radiation with energy greater than a few keV. Because of this extinction and scattering of light along the way from the galactic center to the Earth, our observations of the region are only limited to observations beyond infrared wavelengths or beyond X-ray energy levels. Still, even with this limited portion of the spectrum to work with, we can still get a very good picture of the exciting dynamics and properties of the galactic center.

An interesting and mysterious phenomena is revealed by observations in X-ray. The spectra collected by X-ray photons displays He-like and H-like lines that are estimated to originate in a plasma at two different energies (i.e. two different temperatures)2: 0.8 keV (“soft” plasma) and 8 keV (“hard” plasma).

Plasma! Hot plasma!

So, before diving into what makes these two types of plasma found in the galactic center interesting, it’s useful for an overview of plasmas. Plasma is often described as the fourth state of matter. Plasmas are like gasses, but with one major exception: the atoms are separated into charged particles, with electrons ionized from their atoms, creating a collection of negatively charged electrons and positively charged ions. This essential difference results in the main difference between a plasma and a gas. Plasmas act differently than gasses because the charged particles in a plasma cause additional phenomena beyond that exhibited by uncharged particles in a gas and described by statistical mechanics. The galactic center contains a large amount of hot plasma, and as revealed by X-ray observations, this plasma is found in two different temperatures. Now, an obvious question is how and where the hot plasma in the galactic center originated.

Supernovae and their remnants are excellent sources for energy that help generate heat and plasma on their shock fronts. These supernovae can help explain the presence of the soft plasma. The galactic center hosts a huge number of supernova events, with estimates of 0.04 supernovae per century. This rate is 2000 times greater than the average rate of the rest of the galaxy.

For the warmer, 8 keV, component of the plasma, there’s isn’t yet a satisfactory explanation for a heating mechanism. Supernova remnants are only known to produce plasma that reach energies approaching 3 keV. Supernovae themselves are hotter, but not long enough to heat the plasma found in the galactic center.

Without a heating mechanism, we start running into a problem3. The temperature of a substance corresponds to the kinetic energy of the particles in that substance. So a substance with a high temperature means that the atoms and molecules in the substance have a high kinetic energy. This can be quantified by the mean, or average, thermal velocity, \(v_\text{th}\):

$$v_\text{th} = \sqrt{\frac{k_b T}{\mu m_p}}$$

Here, \(\mu\) is the mean molecular weight. For a pure hydrogen plasma \(\mu = 0.5\), and at the hard plasma temperatures at the galactic center, we get that the thermal velocity is 1250 km/s. This is a problem when we compare this thermal velocity with the escape velocity required for a particle to escape from the galactic center. Estimates of the escape velocity for the galactic center are typically in the range between 1000–1200 km/s, so this means that we expect plasma at this temperature to escape and to not be confined in the galactic center.

Overall, we need a source that can heat the plasma found at the galactic center at the hard plasma temperatures in less time than it takes for the plasma to escape from the galactic center. Unfortunately, we don’t know of any astrophysical mechanisms that can satisfy both these requirements.

But it may not be completely depressing…

Currently, there are theories proposing workarounds to help explain the presence of hard plasma in the galactic center. Some have proposed a magnetic field that may be helping confine the hard plasma in the galactic center and preventing it from escape. However, detection of magnetic filaments in radio observations of the galactic center seem to suggest that this magnetic field that could sufficiently confine the plasma is not present.

It has also been suggested that the assumption of a nearly completely hydrogen plasma may be incorrect. Above, the thermal velocity we calculated relied on an assumption of \(\mu = 0.5\). But this value is higher if the plasma is no longer predominantly hydrogen and instead consisting of heavier elements like helium, subsequently making the thermal velocity much lower. The thermal velocity for a predominantly helium plasma would be about 750 km/s, much less than the estimated escape velocity. Further, the hard helium plasma could be sufficiently heated by friction in molecular clouds found in the galactic center. This proposition of a helium plasma still requires further observational evidence to support, but it might offer a path to understanding where the hot hard plasma in the galactic center came from and how it is regulated.

Sources and Further Exploration

  • Belmont, R., M. Tagger, M. Muno, et al. “A Hot Helium Plasma in the Galactic Center Region”, 20 September 2005, The Astrophysical Journal, 631:L53–L56.
    • A fantastic outline of the problem of explaining the hard plasma in the galactic center, and the proposal of the helium plasma solution to the problem.
  • Carroll, Bradley W. and Dale A. Ostlie. “An Introduction to Modern Astrophysics”, 2007 (2nd Ed.), Pearson Education, Inc: 398–405, 922–932.
    • Good introduction to extinction and the galactic center.
  • Goto, Miwa, Nick Indriolo, T. R. Geballe, and T. Usuda. “H3+ Spectroscopy and the Ionization Rate of Molecular Hydrogen in the Central Few Parsecs of the Galaxy”, 2013, The Journal of Physical Chemistry A, 117: 9919–9930.
    • A short overview of the importance of ionization (e.g. plasmas) in understanding interstellar molecular clouds, and then a discussion of ionization in the galactic center regions.
  • Morris, Mark, and Eugene Serabyn. “The Galactic Center Environment”, 1996, Annu. Rev. Astron. Astrophys., 34: 645–701.
    • A review article with a huge amount of detail about the constituents of the galactic center.
  • Muno, M. P., F. K. Baganoff, M. W. Bautz, et al. “Diffuse X-ray Emission in a Deep Chandra Image of the Galactic Center”, 20 September 2004, The Astrophysical Journal, 613: 326–342.
    • Observations of the diffuse X-ray emission in the galactic center, and an overview of the problem of explaining the hard plasma.
  • Muno, M. P., J. S. Arabadjis, F. K. Baganoff, et al. “The Spectra and Variability of X-ray Sources in a Deep Chandra Observation of the Galactic Center”, 1 October 2004, The Astrophysical Journal, 613: 1179–1201.
    • Details about the spectra of X-ray observations of the galactic center.
  • Skinner, G. K, A. P. Willmore, C. J. Eyles, et al. “Hard X-ray images of the galactic centre”, 10 December 1987, Nature, 330: 544–547.
    • Some early observations of the galactic center in X-ray, and detections of diffuse X-ray emission.

See also:


  1. This can be explained, in a simple way, by Mie scattering. Essentially, Mie scattering explains that a spherical (hehe…) dust grain will scatter electromagnetic wavelengths that are smaller than the order of size of the dust grain. Here’s a helpful analogy (adapted from one provided by Carroll and Ostlie): if the waves on an ocean are much smaller than an obstructing island, they get blocked. However, if they are much larger in size than the island, they pass by mostly unscathed. Mie scattering’s predictions break down for high energy radiation, though, in and beyond the ultraviolet region. 

  2. The energies (\(kT \approx 0.8 \text{ keV}\) and \(kT \approx 8 \text{ keV}\)) easily convert to temperatures. It is convenient to share temperature information in terms of energy since it can be measured directly from the observed X-ray photons. 

  3. Or rather, as demonstrated, the plasma starts running into a problem. (Yes, I know everyone here comes for the subtle jokes. They’re the best kind.) 

ESA’s Rosetta spacecraft became the first spacecraft to rendezvous with a comet earlier today! And we now have some fabulous and stunning images of Comet 67P/Churyumov–Gerasimenko thanks to the OSIRIS (Onboard Scientific Imaging System) camera onboard.

Image: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

ESA’s Rosetta spacecraft became the first spacecraft to rendezvous with a comet earlier today! And we now have some fabulous and stunning images of Comet 67P/Churyumov–Gerasimenko thanks to the OSIRIS (Onboard Scientific Imaging System) camera onboard.

Image: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Gravitational Waves in the Cosmic Microwave Background

Earlier today, astronomers and physicists working on the BICEP2 collaboration announced results from their three year long experiment. Polarization measurements of the Cosmic Microwave Background radiation have revealed signs of gravitational waves. The experiment also provides some of the strongest evidence yet to support the inflation hypothesis first proposed by Alan Guth.

Unfortunately, I’ve been a little short on time this semester (as evidenced by the lack of activity on the blog over the past few months), so I don’t have too much time to dive into this exciting discovery. But I do at least want to point to some of the best explanations of the newly announced results:

  • Physicist Sean Carroll provides an excellent (and technical) overview of the significance of the results here and here.
  • Astrobites also provides a technical but good overview of the results here.
  • The Guardian has a much more accessible article that highlights the significance of the results here.
  • The actual data and results (and pretty plots!) from the BICEP2 collaboration are available here.

Update: It seems that the results are not nearly as significant as initially proposed due to a possibly incorrect accounting of foreground dust. Here’s a revised publication, and an overview of what happened.

Earlier this year, a second reaction wheel (out of the total four) failed in the Kepler spacecraft. Reaction wheels are like gyroscopes, and provided the stability for the precise pointing of the Kepler spacecraft. Three reaction wheels were required to provide the necessary high amount of precision in pointing to enable much of the spacecraft’s exoplanet detection. Without the third reaction wheel, the spacecraft’s primary mission was thought to be finished.

Since then, the (very creative) engineers at NASA have proposed a plan to use photons from the Sun in place of the third reaction wheel. Photons colliding with the surface of the Kepler spacecraft result in a force being applied to the spacecraft, and the resulting radiation pressure could be used to provide the necessary stability for the spacecraft to continue with further exoplanet detection work. The graphic above provides a simple overview of the procedure (click for full size).

Earlier this year, a second reaction wheel (out of the total four) failed in the Kepler spacecraft. Reaction wheels are like gyroscopes, and provided the stability for the precise pointing of the Kepler spacecraft. Three reaction wheels were required to provide the necessary high amount of precision in pointing to enable much of the spacecraft’s exoplanet detection. Without the third reaction wheel, the spacecraft’s primary mission was thought to be finished.

Since then, the (very creative) engineers at NASA have proposed a plan to use photons from the Sun in place of the third reaction wheel. Photons colliding with the surface of the Kepler spacecraft result in a force being applied to the spacecraft, and the resulting radiation pressure could be used to provide the necessary stability for the spacecraft to continue with further exoplanet detection work. The graphic above provides a simple overview of the procedure (click for full size).

FRBs: Mysterious Pulses in Radio

Fast Radio Bursts (FRBs1) are exactly what their name implies: very quick and bright signals visible in the sky at radio wavelengths. Just by that description alone, though, these signals sound a little boring. But when we start to dig a little bit deeper into what makes up a FRB, we find something much more exciting, possibly even originating from very bright cataclysmic events far away.

Dispersion Measure

An especially useful way to look at FRBs is through the lens of the quantity called “dispersion measure”.

The light from any astrophysical source takes a finite amount of time to reach to the Earth. Simply, that time is due to the finite speed of light. However, there can be an additional delay, accounted by the fact that the space that the light travels through is not quite a perfect vacuum. By modeling the space between stars, the interstellar medium, as a cold and ionized plasma, we discover that a signal’s group velocity travels slower than the speed of light. Therefore, a signal traveling through the interstellar medium cannot travel as fast as the speed of light.

This effect can be described in the following equation detailing the time delay that a signal experiences due to dispersion:

$$\Delta t \simeq 4.15 \times 10^6 \text{ ms} \times (f_1^{-2} - f_2^{-2}) \times \text{DM}$$

The DM stands for dispersion measure, and is given by:

$$\text{DM} = \int_0^d n_e dl$$

The main takeaway from these equations is that the time delay is dependent on frequency. Signals at lower frequencies, and therefore at longer wavelengths, arrive later than signals at higher frequencies. This is why this effect becomes important for radio waves, which have long wavelengths. At shorter wavelengths, like optical or even infrared wavelengths, the delay due to dispersion becomes negligible. The time delay is also dependent on the dispersion measure (DM), which in turn is essentially the sum of electrons along our line of sight to the object emitting the signal. If there are more electrons along the way, the DM will be higher, and we expect a longer time delay in the signal.

If there is a short and bright broadband (visible across a wide range of frequencies) signal, the dispersion gives rise to a very characteristic pattern. We see the larger frequencies, or shorter wavelengths, arrive first at the Earth. The shorter frequencies, which have a longer time delay, arrive later. An example is shown in the plots below for DM = 125 and 250 pc cm-3:

Broadband Signal DMs

If there was no dispersion, our broadband signal would just look like the dotted line: all the frequencies would arrive at the Earth at the same time. But since there are electrons in the way, we get dispersion, and the signals we measure start looking like that curved solid line.

The dispersion measure becomes more powerful when you start thinking of it as an analog to distance. Since objects further away emit signals that travel through more of the interstellar medium, they encounter more electrons along their journey to a telescope on the Earth and therefore have higher values for their DM. For example, since pulsars emit quick broadband pulses in radio, we can use their DMs to get a rough estimate of their distances from the Earth.

The dispersion measure is also different when we look out at different areas of the sky. Since the Milky Way is a flat spiral galaxy, most of the dust in the galaxy is confined in a thin plane. When we look at objects in the plane of the galaxy, we expect their DM to be higher because signals emitted by these objects have to travel through more dust, and therefore more electrons that cause dispersion, to reach us. Conversely, looking at objects outside the plane of the galaxy, we expect the DM to be lower since there is not as much dust in the way. We can see this by mapping the DMs of all the pulsars we know of in terms of their positions relative to the galactic plane (or in other words, their galactic latitude):

Pulsars plotted against DM, B

As expected, we can see a peak at a latitude of 0, where the galactic plane is located. Looking away from the plane, the dispersion measures of the pulsars fall away quickly, since there is less dust in the way. The only exception is two small peaks near -30 and -45 degrees latitude: those correspond to the two Magellanic clouds.

The artist formerly known as Lorimer Burst

So now that we understand DMs, we can now properly dive into FRBs. The first FRB was discovered in 2007, and was called a Lorimer Burst. While searching through old archival data of the Small Magellanic Cloud from the Parkes Radio Telescope in Australia, a group of astronomers led by Duncan Lorimer discovered a single very bright burst in radio lasting less than 5 milliseconds. What made the burst notable was its large power, short duration, and especially its high dispersion measure. The burst was located low below the galactic plane, a few degrees away from the Small Magellanic Cloud, yet it still had a DM of 375 pc cm-3.

What this suggested was that the signal was not originating from our Milky Way galaxy. If the signal had originated from within the galaxy, it would likely have had a smaller DM, matching that of those pulsars near the signal. In fact, models of the dust distribution in the Milky Way indicate that only 25 pc cm-3 would be expected for an object located at that particular position in the sky. Instead, the higher DM means that the signal passed through a larger number of electrons before being detected, suggesting much further distances. In the original discovery paper, the authors estimated a maximum distance of about 1 Gpc, which would place it in another distant galaxy. The large distance also suggests that the power of the event that generated the signal would also have to be very large in order to be detected so far away. Unfortunately, no other events in that area of the sky were recorded at the time in other observations. Another simultaneous observation of an event in x-ray or gamma ray, for example, could provide more clues about what caused the burst and where it’s located.

While this burst was fascinating and mysterious on its own, it was still only one event. Without more similar bursts, we could only say a limited amount about what the objects emitting the burst might be. Five more bursts have since been detected and reported: one in 2011 and four more in 2013. Along the way, these bursts lost their original “Lorimer Burst” name and instead gained the name “Fast Radio Burst”. All of these bursts share similar characteristics to the original Lorimer Burst.

The high values for the DM of these bursts is especially notable. Plotting the DM of these FRBs (in red) on our pulsar DM plot from before highlights how much these bursts don’t fit in with the pulsars in the galaxy:

Pulsars plotted against DM, B, with FRBs

The high dispersion measures of these bursts is very convincing in suggesting that FRBs are not from within our own galaxy. This raises many interesting theoretical possibilities. To generate pulses that are powerful enough to be detected over the large distances predicted would require very energetic mechanisms that we do not yet know of or understand.

Currently, the only things visible at radio wavelengths outside our own galaxy include sources like gamma ray bursts and active galactic nuclei. None of them, however, can generate the fast and bright signals that we see for FRBs. FRBs could be leading us towards discoveries of very exciting astronomical phenomena, something that we cannot yet explain theoretically.

A little closer to home?

But throughout this post so far, I may have been a little misleading… When looking at the FRBs, we have been assuming so far that the large DM has been caused by the interstellar medium. Although this is usually the primary cause of the dispersion observed in astronomical objects, it does not necessarily have to be the only cause or even the largest contributor to the dispersion. Any ionized plasma with electrons could theoretically cause the same effect to happen. We cannot necessarily say that the high dispersion measure on FRBs is due to the interstellar medium.

Just a few weeks ago, a group of astronomers proposed that flaring stars within the Milky Way might in fact be the source of FRBs. Examples of small dwarf stars that produce flares in radio with the necessary brightness and time scale have already been found. In order to explain the high DM we observe for FRBs, the astronomers modeled these flaring stars to include a plasma “blanket” surrounding them. Coherent emission in radio could be generated at the bottom of the coronae of the stars, and once it passes through the plasma blanket, a time delay is added on due to dispersion. So if this model turns out to be correct, what astronomers thought might be an effect of the interstellar medium over billions of parsecs may instead just be due to a thick blanket of plasma surrounding flaring stars2.

Small Sample Size

The end of the story about FRBs is that there really is no end right now. We’re just getting started. FRBs are a very new discovery and we haven’t found many examples of them to make convincing statements about what causes them or even where they’re located. We’re working with a very small sample size of 6, and we cannot say very much until we find more examples and perhaps discover interesting events coincident with a FRB at other wavelengths.

The recent discovery of four FRBs allowed astronomers to estimate a rate at which these events may be occurring: about 10000 per day over the whole sky. That’s a big number and you may wonder why we haven’t been able to find more if they’re that frequent. The answer is that the telescopes big enough to detect these signals can only look at small areas of the sky. We have only started to detect FRBs because of large scale surveys searching for pulsars and other radio transients. As we increase these transient surveys over the coming years, we should find more examples of FRBs, and these examples can help us better understand what is actually behind these bright mysterious pulses.

Sources and Further Exploration

  • Keane, E. F., M. Kramer, A. G. Lyne, et al. “Rotating Radio Transients: new discoveries, timing solutions and musings”, 13 April 2011, Monthly Notices of the Royal Astronomical Society, 415, 3065–3080.
  • Keane, E.F., B. W. Stappers, M. Kramer, and A. G. Lyne. “On the origin of a highly-dispersed coherent radio burst”, 19 June 2011, Monthly Notices of the Royal Astronomical Society (pre-print), arXiv:1206.4135v1.
  • Loeb, Abraham, Yossi Shvartzvald, and Dan Maoz. “Fast radio bursts may originate from nearby flaring stars”, 9 October 2013, Monthly Notices of the Royal Astronomical Society (pre-print), arXiv:1310.2419v1.
  • Lorimer, D. R., M. Bailes, M. A. McLaughlin, et al. “A Bright Millisecond Radio Burst of Extragalactic Origin”, 2 November 2007, Science, 318, 777–780.
  • Lorimer, D. R., A. Karastergiou, M. A. McLaughlin, and S. Johnston. “On the detectability of extragalactic fast radio transients”, 25 July 2013, Monthly Notices of the Royal Astronomical Society (pre-print), arXiv:1307.1200v3.
  • Lorimer, Duncan, and Michael Kramer. “Handbook of Pulsar Astronomy”, 2007, Cambridge University Press.
  • Thornton, D., B. Stappers, M. Bailes, et al. “A Population of Fast Radio Bursts at Cosmological Distances”, 5 July 2013, Science, 341, 53–56.

  1. Trust me, it’s a lot more fun if you pronounce FRBs as “Furbies”. Extra points if you imagine small furry/creepy toys actually being responsible for these signals. 

  2. Another way to think of this: These are sick stars that sometimes sneeze. They’re covered in thick plasma blankets in order to stay warm and get better. (My imagination might be a little too active right now…) 

→ The life and death of Buran, the USSR shuttle built on faulty assumptions

Some fascinating details and background about the Soviet shuttle program.

Launching up to 60 times per year with the capacity to lift nearly 25,000 kg into low-Earth orbit meant that the United States could put a lot of hardware into space each year. It seemed plausible that the Americans were planning to launch experimental laser weapons into orbit—and with the shuttle’s capacity to bring 15 tons back from space, these weapons could be tested in orbit and then be brought back for modification. In the long term, this capability would let the Americans build a functioning orbital battle station.

→ An Ode to Kepler, the Planet Hunter

We know trillions of stars, millions of galaxies, and only this one place with this strange accident of self-replicating chemistry. When that sense of singularity ends, as it very well might, how might humans see the cosmos differently? The real space age will have begun.

Alexis Madrigal writes a beautiful tribute to the Kepler spacecraft.

→ Quantum quest

Interesting look at attempts to find more fundamental origins of quantum mechanics.

The lesson, says Fuchs, isn’t that Spekkens’s model is realistic — it was never meant to be — but that entanglement and all the other strange phenomena of quantum theory are not a completely new form of physics. They could just as easily arise from a theory of knowledge and its limits.

The ISS Expedition 36 crew arrived back on Earth on September 10 aboard a Soyuz capsule. This picture in particular beautifully captures the retrorockets being fired to slow down the capsule before its landing.

Soyuz landings are very photogenic.

The ISS Expedition 36 crew arrived back on Earth on September 10 aboard a Soyuz capsule. This picture in particular beautifully captures the retrorockets being fired to slow down the capsule before its landing.

Soyuz landings are very photogenic.