The Hot, Irradiated Center of our Milky Way

Title: The thermal state of molecular clouds in the Galactic Center: evidence for non-photon driven heating
Authors: Y. Ao, C. Henkel, K.M. Menten, M.A. Requena-Torres, T. Stanke, R. Mauersberger, S. Aalto, S. Mühle, and J. Mangum
First Author’s Institution: Max-Planck-Institut für Radioastronomie, Bonn, Germany

The center of the Milky Way In this image of the Galactic center, purple is gas being ionized by many massive stars and star clusters, as well as energetic synchrotron emission near the black hole. Green is hot dust and stars, and orange is cooler dust in the giant clouds of gas discussed here. Photo credit: Adam Ginsburg, NRAO. 


Unlike its candy bar namesake, the center of our Milky Way Galaxy is not actually a very pleasant place to be. In this region, clouds of gas about a million times as massive as our Sun can be stretched like bubblegum by the strongly changing force of gravity as they approach the central black hole. These gas clouds experience radiation from thousands of young, massive stars, and millions of older stars, since our galaxy is a lot more crowded at its center. Lots of stars also means lots of star DEATHS, so these gas clouds also feel the effects of shocks and cosmic-ray radiation from recent supernova explosions.

Perhaps because of the barrage of constant harassment from this environment, gas clouds in the center of the Milky Way are believed to be much hotter than elsewhere in the Galaxy. Studies have shown (e.g., Morris et al, 1983; Hüttemeister et al. 1993) that gas temperatures in these clouds can be as hot as 200 Kelvin, or twenty times hotter than is normal for gas clouds in our neighborhood of the Galaxy. The authors of today’s paper seek to verify these findings by using a new thermometer (a molecule called formaldehyde) to measure the temperature of Galactic center gas clouds.


Hot, Hot, Hot– or Not, Not, Not?

So, how do astronomers measure how hot a gas cloud is?

The figure below illustrates a typical heating cycle for a molecule in a gas cloud. Molecules collide mostly with molecular hydrogen, or H2, the most abundant molecule in space. By colliding frequently, the molecules can reach thermal equilibrium– so that all the molecules have the same temperature, and rate at which they heat up is balanced with the rate at which they cool down. A molecule which is cooling down will emit a photon whose wavelength corresponds closely to its temperature. The average photon temperature from the emission of millions of these molecules tells us what the gas temperature is.

Using molecules as a thermometer

How do we know the temperatures of gas in space? We can measure the gas temperatures from spectral lines of molecules like carbon monoxide, ammonia, and in this paper, formaldehyde. Nearby gas clouds tend to be very cold, around 10 degrees above absolute zero. Galactic center gas clouds are much hotter.

In this paper, the authors use observations of three spectral lines from formaldehyde (H2CO), observed with the APEX millimeter telescope in Chile, to measure the temperatures of clouds in the central 100 parsecs of the galaxy. The authors also map the emission from each line over this entire area, allowing them for the first time to study the temperature structure of these clouds, rather than just deriving a single temperature for each cloud.


Widespread hot gas– Can’t blame it on the photons

The result of this paper is that gas in Galactic center gas clouds really is HOT. This study of 22 different positions in gas clouds in the central 100 parsecs finds gas temperatures from 50 K to over 100 K, with a typical temperature of 85 K. Even outside of the central parts of these clouds, the average gas temperature is 65 K.  The authors also note that the hottest gas is located in clouds close to the very center of the Galaxy.

Figure 3: formaldehyde maps

By comparing the ratio of these three observed lines of formaldehyde, the authors determine gas temperatures at the 22 positions marked on the maps. The location of the central black hole is marked with a white X. Formaldehyde must be quite hot to emit a photon at the energy of the transition shown on the far right. So, the brightest (green) regions in righthand map, at positions 3, 8, and 15, are where some of the hottest gas is found (Ao et al. 2013).

Because formaldehyde traces dense gas in the centers of clouds, this gas cannot be heated by far-ultraviolet photons from massive stars, which do not penetrate deeply into cloud interiors. Additionally, although X-ray photons  are capable of penetrating more deeply into the clouds, there are not enough of them in the Galactic center to explain these high temperatures. The heating of these gas clouds is then not due to photons from massive stars in this region. However, it could still be due to irradiation from cosmic rays from these stars, or due to turbulent shocks, both of which are likely also common in the environment of intense star formation, or a starburst. It is not possible to distinguish between these two heating sources with the current observations of the Galactic center, but the authors suggest that higher resolution observations of cloud structure (to determine whether all of the hot gas is turbulent), or observations of the ionization fraction of the gas (which would be higher in the presence of cosmic rays), could in the future tell us which is more likely.

So, even if the center of our galaxy doesn’t actually make such a good candy bar prototype (craving some hot gas with a density only one-quadrillionth that of air, anyone?), it may be a really good example to use for understanding the processes that heat gas in even more extreme galaxies.

About Betsy Mills

I am a 22nd-grader at UCLA, working with Mark Morris and spending the year at the MPIA in Heidelberg finishing my thesis. I like molecules in space, radio telescopes, the extreme center of our galaxy, getting to look at things no-one else has ever seen before, solving puzzles, and finding creative ways to survive graduate school

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  1. This was a ton of fun to read and very well written. Nice job on your second Astrobite! Also, that “Thermometers in Space” diagram is adorable :3.

  2. You would be an outstanding teacher or professor! Your explanation of collisional heating and radiative de-excitation is fabulous.

  3. Thank you for sharing. I like Milky Way Galaxy.


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