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


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 In this picture of the Galactic facility, purple is gas being ionized by many kind of enormous stars and star clusters, as well as energetic synchrotron emission near the babsence hole. Environment-friendly is hot dust and stars, and oarray is cooler dust in the giant clouds of gas discussed below. Photograph 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 around a million times as huge as our Sun can be stretched favor bubblegum by the strongly transforming force of gravity as they method the central babsence hole. These gas clouds suffer radiation from thousands of young, massive stars, and also countless older stars, since our galaxy is a lot even more crowded at its facility. Lots of stars additionally means numerous star DEATHS, so these gas clouds also feel the impacts of shocks and also cosmic-ray radiation from current supernova explosions.

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Perhaps bereason of the barrage of consistent harassment from this setting, gas clouds in the facility of the Milky Way are believed to be a lot hotter than somewhere else in the Galaxy. Studies have actually displayed (e.g., Morris et al, 1983; Hüttemeister et al. 1993) that gas temperatures in these clouds can be as warm as 200 Kelvin, or twenty times hotter than is normal for gas clouds in our community of the Galaxy. The authors of today’s paper look for to verify these findings by using a brand-new thermometer (a molecule dubbed formaldehyde) to meacertain the temperature of Galactic center gas clouds.

Hot, Hot, Hot– or Not, Not, Not?

So, just how perform astronomers meacertain exactly how hot a gas cloud is?

The number below illustprices a typical heating cycle for a molecule in a gas cloud. Molecules collide mostly via molecular hydrogen, or H2, the a lot of abundant molecule in space. By colliding typically, the molecules have the right to reach thermal equilibrium– so that all the molecules have actually the very same temperature, and rate at which they heat up is balanced through the rate at which they cool down. A molecule which is cooling down will emit a photon whose wavesize synchronizes closely to its temperature. The average photon temperature from the emission of numerous these molecules tells us what the gas temperature is.


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How carry out we recognize the temperatures of gas in space? We have the right to measure the gas temperatures from spectral lines of molecules prefer carbon monoxide, ammonia, and also in this paper, formaldehyde. Nearby gas clouds tfinish to be exceptionally cold, about 10 degrees above absolute zero. Galactic center gas clouds are a lot hotter.


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

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

The outcome of this paper is that gas in Galactic center gas clouds really is HOT. This study of 22 various positions in gas clouds in the main 100 parsecs finds gas temperatures from 50 K to over 100 K, with a typical temperature of 85 K. Even exterior of the central components of these clouds, the average gas temperature is 65 K. The authors likewise note that the hottest gas is located in clouds close to the extremely center of the Galaxy.


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By comparing the ratio of these 3 observed lines of formaldehyde, the authors determine gas temperatures at the 22 positions noted on the maps. The place of the central babsence hole is marked with a white X. Formaldehyde must be rather warm to emit a photon at the energy of the change presented on the far best. 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).


Due to the fact that formaldehyde traces dense gas in the centers of clouds, this gas cannot be heated by far-ultraviolet pholoads from enormous stars, which perform not penetrate deeply right into cloud interiors. Furthermore, although X-ray photons are qualified of penetrating even more deeply into the clouds, tbelow are not sufficient of them in the Galactic facility to define these high temperatures. The heating of these gas clouds is then not due to pholots from huge stars in this region. However before, it could still be because of irradiation from cosmic rays from these stars, or because of stormy shocks, both of which are most likely additionally common in the environment of intense star development, or a starburst. It is not feasible to distinguish between these two heating resources through the current monitorings of the Galactic center, but the authors imply that higher resolution monitorings of cloud structure (to identify whether all of the hot gas is turbulent), or observations of the ionization fractivity of the gas (which would be higher in the presence of cosmic rays), could later on tell us which is even more most likely.

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So, also if the center of our galaxy doesn’t actually make such a good candy bar protoform (craving some warm gas via a thickness just one-quadrillionth that of air, anyone?), it might be a really good instance to usage for understanding the procedures that heat gas in also more extreme galaxies.