Cloud atlas reshapes astronomers' views of stellar birthplaces
A multi-year study of the Whirlpool galaxy (M51) has shaken up astronomers' views of the properties of giant molecular clouds. The new study, which mapped 1,500 such clouds, shows that, instead, they are embedded in a kind of fog of molecular hydrogen much more dense than anyone expected, which permeates the whole of the galactic disc. Pressure exerted by this fog is crucial in determining whether or not new stars will form within the clouds. The study, led by Eva Schinnerer from the Max Planck Institute for Astronomy, made extensive use of the millimeter telescopes of IRAM, the Institut de Radioastronomie Millimétrique.
Most of a galaxy's stars are born within giant molecular clouds – accumulations of hydrogen molecules with total masses between a thousand and several million times that of our Sun. As a region within such a cloud collapses under its own gravity, it contracts until pressure and temperature are high enough for nuclear fusion to set in: a new star is born.
Now, a new study challenges astronomers' traditional views about these stellar birthplaces. Study leader Eva Schinnerer (Max Planck Institute for Astronomy) explains: "Over the past four years, we have created the most complete map yet of giant molecular clouds in another spiral galaxy similar to our own Milky Way, reconstructing the amounts of hydrogen molecules and correlating them with the presence of new or older stars. The picture that is emerging is quite different from what astronomers thought these clouds should be like." The survey, known as PAWS, targeted the Whirlpool galaxy, also known as M51, at a distance of about 23 million light-years in the constellation Canes Venatici ("Hunting dogs").
Annie Hughes, a post-doctoral researcher at MPIA involved in the study, says: "We used to think of giant molecular clouds as solitary objects, drifting within the surrounding interstellar medium of rarified gas in isolated splendor; the main repository of a galaxy's supply of hydrogen molecules. But our study shows that 50% of the hydrogen is outside the clouds, in a diffuse, disk-shaped hydrogen fog permeating the galaxy!"
This "fog" of surrounding gas plays an important role in star formation. So does a structural feature characteristic of spiral galaxies: the spiral arms, which slowly move through the galaxy like ripples on a lake, and are somewhat more densely filled with stars and gas than the rest of the galactic disk. Sharon Meidt, another MPIA post-doctoral researcher involved in the study, says: "These clouds are definitely not isolated. On the contrary, interactions between clouds, fog, and overall galactic structure appear to hold the key to whether or not a cloud will form new stars. When the molecular fog moves relative to the galaxy's spiral arms, the pressure it exerts on any clouds within is reduced, in line with a physical law known as Bernoulli's principle. Clouds feeling this reduced pressure are unlikely to form new stars."
Incidentally, Bernoulli's law is also thought to be responsible for part of the well-known shower-curtain effect: shower curtains blowing inward when one takes a hot shower, another display of reduced pressure.
Jerome Pety of the Institut de Radioastronomie Millimétrique (IRAM), which operates the telescopes used for the new observations, says: "It's good to see our telescopes live up to their full potential. A study that needed such extensive observation time, and required both an interferometer to discern vital details and our 30 m antenna to put those details into a larger context, would not have been possible at any other observatory."
Schinnerer concludes: "So far, the Whirlpool galaxy is one example which we have studied in depth. Next, we need to check that what we have found also applies to other galaxies. For our next steps, we hope to profit from both the extension NOEMA of the compound telescope on the Plateau de Bure and from the newly opened compound telescope ALMA in Chile, which will allow in-depth studies of more distant spiral galaxies."
Eva Schinnerer (study lead)
Max Planck Institute for Astronomy
Phone: (+49|0) 6221 – 528 293
Markus Pössel (Public Information Officer)
Max Planck Institute for Astronomy
Phone: (+49|0) 6221 – 528 261
Max Planck Institute for Astronomy
Phone: (+49|0) 6221 – 528 428
Max Planck Institute for Astronomy
Phone: (+49|0) 6221 – 528 360
PAWS is short for the PdBI Arcsecond Whirlpool Survey; an astronomical survey targeting the Whirlpool Galaxy with a resolution on the arcsecond-level, using the Plateau de Bure Interferometer (PdBI).
The PAWS team consists of Eva Schinnerer (Principal investigator; Max Planck Institute for Astronomy [MPIA]), Dario Colombo (MPIA), Clare Dobbs (University of Exeter), Gaëlle Dumas (IRAM Grenoble), Santiago García-Burillo (Observatorio Astronómico Nacional, Madrid), Annie Hughes (MPIA), Carsten Kramer (IRAM Granada), Adam Leroy (National Radio Astronomy Observatory), Sharon Meidt (MPIA), Jerôme Pety (IRAM Grenoble), Karl Schuster (IRAM Grenoble), and Todd Thompson (Ohio State University).
More information about the project can be found on the PAWS home page, http://www.mpia.de/home/PAWS
The results described here are presented in a series of five articles in the December 10, 2013, Volume 1 issue of the Astrophysical Journal:
Questions and Answers
Which telescopes and instruments were used in this study?
Data for PAWS was taken between August 2009 and June 2010 with the Plateau de Bure Interferometer (PdBI) in the French Alps and the IRAM 30 m telescope in the Spanish Sierra Nevada. Both are operated by the Institut de Radioastronomie Millimétrique, which is headquartered in Grenoble. The PdBI consists of six radio antennas, each 15 meters in diameter, which can be placed up to 760 meters apart and are wired together so as to act as a single, gigantic radio telescope. The size of this virtual telescope is such that it allows astronomers to observe minute details even in a galaxy like M51, which is 23 million light-years away. The IRAM 30 m radio telescope was used to complement the small-scale details with maps showing the galaxy as a whole.
Observations were made of the CO(1-0) transition of carbon monoxide (CO). CO molecules are standard tracers for hydrogen molecules: Hydrogen molecules hardly emit any electromagnetic radiation, and thus are difficult to observe. But they are usually found in the company of CO molecules, which can be readily detected in the radio and infrared region of the spectrum. The CO data were used to create a complete atlas of giant molecular clouds, with the smallest features that could be differentiated around 50 light-years in size.
The analysis also included existing data about ionized and atomic gas, the different types of stars and in particular the presence of young, newly-formed stars in M51. This data was taken from archived observations from the 2MASS survey, with the Hubble Space Telescope, the infrared space telescope Spitzer, the infrared Herschel Space Observatory, the GALEX ultraviolet space telescope, and the THINGS survey of atomic hydrogen.
What is new and important about these results?
This is the first time such a complete and detailed inventory of giant molecular clouds has been compiled for any galaxy. M51 is a particularly suitable candidate: It is not overly far away (around 23 million light-years), so that individual giant molecular clouds can be detected, and it has been observed previously in many different wavelengths. Also, we see the galaxy face-on which, given that stars and gas are distributed within a rather thin disk in such spiral galaxies, means an optimal overview.
The cloud atlas was only possible because of an unprecedented 169 hours of observations with the Plateau de Bure Interferometer – only with a compound telescope such as the PdBI, where separate antennae are combined to give a resolution of about 1" – at the distance of M51, this corresponds to around 130 light-years, or 8 million times the Sun-Earth distance – are the separate giant molecular clouds visible as distinct entities. Reconstructing the overall distribution of molecular hydrogen (deduced from the distribution of CO), correlating with the known distribution of atomic hydrogen, dust, and the different types of stars, and identifying individual giant molecular clouds, listing more than 1,500 separate clouds, was a massive feat of analysis which took more than three years.
The effort was rewarded by a few surprises that have reshaped the view of what giant molecular clouds are, and under what conditions they form stars. The first surprise was that so much of the molecular hydrogen – about half! – is not in giant molecular clouds, but in a diffuse disk – not delineated clouds, but a "fog" pervading the galaxy.
With this unexpected distribution, the influence of external pressure from this fog on the molecular clouds, previously thought to be completely unimportant, turns out to play a role in whether or not a cloud begins to collapse to form new stars: reduce the external pressure, and there will be no significant star formation.
This, in turn, explains another unusual result of the study. Previous thought held that the key to star formation in giant molecular clouds was density: Make a cloud dense enough, and you will form stars. In spiral galaxies, the zones of higher density are the spiral arms – density patterns moving through the galactic disk similar to sound waves. So by the old way of thinking, you would expect to find increased star formation everywhere within the spiral arms, and less star formation everywhere else.
The new study, however, shows that there is a blatant exception in M51: In one large region in both spiral arms of M51, there is hardly any star formation at all. The explanation lies in the role of external pressure: Reconstructing the galactic dynamics, this turns out to be a region in which the fog, and the molecular clouds contained within, are moving relative to the spiral arms at a speed of 15-20 km/s. By Bernoulli's principle of fluid dynamics, this relative motion reduces the inner pressure of the fog; this, in turn, decreases the external pressure on embedded clouds, which in turn inhibits star formation. External pressure, exerted by the surrounding molecular "fog", is a key factor in determining star formation rates – and it is one that, before this study, was sadly neglected.
All in all, the study is an excellent example of how a systematic, long-term study can serve to change the way we see and understand an important class of astronomical objects – in this case, the birthplaces of stars.
What are the next steps?
Next up: studies to find out whether the results obtained by PAWS for the Whirlpool galaxy M51 generalize to other spiral galaxies – or whether there are interesting differences. An obvious next target are barred spiral galaxies, which have an oblong stellar feature (the "bar") in their central regions, as the physical conditions in the gas flowing along the stellar bars are thought to be even more extreme compared to the one in the spiral arms.
For future studies, two future telescopes are of particular interest: The NOEMA extension of the Plateau de Bure interferometer will allow for more detailed studies. And with the recently inaugurated ALMA compound telescope, which consists of more than 60 separate antennas, studies similar to PAWS will be possible in a fraction (∼ 10%) of the time.
Figure 2: Molecular hydrogen in the Whirlpool Galaxy M51. The blueish features show the distribution of hydrogen molecules in M51, the raw material for forming new stars. The PAWS team has used this data to create a catalogue of more then 1,500 molecular clouds. The reddish structures show the distribution of hydrogen atoms.
The background is a color image of M51 by the Hubble Space Telescope. Superimposed in blue is the CO(1-0) radiation emitted by carbon monoxide (CO) molecules, as measured for the PAWS study using the millimeter telescopes of the Institut de Radioastronomie Millimétrique. The CO molecules are used as tracers for molecular hydrogen. The red structures show the HI line emissions of atomic hydrogen.
Credit: PAWS team/IRAM/NASA HST/T. A. Rector (University of Alaska Anchorage)
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Press Releases 2013