I’d never seen galaxy images like this before. Nobody had! These images highlight star forming regions in nearby(ish) galaxies. There are still a number of unanswered questions surrounding how star formation actually occurs. To answer those questions, we are observing galaxies that are actively forming stars within giant clouds of gas. Until recently, we didn’t have the resolution needed to clearly image the individual gas clouds themselves. But images released by a project called PHANGS (Physics at High Angular resolution in Nearby GalaxieS) in a collaboration between the European Southern Observatory Very Large Telescope and the Atacama Large millimeter/submillmeter Array (ALMA) have provided never before seen detail of star forming clouds in other galaxies.
Stars form from Giant Molecular Clouds (GMCs) which are mainly comprised of molecular Hydrogen (H2). Gas within these clouds collapses under gravity eventually becoming dense spheres. With the increase in density and pressure, heat within these spheres makes nuclear fusion possible fusing hydrogen into helium – a star is born! But what triggers the initial collapse of the gas? Does the star formation rate vary between different clouds in the same galaxy? How varied are the clouds themselves? These are all chapters of star formation we’re not entirely certain about. Enter PHANGS.
PHANGS researchers chose target galaxies using a number of preconditions. The galaxies had to be close enough so they could be imaged at the required resolution to see individual GMCs. All the targets are therefore within 17 million parsecs of the Milky Way (about 55 million light years). The galaxies are also not too inclined as to provide a clear line of sight into the disks of the target galaxies. And, perhaps most importantly, the target galaxies are actively forming stars. As “Main Sequence Galaxies” these galaxies are forming stars in their disks without the external gravitational interaction of a nearby galaxy or as a result of galaxy mergers both of which can trigger intense periods of star formation call star bursts. Rather these galaxies are forming stars through processes internal to the galaxy. 90 such galaxies met the criteria and were selected for the survey.
Discovering star forming regions in the targets galaxies is achieved through a combination of finding cold gas as well as hot gases heated by newly formed stars. Cold GMCs birthing new stars are called stellar nurseries. They can range from tens to hundreds of lightyears in diameter with the mass equivalent to thousands of suns. However, the hydrogen these clouds are made from is difficult to see. When hydrogen is exposed to energy, it glows and is easily detectable while cold hydrogen hides in the darkness of space. But GMCs also contain carbon monoxide (CO) which in a cold state is easier to detect than hydrogen. The ratio of CO to hydrogen in GMCs is understood to be a constant and so the amount of detected CO molecule can tell us how much hydrogen is present in a given cloud. It’s this CO signal that ALMA hunts for.
Once hydrogen is excited by the energies of newly forming and young stars, it releases a light known as Hydrogen Alpha. H-alpha is the brightest feature in the spectrum of glowing hydrogen and is how we observe much of the Universe. Combining the hot and cold maps of these GMCs in other galaxies reveals the environment in which stars are forming. An instrument called MUSE on the Very Large Telescope maps the glowing H-Alpha where ALMA detects the cold CO emissions. The finest details resolved by ALMA in the target galaxies are approximately 100 parsecs in diameter (about 326 light years). The researches note this is “cloud-scale” resolution as the target GMCs are also about 100 parsecs in diameter. At this resolution, the clouds can be distinguished as individual structures separate from the structures of the rest of their home galaxies.
While VLT images in optical light, ALMA sees distant galaxies in infrared and radio wavelengths. These wavelengths are helpful for peering at structures that wouldn’t be visible in optical wavelengths like cold gas. But there is a disadvantage. Optical wavelengths can typically provide finer resolution for imaging creating a tradeoff between visibility and resolution. The impressive accomplishment of this initiative is that these new ALMA images achieved resolution in infrared and radio that near optical resolutions. The images are further enhanced by adding the optical resolutions of VLT as well as data from the Hubble Space Telescope in other images.
The amount of detail is mind blowing. These aren’t singular photos taken of each galaxy but rather mosaics. For comparison, the Trottier Observatory where I work could image Andromeda, a much closer galaxy at 2.4 million light years, in a six photo mosaic. Each galaxy imaged in the PHANGS-ALMA project, despite being tens of millions light years away, are mosaics comprised of up to two HUNDRED individuals images. The process of imaging all 90 galaxies at that level of detail spanned a total of 6 years resulting in an new atlas of stellar nurseries – the next generation of stars being born in the Universe.
100,000 stellar nurseries were imaged between the 90 target galaxies. Findings show that location in a galaxy can change the nature of star formation. Clouds in central regions of the galaxy are more massive, denser, and turbulent than those that lie in the far reaches of the galaxy’s disk. The rate at which the clouds form stars, and the resulting final dissipation of the cloud by those new stars blowing the gas away, all seem to vary depending on where the cloud resides in its home galaxy.
The facilities needed to capture these images are powerful, and quite picturesque themselves. Rather than one large telescope, ALMA is an array of 66 dishes spread across the Chajnantor Plateau of the Atacama Desert, Chile. Signals collected by the array are combined effectively creating one giant dish. The individual dishes can also be rearranged depending on the needs of a given project. The Very Large Telescope, also located in the Atacama, is comprised of four telescopes, two with 8.2m mirrors and two smaller 1.8m mirrors. Like ALMA, the telescopes work in concert effectively creating one larger telescope.
Although these are the most detailed images of their kind, the resolution achieved by PHANGS-ALMA is still just barely at the threshold needed to image individual GMCs in the targets galaxies. However, now that the these star forming regions have been mapped, future telescopes such as the soon-to-be-launched James Webb Space Telescope and the Extremely Large Telescope (the next after will be Ultra Large I’m guessing) will be able to revisit these GMCs with enough resolution to peer inside the clouds themselves providing even more insights into star formation.
We were once part of a giant molecular cloud you and I. Everything comprising your body, the computer you’re reading this on, and the planet we inhabit all began from an enormous cloud of stardust. The future of telescopic space exploration is so enticing. James Webb will not only have the capability to study star formation in nearby galaxies, but image some of the first stars ever born in the Universe. The ELT’s primary mirror will be 39 meters in diameter! Its enclosing dome the size of a football field. We’re on the precipice of views of the Universe unlike we’ve ever seen before and ultimately new understandings of our own origins in the cosmos.
Feature Image: Image of Galaxy NGC 3627 located in the constellation LEO. The golden gas glow corresponds to clouds of ionized hydrogen, while the bluish regions reveal the distribution of slightly older stars. Credit: ESO/PHANGS
Galactic fireworks: new ESO images reveal stunning features of nearby galaxies | ESO (Featuring the Original Images)
[2104.07739] PHANGS-ALMA: Arcsecond CO(2-1) Imaging of Nearby Star-Forming Galaxies (arxiv.org) (Open Access Originating Research Paper)
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