Az9: a stable disc in rotation

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What is the structural support of distant star-forming galaxies? Rotation like in the Milky Way? Or radial random motions, as in elliptical galaxies? This technical article touches on this issue 👉 and an explanation for all audiences  👇 1/7

Az9 is a dusty galaxy found by the Alfonso Serrano Large Millimeter Telescope, which the Hubble Space Telescope records as 3 independent images, due to distortions caused by a cluster of galaxies located in the line of sight between it and us (gravitational lensing effect). ). 2/7

Shown in black and white is the HST image in light at 1.6 microns (infrared). The red outlines mark the detection of the LMT at 1.1mm. The multiple images of Az9 are due to the distortion of light exerted by gravity from the galaxy cluster MACS0717.5+3745, which lies in the line of sight between us and Az9. Credit:  ESA/Hubble and NASA, Alexandra Pope (UMass-Amherst). Figure published in Pope et al. 2017, ApJ, 838, 137.

With ALMA observatory we measure the distance, morphology, rotation and turbulence of the interstellar medium of Az9, which forms stars at a rate of 30 solar masses per year (30 times more than the Milky Way). For that, it was necessary to reconstruct the image of the galaxy without distortions. 3/7

The image of gravitationally distorted galaxies in clusters can be reconstructed through gravity models of the clusters. This image shows another galaxy, distorted by another cluster, and its direct telescope image (on the left) and the reconstructed one, in black and white (on the right). Credits: NASA, ESA, and T. Johnson (University of Michigan).

The light from the galaxy is shifted to smaller frequencies by a factor of 5.274 (redshift z=4.274), which puts it about 25 billion light-years away (proper distance), and a mass of stars already formed of two billion solar masses. 4/7

Spectrum of the galaxy Az9, showing the deexcitation line of carbon once ionized [CII]158μm shifted by a factor 5.274 in frequency. Credits: Pope et al. 2023, ApJ, in press

The emission line [CII] makes it possible to measure the relative velocity of different parts of the galaxy with respect to its center. We detect ordered rotational motions of a disk ~6000 light-years in diameter at ~139 km/s, and also turbulent motions (~29 km/s). 5/7

The color map shows the difference in speeds of the different areas of the galaxy with respect to its center: negative values ​​represented in blue indicate movements towards the observer and positive values ​​represented in red movements that move away from the observer. The arrangement of speeds are indicative of a rotating disk. Superimposed on the velocity map are shown in contours the position of the image recorded by HST in the near infrared and by ALMA at 1.1mm. Credit: Pope et al. 2023, ApJ, in press.

The galaxy, therefore, is mainly supported by rotation: it is a stable disk with stars forming, where the centrifugal force balances the tendency of gas and stars to fall towards the center by gravity. Galaxy formation models do not predict disks as stable as this one. 6/7


Scheme in which a mass supported by a rope that revolves around an observer keeps tracing circular orbits. Credits: Wiki.

The article is headed by my colleague Alexandra Pope (UMass-Amherst , USA) and it is signed by 15 other researchers from Australia, Denmark, Spain, the United States and Mexico, including myself. 7/7

Copy of my astro-thread on Twitter, tweeted on  23/07/31













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