File:GalacticRotation2.svg

    Caption: A Hubble Space Telescope (HST) image of galaxy Arp 220 in constellation Serpens. The radio lobes are a false color artist's conception.

    Features:

    1. Arp 220 is an ultraluminous infrared galaxy (ULIRG) whose heightened luminosity is probably due to a starburst (a period of intense star formation) triggered by a galaxy merger that started ∼ 0.7 Gyr ago (see Wikipedia: Arp 220: Features).

    2. The galaxy merger caused interstellar medium (ISM) from both progenitor galaxies to clump in dense regions which became intense star formation regions which give rise to the starburst.

    3. The galaxy merger also disrupted the galaxy disks and randomized the orientation of the orbits of the stars.

    4. A new galaxy disk with spiral arms might form, unless the active galaxy nucleus (AGN) activity converts Arp 220 into quenched galaxy (see explication below) on the time scale of gigayear.

    5. To explicate active galaxy nuclei (AGNs): Large galaxies (like Arp 220) are all thought to a supermassive black hole (SMBH) near their center of mass. If there is a large accretion disk surrounding the SMBH, then intense electromagnetic radiation (EMR) (dominantly in the ultraviolet band (fiducial range 0.01--0.4 μm) and visible band (fiducial range 0.4--0.7 μm = 4000--7000 Å)) will be emitted from the accretion disk and relativistic jets of matter (of interstellar medium (ISM) composition) will be launched perpendicular to the accretion disk (which for disk galaxies is usually aligned with the galaxy disk). The relativistic jets slow and expand to become giant radio lobes (radio nebulae) extending outside of the luminous part of the galaxy.

      The SMBH and its accretion disk constitute an active galaxy nucleus (AGN). A galaxy with AGN is called an active galaxy.

    6. It turns out that if the total mass of a galaxy (including dark matter) exceeds ∼ 10**12 M_☉, then the galaxy is or is becoming a quenched galaxy. The quenching is due to the heating of the galaxy ISM by EMR from the accretion disk around the SMBH whether or NOT the galaxy is recognized as an active galaxy.

      The heating causes all the gas in the galaxy to be too hot to lose pressure support and and collapse into star formation regions

      So star formation turns off and the galaxy becomes a quenched galaxy.

    7. The ∼ 10**12 M_☉ amount has been called the golden mass (see Dekel et al. (2019)). The time scale for quenching after reaching the golden mass 10**12 M_☉ is of order 1 to 2 to 3 Gyr.

    8. In fact, typically a galaxy reaches the golden mass 10**12 M_☉ following a galaxy merger as is happening to Arp 220. The merged galaxy of the golden mass 10**12 M_☉ or greater typically turns into a giant elliptical galaxy. The time scale for galaxy merging and galaxy quenching is of again of order 1 to 2 to 3 Gyr.

      The upshot is that most, maybe nearly all, quenched galaxy by golden mass quenching are giant elliptical galaxies.

    9. EOF

    10. There are, in fact, may possible galaxy quenching processes all of which may act to one degree or another. See file galaxy_quenching.html and references therein.

    Credit/Permission: NASA, 2012 (uploaded to Wikimedia Commons by User:User:Dipankan001, 2012) / Public domain.
    Image link: Wikimedia Commons: File:Active Black Hole Squashes Star Formation.jpg.
    Local file: local link: galaxy_quenching_golden_mass.html.
    File: Galaxies file: galaxy_quenching_golden_mass.html.