Astro 101 Slide Set: Finding the Most Luminous Galaxy Developed by the WISE team 0 Topic: Galaxies Concepts: Galaxies, black holes, early universe, infrared studies Mission: WISE Coordinated by: the NASA Astrophysics Forum An Instructor’s Guide for using the slide sets is available at the ASP website ducation/resources-for-the- higher-education-audience/ ducation/resources-for-the- higher-education-audience/

The Discovery 1 Artist’s conception of the current record holder for most luminous galaxy in the universe: WISE Credit: NASA/JPL-Caltech. In data from the Wide-field Infrared Survey Explorer (WISE) scientists have discovered the current record-holder for most luminous galaxy in the universe, designated WISE It shines with the light of more than 300 trillion suns, the brightest of a newly-identified class of extremely luminous galaxies shining early in the history of the cosmos.

How was the Discovery Made? 2 The ELIRGs also have high “redshifts”: they are very far away and their light has traveled very great distances. Therefore, these galaxies are observed as they were very early in the history of the universe. The light from WISE has traveled 12.5 billion light years from an era when the universe was only about 10% of its present age. Artist’s conception of WISE in space. Credit: NASA/JPL-Caltech. The WISE spacecraft surveyed the sky in the infrared light, creating an extensive catalog of infrared- emitting sources. Scientists studying these sources identified a group of 20 “extremely luminous infrared galaxies” (ELIRGs) shining in infrared light with brightness equivalent to more than 100 trillion suns. The brightest of these is WISE

The Big Picture 3 Galaxies with high luminosity are usually powered by supermassive black holes in their cores. These black holes accrete matter into disks before swallowing it; the disks heat up in the process and emit vast amounts of energy. ELIRGs may be the result of galaxy mergers in which the central black holes combine and clouds of dust and gas collide, lose momentum, and fall into the new black hole. Artist’s conception of a supermassive black hole at the center of a galaxy. Credit: NASA/JPL-Caltech. The cores of these galaxies are obscured by dust. The tremendous energy from the central black hole heats the dust, and the hot dust emits the infrared light that WISE recorded.

What are the Implications? 4 The new finding provides insight into a major growth stage in the history of the supermassive black holes at the centers of these young galaxies. These ELIRGs appear to be producing so much energy that their black holes must already be very massive, and accreting matter faster than normally thought possible--because that energy starts to drive away the matter falling in. Scientists are investigating how so much energy can be produced. It may be that the central black holes formed in chaotic environments that produce slowly spinning black holes. Slow spinners generate less energy per unit mass accreted, allowing them to grow much more quickly than fast spinners, and thus produce the incredibly powerful emissions. The Hubble Ultra Deep Field shows 10,000 galaxies, including many whose light comes from the early days of the universe and helps scientists understand those earlier eras. Credit: NASA/ESA/S. Beckwith and the HUDF Team.

Resources 5 Paper: ( Chao-Wei Tsai et al.) ( Chao-Wei Tsai et al.) Related Paper: (Roberto Assef et al.) Press Release: most-luminous-galaxy-in-universe

Finding the Most Luminous Galaxy BONUS CONTENT 6

Galactic Close Encounters Galaxies evolve in part through interactions with each other, including galactic “mergers.” Such mergers were common in the early universe where WISE finds the ELIRGs, when the universe was smaller. But such interactions continue in the larger universe today. 7 The Antennae Galaxies—colliding spirals where great bursts of new star formation are taking place. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration. In encounters between large and small galaxies, the large “eats” the small. When galaxies are of similar size, collisions can trigger: a) collapse of gas and dust clouds toward the core; b) active galactic nuclei (with central black holes absorbing material); c) bursts of star formation; d) chaotic star motions; e) loss of stars, gas and dust; f) mergers creating a single large galaxy. The Milky Way and Andromeda spiral galaxies are currently on such a collision course, and the disruptions may end up producing a single, large elliptical galaxy when the “dust clears!”

WISE to NEOWISE The WISE spacecraft made a complete survey of the sky at four infrared wavelengths in 2010, taking almost 1.5 million sets of images in the four bands, and creating a catalog of over 560 million objects. Its instrument was cryogenically cooled to -438 degrees F inside a tank of frozen hydrogen to prevent the spacecraft from picking up its own infrared (heat) signature. When the cryogen was depleted after nine months, the mission continued to search for asteroids for four months, completing a second full survey of the sky in two of its infrared bands before the spacecraft was put into hibernation. 8 Asteroid 872 Holda tracked by NEOWISE. Credit: NASA/JPL- Caltech. In 2013, the spacecraft was switched back on specifically to search for and track “Near-Earth Objects” or NEOs– asteroids whose orbits bring them close to the orbit of Earth and which may one day pose a risk of collision. The new NEOWISE mission helps to increase understanding of the asteroids and comets it studies and tracks, and helps to refine concepts for future NEO cataloging missions.