An Extreme, Distant Quasar in the Cosmic House of Mirrors

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Title: ALMA Observations of the Sub-kpc Structure of the Host Galaxy of a z = 6.5 Lensed Quasar: A Rotationally-Supported Hyper-Starburst System at the Epoch of Reionization
Authors: Minghao Yue et al.
First Author’s Institution: Steward Observatory, University of Arizona
Status: Published in ApJ

The party starts at dawn, and you all are invited! You’ll be surrounded by new, up-and-coming stars and a big fireworks show! There’s even a house of mirrors, where you can see a distorted, magnified version of yourself. Be careful of the action in the very center — it’s bright and shiny, but if you get too close, you could get ejected from the party entirely. Looking back, your view of the party might get a little warped, but it will surely be a bright point. Oh, and it’s hosted by a luminous quasar (z = 6.5) that is powered by a supermassive black hole — you can’t miss it!

Into the Cosmic House of Mirrors

In today’s paper, the authors target a gravitationally lensed quasar and its host galaxy in the early universe, the first of its kind to be observed so far back in spacetime (during cosmic dawn). Gravitational lensing occurs when mass along the observer’s line of sight distorts light before it reaches the observer, turning the universe into a sort of cosmic house of mirrors. During the process, some clump of mass (in this case, another galaxy) in the foreground magnifies and warps the light — a consequence of general relativity — coming from the background object (here, the quasar and its host galaxy). To get a sense of how this works, see Figure 1 below, or check out the iOS app GravLens3!

Schematic diagram illustrating how gravitational lensing works.

Figure 1: Diagram demonstrating gravitational lensing. Light from the more distant quasar is intercepted by a massive foreground galaxy, causing the light to be bent and magnified. The resulting image seen by the telescope shows the foreground galaxy surrounded by multiple images of the lensed quasar. [NASA/ESA/D. Player (STScI)]

This lensing is like having an extra, natural telescope amplifying the signal and providing a higher resolution image. For this dataset, the galaxy’s light is boosted by about 4 times, and stretched out enough to resolve structure on small scales. But the amplification doesn’t come for free — the lensing also manipulates the image from the background source. While the lensed object is actually a single blob, the lensed image we get is stretched out and appears multiple times, depending on how the mass of the foreground lens is distributed and positioned with respect to the background lensed object.

Hyper-Starbursting and Supersized

Quasars, some of the brightest and most extreme objects in the universe, are found in galaxies during a temporary mega-bright phase (see this astrobite for more about how they grow). During this phase, their central supermassive black holes vigorously funnel in material, building a disk of hot, luminous material around it and ejecting material via energetic jets. While quasars are bright and easy to detect, the galaxies that host and fuel them are often hidden behind the shining quasar in their active centers. However, if we can peer behind the quasar and target the gas and dust in the host galaxy, we can better understand the nature and origin of the galaxy–quasar system.

Today’s authors use millimeter wavelength observations from the Atacama Large Millimeter/submillimeter Array (ALMA) to measure the host galaxy’s dust and [CII] emission. [CII] is a molecular emission line that is commonly used to trace the motions and content of gas, the main fuel source for luminous galaxies. These observations provide clues about the structure, motions, and star formation activity within the host galaxy. After building a model of the lens to disentangle the multiple images into the original single source, the authors were able to confirm their previous lens model based on HST data (see the group’s earlier paper), and model the new ALMA data.

Based on their reconstruction of the background host galaxy (see Figure 2), the light profile looks like your everyday disk galaxy, and the kinematic tracers show a smooth velocity structure. These clues together suggest that the host galaxy is a regular, rotating disk, like a vinyl spinning around on a record player. However, the dynamics are a bit more complicated, as the galaxy doesn’t appear to be symmetric along its axes, nor does it seem to be a thin disk — so, not quite a vinyl on a record player. It could just be clumpy, but more likely it has a thick geometry and is more spheroidal than disky. Higher resolution observations are needed to better understand the complex kinematics going on here, which are a common feature of high redshift quasars.

Two plots describing velocity structure of the quasar's galactic host, first as a Dec vs. RA plot with velocity represented by colors, and then as a velocity vs. position plot.

Figure 2: Left: velocity structure of the host galaxy, which shows a smooth rotation with blue moving towards and red moving away from the observer. The red curves trace the caustic, which describes the gravitational lensing and marks the curve of highest magnification. The black line corresponds to the right panel. Right: breakdown of the position and velocity across the galaxy. This also shows generally smooth rotation, with a minor irregular feature in the yellow box (region corresponding to black dots in the left panel). [Yue et al. 2021]

The kinematics also provide key measurements that can be used to estimate properties of the galaxy’s supermassive black hole. In the local universe, the mass of a central supermassive black hole and the mass of its host galaxy follow a simple positive relation. In this paper, the authors describe their galaxy’s black hole as oversized, as it’s relatively large compared to the host galaxy. This has been found for other similar distant quasars, and suggests a fundamental difference in the coevolution of black holes within quasar systems and their host galaxies at early times.

A Case Study for the Extreme

This system is the farthest lensed quasar in the universe (redshift z = 6.5) discovered so far, among the very first stars and galaxies (to learn about the most distant quasar currently known, which is non-lensed, check out this astrobite.) It’s a pretty intense system, with an extreme star formation rate and efficiency, and an overly massive black hole in the middle — definitely a real rager of a party. Yet these extreme characteristics are not necessarily unique, and they have been observed for the small sample of (non-lensed) quasars at cosmic dawn with ALMA. What makes this galaxy in particular so special is the fact that it’s gravitationally lensed, allowing for observations with much higher detail and signal. Moreover, the authors refer to this quasar as a case study for probing the nature, origin, and structures of quasars at cosmic dawn, and for understanding how supermassive black holes and their host galaxies co-evolve. Looking ahead to future studies, the authors hope to use higher resolution data to measure the influence of the central black hole more directly, confirm the geometry of the galaxy, and more precisely map its velocity field. Back into the cosmic house of mirrors at the party at dawn!

Original astrobite edited by James Negus.

About the author, Olivia Cooper:

I’m a first year grad student at UT Austin studying the obscured early universe, specifically the formation and evolution of dusty star-forming galaxies. I recently graduated from Smith College where I studied astrophysics and climate change communication. Besides doing science with pretty pictures of distant galaxies, I also like driving to the middle of nowhere to take pretty pictures of our own galaxy!