New Answers on the Horizon
Black hole science asks many questions, some of which we’ve talked about in this blog: Where do we find black holes? How “big” are they? How old? And how do they grow and change over time? Now, in our final article about these cosmic leviathans, we'll look at how TMT will help advance our understanding. As we’ll see, TMT will be an important part of the quest to learn more about these unfathomably massive, yet elusive, objects. Working on its own and in conjunction with other observatories, TMT will use its next-generation capabilities to bring us new answers to these questions—and more.
TMT Enters the Picture
Let’s look closer at some of those capabilities, and then explain how they’ll help us find out more about black holes than ever before.
The Enormous “Retina”
I’ve compared TMT to a giant eye before, and to return to that metaphor once again, we start with the first and most obvious among TMT’s important capabilities: its huge primary mirror. As I’ve noted in previous articles, the main advantage of such a large mirror is that it simply gathers more light than existing optical telescopes. Taking in more light means high sensitivity (we can see faint objects) and high spatial resolution (we can resolve individual features in what we’re looking at, compared to just seeing a fuzzy blob with no details). Here’s an example of looking at a galaxy in the young Universe (more about the young Universe later), first with a current 10m diameter mirror optical-infrared telescope (on the left), and then as it will appear when seen through TMT (on the right):
It’s easy to see, no pun intended, how more light will improve our ability to examine distant objects.
TMT will also have specially-designed science instruments, some of which I’ve touched on in previous articles, and some of which will be described in upcoming articles. The main purpose for this class of instruments is to make maximum use of the light TMT’s big primary mirror brings in, just as the lenses of our eyes, or of the eyeglasses we might wear, help our eyes make maximum use of the light they gather.
Our science instruments, by and large, are designed to provide enhanced sensitivity and broad wavelength coverage, with quick turnaround time. Enhanced sensitivity means most of the light collected by the telescope mirrors is captured and converted into a strong signal that we can detect. Broad wavelength coverage means we can see a wide range of colors and split up the light using spectroscopy into its component color signatures.
You may recall colors let us identify different chemicals in the object at which we’re looking. Splitting up that light more and more finely makes it possible for us to pick up subtle red-blue shifts in the spectral signatures, and measure objects’ velocities with high precision. Finally, rapid turnaround time from one instrument to the next is important in making sure we can use the most suitable instrument for a particular set of observations; for example, before an object we want to study fades away.
One of the first science instruments deployed on TMT will be the Wide Field Optical Spectrograph (WFOS). Below is a current conceptual view of what the instrument will look like. This is currently early in its design phase, and, as an aside, you can see how big it is in comparison to the people shown for scale in the image.
We use adaptive optics to factor out the distortion produced by the Earth’s atmospheric turbulence and to improve the accuracy and quality of our images. Likewise, TMT's astrometric capabilities, also the subject of a prior set of articles, will help us pinpoint the location of objects in the night sky.
Another science instrument TMT will have from inception will be the Infrared Imaging Spectrograph (IRIS), which will take advantage of this adaptive optics system to provide high-precision imaging, spectroscopy and astrometry. The instrument is in its final design phase, and the team recently met in Japan, hosted by team members at the National Astronomical Observatory of Japan.
Below is an image of what the instrument will look like. We'll look more closely at the Infrared Imaging Spectrograph in a future blog entry.
One Eye Among Many
To make the best use of TMT’s specialized capabilities, we’ll partner with other optical observatories to zero in on phenomena that look interesting, but can’t be explored in as much detail with other telescopes. Those other observatories are designed to perform wide surveys of the sky, so they have a greater chance of noticing novel objects that have appeared, or unusual behavior of known objects in the sky. For example, when a star gets too close to a supermassive black hole and is ripped apart, a flare might appear in the sky and be seen by another observatory, such as the Large Synoptic Survey Telescope. When that happens, TMT will then focus in on the very small part of the sky where the interesting object has appeared, and use its high sensitivity to discern details of that object. In our example, those details would include the properties of the star’s aftermath, such as the physical structure and chemical makeup of its residue.
A Partner in Perception
TMT will also be part of the movement, called “multi-messenger” astronomy, to collect different kinds of information—for example, light waves and gravitational waves—from astronomical objects and events. Being able to pick up both light and gravitational waves is like having eyes (to detect light) and ears (to detect the gravitational waves), and greatly expands what scientists can learn. One example of multi-messenger astronomy is a scenario where two supermassive black holes (each at the center of a galaxy) are in the process of merging. In this scenario we can envisage LISA, an upcoming space-based version of LIGO, detecting the gravitational waves from the merger. Then, TMT could follow up by focusing in on the host galaxies to observe the light coming from the individual galactic cores.
How TMT Will Build Our Knowledge
Now that we’ve reviewed TMT’s capabilities, let’s look at how they’ll enable specific science contributions to black hole research—what we have compared to cosmic “whale watching”—within our own galaxy and even in other galaxies.
The Milky Way’s Resident Supermassive Black Hole
Within our Milky Way galaxy, TMT will more accurately pinpoint and “weigh” our own supermassive black hole, properly known as Sagittarius A*, but which we nicknamed “Moby” in the previous articles in this series. By gathering more light and letting us see greater detail, TMT will reveal fainter stars, closer to the Milky Way’s center, and thus closer to Moby. TMT will track the motion of those fainter stars as they make their rapid orbits around Moby, and those measurements will tell us more accurately where Moby is and how massive it is.
Here's a comparison between what we can see using adaptive optics with the W.M. Keck Observatory, compared to what we will be able to see with TMT. It’s easy to see how much of an improvement we will get, even over an observatory as scientifically productive as Keck.
As another way of demonstrating this improvement dynamically, here's a movie showing these orbits.
Even beyond what we learn about Moby, watching those innermost, fastest-orbiting stars—the ones closest to Moby—will give us information we need to figure out whether Einstein’s theory of general relativity holds under such extreme conditions of high speed and gravitational pull.
Leviathans in Galaxies Far, Far Away
Outside the Milky Way, our ability to gather information has been constrained by the limitations of existing observatories, and much creativity and cleverness has been needed to be able to draw conclusions about what’s going on out there. As a result of those efforts, scientists have been able to deduce many intriguing things about other galaxies and their resident black holes, despite the limitations of existing equipment.
For example, the most powerful optical telescopes that currently exist can zoom in and see some details of a distant galaxy. But when it comes to resolving features close to the supermassive black hole at the center, these telescopes run up against limitations. With TMT, we’ll be able to focus in on narrow areas inside a galaxy, and for the first time, be able to distinguish motions of individual stars near the center—at least for galaxies that are relatively close to us. From watching individual stars, we’ll be able to reconstruct their stellar orbits, and triangulate more precisely on where we think the supermassive black hole is located within its galaxy—just as was done to find Moby in our own galaxy (as described in Part 2 of this series).
We’ll also be able to expand on the technique of tracking the speed with which matter rotates within the galaxy. From this, we will estimate the corresponding supermassive black hole mass, not just for galactic near-neighbors, but for galaxies more than 20 times farther than currently possible. Being able to see 20 times farther will multiply by about 1000 times the number of galaxies where these direct black hole mass measurements can be performed.
Here's an image comparing what we can see in the center of the Andromeda galaxy, with Hubble on the left and with TMT on the right. You can see how many more individual stars we will be able to make out with TMT.
From Part 3 of this series, you may also recall the correlation between supermassive black hole mass and velocity dispersion in the central bulge of the galaxy, and how this is an indirect way of determining supermassive black hole mass. TMT will let us measure velocity dispersion within fainter galaxies, more distant galaxies, and simply more galaxies. These measurements will yield better assessments of supermassive black hole mass for those galaxies—even galaxies in which the “supermassive” black hole is actually just a junior-leviathan, intermediate-mass, black hole. Some of the theories about how black holes are formed and grow predict that such intermediate mass black holes should be abundant in the universe. But so far, because of limitations on what we can see with existing telescopes, it’s been very difficult to find them, or even evidence that would let us deduce their existence.
Beyond “Whale Watching”: Exploring the Depths
Searching out these cosmic leviathans will teach us more about their surroundings, too—the galactic “oceans” where they live—and the other inhabitants of those environments. Some of the questions we ask about black holes—things like how old they are, and how they change over time—are essentially questions about their host galaxies, because a galaxy and its central black hole are thought somehow to evolve together. So the information TMT provides will help us understand galaxies and black holes better.
Being able to resolve individual features within a galaxy won’t just help us find black holes, it will also show us more than ever before about how stars form, especially those in the presence of supermassive black holes. We hope to explain why we find unexpectedly-young stars close to Moby. This means we’ll want to get a firmer understanding of star-formation conditions, and try to figure out whether stars form near a supermassive black hole or migrate there. And, of course, we want to figure out how a nearby supermassive black hole affects the properties of stars.
TMT will also let us more closely examine so-called active galactic nuclei (black hole jets, described in Part 4) and the surrounding accretion disks where we find a black hole feeding on a close-by star or gas cloud.
In previous articles I talked about redshift (the stretching-out of light waves caused by expansion of the Universe), and the now-familiar concept that when we look into the deep Universe, we’re actually looking back in time, because the light we see necessarily left its source many eons ago. Some light comes to us from galaxies so far away, it had to originate—and leave its source galaxy—way back when the Universe was still in its infancy, and consequently, when the galaxy itself was also young. Thus, the light as it comes to us now would show us how the galaxy looked in the blush of youth—if we could see it well enough. But because the light is coming from such a tremendous distance, it is exceedingly faint—like a ripple that begins in the ocean on the other side of the Earth and washes up almost undetectably on a shore half a world away—and fainter even than that.
TMT’s enhanced light-collecting power will give it the sensitivity to observe even these faint galaxies. In fact, TMT’s optical (visible-light) sensitivity is ideal for studying these most distant galaxies. When a galaxy forms, it generates ultraviolet light. The wavelength of ultraviolet light is shorter than for optical light, so we can’t see ultraviolet light with optical instruments. But because of redshift, the waves of ultraviolet light generated when the galaxy formed get stretched out, and by the time those light waves reach us, they’re long enough to fall into the optical and infrared parts of the spectrum—precisely what TMT is able to detect.
In addition to simply being able to gather more light, TMT will also let us gather more information from the light we’re seeing. As I mentioned at the beginning of this article, TMT’s high resolution and high-sensitivity spectroscopy will let us map out the physical structure and chemical profiles inside a galaxy, and see how these things vary within the galaxy.
TMT will let us look at many different types of galaxies at different redshifts (ages). As we do this over and over, we’ll begin to be able piece together how the environments hosting black holes have changed over time, and how consistent things like the relationship between velocity dispersion and black hole mass are from one galaxy to the next. We’ll also help solve the mystery of why some of the heftiest supermassive black holes we’ve found so far reside in the youngest galaxies we know of (see here for further details), and how such enormous black holes could have formed in such a short time.
In talking about how TMT will work together with other observatories to let us peer across the cosmos in search of its mysterious “white whales”—black holes, and especially supermassive black holes—what we’re actually talking about is why humanity constructs technological wonders like TMT in the first place. Human beings want to learn, know, explore, and explain. Gargantuan yet invisible to our eyes, black holes are some of the most unfathomable phenomena imaginable—yet with TMT, we expect to be able to answer in more detail and with greater accuracy than ever before many of the most perplexing questions we can ask about them. In the process, TMT will also teach us more than has ever been understood about the galaxies they inhabit, and ultimately tackle the question of how black holes and galaxies, and our Universe, change through cosmic history.
This brings to an end this five-part series of articles on supermassive black holes. In future articles, we'll address other compelling science topics for TMT, such as exoplanet characterization and the search for possible signs of life.
Key Reference: The TMT Detailed Science Case
This is the highest-level statement of the science that we wish to pursue with TMT. It provides examples of the kinds of exciting, groundbreaking science that will be enabled by our telescope. The document serves as the basis for the formal scientific requirements that we need to meet.
Skidmore, Warren et al., "Thirty Meter Telescope Detailed Science Case: 2015", Research in Astronomy and Astrophysics, Volume 15, Issue 12. DOI: 10.1088/1674-4527/15/12/001 (December 2015).
Tuan Do & Andrea Ghez, “Envisioning the next decade of Galactic Center science: a laboratory for the study of the physics and astrophysics of supermassive black holes.”
Astro2020 Science White Paper, arXiv:1903.05293 (2019)
M. Parsa, A. Eckart, B. Shahzamanian, V. Karas, M. Zajacek, J. A. Zensus and C. Straubmeier, Investigating the Relativistic Motion of the Stars Near the Supermassive Black Hole in the Galactic Center, arXiv:1708.03507v1 [astro-ph.GA] (11 Aug 2017)
Shelley A. Wright, James E. Larkin, Anna M. Moore, Tuan Do, Luc Simard, Mate Adamkovics, Lee Armus, Aaron J. Barth, Elizabeth Barton, Hope Boyce Jeffrey Cooke, Patrick Cote, Timothy Davidge, Brent Ellerbroek, Andrea Ghez, Michael C. Liu, Jessica R. Lu, Bruce A. Macintosh, Shude Mao, Christian Marois, Mathias Schoeck, Ryuji Suzuki, Jonathan C. Tan, Tommaso Treu, Lianqi Wang, Jason Weiss (and IRIS team), “The InfraRed Imaging Spectrograph (IRIS) for TMT: Overview of innovative science programs”, Proceedings of the SPIE, 9147-369, Instrumentation and Methods for Astrophysics, arXiv:1407.2996 (2014)
A. Hees, T. Do, A. M. Ghez, G. D. Martinez, S. Naoz, E. E. Becklin, A. Boehle, S. Chappell, D. Chu, A. Dehghanfar, K. Kosmo, J. R. Lu, K. Matthews, M. R. Morris, S. Sakai, R. Schödel, G. Witzel, “Testing General Relativity with stellar orbits around the supermassive black hole in our Galactic center”, arXiv:1705.07902 (2017)
A. M. Ghez, S. Salim, N. N. Weinberg, J. R. Lu, T. Do, J. K. Dunn, K. Matthews, M. Morris, S. Yelda, E. E. Becklin, T. Kremenek, M. Milosavljevic, J. Naiman, “Measuring Distance and Properties of the Milky Way's Central Supermassive Black Hole with Stellar Orbits” arXiv:0808.2870 (2008)
T. Do, G.D. Martinez, S. Yelda, A. Ghez, J. Bullock, M. Kaplinghat, J.R. Lu, A.H.G. Peter and K. Phifer, “Three-dimensional stellar kinematics at the galactic center: measuring the nuclear star cluster spatial density profile, black hole mass, and distance”, The Astrophysical Journal Letters, 779:L6 (2013 December 10), arXiv:1311.0886
Tuan Do, Shelley A. Wright, Aaron J. Barth, Elizabeth J. Barton, Luc Simard, James E. Larkin, Anna M. Moore, Lianqi Wang and Brent Ellerbroek, “Prospects for measuring supermassive black hole masses with future extremely large telescopes”, The Astronomical Journal, 147:93 (2014 April), arXiv:1401.7988
Tuan Do, Aurelien Hees, Arezu Dehghanfar, Andrea Ghez, and Shelley Wright, “Measuring the effects of General Relativity at the Galactic Center with Future Extremely Large Telescopes”, arXiv:1711.06389v1 (17 Nov 2017)
GRAVITY Collaboration: R. Abuter et. al, “Detection of the gravitational redshift in the orbit of the star S2 near the Galactic centre massive black hole?” Astronomy & Astrophysics, 615, L15 (2018), arXiv:1807.09409
C. M. Harrison, “Impact of supermassive black hole growth on star formation”, Nature Astronomy 1, 0165 (2017), DOI: 10.1038/s41550-017-0165, arXiv:1703.06889
Nicholas J. McConnell, Chung-Pei Ma, Karl Gebhardt, Shelley A. Wright, Jeremy D. Murphy, Tod R. Lauer, James R. Graham & Douglas O. Richstone, “Two ten-billion-solar-mass black holes at the centres of giant elliptical galaxies”, Nature (08 December 2011), arXiv:1112.1078
Jonelle Walsh, “Exploring the Supermassive Black Hole - Galaxy Connection”, TMT Science Forum (Dec 10, 2018)
LSST Science Collaborations and LSST Project 2009, LSST Science Book, Version 2.0, arXiv:0912.0201
Zoltán Haiman, “The Origin and Detection of High-Redshift Supermassive Black Holes”, Proceedings of "The First Stars and Galaxies: Challenges for the Next Decade", Austin, TX, March 8-11, 2010, arXiv:1007.4741
Roger Blandford, David Meier and Anthony Readhead, “Relativistic Jets from Active Galactic Nuclei”, Annual Review of Astronomy and Astrophysics, Vol. 57:467-509 (Volume publication date August 2019), https://doi.org/10.1146/annurev-astro-081817-051948
Jenny E Greene et. Al,. “Origins and Fundamental Physics of Supermassive Black Holes”, US ELT Program presentation from AAS #233, Seattle (7 January 2019)
Law, D. R., Wright, S. A., Ellis, R. S., Erb, D. K., Nesvadba, N., Steidel, C. C., and Swinbank, M., “Kinematics and Formation Mechanisms of High-Redshift Galaxies,” in [astro2010: The Astronomy and Astrophysics Decadal Survey], https://arxiv.org/abs/0902.2567 (2009)
Many thanks to my colleagues Matthias Schöck (TMT System Scientist), Warren Skidmore (TMT Instrument System Scientist) and Nicholas McConnell (STEM Workforce Development and Program Manager at The Institute for Scientist & Engineer Educators), for helping with this blog entry. My thanks also to Tuan Do, Associated Research Scientist and Deputy Director of the Galactic Center Group at UCLA, and to Devin Chu, graduate student with the UCLA Division of Astronomy & Astrophysics.