Key Science Goals for the Next-Generation Event Horizon Telescope
Abstract
1. Introduction
- EHT images are effectively monochromatic. The currently published EHT measurements sample only 4 GHz of bandwidth, centered on 228 GHz. BH images are expected to have a complex structure in frequency, with changing synchrotron emissivity, optical depth, and Faraday effects, making multi-frequency studies a powerful source of physical insight (see, e.g., [29,30,31,32]). The EHT has successfully completed commissioning observations at 345 GHz [33], which is now a standard observing mode. However, 345 GHz observations will be strongly affected by atmospheric absorption, severely affecting sensitivity and likely restricting detections to intermediate baseline lengths among the most sensitive sites (e.g., [34]).
- EHT images have severely limited image dynamic range. Current EHT images are limited to a dynamic range of only [4,11], providing only modest information about image signatures that are related to the horizon and limiting the ability to connect the event-horizon-scale images to their relativistic jets seen until now only at larger scales, via lower wavelength observations.1For comparison, VLBI arrays operating at centimeter wavelengths routinely achieve a dynamic range of on targets such as M87 (e.g., [35]).
- EHT observations have only marginally resolved the rings in Sgr A and M87. The EHT only samples a few resolution elements across the images. For instance, the EHT has only determined an upper limit on the thickness of the M87 ring [6], and the azimuthal structure of the rings in both sources is poorly constrained.
- EHT images cannot yet study the dynamics of M87 or Sgr A. The gravitational timescale is for M87 and is for Sgr A. In each source, the expected evolution timescale is (e.g., [36])—approximately for M87 and for Sgr A. Current EHT campaigns consist of sequential observing nights extending for only ∼1 week, which is too short to study the dynamical evolution of M87. Moreover, the current EHT baseline coverage is inadequate to meaningfully constrain the rapid dynamical evolution of Sgr A, which renders standard Earth-rotation synthesis imaging inapplicable [11,12].
2. Key Science Goals of the ngEHT
2.1. Fundamental Physics
2.1.1. Existence and Properties of Horizons
2.1.2. Measuring the Spin of a SMBH
2.1.3. Constraining the Properties of a Black Hole’s Photon Ring
2.1.4. Constraining Ultralight Fields
2.2. Black Holes and Their Cosmic Context
2.2.1. Understanding Black Hole-Galaxy Formation, Growth and Coevolution
2.2.2. Understanding How SMBHs Merge through Resolved Observations of Sub-Parsec Binaries
2.2.3. Multi-Wavelength and Multi-Messenger Studies of SMBHs and Their Relativistic Outflows
2.3. Accretion
2.3.1. Revealing the Driver of Black Hole Accretion
2.3.2. Localized Heating and Acceleration of Relativistic Electrons
2.3.3. Dynamical Signatures of Frame Dragging near a Rotating Black Hole
2.4. Jet Launching
2.4.1. Jet Power and Black Hole Energy Extraction
2.4.2. Physical Conditions and Launching Mechanisms for Relativistic Jets
2.5. Transients
2.5.1. Dynamics of Black Hole X-ray Binaries
2.5.2. Extragalactic Transients
2.6. New Horizons
2.6.1. Proper Motions and Secular (CMB) Parallaxes of AGN
2.6.2. Studies of Black Hole Masses and Distances with Megamasers
2.7. Algorithms and Inference
2.8. History, Philosophy, and Culture
2.8.1. Responsible Siting
2.8.2. Algorithms, Inference, and Visualization
2.8.3. Foundations
2.8.4. Collaborations
3. Summary
- Improved angular resolution and image fidelity through increased sensitivity and baseline coverage. These enhancements are the most significant requirements for studies of fundamental physics with the ngEHT.
- Expanding from independent multi-band observations to simultaneous multi-band observations at 86, 230, and 345 GHz. This upgrade will substantially improve the EHT’s sensitivity to observe faint sources, dim extended emission, and compact structure on the longest baselines at 345 GHz, especially through the use of multi-frequency phase transfer.
- Adding more sites to enable “snapshot” imaging of variable sources including Sgr A, and extending observing campaigns over multiple years. Together, these upgrades will improve the temporal sensitivity of current EHT observations by 5 orders of magnitude, enabling a wealth of new variability studies (see Figure 2).
Author Contributions
Funding
Data Availability Statement
Acknowledgments
Conflicts of Interest
1 | Since its first observing campaign, three sites have joined the EHT (see Figure 1). These additions are expected to substantially improve upon the dynamic range of published EHT images. |
2 | In contrast, most telescopes of the present EHT are astronomical facilities that only commit a small fraction of their total observing time to VLBI. |
3 | https://www.ngeht.org/ngeht-meeting-2021 (accessed on 20 April 2023). |
4 | https://www.ngeht.org/ngeht-meeting-november-2021 (accessed on 20 April 2023). |
5 | https://www.ngeht.org/ngeht-meeting-june-2022 (accessed on 20 April 2023). |
6 | https://www.ngeht.org/broadening-horizons-2022 (accessed on 20 April 2023). |
7 | https://www.mdpi.com/journal/galaxies/special_issues/ngEHT_blackholes (accessed on 20 April 2023). |
8 | |
9 | https://challenge.ngeht.org/ (accessed on 20 April 2023). |
10 | Two excellent doctoral dissertations offer fine-grained analysis of the mountaintop dispute, and are a good entry point into this issue. Ref. [249] focuses on the triply conflicting astronomical, environmental and indigenous narratives that collided at Mt. Graham, Mauna Kea, and Kitt Peak; Ref. [250] addresses the Kanaka rights claim, specifically about the Thirty Meter Telescope (TMT), in opposition to a framing of the dispute as one of “stakeholders” or a “multicultural” ideal. Ref. [251] focuses on Mauna Kea in a subsequent article, also on the TMT. An important current Hawaiian-led impact assessment of the TMT, including further links, is [252]; other Native Hawaiian scientists, including [253] have spoken for a much-changed process and against the notion that opposition to the TMT is against science. |
11 | The workshop was held on the 4th of November 2022. Workshop Speakers included C. Prescod-Weinstein, K. Kamelamela, H. Nielson, M. Johnson, J. Havstad, T. Nichols, R. Chiaravalloti, S. Doeleman, G. Fitzpatrick, J. Houston, A. Oppenheimer, P. Galison, A. Thresher and P. Natarajan. Much of the work being performed by the responsible siting group owes its genesis in the excellent contributions of the speakers and attendees of the workshop and we are grateful for their past and ongoing contributions. |
12 | For a detailed discussion of siting and community guidelines for gene-drive technology, for example, see Singh [254]. |
13 | |
14 | An outstanding example of joint concern crossing environmental, cultural, epistemic, and technical concerns, in the case of LIGO, can be found in Nichols [257]. Another instanced of community participation by (here in relation to NASA for their Asteroid Redirect Mission): Tomblin et al. [258]. On the siting of the Superconducting Supercollider, Riordan et al. [259]; an historical-anthropological study of the placement of the French/European launch center, Redfield [260]. |
15 | Consent, and environmental justice, have been at the center of siting nuclear facilities, including power generation, weapons testing, accident sites, and waste disposal. The literature is vast, but a starting point with many further references can be found in sources including: Gerrard [261] addresses community concerns about siting from the perspective on an environmental lawyer; Kuletz [262] focuses on Western US nuclear sites of waste; Masco [263] attends to the quadruple intersection of weapons scientists, Pueblo Indian nations, nuevomexicano communities, and activists as they live amidst and confront the legacy of Los Alamos. On consent-based siting rather than top-down imposition, see Hamilton et al. [264]; and for a recent development and analysis of consent-based siting, Richter et al. [265]. |
16 | For lessons learnt regarding knowledge formation, governance, organisational structure, decision-making, diversity, accountability, creativity, credit assignment and the role of consensus, from a range of perspectives across the humanities and social sciences, see e.g., (a) in general: Galison and Hevly [272], Knorr Cetina [273], Sullivan [274], Shrum et al. [275], Boyer-Kassem et al. [276] and references therein; (b) for specific collaborations and institutions: Collins [277], Nichols [278] on LIGO; Boisot et al. [279], Ritson [280], Sorgner [281], Merz and Sorgner [282] on ATLAS and/or CERN; Jebeile [283] on the IPCC; Smith et al. [284], Vertesi [285] on NASA; and Traweek [286] on SLAC and KEK. |
17 | |
18 | Regarding authorship challenges and possible solutions relevant to the ngEHT context, see e.g., Boyer-Kassem et al. [276], Resnik [301], Rennie et al. [302], Cronin [303], Galison [304], Wray [305], McNutt et al. [306], Bright et al. [307], Heesen [308], Dang [309], Nogrady [310], Habgood-Coote [311] and www.icmje.org/icmje-recommendations.pdf (accessed on 20 April 2023). |
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Johnson, M.D.; Akiyama, K.; Blackburn, L.; Bouman, K.L.; Broderick, A.E.; Cardoso, V.; Fender, R.P.; Fromm, C.M.; Galison, P.; Gómez, J.L.; et al. Key Science Goals for the Next-Generation Event Horizon Telescope. Galaxies 2023, 11, 61. https://doi.org/10.3390/galaxies11030061
Johnson MD, Akiyama K, Blackburn L, Bouman KL, Broderick AE, Cardoso V, Fender RP, Fromm CM, Galison P, Gómez JL, et al. Key Science Goals for the Next-Generation Event Horizon Telescope. Galaxies. 2023; 11(3):61. https://doi.org/10.3390/galaxies11030061
Chicago/Turabian StyleJohnson, Michael D., Kazunori Akiyama, Lindy Blackburn, Katherine L. Bouman, Avery E. Broderick, Vitor Cardoso, Rob P. Fender, Christian M. Fromm, Peter Galison, José L. Gómez, and et al. 2023. "Key Science Goals for the Next-Generation Event Horizon Telescope" Galaxies 11, no. 3: 61. https://doi.org/10.3390/galaxies11030061
APA StyleJohnson, M. D., Akiyama, K., Blackburn, L., Bouman, K. L., Broderick, A. E., Cardoso, V., Fender, R. P., Fromm, C. M., Galison, P., Gómez, J. L., Haggard, D., Lister, M. L., Lobanov, A. P., Markoff, S., Narayan, R., Natarajan, P., Nichols, T., Pesce, D. W., Younsi, Z., ... Wielgus, M. (2023). Key Science Goals for the Next-Generation Event Horizon Telescope. Galaxies, 11(3), 61. https://doi.org/10.3390/galaxies11030061