Authors: Kevin C. Harrington, Román Fernández Aranda, Leindert Boogaard, Axel Weiß, Tanio Diaz Santos, Manuel Aravena, Roberto J. Assef, Chao-Wei Tsai, Peter Eisenhardt, Daniel Stern
First Author’s Institution: Joint ALMA Observatory, Santiago, Chile
Status: Submitted to Astronomy & Astrophysics [open access]
The spectacular deaths of the most massive stars leave behind black holes. These remnants weigh a few times as much as the Sun. A very different kind of black hole, known as a supermassive black hole (SMBH), can be hundreds of billions of times more massive. In some cases, a SMBH is large enough to exert a significant influence on its entire host galaxy.
An active galactic nucleus (AGN) forms when the SMBH at the center of a massive galaxy starts rapidly accreting material from its surroundings. The infalling gas heats up and shines brightly. In fact, the most luminous AGN can easily outshine their entire host galaxies. Early work on AGN examined the brightest sources, which could be seen at staggering distances. Detailed studies of their fainter cousins were possible for only nearby objects. In this regard, the James Webb Space Telescope (JWST) has been absolutely groundbreaking, unlocking insight into faint AGN (including the famous little red dots) in the early universe. Studies of these sources have presented a seemingly endless gauntlet of puzzles.
A class of AGN known as hot dust-obscured galaxies (Hot DOGs) lie at the other extreme, as they’re among the most luminous sources ever observed. The black holes at their centers are whipping up a frenzy, violently driving outflows of gas at up to a few percent the speed of light. Despite their brightness, these dramatic sources remain mysterious because they’re cloaked in dust. The authors of today’s paper leverage observations by the Atacama Large Millimeter Array (ALMA) and the Very Large Array (VLA) to pierce the veil and characterize the substantial mass of gas and dust in one of the most breathtaking Hot DOGs.
Dissecting a Hot Dog
Dust scatters and absorbs the light that human eyes can see, making it very difficult to study it with visible-light telescopes; however, dust glows brightly in the far-infrared (FIR) part of the spectrum. So does warm molecular gas, the raw material required to build new stars. Observatories that probe the FIR, like ALMA and VLA, are thus excellent tools to probe large reservoirs of mass that would otherwise be invisible. The authors of today’s paper use data from these observatories to study W2246-0526, the most luminous known Hot DOG (and one of the brightest known galaxies in the entire universe)!
Carbon monoxide (CO) is known to be an excellent tracer of molecular gas. It can emit light at specific frequencies in the FIR due to a series of “ro-vibrational” transitions. Examining CO emission lines can tell us a lot about the state of the molecular gas, including its excitation, density, and temperature. Moreover, the continuum around these emission lines can be used to characterize the dust content of the source. These measurements all come together to form a spectral energy distribution (SED).
The authors employ a TUrbulent Non-Equilibrium Radiative transfer (TUNER) model to simultaneously fit the observed continuum and line observations. This modeling allows the total FIR SED to be broken up into components, revealing reservoirs of material at a variety of densities and temperatures. The results present a fascinating inventory of molecular gas and dust in W2246-0526.
An Extra-Hot Hot Dog
The SED of W2246-0526 is perplexing (see Figure 1). An unexpected emission line from CO molecules, specifically CO(12-11), is the brightest. In more typical sources, the lower-excitation lines of CO dominate (smaller numbers inside the parenthesis). In fact, the authors state that W2246-0526 may be more highly excited than any other known galaxy! In order to create these conditions, the gas must be at incredibly high temperatures and densities. In their models, the hottest, most dense gas seems to be concentrated in just the central 100 parsecs of the galaxy. For reference, the Milky Way is about 50,000 parsecs across. The incredibly small physical size of this region, together with its extreme conditions, strongly suggest that this emission is being driven by the central AGN.
Further evidence for this claim comes from the observed dust properties. The average dust temperature appears to be a toasty 95 K–warm compared to typical galaxy dust temperatures of 10-30 K. Even still, the inferred ratio of the gas temperature to the dust temperature hovers around 4. This ratio is very high if the heating is coming from energetic radiation (like how a microwave heats up food or the Sun heats the Earth). Instead, this high ratio of gas temperature to dust temperature suggests the presence of substantial “mechanical feedback”, which occurs when fast-moving material crashes into gas and heats it up by compressing it (similar to how a diesel engine works). The AGN in this source is injecting a massive amount of energy into the gas around it.
The inferred molecular gas mass within the inner thousand parsecs is almost a hundred billion solar masses; such a staggering amount of material near the central AGN may explain the extreme “outflows”, material being ejected from the core at high speed. The authors also derive a dust mass between 100 million and 1 billion solar masses. This is certainly a large amount of obscuring material, but it’s actually less than expected for the amount of gas present!
The authors note that the highly excited conditions in this enigmatic source may actually cause the typical relations between CO line intensity and molecular gas mass to break down. To better understand this intriguing source, it appears that we may need to improve our calibrations, motivating the need for larger and more detailed samples of nearby AGN. Finally, the authors note that next-generation observatories may be able to spatially resolve the source of extreme emission in sources like W2246-0526 to even smaller scales. Dissecting Hot DOGs may sound mundane, but doing so will yield crucial insights into the conditions around some of the most captivating galaxies in the universe.
Astrobite edited by Brandon Pries
Featured image credit: NASA/CXC/Ohio State Univ./J.Eastman et al (AGN); Andy Li (Hot dog bun)
