Galaxy light show

NGC 1672

A spiral galaxy with an oval-shaped disc. Two large arms curve out away from the ends of the disc. The arms are traced by bright pink patches where stars are forming and by dark reddish threads of dust. The core is very bright and star-filled. Some large stars appear in front of the galaxy. Directly under the point where the right arm joins the disc, a fading supernova is visible as a green dot. Credit: ESA/Hubble & NASA, O. Fox, L. Jenkins, S. Van Dyk, A. Filippenko, J. Lee and the PHANGS-HST Team, D. de Martin (ESA/Hubble), M. Zamani (ESA/Hubble)

This Hubble Picture of the week features NGC 1672, a barred spiral galaxy located 49 million light-years from Earth in the constellation Dorado. This galaxy is a multi-talented light show, showing off an impressive array of different celestial lights. Like any spiral galaxy, its disc is filled with billions of shining stars that give it a beautiful glow. Along its two large arms, bubbles of hydrogen gas are made to shine a striking red light by the powerful radiation of newly-forming stars within. Near to the centre lie some particularly spectacular stars; newly-formed and extremely hot, they are embedded in a ring of hot gas and are emitting powerful X-rays. And in the very centre sits an even more brilliant source of X-rays, an active galactic nucleus created by the heated accretion disc around NGC 1672’s supermassive black hole; this makes NGC 1672 a Seyfert galaxy.

But a highlight of this image is the most fleeting and temporary of these lights: supernova SN 2017GAX, visible in just one of the six Hubble images that make up this composite image. This was a Type I supernova caused by the core-collapse and subsequent explosion of a giant star, going from invisibility to a new light in the sky in just a matter of days. In that image from later that year, the supernova is already fading, and so is only just visible here as a small green dot, just below the crook of the spiral arm on the right side. In fact this was on purpose, as astronomers wanted to look for any companion star that the supernova progenitor may have had — something impossible to spot beside a live supernova! For a closer look at the supernova’s appearance, you can compare the two images with this slider tool.

Recently, NGC 1672 was also among a crop of galaxies imaged with the NASA/ESA/CSA James Webb Space Telescope, showing the ring of gas and the structure of dust in its spiral arms. A Hubble image was also released previously in 2007.

Links

• Slider tool
• Image B (Collage of galaxy with / without supernova)
• Image C (NGC 1672 without supernova)

Source: ESA/Hubble/potw

---

Little Red Dots and Big Black Holes

Images of six "little red dot" galaxies from JWST.
Credit: NASA/ESA/CSA/I. Labbe

Title: A Census of Photometrically Selected Little Red Dots at 4 < z < 9 in JWST Blank Fields
Authors: Vasily Kokorev et al.
First Author’s Institution: Kapteyn Astronomical Institute
Status: Accepted to ApJ

Imagine you peek into a kindergarten class and, to your shock, you see that all the children are well over 6 feet tall. That is precisely how astronomers felt when data from JWST showed galaxies with massive black holes just a few hundred million years after the birth of the universe. Some of these black holes have been measured to be a million times more massive than the mass of our Sun, and astronomers are puzzled as to how they could have gained so much mass in such a short time.

The earliest galaxies likely to host these black holes show up as little red dots in the images JWST took of the early universe (as seen in the long exposure images; see the bottom image in Figure 1). They are believed to be compact (with a small radius) galaxies with a Type I active galactic nucleus and obscured (covered by dust), which accounts for their red color and why they are easily observed in the infrared. The spectrum of the galaxy has a “V” shape, a blue continuum from the unobscured part of the active galactic nucleus in the galaxy, and a red continuum from the obscured part (see Figure 1). These little red dots have appeared in several images taken by JWST, hinting that plenty of massive black holes are lurking in the early universe.

Figure 1: The characteristic spectrum of little red dots (on the left), with the compact source contributing to the dust-free blue color in the continuum (top right) and the dust-reddened part (bottom right). Adapted from Kokorev et al. 2024

Where You Look Matters!

It is vital to systematically look at these little red dot galaxies to understand how many massive black holes were in the early universe. Two factors can introduce biases in the counting of these galaxies. One is the phenomenon of cosmic variance: is JWST just preferentially looking in a direction with many little red dots, or should we expect the same number even when it looks at different parts of the universe? The other is how crowded it is in the direction in which you are observing: if you have a lot of stars in the direction you are looking, they could be misclassified as little red dots or vice versa. If you happen to have plenty of massive galaxy clusters in the direction you are looking, they may create an illusion that more of these little red dots exist than their true numbers (a phenomenon known as gravitational lensing).

To minimize the errors caused by these effects in determining the number of little red dots in the early universe, the authors of today’s research article specifically look at large areas (640 arcminutes2) of the sky by combining JWST data from various programs. This would minimize the effects of cosmic variance as you can measure the numbers over a bigger area of the sky. They also look specifically at data in blank fields (defined here as areas on the sky without galaxy clusters), which helps them determine the accurate number of objects per unit volume. All these galaxies are photometrically selected (i.e., chosen only from looking at images rather than spectra), meaning there is limited spectroscopic data to help confirm what kind of objects they are. Galaxies are determined to be little red dots based on their red colors and how compact they are in the images. Using the obtained fluxes, the authors then construct spectral energy distributions to determine the redshifts (z) of the sample. Limiting the sample to z > 4 (for the early universe), the authors end up with 260 little red dot galaxies.

Do Not Judge a Galaxy by Its Size (in Your JWST Image!)

On calculating the total luminosity of the little red dots and comparing it to their redshifts, the authors find a large number of bright little red dots at redshift of z = 5 (around a billion years after the beginning of the universe). The number of little red dots is almost 100 times more than the number of ultraviolet-selected quasars, which are active galactic nuclei identified using another method. They also find that computer models are unable to reproduce the high fraction of the bright galaxies they uncover at high redshifts (left side of Figure 2). The authors derive the mass of the black holes at redshifts of z = 4.5–6.5 to be around 106–108 solar masses, indicating that these black holes were already massive a few hundred million years after the Big Bang. They find deviations from the predicted number densities of massive black holes at these redshifts from galaxy simulations. This is likely because the galaxies that host more gigantic black holes are very dusty, and thus, their spectra do not have any blue continuum. They may then be missed from selections of little red dots as one of the factors it depends on is the characteristic “V”-shaped spectrum, which would need a contribution from the blue continuum (right side of Figure 2).

Figure 2: Left: The number density of the little red dots as a function of luminosity at 6.5 < z < 8.5 with the predicted values from simulations indicated by the blue solid line. Right: The number density as a function of black hole mass at 4.5 < z < 6.5. The observed number density of more massive black holes is lower than the values predicted by simulations. Adapted from Kokorev et al. 2024

While spectroscopy is a more reliable method to identify massive black holes, many galaxies that host black holes can still be picked out using near-infrared colors and photometry, which is a much less expensive technique. The challenge lies in ensuring that the photometrically selected sources are reliable, and the authors of today’s article made great use of this technique. Follow-up spectroscopic studies of these photometrically selected samples can help us further understand the exact nature of the black holes. Such studies are already underway, and we can look forward to finally making sense of how these black holes became so massive in such a short time after the formation of the universe!

Original astrobite edited by Delaney Dunne.

Source: American Astronomical Society/AASNova

---

About the author, Archana Aravindan:

I am a PhD candidate at the University of California, Riverside, where I study black hole activity in small galaxies. When I am not looking through some incredible telescopes, you can usually find me reading, thinking about policy, or learning a cool language!

---

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

---

Pre-Supernova Burps and Red Supergiant Reflux

SN 1987A is an example of a supernova that collided with circumstellar material as it expanded.
Credit: ESA/Hubble, NASA

Title: 3D Hydrodynamics of Pre-supernova Outbursts in Convective Red Supergiant Envelopes
Authors: Benny T.-H. Tsang, Daniel Kasen, and Lars Bildsten
First Author’s Institution: University of California, Berkeley
Status: Published in ApJ

All right, I know what you’re thinking: “What do my digestive problems (for the hopefully very few of you) have to do with a star’s last, quite spectacular, goodbye?” Unlike most humans, stars don’t possess working intestines, and their outbursts — supernovae — are far more impressive than anything we can manage. There are many different kinds of supernovae, but they are broadly split into two categories: Type I and Type II. We typically see no emission lines of hydrogen in the spectra of Type I supernovae (here’s an example of why), while the spectra of Type II supernovae do contain hydrogen lines. We will talk about this latter type in today’s bite.

Red Supergiant Burps and Interacting Supernovae

Before red supergiants go supernova, they are prone to breathtaking belches called pre-supernova outbursts. These outbursts push huge amounts of gas from the star out into the so-called circumstellar medium — the material in the star’s direct neighborhood. If this neighborhood is filled with enough gas when a star dies, the expanding supernova will push against and interact with this material. This interplay between the material around the star and the supernova can actually be observed from Earth, giving rise to what is known as a Type IIn or interacting supernova.

How visible this interaction is depends mostly on how much material is in the stellar neighborhood. This depends again on how much gas the star decides to throw out, and also on how these pre-supernova outbursts (or burps) are actually formed. The authors of today’s article show that this has a lot to do with convection in the red supergiant.

Red Supergiant Boiling Pot

Simulating these red supergiant outbursts shortly before they go supernova is not new, so we already know the causes of these pre-supernova outbursts:

• Increasingly unstable nuclear fusion in the core of the star causes powerful gravity waves (not to be confused with gravitational waves)
• Large-scale convection in the red supergiant carries around material in the star, which can destabilize the nuclear fusion in the core, giving a very variable energy output
• Pair instability can cause the core’s energy output to go through cycles of drops and spikes
• A binary companion star can disturb the red supergiant enough to cause the star to temporarily become unstable

The bottom line is that some process releases a large amount of extra energy inside the star, which, depending on how the star reacts to this energy release, can lead to different outbursts of gas. Until now, the simulations of these outbursts have usually been spherically symmetric, meaning that the simulation of the outburst looks exactly the same from any direction. You can also see this as a simulation along a single line of sight from the outside of the star inwards (i.e., one-dimensional).

The problem with this approach is that you cannot simulate convection this way. To deal with convection, the authors of today’s article took the brute-force approach and did a fully 3D simulation. They simulated the region of the star outside the nuclear core (called the envelope) and started with a large energy release at the innermost part of their simulation. The authors considered different styles of energy release in the envelope. These included:

• A large, sudden energy release, comparable to the energy needed to keep the star together by gravity. This can cause a mass ejection, quite like the Sun but on much larger scales.
• A slow release of energy, which causes a much steadier stream of mass flowing away from the star instead of an explosive loss of mass.
• Varying direction of energy release, which influences how (and where to) the pre-supernova outburst will occur.

A snapshot of the authors’ simulation is shown in Figure 1. Here, we see both the envelope density on the left and the velocity of the envelope gas in the radial direction on the right. In the velocity graph, we can see zones both moving away from the star and falling back towards the core. These are the same as convection cells we can find in daily life — like in a pot of boiling water.

Figure 1: Left: Density slice of the star’s outer layers, with radius (R) vs. the distance from the core to the pole (z). Right: Velocity in the radial direction (away from the core) slice with the same axes as on the left. Credit: Tsang et al. 2022

The convection cells leave “holes” or channels of lower density in the envelope from the outside to inner parts of the star. Through these channels, much more gas can escape than would be possible without convection.

We can also see this in Figure 2: the simulation in the left panel, which included the convection, resulted in much more mass loss than the simulation in the right panel, which did not. These channels of low density appear where most of the mass escapes in the convection simulation.

Figure 2: Two images of the star’s surface in Mollweide projection, showing how much mass has escaped. On the left is a model with convection, where the colors indicate the amount of mass lost per direction (or, specifically, solid angle). On the right is a simulation without convection. Credit: Tsang et al. 2022

This article shows the necessity of taking convection in 3D into account, where the loss of mass from the pre-supernova outbursts has mostly been underestimated. This increases the amount of gas in the neighborhood of the red supergiant, ultimately affecting how the interacting supernova will look to us on Earth.

Source: American Astronomical Society - AAS Nova 

---

By Astrobites

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

---

About the author, Roel Lefever:

Roel is a first-year PhD student at Heidelberg University, studying astrophysics. He works on massive stars and simulates their atmospheres/outflows. In his spare time, he likes to hike/bike in nature, play (a whole lot of) video games, play/listen to music (movie soundtracks!), and to read (currently The Wheel of Time, but any fantasy really).

---

NuSTAR Finds New Clues to 'Chameleon Supernova'

Supernova SN 2014C (X-ray) [annotated]

This image from NASA's Chandra X-ray Observatory shows spiral galaxy NGC 7331, center, in a three-color X-ray image. Red, green and blue colors are used for low, medium and high-energy X-rays, respectively. An unusual supernova called SN 2014C has been spotted in this galaxy, indicated by the white box in the image.  Credit: NASA/CXC/CIERA/R.Margutti et al. Annotated image

Supernova SN 2014C

This visible-light image from the Sloan Digital Sky Survey shows spiral galaxy NGC 7331, center, where astronomers observed the unusual supernova SN 2014C . The inset images are from NASA's Chandra X-ray Observatory, showing a small region of the galaxy before the supernova explosion (left) and after it (right). Red, green and blue colors are used for low, medium and high-energy X-rays, respectively. Credit: X-ray images: NASA/CXC/CIERA/R.Margutti et al; Optical image: SDS.  Hi-res image

Annotated image

We're made of star stuff," astronomer Carl Sagan famously said. Nuclear reactions that happened in ancient stars generated much of the material that makes up our bodies, our planet and our solar system. When stars explode in violent deaths called supernovae, those newly formed elements escape and spread out in the universe.

One supernova in particular is challenging astronomers' models of how exploding stars distribute their elements. The supernova SN 2014C dramatically changed in appearance over the course of a year, apparently because it had thrown off a lot of material late in its life. This doesn't fit into any recognized category of how a stellar explosion should happen. To explain it, scientists must reconsider established ideas about how massive stars live out their lives before exploding.

"This 'chameleon supernova' may represent a new mechanism of how massive stars deliver elements created in their cores to the rest of the universe," said Raffaella Margutti, assistant professor of physics and astronomy at Northwestern University in Evanston, Illinois. Margutti led a study about supernova SN 2014C published this week in The Astrophysical Journal.

A supernova mystery

Astronomers classify exploding stars based on whether or not hydrogen is present in the event. While stars begin their lives with hydrogen fusing into helium, large stars nearing a supernova death have run out of hydrogen as fuel. Supernovae in which very little hydrogen is present are called "Type I." Those that do have an abundance of hydrogen, which are rarer, are called "Type II."

But SN 2014C, discovered in 2014 in a spiral galaxy about 36 million to 46 million light-years away, is different. By looking at it in optical wavelengths with various ground-based telescopes, astronomers concluded that SN 2014C had transformed itself from a Type I to a Type II supernova after its core collapsed, as reported in a 2015 study led by Dan Milisavljevic at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. Initial observations did not detect hydrogen, but, after about a year, it was clear that shock waves propagating from the explosion were hitting a shell of hydrogen-dominated material outside the star.

In the new study, NASA's NuSTAR (Nuclear Spectroscopic Telescope Array) satellite, with its unique ability to observe radiation in the hard X-ray energy range -- the highest-energy X-rays -- allowed scientists to watch how the temperature of electrons accelerated by the supernova shock changed over time. They used this measurement to estimate how fast the supernova expanded and how much material is in the external shell.

To create this shell, SN 2014C did something truly mysterious: it threw off a lot of material -- mostly hydrogen, but also heavier elements -- decades to centuries before exploding. In fact, the star ejected the equivalent of the mass of the sun. Normally, stars do not throw off material so late in their life.

"Expelling this material late in life is likely a way that stars give elements, which they produce during their lifetimes, back to their environment," said Margutti, a member of Northwestern's Center for Interdisciplinary Exploration and Research in Astrophysics.

NASA's Chandra and Swift observatories were also used to further paint the picture of the evolution of the supernova. The collection of observations showed that, surprisingly, the supernova brightened in X-rays after the initial explosion, demonstrating that there must be a shell of material, previously ejected by the star, that the shock waves had hit.

Challenging existing theories

Why would the star throw off so much hydrogen before exploding? One theory is that there is something missing in our understanding of the nuclear reactions that occur in the cores of massive, supernova-prone stars. Another possibility is that the star did not die alone -- a companion star in a binary system may have influenced the life and unusual death of the progenitor of SN 2014C. This second theory fits with the observation that about seven out of 10 massive stars have companions.

The study suggests that astronomers should pay attention to the lives of massive stars in the centuries before they explode. Astronomers will also continue monitoring the aftermath of this perplexing supernova.

"The notion that a star could expel such a huge amount of matter in a short interval is completely new," said Fiona Harrison, NuSTAR principal investigator based at Caltech in Pasadena. "It is challenging our fundamental ideas about how massive stars evolve, and eventually explode, distributing the chemical elements necessary for life."

NuSTAR is a Small Explorer mission led by Caltech and managed by JPL for NASA's Science Mission Directorate in Washington. NuSTAR was developed in partnership with the Danish Technical University and the Italian Space Agency (ASI). The spacecraft was built by Orbital Sciences Corp., Dulles, Virginia. NuSTAR's mission operations center is at UC Berkeley, and the official data archive is at NASA's High Energy Astrophysics Science Archive Research Center. ASI provides the mission's ground station and a mirror archive. JPL is managed by Caltech for NASA.

Source: NASA's Nuclear Spectroscopic Telescope Array (NuSTAR)