An unassuming galaxy hides a secret 15 million light-years long.
If we could get high-quality optical images of J1420?0545, they might look like this photograph of its closer cousin, the giant radio galaxy 3C 236. This Hubble image only shows the galaxy?s core; radio telescopes reveal a much larger structure. Image credit: NASA/ESA.
The Milky Way is about 50 to 60 kiloparsecs in diameter ? a moderately sized spiral galaxy. It?s a few orders of magnitude larger than the smallest galaxies, ultra-compact dwarfs like M60?UCD1 that have most of their stars clustered in a sphere less than 50 to 100 parsecs across. At the extreme opposite end of the spectrum lie supergiant ellipticals, more formally known as cD galaxies, whose diffuse halos can be up to 1?2 megaparsecs wide. To put this in perspective, the Andromeda galaxy is 0.78 Mpc away. This means that the 2-megaparsec-long stellar halo of IC 1101 ? sometimes hailed as the largest known galaxy in the observable universe ? could stretch from the Milky Way to Andromeda and then some.
IC 1101, possibly the largest known galaxy in the universe. Its diffuse halo might not look like much, but it extends about one megaparsec in each direction. Image credit: NASA/ESA/Hubble Space Telescope
Yet IC 1101 pales in comparison to another class of objects: radio galaxies. Radio galaxies are sources of strong synchrotron emission, radiation from particles being accelerated along curved paths by magnetic fields. Active galactic nuclei are the culprits, supermassive black holes accreting matter and sending out jets of energetic electrons. In most cases, these jets are hundreds of kiloparsecs in length, and some are even longer.
This week?s blog post talks about J1420?0545, currently the largest-known radio galaxy. To be more specific, it has the largest radio ?cocoon? ever observed. These cocoons are structures formed by shocked plasma from the jets, which expands outward into the intergalactic medium (IGM) and encases the jets and the lobes they form. The entire radio structure around J1420?0545 is enormous, stretching 4.69 Mpc ? 15 million light-years ? from end to end. Read on to find out just how extraordinary this galaxy is and how we know so much about its enormous cocoon, despite knowing so little about the host galaxy itself.
Initial observations and slight surprise
J1420?0545 was discovered, like many unusual galaxies, in a survey scanning the sky. In particular, it showed up as two large radio lobes spaced 17.4′ apart on the FIRST and NVSS surveys observing at 1.4 GHz using the Very Large Array (VLA). Follow-up observations made at Effelsberg and the Giant Metrewave Radio Telescope (GMRT) (Machalski et al. 2008) then confirmed that there was a radio-loud core located midway between them, and that it corresponded to a previously-known dim galaxy.
Fig. 1, Machalski et al. 2008. VLA/Effelsberg observations of J1020?0545 showed that the main sources of 1.4 GHz emission were two large radio-loud lobes and a weaker central source. The galaxy itself is in the crosshairs in the image, a speck among specks.
Redshift values for that galaxy were available (z~0.42?0.46), but had large uncertainties, so the team performed their own optical photometry at the Mount Suhora Observatory. The spectra derived from this proved useful in two ways. First, the spectroscopy allowed the team to figure out what sort of galaxy they were looking at. Unlike the radio lobes, the optical emission from the center couldn?t be resolved, and it wasn?t possible to image the galaxy in the same way that we could take a picture of, say, our neighbor Andromeda. Fortunately, there was a solution: The 4000 discontinuity.
Elliptical galaxies are typically old, having formed over time from mergers and collisions of smaller galaxies of varying types. Star formation levels are low, meaning that there are relatively few young, hot, blue stars compared to star-forming spiral and lenticular galaxies. Now, at wavelengths a bit shorter than 4000 , there is a drop-off in emission thanks to absorption by metals in stellar atmospheres. In most galaxies, hot stars fill in this gap, when present. However, in elliptical galaxies, there are few hot stars, and so there is a ?discontinuity? in the spectra around 4000 .
Fig. 3, Kennicutt 1992. The steep dropoff in the spectra of NGC 4889, a supergiant elliptical galaxy, is characteristic of the 4000 discontinuity. As an E4 galaxy, it?s moderately ellipsoidal in shape.
The team found other spectral features corroborating the hypothesis that J1420?0545 is an elliptical galaxy. Now that they knew the sort of spectrum they expected to see, they could fit a model to it. Measurements of [O II] and Ca II absorption lines yielded a new redshift of z~0.03067, placing the object closer than originally thought. Since the redshift (and therefore the distance) was known, as well as the angular size of the radio cocoon, its size could be estimated ? assuming that the inclination angle was 90, as suggested by the weak emission from the core. A simple calculation showed that the jets must be 4.69 Mpc long.
How did it get so big?
A radio structure of this size isn?t unprecedented. The giant radio galaxy 3C 236 had already been discovered, and found to have a radio cocoon 4.4 Mpc in length. However, what was surprising about J1420?0545 wasn?t just its size, but its age. Best-fit models of the jet and ambient medium found the structure to have an age of about 47 million years; 3C 236, on the other hand, is thought to have been active for 110 million years ? more than double that. So why is J1420?0545, a relatively young radio galaxy, so large?
Fig. 1, Carilli & Barthel 1995. A radio galaxy?s narrow jets are surrounded by a bow shock at the boundary with the intergalactic medium, as well as a radio cocoon.
The answer turned out to be the intergalactic medium itself, the hot plasma that fills the spaces between galaxies. The IGM at the center of the galaxy is lower than at the center of 3C 236 by about a factor of 20, meaning that the gas pressure opposing the jets? expansion was correspondingly lower. The power of the AGN in J1420?0545 is also 50% greater than the AGN in 3C 236; this, combined with the substantially lower ambient IGM density, meant that the jets experienced much less resistance as they plowed into intergalactic space, and could therefore expand faster and farther in a shorter amount of time.
This of course just begs the question: Why is the local IGM so rarefied on so large a scale? Originally, the group thought that it was simply a naturally under-dense region of space, similar to a void ? an underdensity dozens of megaparsecs across that formed shortly after the Big Bang. However, after additional VLA and GMRT measurements (Machalski et al. 2011), they considered an alternative possibility: that the jets were the result of more than one round of AGN activity.
Double, double, radio bubbles
The team suggested classifying J1420?0545 as a double-double radio galaxy (DDRG). DDRGs exhibit two pairs of lobes that are aligned to within a few degrees, indicating that the central AGN underwent a period of activity, shut down, and then restarted. The key piece of information from the old VLA and GMRT data that suggested that J1420?0545 might be an extreme DDRG was the shape of its jets. The narrow jets are characteristic of double-double radio galaxies undergoing their second period of activity.
If the DDRG hypothesis is true, there should be a second faint outer radio cocoon surrounding the structure. After the first period of AGN activity, once the jets ceased, the cocoon should have quickly cooled through energy losses by synchrotron radiation and inverse-Compton scattering; with a suitable choice of parameters, it would be quite possible for it to be below the sensitivity of the VLA and GMRT. However, the team is hopeful that higher-sensitivity measurements in the future might be able to discover it.
In an interesting twist, it was suggested around the same time that 3C 236 is also a DDRG ? albeit one in the very early stages of its second period of AGN activity (Tremblay et al. 2010). A group observed four bright ?knots? near its core that were visible in the far ultraviolet. They appear to be associated with the AGN?s dust disk, and are about ten million years old.
Fig. 4, Tremblay et al. The star-forming knots in the core of 3C 236. The nucleus itself, hiding a supermassive black hole, is surrounded by dust lanes
3C 236?s two large radio lobes appear to be relic, and it has a smaller (~2 kpc) compact structure that seems to be much more recent. This is the key bit of evidence suggesting that it, too, might be a DDRG: The compact radio structure appears to be the same age as the knots, meaning that whatever event caused one likely caused the other. For instance, if a new reservoir of gas became available, it could fuel both AGN activity and a new round of star formation. If this is true, and the compact source ends up resulting in jets, it?s possible that 3C 236 could end up the size of J1420?0545 ? or larger.
I?ll end this post by discussing the question I posed in the title: Does J1420?0545 deserve to be called the largest known galaxy? We don?t know quite how large its stellar halo is, but it?s assuredly much smaller than the giant radio cocoon that surrounds it. At the same time, the cocoon represents a very distinct boundary between the galaxy and the intergalactic medium, and the shocked plasma inside it should behave quite differently from plasma in the IGM. Ironically, unlike normal elliptical galaxies that have diffuse halos, we can place a finger on where this giant ends and where intergalactic space begins.
One day, perhaps, we?ll find a giant radio galaxy even larger than J1420?0545, and the question will be moot. For now, though, I leave the question open? and I?ll wait for more VLA data. Clinching evidence of an outer cocoon could be around the corner. All we have to do is wait and see.