Authors: Dennis Zaritsky et al.
First Author’s Institution: Steward Observatory, University of Arizona, Tucson, AZ
Status: Submitted to MNRAS
When studying the histories and interactions between the galaxies in our universe, the structures known as galactic tails can often be key signatures in uncovering the pasts of the island universes that make up the cosmos we observe. Studies of the trailing tails of gas and dust that galaxies sometimes leave behind have triggered revolutions in our theories of galactic mergers and galaxy evolution, with the most extreme examples pushing the boundaries of our understanding of these mechanisms.
The authors of today’s paper report the discovery of perhaps the longest optically-visible galactic tail ever detected – which, besides its length, contains a number of enigmatic properties and interesting features that make explaining its origins challenging – but not impossible.
The Kite with the Longest Tail
The galaxy which this paper has named “Kite” (due to its tail) was initially discovered serendipitously by the authors, as they searched for low surface brightness galaxies in the Legacy Survey images. The algorithm they used to filter for ultra-diffuse galaxy candidates selected the feature labeled ‘i’ in Figure 1 as a candidate – upon visual inspection, they identified this feature as part of a long galactic tail that led back to Kite. Figure 1 shows the galactic system in full detail: two galaxies (Kite and a companion galaxy called Mrk 0926), 10 salient features along the tail (labeled a through j), and a candidate disrupted galaxy named ‘Kite A’ halfway along the tail, which will be discussed later on.
The full extent of the tail covers a region of 7 arcminutes in the sky; at Kite’s projected distance of 187 Mpc (187 million parsecs) from us, this corresponds to a tail length of 380 kpc (380,000 parsecs, or ~1.2 million light years), the longest optical tail of which the authors are aware. Furthermore, the tail keeps its length-to-width ratio the same throughout itself, rather than spreading out or diffusing outwards as it extends, and is also remarkably linear, with almost all the labeled features falling neatly into a straight line with each other and with Kite.
As for the features themselves: the tail contains several well-defined areas of brighter emission (labeled a-j) interspersed with very diffuse emission along the tail, as well as a smaller galaxy named ‘Kite A’ which appears to be projected onto the tail from our point of view. Near-ultraviolet emission (from the GALEX Medium Imaging Survey) indicated that these features also have young stars within them, which we wouldn’t expect from Kite – a type of galaxy known as an S0/a galaxy, which is gas-poor, and thus wouldn’t have enough material to make stars along the full extent of the tail.
Further observations of Kite and its tail were taken by the authors using the Baade Magellan telescope and the IMACS spectrograph, producing spectroscopic observations of the region. The observations were done in the wavelengths around the H-alpha line, tracing the emission from molecular clouds and star-forming regions, and revealed that the tail’s features indeed showed signs of star formation. The redshift measured in the spectra of each feature also showed that these features, as well as the smaller Kite A, were at the same distance from Earth as Kite, and thus squarely within the tail.
Drawn Out by the Tides?
There are multiple interpretations for the origin of Kite’s tail, each of which have their issues. The typical source for galactic tails is thought to be ram pressure stripping – in essence, as a galaxy moves through a denser galactic medium (such as within a galaxy cluster), the gas and dust within the galaxy feel drag pressure as they move through the medium. However the intergalactic medium around Kite is actually quite sparse – too sparse for such a strong tail to be the result of this effect.
Another explanation for Kite’s tail could be tidal interactions during a close encounter between Kite and a neighboring galaxy. Normally such interactions would result in an elliptical tail following the galaxy’s orbit, and if Mrk 0926 would be the obvious candidate for a tidal interaction, then also likely lead to a bridge of material between the two galaxies. It could be a coincidence in the orientation of the interaction relative to our point of view, but neither of these effects is observed.
Alternatively, a tidal origin for the tail could result from a near destruction of a smaller satellite galaxy – such as Kite A. This would explain where all the extra gas in the tail (enough to ignite star formation) came from, as well as the lack of disturbances in Kite’s and Mrk 0926’s morphology. However this interpretation has its own issues – it would require the satellite galaxy to orbit Kite A in the same plane as its disk (in order to reproduce the alignment seen in observations), and the large distribution of material along 380 kpc would not only imply a direct collision between the two galaxies, but also a very tight matter distribution for Kite, such that material from Kite A that collides at a slightly further distance from the center of interaction than neighboring material would be scattered a much larger distance.
Regardless of the origin method, there are additional puzzles enveloped in the tail. Estimating the age of the tail by dividing its length by the transverse velocity away from Kite at its tip gives age estimates of one to several Gyr for the tail (1 Gigayear = 1 billion years). However, given that there is active star formation and young stars across the tail, an age of >1 Gyr implies that either star formation has been actively maintained for this long, or that some effect triggered star formation throughout the entire thing recently. Both scenarios are unlikely, though not impossible. Additionally, the fact that the tail is both so thin and has maintained its width implies that either some mechanism is actively maintaining the tail’s collimation, or the tail is much younger than estimated – implying its velocity with respect to Kite is much faster than estimated, too.
A Tail as Young as Time
Given reasons to favor a shorter lifetime for the tail, as well as the presence of Kite A within it, the authors propose that the tail formed from a hyperbolic ejection of Kite A out of the Kite-Mrk 0926 system after a three-body gravitational interaction. Under this explanation, Kite A would have been ejected at velocities significantly higher than the escape velocity for the system, allowing for a much younger and faster moving tail, as well as explaining why the tail is linear and thin, rather than elliptical and diffused outwards. Tidal interactions from this kind of event would leave a trail of material both in front and behind Kite A, as is observed. Furthermore, an age of around 300 Myr (1 Megayear = 1 million years) instead of >1 Gyr, estimated from the youngest stellar populations in Kite A, would imply a mean transverse velocity for Kite A that is certainly larger than its escape velocity at its current position, consistent with the hypothesis.
In order to verify the accuracy of this proposal, detailed simulations of the system will need to be made. Regardless whether the author’s hypothesis is correct, however, the observational facts of Kite’s tail reveal a curious, enigmatic system that pushes the limits of current theories of galactic tail formation and galaxy interactions; testing the limitations of our theories and pushing the boundaries on how we come to understand these aspects of our universe.
Astrobite edited by Mark Popinchalk
Featured image credit: NASA, H. Ford (JHU), G. Illingworth (USCS/LO), M. Clampin (STScI), G. Hartig (STScI), the ACS Science Team, and ESA.