Title: An enigmatic 380 kpc long linear collimated galactic tail
Authors: Dennis Zaritsky, Jacob P. Crossett, Yara L. Jaffé, Yara L., Richard Donnerstein, Ananthan Karunakaran, Donghyeon J. Khim, Ana C. C. Lourenço, Kristine Spekkens, Ming Sun, Benedetta Vulcani
First Author’s Institution: Steward Observatory, University of Arizona, Tucson, AZ, 87521, USA
Status: Published in MNRAS [open access]
The interactions between galaxies and between a galaxy and its surrounding intergalactic medium (IGM) will leave signatures of the interaction on the surviving structures. Major interactions like galaxy mergers and accretion are central to hierarchical galactic formation theories. In recent years a number of new ‘tail’ or ‘tendril’ structures have been uncovered by our improving detector technology and new large all-sky surveys. Tail structures can be useful to explore the local dark matter distribution, the formation of dark-matter-free dwarf galaxies, star formation in extragalactic environments, and the characteristics of the IGM.
In this paper, the authors discuss a unique tail structure that appears to extend from an edge-on galaxy, which they nickname the Kite. The Kite is a type S0/a galaxy, also known as a lenticular galaxy, which spans the gap between orderly spiral and blobby elliptical galaxies. These kinds of galaxies have used up their interstellar gas in star formation and don’t have a lot of free gas or star formation going on anymore. The Kite’s tail was discovered in the optical band by luck during the search for low surface brightness galaxies in the DESI Legacy Survey; the full system is shown in Figure 1.
The Kite is itself interesting because it is one of a pair of binary galaxies, both with active galactic nuclei (AGN) at their center. The companion galaxy, Mrk 0926, is 57 kpc away and sits at the same redshift of z=0.0470. The region is not part of any known cluster or group and the background IGM is likely very low density, which is important when considering formation methods later in the paper. Along the tail there is a small galaxy, nicknamed Kite A, that is part of the local system. There is also another galaxy 300 kpc away that the authors discount as being relevant.
The tail itself has an estimated linear extent of 380 kpc based on the adopted distance to the system of 187 Mpc. This is one of the longest known tails, more than 6 times longer than the tendrils in the largest jellyfish galaxy. It has defined knots of emission amidst the diffuse emission, which the authors pick out as unique features a-j in Figure 1. The width of the tail is about the same as the Kite, giving it a length to width ratio of ~40. Near-UV observations from GALEX show emission from some of the knots in the tail, which usually indicates the presence of young (~few 100 Myr old) stellar populations. This suggests that star formation is occurring outside of any galaxy in the tail.
Spectroscopic follow-up observations confirmed that the features all have consistent redshifts and are local to one another. Kite A is certainly in the vicinity, even if they cannot definitively place it within the tail feature. The spectra also show a velocity gradient along the tail, suggesting that it is not flat on the sky and the 380 kpc is actually under-estimating its length.
Formation Methods
There are two major methods by which these kinds of tail structures are thought to form: ram pressure stripping and tidal deformation.
In ram pressure stripping, the pressure due to the denser external medium rips gas away from the galaxy, forming tails or tendrils. This is how jellyfish galaxies in cluster environments are thought to form. This is an obvious interpretation for the Kite, but the surrounding environment is not likely to be dense enough to produce the observed structure.
Tidal disruptions are due to the gravitational interactions between two bodies, and are known to form tails (Fig. 3). If the Kite’s tail is the result of tidal forces, where does it originate from? The Kite galaxy shouldn’t have enough gas in it to produce such an extended feature with ongoing star formation. If it is due to the interaction of Mrk 0926 and the Kite, then we would expect to see a kind of bridge form between the two galaxies, disruptions to their morphologies, and likely a second tail extending from Mrk 0926.
It might be possible that the Kite shredded a low mass satellite galaxy, which Kite A might be the last remaining piece of. Ripping up another galaxy could explain where all of the tail gas came from, but would require specific circumstances. The satellite would have to essentially hit the Kite head-on and the Kite would need to have a very compact gravitational potential for the energy to work out.
A major challenge in this analysis is determining the age of the tail. Based on simple kinematic arguments (time = distance / velocity) and known galactic gas speeds of a few hundred km/s, they estimate a formation timescale of a ~Gyr. However, estimates of the star formation history of the clumps in the tail using ultraviolet measurements of Kite A suggest a young, <300 Myr old stellar population. If the tail is ~Gyr old, then either something needs to have triggered the star formation long after the gas moved away from the Kite, or the star formation rate was somehow maintained over that time. Additionally, measuring the dispersion of features in the tail shows that either something is actively maintaining the collimation of the tail, or the tail has to be much younger. If it is younger, then the velocity in the gas must be much higher than typical galactic gas velocities. They also expect older tails to have experienced more torques, so the linearity of the tail is in favor of a younger tail.
The authors suggest that the system is due to a tidally disruptive three-body problem between the Kite, Kite A, and Mrk 0926. The two more massive galaxies ejected Kite A from their system, which caused it to tidally disrupt and form two tails with itself in the center, consistent with the observational positions. Assuming the 300 Myr age is accurate, Kite A would have traveled at 460 km/s, which is faster than the necessary escape velocity from the system. It also explains where the gas came from, since the main Kite galaxy is too gas depleted. This interaction may also have been the kickstarter to activate the AGN in Mrk 0926 and Kite.
While this system is in and of itself interesting, it does have larger reaching implications for these types of tails. Star forming clumps in the tail are expected to not contain any dark matter, and if they remain gravitationally tied together, they will become new members of the population of dark matter free galaxies. Dark-matter-free dwarf galaxies have stumped astronomers for some time, and this is a potential formation mechanism. This kind of satellite ejection could also explain some observed diffuse isolated stellar systems in the Virgo cluster. If those populations have a high metallicity, it could indicate a tidally disrupted galactic origin.
The Kite is just one of a new class of tail and tendril systems being uncovered by major sky surveys. Its unusual size, configuration, and star formation abilities make it a valuable new piece of evidence in the study of galactic evolution and galaxy-IGM interactions.
Astrobite edited by Samantha Wong
Featured image credit: Zarinsky+23 Figure 1, Ammar Ahmed
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