Cold Horseshoes in Fast Outflows

The dynamic environments around active galaxies often exhibit delicate filaments of cold gas. In a new study, scientists have explored how these fragile structures are able to form and survive within their hot, fast-moving surroundings.

Curious Structures

Image of NGC 1275 that contains outlines and insets identifying two particular filamentary structures

In this annotated image of NGC 1275 (click to enlarge), outlines and insets identify two filamentary structures: the blue loop (dotted outline and bottom left inset) and the horseshoe filament (dashed outline and top right inset). These two strikingly shaped filaments may both have been created during the same outburst. [Annotations: Yu Qiu]

The Perseus cluster, located more than 200 million light-years away, is a collection of thousands of galaxies embedded in a cloud of hot gas. At the cluster’s heart lies NGC 1275, an active galaxy that’s rapidly forming stars and contains an accreting supermassive black hole — two factors that result in outbursts of hot, fast outflows that are spewed into the intracluster medium.

In the midst of all this action, there’s a conundrum: we also see cold, outflowing gas that forms slender, elongated filamentary structures extending tens of thousands of light-years. Where does this cold gas come from, and how is it not heated or destroyed by the fast, hot outflows of the active galaxy?

Sweeping Up Old or Forming New?

Two explanations have been proposed for these cold outflows:

  1. The hot winds flowing from the active galaxy sweep up existing cold gas and carry it along, drawing it out into filaments.
    This idea has a challenge: long before the cold gas manages to reach the speeds we observe — more than 100 km/s! — it would likely be destroyed by shocks, preventing the formation of filaments.
  2. The cold gas forms within the hot outflows as these winds slow, cool, and fragment into filaments.
    This idea shows promise! In a new study, a team of scientists led by Yu Qiu (邱宇; Peking University, China) has explored this possibility further using a set of detailed simulations of an outbursting active galaxy.

Threads, Loops, and Horseshoes

Plots showing the shapes and speeds of cold gas in two simulations at three times. The shapes of the resulting filaments are very different in the two simulations.

The shape and speed of cold gas that forms within the outflows in two of the authors’ simulations (top and bottom) at three different times (left, middle, and right). The two simulations, which had different starting conditions, produce very different shapes of filaments: the top is long and threadlike, whereas the bottom is a perpendicular ring structure. [Qiu et al. 2021]

Qiu and collaborators’ 3D hydrodynamic simulations model a hot, radial outflow erupting from the center of a cluster similar to Perseus. From these simulations, the authors show how gravity and pressure from the surroundings cause the hot outflow to slow and cool. They confirm that this process eventually leads to fragmentation, forming filaments of cold gas that move at high speeds consistent with what we observe.

One especially interesting result of the authors’ work: the shapes of the resulting filaments depend strongly on the starting conditions of the outflow. This could explain some particularly striking shapes that we observe in Perseus — there are not only radial threads, but also a loop and a horseshoe at opposite sides of the central galaxy.

The authors show that a bipolar outburst with specific physical conditions can create two perpendicular rings of cold gas instead of long filaments — which could easily reproduce the loop and horseshoe we see in Perseus.

Qiu and collaborators demonstrate how we can use the morphology and locations of the filaments to probe the history of the active galaxy’s outbursts, inferring their energetics and properties. Further study of these delicate threads, loops, and horseshoes is sure to provide a wealth of new information about distant, active galaxies and clusters.


“Dynamics and Morphology of Cold Gas in Fast, Radiatively Cooling Outflows: Constraining AGN Energetics with Horseshoes,” Yu Qiu et al 2021 ApJL 917 L7. doi:10.3847/2041-8213/ac16d9