Ankit Singh

Galaxies convert gas into stars while embedded inside dark matter halos formed by gravity aggregating primordial fluctuations. They can be broadly classified into two types: actively star-forming (blue in colour) and passively star-forming (red in colour). One way to trace the star formation history of the Universe is to measure cosmic star formation rate density (CSFRD) which measures stellar mass formed per Myr in per cubic Mpc in the Universe. CSFRD peaks after 3.5 Gyr from the Big Bang and decreases to its present value. What leads to this decrease is still a topic of research.

At large-scale galaxies appear to be decorating intricately woven linear structures giving rise to colossal collection of mass wherever they intersect. The vicinity of a galaxy, called the environment, can be classified depending upon the number density of galaxies surrounding it. Galaxies residing in the high-density environment called groups or clusters, form less stars compared to similar galaxies in the low-density environment called field. Observations point out that half of the baryons in the Universe are expected to reside in the intermediate density environment called filament in the form of the warm-hot intergalactic medium (WHIM). Thus, the story of galaxy evolution is incomplete without knowing how the filaments affect galaxy properties. Our study focuses on the filament environment and how it affects the properties of galaxies residing in it.

In simulations, one has all the information about the galaxies formed, and thus it enables us to do mock-observations. We use Evolution and Assembly of GaLaxies in their Environment (EAGLE) simulations. We select two clusters with a total mass M ≥ 10^{14} solar mass at a time and cut a slice of thickness 60 Mpc from the simulation box. We project this 3D slice to a 2D plane and characterize the environment of galaxies. We make use of the friends-of-friends algorithm to identify galaxies belonging to group or cluster environment and use Discrete Persistent Structures Extractor (DisPerSE) to identify galaxies belonging to filament environment. The remaining galaxies are taken as belonging to the field. As there are 9 clusters which satisfy our criterion, we get 36 mock observations. We study the variation in the properties of galaxies in filaments as a function of their perpendicular distance from the spine of the filaments (d_{per}).

We find that median colour of the galaxies becomes redder as the d_per decreases and saturates around 1 Mpc with a decrease in colour very close to the spine. We define passive galaxies having specific star formation rate (sSFR = star formation rate/ stellar mass) less than 0.01 Gyr^{-1}. The fraction of passive galaxies in a bin of d_{per} increases with decreasing d_{per}. The median sSFR and gas mass fraction (µ_{gas} = gas mass/total mass) decrease on decreasing d_{per}. There is an increase in stellar mass fraction (µ_{star} = stellar mass/total mass) with decreasing d_{per}. The star-forming gas (with Temperature ≤ 10^4 K) metallicity (Z_{SF}) within d_{per} ≲ 2 Mpc is elevated relative to gas further away. The non-star forming gas metallicity (Z_{NSF}) increases smoothly with decreasing d_{per} , indicating accretion of non-star-forming gas on the filament galaxies.

To conclude, in this study we observe a colour trend is observed within a radius of 2 Mpc around the spine of the filament, thus providing an upper limit for the filaments’ radius. The intrafilamentary gas condenses into the filament galaxies thus fueling star formation in them. The trends in galaxy properties are a consequence of increased gravitational interactions caused by an increased number density of galaxies between filament galaxies closer to the spine of the filament.