Genetically modified galaxies: The impact of different merger histories on stellar haloes

Title: How cosmological merger histories shape the diversity of stellar haloes

Authors:  Martin P. Rey, Tjitske Okay. Starkenburg

First Writer’s Establishment: Lund Observatory, Lund, Sweden

Standing: Submitted to MNRAS

Observations have proven that the celebrities in our Universe should not at all times gravitationally sure to a galaxy. Along with being surrounded by an invisible halo of dark matter, galaxies can be surrounded by stellar haloes. Stellar haloes are a roughly spherical, diffuse element of matter and are composed of stars which were displaced from their mum or dad galaxies because of the violent means of galaxy mergers (see this post for latest work on stellar haloes). Whereas the entire mass of stellar haloes surrounding galaxies tends to be a lot lower than the mass of the galaxies themselves, stellar haloes are thought to encode vital details about the merger historical past of a galaxy and its darkish matter halo, and have the potential to supply clues on how a galaxy has fashioned. 

The authors of as we speak’s paper use cosmological simulations to analyze whether or not totally different merger histories of galaxies can clarify the big variety noticed within the properties of current day stellar haloes. 

Modifying start circumstances to check totally different formation pathways

Testing whether or not a system’s merger historical past has an impact on its current day stellar halo is difficult to do in isolation. To cleanly isolate the impact of a galaxy’s formation historical past, as we speak’s authors make use of a method referred to as ‘genetic modification’ (see this astrobite for a complete overview of the method). Genetic modification is a method that’s utilized in mixture with cosmological simulations, and basically permits for a managed experiment to be run which might cleanly take a look at the impact of various merger pathways on a ensuing galaxy’s properties.

Briefly, this methodology works by deciding on a darkish matter halo that has fashioned in a cosmological simulation and has been advanced to the current day. This darkish matter halo’s historical past is then traced again to the start of the simulation, permitting the authors to establish the overdense area from which the halo initially fashioned. As soon as this preliminary area is discovered, it’s remoted and altered. By barely altering a small spatial area of the preliminary circumstances after which rerunning the simulation, the authors are capable of take a look at the impact of fixing the formation pathway of the person halo on the current day galaxy properties, since this preliminary modification can have an effect on the properties of the mergers that the system experiences over cosmic time. The ultimate mass of the system and the encompassing giant scale construction stays largely unaffected for all the totally different modified instances–all that adjustments is the precise path the halo took to succeed in its last mass!

The particular modifications examined in as we speak’s paper are utilized to 2 totally different reference methods: one late forming darkish matter halo and one early forming (known as Late Former and Milky Approach-like respectively within the paper). The modifications utilized to the Late Former object end in totally different timings of its main mergers, whereas the modifications utilized to the Milky Approach-like system end in adjustments to the plenty of the haloes that it has merged with prior to now. The thought right here is to check what impact these numerous merger eventualities have on the current day stellar haloes of those methods, with the whole lot else held equal.

Determine 1 shows the impact of the creator’s modifications on the evolution of mass over time, with the left panel exhibiting the Late Former system and the precise panel exhibiting the Milky Approach-like system. The black line within the prime panels reveals the common darkish matter mass progress over time for all darkish matter haloes of comparable last mass within the simulation. The orange line (left panel) refers back to the unique, unaltered Reference circumstances for the Late Former halo and the blue line (proper panel) refers back to the unique, unaltered Reference circumstances for the Milky Approach-like system. The opposite colored traces confer with modified histories that change the properties of the previous mergers, as evidenced by the totally different areas and trajectories of the sharp will increase in plenty at totally different instances. 


Determine 1. Mass progress over cosmic time for the 2 methods examined on this work and their numerous genetically modified (GM) histories (Late Former system on the left panel, Milky Approach-like on the precise). The darkish matter mass evolution is proven within the prime panel whereas the stellar mass evolution is plotted on the underside panel. The Reference, unaltered mass progress for every system is proven by the orange (left panel) and blue (proper panel) traces, with the median mass progress for darkish matter haloes proven in black. Variations to those reference circumstances utilizing the genetic modification method are plotted within the different colors, with the small print of the modifications listed in every legend. Tailored from Figures 2 and three of the paper.

Impact on the ensuing stellar halo

Now that the authors have chosen and modified their chosen methods, they will look into how adjustments to merger histories have an effect on the current day stellar haloes. Particularly, they examine how the fraction of mass within the stellar halo varies for every of their methods, after which examine these to noticed stellar halo fractions of comparable mass galaxies within the Native Quantity (a quantity that incorporates round 500 galaxies inside 10 Mpc of the Milky Approach).


Determine 2. The fraction of mass within the stellar halo plotted towards the stellar mass of the galaxy. Observations of stellar halo mass fractions for galaxies of comparable stellar plenty are plotted in gray, and the simulated methods are plotted in blue-green (Milky Approach-like) and brown-pink (Late Former), with the color scheme being in keeping with Determine 1. The gray arrows present the rising formation time of the darkish matter halo in every system, which is altered by the genetic modifications. For the Late Former, this can be a clear correlation between the stellar halo mass fraction and the formation time of the darkish matter halo. Determine 4 within the paper.

The primary takeaway from Determine 2 is that the unfold in stellar halo fraction seen in noticed Milky Approach-like methods (gray factors), will be recovered in simulated methods by various their merger histories. Moreover, for the Late Former system, a development is seen between the stellar halo mass fraction and the time {that a} galaxy skilled its main mergers, with early main mergers usually resulting in a decrease stellar halo fraction than later main mergers. Nevertheless, as will be seen from the upturn within the stellar halo fraction for the Milky Approach-like system, this development doesn’t at all times maintain. The authors be aware that summarising the scatter within the stellar halo mass fraction with one statistic, such because the formation time is difficult because of the advanced nature of a galaxy’s merger historical past, and that extra detailed metrics could also be required to seize the sensitivity of the stellar halo to the complete particulars of the system’s merger historical past.

In abstract, as we speak’s authors have leveraged the genetic modification method to indicate that systematically various merger eventualities can reproduce the scatter seen in observations of the stellar halo mass fraction. Whereas this outcome helps the concept that there’s a basic hyperlink between stellar haloes and the merger historical past of methods, the outcomes additionally spotlight the problem in summarising this scatter by means of a one-point statistic. Galaxy formation is a posh course of, and stellar haloes seem like delicate to all the vary of previous merger interactions, making characterising the variety of stellar haloes a enjoyable (however difficult) pursuit!

Astrobite edited by Alex Pizzuto

Featured picture credit score: NASA, ESA


About Katy Proctor

I’m a first-year PhD pupil on the Worldwide Centre for Radio Astronomy Analysis on the College of Western Australia. My analysis is concentrated on utilizing cosmological simulations to review the construct up of stellar halos. Exterior of analysis, I can often be discovered climbing up partitions or taking part in guitar.

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Genetically modified galaxies: The impact of different merger histories on stellar haloes


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