Cressida Cleland
It has been established that as galaxies evolve, their optical colours change from blue to red. Blue galaxies are usually actively forming stars, while red galaxies are considered dead, or `quenched’. Thus, an important aspect of galaxy evolution is the reduction or quenching of star-formation. Many authors have researched the mechanisms that cause quenching, however, it is difficult to determine which (if any) of these processes are dominant, as they are heavily dependent on each other. One important factor to consider is how the local environment of a galaxy affects quenching, i.e., if the galaxy is in a dense environment or not. This work investigates recently-quenched galaxies in groups, using central/satellite distinction. To constrain the timescale of the quenching we combine H-alpha flux and UV flux, noting that H-alpha traces star-formation on timescales of 10 million years and UV traces star-formation on timescales of 100 million years. This implies that galaxies with significant UV but negligible H-alpha have ceased star-formation in the past ~100 million years. We define these galaxies as `transient’ galaxies. By exploring the properties of these galaxies, we get insight into how the environment of galaxies directly affects their star-formation. We find that transient galaxies occur as strong functions of stellar mass and group halo mass, whereby galaxies at lower stellar mass are more susceptible to environmental effects, and this effect is stronger when the group has a larger halo mass.
Here, we look at the environmental effects on galaxy quenching. If a galaxy is a member of a group of galaxies, it is called a satellite galaxy. If a galaxy is on its own, or at the centre of the group, it is called a central galaxy. Satellite galaxies feel environmental effects more than centrals.
The emission line H-alpha can identify star-formation that occurred within the last 10 million years, while UV light can identify star-formation that occurred within the last 100 million years.
This means if a galaxy that has strong UV, but negligible H-alpha, it must have stopped forming stars in the last 100 million years or so. We call these galaxies that have recently quenched ‘transient’ galaxies. Rejuvenating galaxies are defined as galaxies which have recently begun star-forming again after being quenched.
We explore properties of transient galaxies to try determine which factors are dominant in galaxy quenching.
Figure 1 shows a plot of the fraction of transient galaxies versus stellar mass for both central galaxies and satellite galaxies. At low stellar mass, the fraction of transient galaxies for satellites is higher than centrals. At high stellar mass, both fractions increase at roughly the same pace. This implies that the environmental factor is what causes the quenching at low mass. At high mass, the increase in mass is the more important factor.
Figure 2 shows a plot of the fraction of rejuvenating galaxies versus stellar mass for both central galaxies and satellite galaxies. In both satellites and centrals, the fraction is close to 0. This shows that when transient galaxies quench, they do not tend to rejuvenate periodically. The quenching event is one-off.
Figure 3 shows the fraction of transient galaxies over star-forming galaxies and the fraction of transient galaxies over quenched galaxies as a function of radius, binned by halo mass. In this figure, we see that more transient galaxies are found close to the group centre. We also see that the transient fraction increases with group halo mass. This implies a larger group imparts a stronger gravitational pull on satellite galaxies, which would increase the possibility of ram-pressure stripping or violent mergers.
Figure 4 shows histograms of low and high stellar mass transient and star-forming galaxies as a function of radius. We see that low stellar mass transients are distributed towards low group radii with a gentle slope, whereas high stellar mass transients have a steep slope, closer to the group centre. This implies that low stellar mass satellites are more easily quenched by the environment, but high stellar mass satellites only quench after spending some time in the group.
Figure 5 shows histograms of low and high group halo mass transient and star-forming galaxies as a function of velocity offset. Here, we find that satellites in groups of higher mass have larger velocity offsets compared to the velocity of the group. This supports the possibility that higher mass groups would have a higher chance of causing ram-pressure stripping.
It seems plausible that environmental processes are dominant for low stellar mass satellites and high halo mass groups in particular. However, we also find that the transient distributions are identical to the star-forming distributions. If environmental processes were dominant, then those star-forming galaxies would also be starting to quench. This leaves one (or a combination) of two possibilities: the group isn't totally efficient at quenching, or the properties of the satellite are involved in the quenching. This is the topic of future research.