In their study, O'Neil et al. investigate the impact of galaxy selection on the splashback boundaries of galaxy clusters using the IllustrisTNG cosmological galaxy formation simulation. The splashback radius ($R_{\rm sp}$) is measured by analyzing the number density of the subhalo population in stacked clusters within specific mass ranges. The authors focus on clusters with halo masses ranging from $10^{13} \leq M_{\rm 200, mean} / {\rm M}_\odot \leq 10^{15}$ and apply various selection criteria including cuts in subhalo mass, galaxy stellar mass, $i$-band absolute magnitude, and specific star formation rate. By examining these different criteria, they aim to understand how the measured splashback radius changes relative to the intrinsic dark matter radius. The results show that galaxies with higher masses and luminosities tend to trace smaller splashback radii compared to the dark matter radius. This suggests that changes in the galaxy population play a role in determining the observed trends. Interestingly, the study also reveals that quenched galaxies can be used to reliably reconstruct the dark matter splashback radius. Furthermore, O'Neil et al. address discrepancies between observational predictions of $R_{\rm sp}$ based on galaxy number counts and dark matter measurements. They attribute these differences to variations in selection functions employed by different studies, such as those using optically- or SZ-selected clusters. By considering these selection effects, they are able to reconcile conflicting predictions. The authors demonstrate that changes in $R_{\rm sp}$ measured through number counts cannot be solely attributed to a simple change in galaxy abundance inside and outside of the cluster. This finding highlights the complex nature of splashback boundaries and emphasizes the importance of considering multiple factors when studying them. Overall, this study provides valuable insights into how galaxy selection affects our understanding of splashback boundaries in galaxy clusters. The findings contribute to ongoing efforts to improve our knowledge of the relationship between galaxies and dark matter structures in the universe.
Summary- The researchers studied how different ways of choosing galaxies affect the boundaries of galaxy clusters.
- They measured a special distance called the splashback radius by looking at groups of galaxies in the clusters.
- They used different rules to decide which galaxies to include, like their size and brightness.
- Bigger and brighter galaxies tend to have smaller splashback radii compared to the dark matter around them.
- Galaxies that are not forming new stars can help us figure out the size of the dark matter splashback.
Definitions
- Galaxy selection: Choosing which galaxies to study or include in a group.
- Splashback radius: A special distance that shows where galaxy clusters end.
- Subhalo population density: How many smaller groups of galaxies there are in a cluster.
- Mass range: A specific range or amount of mass (how much stuff) something has.
- Luminosity: How bright an object is.
Exploring the Impact of Galaxy Selection on Splashback Boundaries in Galaxy Clusters
Galaxy clusters are large collections of galaxies bound together by gravity. They are among the largest structures in the universe and provide valuable insights into how galaxies interact with dark matter. In a recent study, O'Neil et al. investigate how galaxy selection affects our understanding of splashback boundaries in galaxy clusters using the IllustrisTNG cosmological simulation. The authors focus on clusters with halo masses ranging from $10^{13} \leq M_{\rm 200, mean} / {\rm M}_\odot \leq 10^{15}$ and apply various selection criteria including cuts in subhalo mass, galaxy stellar mass, $i$-band absolute magnitude, and specific star formation rate. By examining these different criteria, they aim to understand how changes in the observed population can influence measurements of splashback radii ($R_{\rm sp}$).
Measuring Splashback Radii
The authors measure $R_{\rm sp}$ by analyzing the number density of subhalos within stacked clusters within specific mass ranges. This method allows them to compare their results to previous observational predictions based on galaxy number counts as well as dark matter measurements. By considering both types of data sets, they are able to gain a more complete picture of how changes in galaxy population affect our understanding of splashback boundaries.
Results
The results show that galaxies with higher masses and luminosities tend to trace smaller splashback radii compared to those measured through dark matter simulations alone. This suggests that variations in selection functions employed by different studies can have an impact on our understanding of these boundaries. Furthermore, quenched galaxies were found to be reliable proxies for reconstructing dark matter splashback radii accurately when compared with other methods such as optical or SZ-selected cluster samples.
Implications
Overall, this study provides valuable insights into how changes in galaxy populations can influence our understanding of splashback boundaries in galaxy clusters. The findings contribute to ongoing efforts to improve our knowledge about the relationship between galaxies and dark matter structures in the universe and emphasize the importance of considering multiple factors when studying them.. Additionally, it highlights discrepancies between observational predictions based on number counts versus those derived from dark matter simulations which may be attributed to differences between selection functions used by different studies
We explore how the splashback radius ($R_{\rm sp}$) of galaxy clusters, measured using the number density of the subhalo population, changes based on various selection criteria using the IllustrisTNG cosmological galaxy formation simulation. We identify $R_{\rm sp}$ by extracting the steepest radial gradient in a stacked set of clusters in 0.5 dex wide mass bins, with our clusters having halo masses $10^{13} \leq M_{\rm 200, mean} / {\rm M}_\odot \leq 10^{15}$. We apply cuts in subhalo mass, galaxy stellar mass, $i$-band absolute magnitude and specific star formation rate. We find that, generally, galaxies of increasing mass and luminosity trace smaller measured splashback radii relative to the intrinsic dark matter radius. We also show that quenched galaxies may be used to reliably reconstruct the dark matter splashback radius. This trend is likely due to changes in the galaxy population. Additionally, we are able to reconcile different observational predictions that $R_{\rm sp}$ based upon galaxy number counts and dark matter may either align or show significant offset (e.g. those using optically- or SZ-selected clusters) through the selection functions that these studies employ. Finally, we demonstrate that changes in $R_{\rm sp}$ measured through number counts are not due to a simple change in galaxy abundance inside and outside of the cluster.
