OTROS MATERIALES PREFABRICADOS

It has been well known for some time that galaxy environments play a role in the structural evolution of galaxies. Dressler (1980) established the Morphology-Density relation, which

shows that in dense environments, those within galaxy groups and clusters, galaxies are more likely to have early-type morphologies. Over the decades many mechanisms have been theorised to explain these relationships. Major and minor mergers provide pathways for transformative, and in some cases destructive, changes to a galaxy’s shape, mass and size by adding new material. Conversely, tidal stripping, ram pressure stripping, and galaxy-galaxy harassment all provide ways for mass to be removed from a galaxy. Depending on a galaxy’s environment, they may be more likely to be affected by one set of processes than another.

Many authors have established that the Size-Mass relation for galaxies was independent of environment (Baldry et al., 2006; Huertas-Company et al., 2013; Kauffmann et al., 2004; Shankar et al., 2014). That is, galaxies at fixed mass have the same average size regardless of their local environment. This is interesting, because as I stated above, galaxies in different environments are subject to different structurally transformative processes. Central galaxies, in all but the largest halos, are typically stationary in their gravitational potential wells and as such are not subject to ram pressure stripping or tidal stripping, and as the largest galaxies in the group they do not typically experience harassment. However, due to their prime position in the halo, they are perfectly poised to experience numerous mergers, and cannibalise smaller satellite galaxies to increase their size and mass (Hirschmann et al., 2013; McCavana et al., 2012). Satellites are not typically subject to minor mergers, but do experience ram pressure stripping due to their motion through intracluster medium, tidal stripping due to their offset from the centre of the gravitational potential and harassment from interactions with other satellites (Abadi et al., 1999; Balogh et al., 2004; Dekel et al., 2003; Gunn and Gott, 1972; Larson et al., 1980; Pasquali, 2015; van den Bosch et al., 2008b).

A key point to all the above processes, except major mergers, is that in theory they should preferentially act on the outskirts of a galaxy. In elliptical galaxies in particular, the galaxy core is somewhat shielded from the effects of the outer environment. Adding and removing stars and gas from the galaxy is like peeling or adding layers to an onion. In addition, these processes explicitly add or take away stellar mass, and as such pose an interesting point when you consider the Size-Mass relation. If stars are removed from a galaxy by tidal stripping, the galaxy’s size must decrease as well, and vice versa for minor mergers. If the masses of galaxies are changing, is it even valid to compare galaxies in different environments based on their mass? Two galaxies that begin their lives in similar conditions may have the same mass, but if one is accreted onto a larger dark matter halo it becomes subject to dramatically different forces on its evolution. Two galaxies with similar masses now, may have had significantly different masses in the past, and so comparing galaxies at fixed mass doesn’t necessarily capture the environmental aspect of the evolution. For this, I require a galaxy property that is invariant to environment.

In Chapter 2, I chose to study the relationships between core velocity dispersion (σ0),

size, mass and metallicity. σ0is the measure of the motion of stars in the very core of the

galaxy. With the exception of major mergers, it is likely much less affected by environmental processes than stellar mass. It is perfect, then, for studying any differences in the structural evolution of central and satellite galaxies.

For star forming galaxies, I found very little difference in the masses and sizes of centrals and satellites at fixed σ0. This suggests that star forming satellites may not have been

satellites for very long. Recent studies have shown that satellites may quench their star formation quite rapidly once they are accreted onto a new dark matter halo, as such it may be the case that these satellites have not had the time to be stripped or harassed yet (Bluck et al., 2016; Knobel et al., 2015; Oman and Hudson, 2016; Smethurst et al., 2017; Wetzel, 2011; Wetzel et al., 2013). Star forming galaxies are also typically disks, which may affect how the environmental processes act on their mass and size.

For quiescent galaxies however, I see that there is a significant difference in the Mass-σ0

and Size-σ0 relations for centrals and satellites. At low σ0I find that centrals are around

5% larger than satellites, but at the high σ0end the centrals are about 30% larger. Centrals

are also consistently more massive than satellites, but the degree to which this is the case is smaller. This is due to the shape of the Size-Mass relation; small changes of mass in high mass galaxies causes larger changes in size than in low mass galaxies. We also show that these differences are not due to residual differences in the star formation and Sérsic indices of the centrals and satellites.

We took this research a step further by investigating the radial mass profiles of centrals and satellites. We constructed mass profiles from the r-band and g-band colour profiles from SDSS photometry. In the cores of both star forming and quiescent galaxies, the stellar mass density is the same for central and satellite galaxies. However as I study the outer regions of these profiles, I see that centrals have higher mass and mass density than satellites. These profiles are crucial to the results of this study, for they confirm two things. Firstly, that the cores of central and satellite galaxies are unaffected by the different environmental processes. Secondly, that the environment acts to add and remove mass preferentially from the outer regions of centrals and satellites, respectively. The differences between the profiles follow the same trends and the Size-σ0and Mass-σ0profiles, they are stronger for quiescent galaxies

than in star forming ones, and there is a dependence on σ0for the strength of the difference.

Finally, I studied the metallicity of central and satellites, as measured within the 3" fibre in SDSS. As this metallicity is measured within the inner regions of the galaxy, I expected to find little difference in the Mass-Metallicity and σ0-Metallicity relations. Indeed, I found

in metallicity between centrals and satellites in the mass-metallicity relationship. This differences was found to be entirely consistent with the Mass-σ0relation. Metallicity then, at

least in the core of the galaxy, is also unaffected by environmental processes.

Taken together, these results suggest three possibilities. That centrals are growing due to mass deposition from minor mergers, that satellites are shrinking due to the removal of mass via ram pressure, tidal stripping and harassment, and that satellites that quench after being accreted onto new halos have different masses and sizes than those that quench as centrals.

We expect that, due to the stellar-to-halo mass relation, that more massive (higher σ0)

galaxies would accrete more mass via minor mergers than low mass (low σ0) galaxies. This

explains why I see that the differences in mass and size are larger for high σ0. These galaxies

are subject to a higher rate of mergers, and as such more of their mass comes from this process (Naab et al., 2009). Tidal stripping is perhaps consistent with our results, as the expected mass loss, between a few percent up to 20%, agrees with our results. It is expected that there would be a dependence on stellar mass, with smaller galaxies in higher mass halos losing more mass. However, the difference between the Mass-σ0relationships is fairly flat,

which seems to be inconsistent with the mass dependent stripping fraction seen in galaxy cluster simulations (Bahe et al., 2016). Fianlly, I expect that the corrections I have made to ensure the star formation rate and Sérsic index distributions are the same in all of our samples would cancel out any differences caused by where and when satellite galaxies quench. It remains the case that different quenching mechanisms between centrals and satellites may mean that satellite galaxies that quenched as satellites are larger and more massive than satellites quenched as centrals. This is due to star forming galaxies being larger at fixed σ0

than quiescent galaxies.

Ultimately, it appears that minor mergers providing a way for quiescent central galaxies to continue growing is the main driver for the differences in stellar mass and half-light radius I see at fixed σ0. We expect that tidal stripping plays a small role at low σ0, but due to the

relationship between expected mass loss and total stellar mass, at high σ0essentially all of

the differences in size and mass can be attributed to minor mergers that have occurred over the last 3 Gyr, or the average time-scale for satellite in fall (Wetzel et al., 2013).