ALBERT

Description: We present a study of the physical properties of the disc and tail of ESO137–001, a galaxy suffering from extreme ram-pressure stripping during its infall into the Norma cluster. With sensitive and spatially resolved MUSE (Multi Unit Spectroscopic Explorer) spectroscopy, we analyse the emission line diagnostics in the tail of ESO137–001, finding high values of [N ii ]/Hα and [O i ]/Hα that are suggestive of the presence of shocks in turbulent gas. However, the observed line ratios are not as strong as commonly seen in pure shock heating models, suggesting that other emission mechanisms may contribute to the observed emission. Indeed, part of the observed emission, particularly at close separations from the galaxy disc, may originate from recombination of photoionized gas stripped from the main body of ESO137–001. We also identify a large number of bright compact knots within in the tail, with line ratios characteristic of H ii regions. These H ii regions, despite residing in a stripped gas tail, have quite typical line ratios, densities, temperatures, and metallicity (~0.7 solar). The majority of these H ii regions are embedded within diffuse gas from the tail, which is dynamically cool ( ~ 25–50 km s – 1 ). This fact, together with a lack of appreciable gradients in age and metallicity, suggests that these H ii regions formed in situ . While our analysis represents a first attempt to characterize the rich physics of the ESO137–001 tail, future work is needed to address the importance of other mechanisms, such as thermal conduction and magnetohydrodynamic waves, in powering the emission in the tail.

Description: We examine galaxy groups from the present epoch to z  ~ 1 to explore the impact of group dynamics on galaxy evolution. We use group catalogues from the Sloan Digital Sky Survey (SDSS), the Group Environment and Evolution Collaboration (GEEC) and the high-redshift GEEC2 samples to study how the observed member properties depend on the galaxy stellar mass, group dynamical mass and dynamical state of the host group. We find a strong correlation between the fraction of non-star-forming (quiescent) galaxies and galaxy stellar mass, but do not detect a significant difference in the quiescent fraction with group dynamical mass, within our sample halo mass range of ~10 13 –10 14.5 M , or with dynamical state. However, at z  ~ 0.4 we do find some evidence that the quiescent fraction in low-mass galaxies [log 10 ( M star /M )  10.5] is lower in groups with substructure. Additionally, our results show that the fraction of groups with non-Gaussian velocity distributions increases with redshift to z  ~ 0.4, while the amount of detected substructure remains constant to z  ~ 1. Based on these results, we conclude that for massive galaxies [log 10 ( M star /M )  10.5], evolution is most strongly correlated to the stellar mass of a galaxy with little or no additional effect related to either the group dynamical mass or the dynamical state. For low-mass galaxies, we do find some evidence of a correlation between the quiescent fraction and the amount of detected substructure, highlighting the need to probe further down the stellar mass function to elucidate the role of environment in galaxy evolution.

Description: We analyse the sizes, colour gradients and resolved stellar mass distributions for 36 massive and passive galaxies in the cluster XMMUJ2235-2557 at z  = 1.39 using optical and near-infrared Hubble Space Telescope ( HST ) imaging. We derive light-weighted Sérsic fits in five HST bands ( i 775 , z 850 , Y 105 , J 125 , H 160 ), and find that the size decreases by ~20 per cent going from i 775 to H 160 band, consistent with recent studies. We then generate spatially resolved stellar mass maps using an empirical relationship between $M_{{\ast }}/L_{H_{160}}$ and ( z 850  –  H 160 ) and use these to derive mass-weighted Sérsic fits: the mass-weighted sizes are ~41 per cent smaller than their rest-frame r -band counterparts compared with an average of ~12 per cent at z  ~ 0. We attribute this evolution to the evolution in the $M_{{\ast }}/L_{H_{160}}$ and colour gradient. Indeed, as expected, the ratio of mass-weighted to light-weighted size is correlated with the M * / L gradient, but is also mildly correlated with the mass surface density and mass-weighted size. The colour gradients ( z  –  H ) are mostly negative, with a median value of ~0.45 mag dex –1 , twice the local value. The evolution is caused by an evolution in age gradients along the semimajor axis ( a ), with age  = dlog ( age )/dlog ( a ) ~– 0.33, while the survival of weaker colour gradients in old, local galaxies implies that metallicity gradients are also required, with Z  = dlog ( Z )/dlog ( a ) ~– 0.2. This is consistent with recent observational evidence for the inside–out growth of passive galaxies at high redshift, and favours a gradual mass growth mechanism, such as minor mergers.

Description: A well-calibrated method to describe the environment of galaxies at all redshifts is essential for the study of structure formation. Such a calibration should include well-understood correlations with halo mass, and the possibility to identify galaxies which dominate their potential well (centrals), and their satellites. Focusing on z  ~ 1 and 2, we propose a method of environmental calibration which can be applied to the next generation of low- to medium-resolution spectroscopic surveys. Using an up-to-date semi-analytic model of galaxy formation, we measure the local density of galaxies in fixed apertures on different scales. There is a clear correlation of density with halo mass for satellite galaxies, while a significant population of low-mass centrals is found at high densities in the neighbourhood of massive haloes. In this case, the density simply traces the mass of the most massive halo within the aperture. To identify central and satellite galaxies, we apply an observationally motivated stellar mass rank method which is both highly pure and complete, especially in the more massive haloes where such a division is most meaningful. Finally, we examine a test case for the recovery of environmental trends: the passive fraction of galaxies and its dependence on stellar and halo mass for centrals and satellites. With careful calibration, observationally defined quantities do a good job of recovering known trends in the model. This result stands even with reduced redshift accuracy, provided the sample is deep enough to preserve a wide dynamic range of density.

Description: We present new analysis from the Group Environment Evolution Collaboration 2 (GEEC2) spectroscopic survey of galaxy groups at 0.8 〈  z  〈 1. Our previous work revealed an intermediate population between the star-forming and quiescent sequences and a strong environmental dependence in the fraction of quiescent galaxies. Only ~5 per cent of star-forming galaxies in both the group and field sample show a significant enhancement in star formation, which suggests that quenching is the primary process in the transition from the star-forming to the quiescent state. To model the environmental quenching scenario, we have tested the use of different exponential quenching time-scales and delays between satellite accretion and the onset of quenching. We find that with no delay, the quenching time-scale needs to be long in order to match the observed quiescent fraction, but then this model produces too many intermediate galaxies. Fixing a delay time of 3 Gyr, as suggested from the local Universe, produces too few quiescent galaxies. The observed fractions are best matched with a model that includes a delay that is proportional to the dynamical time and a rapid quenching time-scale (~0.25 Gyr), but this model also predicts intermediate galaxies H strength higher than that observed. Using stellar synthesis models, we have tested other scenarios, such as the rejuvenation of star formation in early-type galaxies and a portion of quenched galaxies possessing residual star formation. If environment quenching plays a role in the GEEC2 sample, then our work suggests that only a fraction of intermediate galaxies may be undergoing this transition and that quenching occurs quite rapidly in satellite galaxies (0.25 Gyr).

Description: We present the data release of the Gemini-South GMOS spectroscopy in the fields of 11 galaxy groups at 0.8 〈  z  〈 1, within the COSMOS field. This forms the basis of the Galaxy Environment Evolution Collaboration 2 (GEEC2) project to study galaxy evolution in haloes with M ~ 10 13 M across cosmic time. The final sample includes 162 spectroscopically confirmed members with R  〈 24.75, and is 〉50 per cent complete for galaxies within the virial radius, and with stellar mass M star 〉 10 10.3 M . Including galaxies with photometric redshifts, we have an effective sample size of ~400 galaxies within the virial radii of these groups. We present group velocity dispersions, dynamical and stellar masses. Combining with the GCLASS sample of more massive clusters at the same redshift, we find the total stellar mass is strongly correlated with the dynamical mass, with log M 200  = 1.20(log M star  – 12) + 14.07. This stellar fraction of ~1 per cent is lower than predicted by some halo occupation distribution models, though the weak dependence on halo mass is in good agreement. Most groups have an easily identifiable most massive galaxy (MMG) near the centre of the galaxy distribution, and we present the spectroscopic properties and surface brightness fits to these galaxies. The total stellar mass distribution in the groups, excluding the MMG, compares well with an NFW (Navarro Frenk & White) profile with concentration 4, for galaxies beyond ~0.2 R 200 . This is more concentrated than the number density distribution, demonstrating that there is some mass segregation.

Description: Galaxies grow primarily via accretion-driven star formation in discs and merger-driven growth of bulges. These processes are implicit in semi-analytical models of galaxy formation, with bulge growth in particular relating directly to the hierarchical build-up of haloes and their galaxies. In this paper, we consider several implementations of two semi-analytical models. Focusing on implementations in which bulges are formed during mergers only, we examine the fractions of elliptical galaxies and both passive and star-forming disc galaxies as functions of stellar and halo mass, for central and satellite systems. This is compared to an observational cross-matched Sloan Digital Sky Survey+Third Reference Catalog of Bright Galaxies z  ~ 0 sample of galaxies with accurate visual morphological classifications and M *  〉 10 10.5 M . The models qualitatively reproduce the observed increase of elliptical fraction with stellar mass, and with halo mass for central galaxies, supporting the idea that observed ellipticals form during major mergers. However, the overall elliptical fraction produced by the models is much too high compared with the z  ~ 0 data. Since the ‘passive’ – i.e. non-star-forming – fractions are approximately reproduced, and since the fraction which are star-forming disc galaxies is also reproduced, the problem is that the models overproduce ellipticals at the expense of passive S0 and spiral galaxies. Bulge growth implementations (tuned to reproduce simulations) which allow the survival of residual discs in major mergers still destroy too much of the disc. Increasing the lifetime of satellites, or allowing significant disc regrowth around merger remnants, merely increases the fraction of star-forming disc galaxies. Instead, it seems necessary to reduce the mass ratios of merging galaxies, so that most mergers produce modest bulge growth in disc galaxy remnants instead of ellipticals. This could be a natural consequence of tidal stripping of stars from infalling satellite galaxies, a process not considered in our models. However, a high efficiency of quenching during and/or subsequent to minor mergers is still required to keep the passive fraction high.

Description: We present deep Gemini Multi-Object Spectrograph-South spectroscopy for 11 galaxy groups at 0.8 〈 z  〈 1.0, for galaxies with r AB  〈 24.75. Our sample is highly complete (〉66 per cent) for eight of the 11 groups. Using an optical–near-infrared colour–colour diagram, the galaxies in the sample were separated with a dust insensitive method into three categories: passive (red), star-forming (blue) and intermediate (green). The strongest environmental dependence is observed in the fraction of passive galaxies, which make up only ~20 per cent of the field in the mass range 10 10.3  〈 M star /M  〈 10 11.0 , but are the dominant component of groups. If we assume that the properties of the field are similar to those of the ‘pre-accreted’ population, the environment quenching efficiency ( ) is defined as the fraction of field galaxies required to be quenched in order to match the observed red fraction inside groups. The efficiency obtained is ~0.4, similar to its value in intermediate-density environments locally. While green (intermediate) galaxies represent ~20 per cent of the star-forming population in both the group and field, at all stellar masses, the average specific star formation rate of the group population is lower by a factor of ~3. The green population does not show strong H absorption that is characteristic of starburst galaxies. Finally, the high fraction of passive galaxies in groups, when combined with satellite accretion models, require that most accreted galaxies have been affected by their environment. Thus, any delay between accretion and the onset of truncation of star formation () must be 2 Gyr, shorter than the 3–7 Gyr required to fit data at z  = 0. The relatively small fraction of intermediate galaxies require that the actual quenching process occurs quickly, with an exponential decay time-scale of q 1 Gyr.

Description: The fraction of galaxies with red colours depends sensitively on environment, and on the way in which environment is measured. To distinguish competing theories for the quenching of star formation, a robust and complete description of environment is required, to be applied to a large sample of galaxies. The environment of galaxies can be described using the density field of neighbours on multiple scales – the multiscale density field . We are using the Millennium simulation and a simple halo occupation distribution (HOD) prescription which describes the multiscale density field of Sloan Digital Sky Survey DR7 galaxies to investigate the dependence of the fraction of red galaxies on the environment. Using a volume-limited sample, where we have sufficient galaxies in narrow density bins, we have more dynamic range in halo mass and density for satellite galaxies than for central galaxies. Therefore, we model the red fraction of central galaxies as a constant while we use a functional form to describe the red fraction of satellites as a function of halo mass which allows us to distinguish a sharp from a gradual transition. While it is clear that the data can only be explained by a gradual transition, an analysis of the multiscale density field on different scales suggests that colour segregation within the haloes is needed to explain the results. We also rule out a sharp transition for central galaxies, within the halo mass range sampled.

Description: We present an analysis of galaxies in groups and clusters at 0.8 〈  z  〈 1.2, from the GCLASS and GEEC2 spectroscopic surveys. We compute a ‘conversion fraction’ f convert that represents the fraction of galaxies that were prematurely quenched by their environment. For massive galaxies, M star  〉 10 10.3 M , we find f convert  ~ 0.4 in the groups and ~0.6 in the clusters, similar to comparable measurements at z  = 0. This means the time between first accretion into a more massive halo and final star formation quenching is t p  ~ 2 Gyr. This is substantially longer than the estimated time required for a galaxy's star formation rate to become zero once it starts to decline, suggesting there is a long delay time during which little differential evolution occurs. In contrast with local observations we find evidence that this delay time-scale may depend on stellar mass, with t p approaching t Hubble for M star  ~ 10 9.5 M . The result suggests that the delay time must not only be much shorter than it is today, but may also depend on stellar mass in a way that is not consistent with a simple evolution in proportion to the dynamical time. Instead, we find the data are well-matched by a model in which the decline in star formation is due to ‘overconsumption’, the exhaustion of a gas reservoir through star formation and expulsion via modest outflows in the absence of cosmological accretion. Dynamical gas removal processes, which are likely dominant in quenching newly accreted satellites today, may play only a secondary role at z  = 1.