Boris Häußler

We introduce the Gini index ( G ) as a metric for studying the emergence of brightest cluster galaxies (BCGs) by quantifying the disparity in the light distribution of galaxy cluster cores. The Gini index collapses the skewness of the bright-end luminosity function to a single statistic, and provides a measure for the light dominance of the BCG over other cluster members that is more robust than the magnitude gap. We argue that the Gini index is far less sensitive to short-term changes in the cluster dynamical state than the magnitude gap and solves its known degeneracies by taking the entire cluster membership into account, making G a more sensitive metric to long-term BCG assembly. We apply the magnitude gap and Gini index to a progenitor-selected sample of 92 South Pole Telescope- and Planck-selected cluster cores at 0 < z < 1 with corresponding Hubble Space Telescope F110W and F200LP and Sloan Digital Sky Survey g and r photometry. Based on both parametric and empirical tests, we find no statistical evidence for evolution in G or M _1,2 over 0 < z < 1, and find that BCGs have not grown significantly (2×) faster than the cluster population for z < 1 at 3 σ confidence. This implies that BCGs achieve dominance and establish defining properties early ( z > 1) in massive clusters. In general, we find that the Gini index is a valuable and relatively straightforward measurement, with well-controlled systematics, that could be applied to much larger samples to study BCG dominance and emergence.

We investigate how the environment affects the galaxy structure in the Spiderweb protocluster at z = 2.16 using JWST/NIRCam F115W, F182M, and F410M imaging (rest-frame ∼3500 Å to 1.4 μm). We performed homogeneous multi-wavelength parametric modelling of the single-Sérsic and bulge–disc decomposition for a sample of 103 Spiderweb member galaxies within the JWST field of view (up to ∼2 × R ₂₀₀ ). Star-forming galaxies (SFGs) follow a mass-size relation broadly consistent with the field, with a mildly steeper trend at the high-mass end, within uncertainties. Passive galaxies exhibit a flatter mass–size relation, and their typical size (intercept at fixed stellar mass) lies between that of the coeval field and cluster passive populations, indicating an intermediate evolutionary stage. In both star-forming and passive systems, bulges are systematically more compact than discs. Galaxy sizes decrease slightly with increasing wavelength, whereas ALMA-detected dusty SFGs exhibit a much steeper wavelength dependence, consistent with centrally concentrated, obscured star formation. The passive fraction depends primarily on local density: it resembles the field at Σ ≲ 100-200 gal Mpc ⁻² and rises to ∼60% at Σ ≳ 1000 gal Mpc ⁻² ; however, the uncertainties are relatively large due to the small number of galaxies in the highest-density bins, and we find no significant additional dependence on clustercentric distance. We also find a weak but significant correlation between local density and Sérsic index (strongest in F410M), but no clear correlation with effective radius or the star formation rate surface density Σ _SFR . These results support a picture in which quenching and structural transformation are already advanced, while size growth is still ongoing.

The background galaxies in Euclid Early Release Observation images of the Perseus cluster make up a remarkable sample for the combination of a 0.57 deg ² area, 25.3 and 23.2 AB mag depth, and angular resolutions in the optical and near-infrared bands of 0 _⋅ ^′′ 1 and 0 _⋅ ^′′ 3, respectively. As part of the effort towards characterising the history of the Hubble sequence, we performed a morphological analysis of 2445 and 12 786 galaxies with I _E ≤ 21 and I _E ≤ 23, respectively. We used single-Sérsic profiles and the sums of a Sérsic bulge and an exponential disc to model these galaxies with SourceXtractor++ and analysed their positional, structural, and flux parameters in order to assess their similarities and differences. The fitted galaxies to I _E ≤ 21 span the various Hubble types with ubiquitous bulge and disc components and a bulge-to-total light ratio ( B/T ) that takes all values from 0 to 1. The effective radius of the single-Sérsic profile is an intermediate estimate of galaxy size (between the bulge and disc effective radii) depending on B/T . The axis ratio of the single-Sérsic profile is higher than the disc axis ratio, and this difference increases with B/T . The choice of model impacts the photometry with −0.08 to 0.01 mag median systematic I _E offsets between single-Sérsic and bulge-disc total magnitudes and a 0.05–0.15 mag dispersion from low to high B/T . We measured a median 0.3 mag bulge-disc colour difference in rest-frame M _g − M _i that originates from the disc-dominated galaxies, whereas bulge-dominated galaxies have median colours similar to those of their components. Remarkably, we also measured redder inside disc colour gradients based on 5 to 10% systematic variations of disc effective radii between the optical and near-infrared bands. This analysis demonstrates the usefulness and limitations of single-Sérsic profile modelling and the power of bulge-disc decomposition for characterising the morphology of lenticulars and spirals in Euclid images. We make available the catalogues of best-fit parameters for the morphological and SED fits.

Context. As galaxies evolve in dense cluster and protocluster environments, they interact and quench their star formation. This gradually transforms the dominant galaxy population from star-forming galaxies to quiescent red galaxies. This transformation is identifiable by observing the galaxy colors and can also be seen in the morphological transformation of late-type galaxies into early-type galaxies, which creates the morphology-density relation observed when populations in clusters are compared to field galaxies at a given epoch. However, high- z ( z > 2) galaxy morphology studies are hindered by the high angular resolution necessary to characterize the morphology. Aims. We studied HST WFC3 F160W observations of protoclusters from the MAMMOTH survey (BOSS1244 and BOSS1542) at z ∼ 2.23 with populations of previously identified H α emitters. Methods. By measuring the Sérsic index of 151 H α emitters, we searched for the early morphological transformation of high- z star-forming galaxies in these well-studied large nonvirialized protoclusters, which we think are the precursors of present-day galaxy clusters. Results. The morphology of the populations of star-forming galaxies in protoclusters does not differ from that of their field counterparts. However, we also identified a population of clumpy potentially merging galaxies, which might increase the population of early-type galaxies within these structures. In BOSS1244, which has two previously identified massive quiescent galaxies including a brightest cluster galaxy (BCG), we additionally found an abundance of early-type galaxies near the BCG and two coeval high- z quasars. Conclusions. Although the morphology of field and protocluster galaxies are very similar, the population of early-type star-forming galaxies surrounding the spectroscopically confirmed quiescent BCG in BOSS1244, which is not observed in BOSS1542, might indicate differences in the evolutionary state of these two coeval protoclusters. It might be a sign of an early forming cluster core in BOSS1244.

The current standard model of cosmology successfully describes a variety of measurements, but the nature of its main ingredients, dark matter and dark energy, remains unknown. Euclid is a medium-class mission in the Cosmic Vision 2015–2025 programme of the European Space Agency (ESA) that will provide high-resolution optical imaging, as well as near-infrared imaging and spectroscopy, over about 14 000 deg² of extragalactic sky. In addition to accurate weak lensing and clustering measurements that probe structure formation over half of the age of the Universe, its primary probes for cosmology, these exquisite data will enable a wide range of science. This paper provides a high-level overview of the mission, summarising the survey characteristics, the various data-processing steps, and data products. We also highlight the main science objectives and expected performance.

We use the Ultraviolet Imaging of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey fields (UVCANDELS) to measure half-light radii in the rest-frame far-UV for ∼16,000 disk-like galaxies over 0.5 ≤ z ≤ 3. We compare these results to rest-frame optical sizes that we measure in a self-consistent way and find that the stellar mass–size relation of disk galaxies is steeper in the rest-frame UV than in the optical across our entire redshift range. We show that this is mainly driven by massive galaxies (≳10¹⁰ M _⊙), which we find to also be among the most dusty. Our results are consistent with the literature and have commonly been interpreted as evidence of inside-out growth wherein galaxies form their central structures first. However, they could also suggest that the centers of massive galaxies are more heavily attenuated than their outskirts. We distinguish between these scenarios by modeling and selecting galaxies at z = 2 from the VELA simulation suite in a way that is consistent with UVCANDELS. We show that the effects of dust alone can account for the size differences we measure at z = 2. This indicates that, at different wavelengths, size differences and the different slopes of the stellar mass–size relation do not constitute evidence for inside-out growth.

The many unique properties of galaxies are shaped by physical processes that affect different components of the galaxy – such as their bulges and discs – in different ways, and they leave characteristic imprints on the light and spectra of these components. Disentangling these spectra reveals vital clues that can be traced back in time to understand how galaxies, and their components, form and evolve throughout their lifetimes. With BUDDI, we have decomposed the integral field unit (IFU) datacubes in SDSS-MaNGA DR17 into Sérsic bulge and exponential disc components and extracted clean bulge and disc spectra. BUDDI-MaNGA is the first large statistical sample of such decomposed spectra of 1452 galaxies covering morphologies from ellipticals to late-type spirals. We derive stellar masses of the individual components with spectral energy distribution (SED) fitting using BAGPIPES and estimate their mean mass-weighted stellar metallicities and stellar ages using PPXF. With this information, we reconstruct the mass assembly histories of the bulges and discs of 968 spiral galaxies (Sa-Sm types). Our results show a clear downsizing effect especially for the bulges, with more massive components assembling earlier and faster than the less massive ones. Additionally, we compare the stellar populations of the bulges and discs in these galaxies, and find that a majority of the bulges host more metal-rich and older stars than their disc counterparts. Nevertheless, we also find a non-negligible fraction of the spiral galaxy population in our sample contains bulges that are younger and more metal-enhanced than their discs. We interpret these results, taking into account how their formation histories and current stellar populations depend on stellar mass and morphology.

The many unique properties of galaxies are shaped by physical processes that affect different components of the galaxy - like the bulges and discs - in different ways, and leave characteristic imprints on the light and spectra of these components. Disentangling their spectra can reveal vital clues that can be traced back in time to understand how galaxies, and their components, form and evolve throughout their lifetimes. With BUDDI, we have decomposed the IFU datacubes in SDSS-MaNGA DR17 into a Sérsic bulge component and an exponential disc component and extracted their clean bulge and disc spectra. BUDDI-MaNGA is the first and largest statistical sample of such decomposed spectra of 1452 galaxies covering morphologies from ellipticals to late-type spirals. We derived stellar masses of the individual components with SED fitting using BAGPIPES and estimated their mean mass-weighted stellar metallicities and stellar ages using pPXF. With this information in place, we reconstructed the mass assembly histories of the bulges and discs of the 968 spiral galaxies (Sa-Sm Types) in this sample to look for systematic trends with respect to stellar mass and morphology. Our results show a clear downsizing effect especially in the bulges, with more massive components assembling earlier and faster than the less massive ones. Additionally, on comparing the stellar populations of the bulges and discs in these galaxies, we find that a majority of the bulges host more metal-rich and older stars than their disc counterparts. Nevertheless, we also find that there exists a non-negligible fraction of the spiral galaxy population in our sample with bulges that are younger and more metal-rich than their discs. We interpret these results, taking into account how their formation histories and current stellar populations depend on stellar mass and morphology.