Sabine Bellstedt

Using data from the Galaxy and Mass Assembly (GAMA) survey, we investigate how galaxy properties correlate with the local environment, focusing on its two southern regions (G02 and G23) that have not previously been examined in this context. We employ two-point and marked correlation functions to quantify the environmental dependence of galaxy color, stellar mass, luminosity across the u , g , r , J , and K bands, as well as star formation rate (SFR) and specific star formation rate (sSFR). Our results show that u − r and g − r colors are most strongly correlated with local overdensity, followed by stellar mass. The SFR and sSFR exhibit a clear inverse relationship with environmental density, consistent with the trend observed for u -band luminosity, which traces young stellar populations. The galaxies brighter in the g , J , and K bands preferentially inhabit denser regions. We interpret the environmental dependence of color using halo occupation distribution (HOD) modeling. The HOD modeling indicates that red galaxies trace dense environments more strongly than blue galaxies, exhibiting significantly higher satellite fractions and occupying more massive halos. We demonstrate that both redshift and stellar mass incompleteness systematically affect clustering measurements, generally enhancing the observed environmental dependence. Comparing our measurements from the southern regions with those from the equatorial GAMA regions, we find that the overall environmental trends remain consistent. The measured correlations provide key constraints for models of galaxy assembly across mass and environment, while environmental trends in color and near-infrared luminosity trace stellar mass growth and quenching with redshift.

We present measurements of galaxy spin-filament alignments at 0.20 < z < 0.42 using the MAGPI Survey, exploiting resolved stellar and ionised gas kinematics to reconstruct 3D spin axes. Cosmic filaments are identified using a stellar mass-matched constant-number-density population from the GAMA, MAGPI Redshift Survey and DESI datasets. We detect a flipping of the statistical alignment signal at z ∼ 0.3 as a function of M⋆: low-mass galaxies preferentially exhibit spins parallel to the direction of their host filaments, while high-mass galaxies tend towards perpendicular orientations. The transition mass is measured to be log (M⋆, t/M⊙) ≃ 10.8, approximately 0.6 dex higher than the value for the SAMI Galaxy Survey at z ∼ 0.05. Bootstrap analysis gives 99.5% confidence that the transition mass is higher for MAGPI than SAMI. At fixed stellar mass MAGPI galaxies exhibit stronger parallel alignments than their SAMI counterparts, suggesting evolution of spin–filament alignments over the past ∼ 3 Gyr. The gaseous component in MAGPI galaxies is more closely aligned with filaments than the stellar component, relative to a M⋆-matched SAMI sample. This indicates a stronger coupling between ionised gas and the large-scale structure at z ∼ 0.3, consistent with coherent gas accretion onto filaments remaining efficient at higher M⋆. At z ∼ 0 we detect a transition toward perpendicular alignments at lower M⋆, while the gas and stellar components exhibit similar alignments relative to filaments, likely due to the cumulative effects of mergers and secular evolution.

Constraining the timescale and manner in which the Epoch of Reionization (EoR) occurred is a major JWST science goal. However, any constraints on the stellar or ionizing parameters ( ξ _ion ) of galaxies in the EoR must contend with biases introduced by both the data and the models used. We explore three techniques that use spectroscopic and photometric data as well as three different spectral energy distribution (SED) fitting codes, each comprising multiple star formation history, stellar population synthesis, dust, and photoionization prescriptions to determine their relative influence on stellar parameters and ξ _ion . We use z ∼ 3 EoR analog galaxies due to their reliable photometric coverage (improved physical constraints) in comparison to direct EoR sources and potential for research into direct Lyman continuum escape. For this population the median stellar mass can vary by over 0.6 dex and the star formation rate by more than 0.9 dex. Further, ξ _ion can vary by over 1.1 dex for individual sources when comparing spectroscopic and photometric derivations, or by more than 0.5 dex when fitting SEDs with different models. As such, the choice of methodology can have significant consequences for the derived ξ _ion and the subsequent sources of reionization. We find that the presence of a redshift evolution for ξ _ion is dependent on the method adopted for its derivation, where a consistent method yields an evolutionary trend with redshift in extreme emitters while an indiscriminant selection of studies does not. The model, method, and data dependence of the ξ _ion parameter are undeniable even for a homogeneous population.

Context. Next-generation photometric and spectroscopic galaxy surveys will enable unprecedented tests of the concordance cosmological model and of galaxy formation and evolution. Fully exploiting their potential requires a precise understanding of the selection effects on galaxies and biases on measurements of their properties, which are required, above all, for accurate estimates of redshift distributions. The forward-modelling of galaxy surveys offers a powerful framework to simultaneously recover galaxy redshift distributions and characterise the observed galaxy population. Aims. We present G AL SBI-SPS, a new stellar population synthesis (SPS)-based galaxy population model developed for cosmological and galaxy evolution studies. The model generates realistic galaxy catalogues, which we use to forward-model Hyper-Suprime Cam (HSC) observations in the COSMOS field. Methods. G AL SBI-SPS samples the physical properties of galaxies from analytical parametrisations informed by GAMA, DEVILS, and literature data, it computes galaxy magnitudes with the generative SED package P RO S PECT , and it simulates HSC images in the COSMOS field with UFig. We measured photometric properties consistently in real data and simulations. We compared redshift distributions and photometric and physical properties to observations and to those from the phenomenological G AL SBI model. Results. G AL SBI-SPS reproduces the observed g , r , i , z , y magnitude, colour, and size distributions down to i ≤ 23 with good accuracy. Median differences in magnitudes and colours remain below 0.14 mag, with the model covering the full colour space spanned by HSC data. Galaxy sizes are overestimated by ∼0.2″ on average and some tension exists in the g − r colour distribution, but the latter is comparable to that seen in the phenomenological G AL SBI model. Redshift distributions show a mild positive offset (0.01 ≲ Δ¯ z ≲ 0.08) in the mean. G AL SBI-SPS qualitatively reproduces the stellar mass–star formation rate and size–stellar mass relations seen in COSMOS2020 data. Conclusions. G AL SBI-SPS provides a realistic, survey-independent description of the galaxy population at a Stage-III-like depth using only literature-based parameters. Its predictive power is expected to improve significantly when constrained against deep observed data using simulation-based inference, thereby providing accurate redshift distributions that satisfy the stringent requirements set by Stage IV surveys.

Galaxies with different morphological characteristics likely have different evolutionary histories, such that understanding the mechanisms that drive morphological change can provide valuable insights into the galaxy evolution process. These mechanisms largely correlate with local environment, ultimately leading to the well-known local morphology–density relation. To explore how the morphology–density relation is produced, we must look to earlier times, and trace the co-evolution of environment and morphology in an un-biased and self-consistent manner. Here we use new environmental metrics from the Deep Extragalactic VIsible Legacy Survey (DEVILS) to explore the spectroscopic morphology–density relation at intermediate redshift (0.3$<$z$<$0.5) and compare directly to the Galaxy And Mass Assembly Survey (GAMA) at 0$<$z$<$0.08. Importantly, both the galaxy morphologies and environmental metrics in DEVILS and GAMA are derived in a very similar manner, reducing any methodology biases. We see a clear evolution in morphological classes between DEVILS and GAMA, which is modulated by environment. These trends are consistent with a scenario where in all environments disc-dominated galaxies are transitioning to classical bulge + disc systems (potentially via minor mergers and/or secular evolution), and in high-density environments there is an increasing prevalence of visually selected elliptical galaxies (potentially via major mergers and/or disc fading), with the fraction of ellipticals increasing by $\sim$0.3 in the most dense regions over the last $\sim$7 Gyr, but remaining largely unchanged in low-density environments.

Internal gas inflows driven by galaxy mergers are considered to enhance star formation rates (SFRs), fuel supermassive black hole growth, and stimulate active galactic nuclei (AGNs). However, quantifying these phenomena remains a challenge, due to difficulties both in classifying mergers and in quantifying galaxy and AGN properties. We quantitatively examine the merger–SFR–AGN connection using Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) galaxies using novel methods for both galaxy classification and property measurements. Mergers in HSC-SSP observational images are identified through fine-tuning Zoobot, a pretrained deep representation learning model, using images and labels based on the Galaxy Cruise project. We use galaxy and AGN properties that were produced by fitting Galaxy and Mass Assembly spectra using the spectral energy distribution fitting code ProSpect, which fits panchromatically across the far-ultraviolet through far-infrared wavelengths and obtains galaxy and AGN properties simultaneously. Small differences are seen in SFR and AGN activity between mergers and controls, with ΔSFR = −0.009 ± 0.003 dex, Δf _AGN = −0.010 ± 0.033 dex, and ΔL _AGN = 0.002 ± 0.025 dex. After further visual purification of the merger sample, we find ΔSFR = −0.033 ± 0.014 dex, Δf _AGN = −0.024 ± 0.170 dex, and ΔL _AGN = 0.019 ± 0.129 dex for pairs, and ΔSFR = −0.057 ± 0.024 dex, Δf _AGN = 0.286 ± 0.270 dex, and ΔL _AGN = 0.329 ± 0.195 dex for postmergers. These numbers suggest secular processes being an important driver for star formation and AGN activity, and present a cautionary tale when using longer-timescale tracers.