In their study of galactic structures, Fraser-McKelvie et al. examine the first 12 galaxies from the GECKOS Survey (Generalising Edge-on galaxies and their Chemical bimodalities, Kinematics, and Outflows out to Solar environments). This survey, part of the VLT/MUSE large program, focuses on edge-on galaxies (those viewed from the side) because their orientation allows for a clearer distinction of internal structures, like bulges and discs, due to their separation in light. The team investigates the kinematic sub-structures within these galaxies, aiming to understand how different galactic components influence galactic evolution.

The GECKOS Survey and Data Collection

GECKOS targets Milky Way-like galaxies using the MUSE instrument at the Very Large Telescope, capturing two-dimensional (2D) data on the motion of stars. The study’s sample includes galaxies at varying distances and star-formation rates to capture a range of structural features. This setup ensures that each galaxy’s bulge (the central part of a galaxy) and disc structures are covered in detail. The nGIST pipeline was introduced to handle data reduction and analysis, enabling detailed maps of stellar velocity and dispersion across the galaxies.

Identifying Boxy-Peanut Bulges and Bars

One primary aim was to distinguish boxy-peanut (BP) bulges in these galaxies, a bulge shape often associated with stellar bars (elongated structures of stars) when seen from certain angles. The authors use visual imaging and unsharp masking techniques to confirm the presence of BP structures in 8 out of the 12 galaxies. Additionally, they find kinematic indicators of bars in their stellar velocity and dispersion profiles, which suggest how these bars are oriented in each galaxy, either end-on (directly facing the viewer) or side-on (at a right angle to the viewer).

Kinematic Maps and 1D Profiles

By examining 2D kinematic maps and 1D line profiles of each galaxy’s velocity and velocity dispersion, the researchers found diverse kinematic patterns in the bulge and disc regions. For example, some galaxies displayed distinct, “double-humped” rotation curves in their velocity profiles, a feature commonly associated with the presence of a bar. These curves reveal how stars move in different sections of the galaxy and how bar orientation affects these movements.

Evidence of Nuclear Discs

Fraser-McKelvie’s team identified nuclear discs in several galaxies based on features such as central depressions in velocity dispersion and unique profiles in their higher-order kinematic moments. Nuclear discs are small, dense discs near the center of a galaxy, and these kinematic signatures suggest how stars in these regions move differently from those in the surrounding bulge and disc.

Implications for Galaxy Morphology

This study concludes that kinematic sub-structures in edge-on galaxies like those in the Milky Way are varied and complex, demonstrating that these structures contribute significantly to the overall shape and evolution of galaxies. The researchers suggest that kinematic mapping, especially in 2D, provides a more nuanced view of galaxy structure than visual imaging alone. Such observations support a modern view of galaxy morphology, which encompasses the diverse and sometimes unexpected structures seen across galaxies.

Conclusion

The GECKOS Survey and its findings present a more detailed picture of how galaxies are structured and the different forces shaping their evolution. The survey’s approach, combining high-resolution imaging and sophisticated kinematic analysis, is poised to enhance our understanding of how galaxies like the Milky Way came to look the way they do today.

Source: Fraser-McKelvie