Galaxy And Mass Assembly (GAMA): mass-size relations of z$<$0.1 galaxies subdivided by Sérsic index, colour and morphology

Abstract: We use data from the Galaxy And Mass Assembly (GAMA) survey in the redshift range 0.01$<$z$<$0.1 (8399 galaxies in $g$ to $K_s$ bands) to derive the stellar mass $-$ half-light radius relations for various divisions of 'early' and 'late'-type samples. We find the choice of division between early and late (i.e., colour, shape, morphology) is not particularly critical, however, the adopted mass limits and sample selections (i.e., the careful rejection of outliers and use of robust fitting methods) are important. In particular we note that for samples extending to low stellar mass limits ($<10^{10}\mathcal{M_{\odot}}$) the S\'ersic index bimodality, evident for high mass systems, becomes less distinct and no-longer acts as a reliable separator of early- and late-type systems. The final set of stellar mass $-$ half-light radius relations are reported for a variety of galaxy population subsets in 10 bands ($ugrizZYJHK_s$) and are intended to provide a comprehensive low-z benchmark for the many ongoing high-z studies. Exploring the variation of the stellar mass $-$ half-light radius relations with wavelength we confirm earlier findings that galaxies appear more compact at longer wavelengths albeit at a smaller level than previously noted: at $10^{10}\mathcal{M_{\odot}}$ both spiral systems and ellipticals show a decrease in size of 13% from $g$ to $K_s$ (which is near linear in log wavelength). Finally we note that the sizes used in this work are derived from 2D S\'ersic light profile fitting (using GALFIT3), i.e., elliptical semi-major half light radii, improving on earlier low-z benchmarks based on circular apertures.

Analysis of Mass-Size Relations in Low-Redshift Galaxies: Insights from the GAMA Survey

This paper presents a detailed exploration of the mass-size relations among galaxies with redshifts less than 0.1, using data from the Galaxy And Mass Assembly (GAMA) survey. The dataset comprises 8399 galaxies, observed across several bands ranging from $g$ to $K_s$, allowing for robust relationships between stellar mass and half-light radius to be established.

Study Overview

The authors classify the galaxy sample into 'early' and 'late'-type systems based on several criteria, including Sersic index, color, and morphological features. The mass-size relations are then derived for these subsets. The motivation behind this exercise is to provide a local benchmark of galaxy mass-size relations that can inform and calibrate ongoing studies at higher redshifts.

Key Observations and Findings

1. Subdivision Methodologies: The study finds that the methodological choice, such as using color or morphology, is less critical to results than careful sample selection and the rejection of outliers. Importantly, low mass limits (less than $10^{10} M_\odot$) blur the Sersic index bimodality that serves to distinguish between galaxy types in more massive samples.

2. Relationships in Different Bands: A significant finding is the reduction of galaxy sizes observed at longer wavelengths, indicating a consistent decrease of 13% in size from the $g$-band to the $K_s$-band for both spirals and ellipticals at a mass of $10^{10} M_\odot$. This observation challenges previously reported larger variations.

3. Size Measurements: The work improves on earlier benchmarks by employing 2D Sersic light profile fitting, thereby enhancing the reliability of size measurements compared to those derived from circular apertures.

4. Comparison with Existing Literature: The results align, in part, with existing studies but correct earlier assumptions about size variation across wavelengths by using deeper imaging data from the VIKING survey rather than the shallower UKIDSS data used in predecessor studies.

Theoretical and Practical Implications

• Wavelength Dependence: Confirming that galaxy sizes are smaller at longer wavelengths underscores the necessity for a uniform rest-frame definition when comparing datasets spanning different redshifts. It reinforces the need to account for bandpass effects when interpreting evolutionary trends.

• Methodological Insight: The work illustrates the importance of robust fitting procedures and sample selection criteria in capturing reliable mass-size relations, serving as a guide for structure in observational datasets.

• Implications for Galaxy Evolution Studies: By offering a comprehensive local framework, the paper informs the interpretation of size evolution in galaxies over cosmic time, particularly the physical mechanisms that enlarge early-type galaxies primarily in size rather than mass since $z\sim2$.

Future Directions

The authors propose to extend their analysis beyond whole galaxies to their components (e.g., bulges and disks), which could provide more granular insights into the evolutionary processes of galaxies. Additionally, exploring angular momentum's relation to galaxy size could offer new avenues for understanding the fundamental mass-spin-morphology relationship.

In conclusion, this paper augments the arsenal of tools and data available for deciphering galaxy evolution within the universe, emphasizing the nuanced understanding required when analyzing such relations across various observational windows. The recalibrations presented serve as a pivotal resource for comparative studies aimed at unraveling the life cycles of galaxies throughout cosmic time.