The baryonic mass-size relation of galaxies. I. A dichotomy in star-forming galaxy disks

TL;DR

The study defines the baryonic mass–size relation $M_{ m bar}$ vs. $R_{ m 50, bar}$ using SPARC data and discovers two distinct sequences: high-surface-density (HSD) star-dominated disks and low-surface-density (LSD) gas-dominated disks. LSD galaxies exhibit a slope near 2 in the $M_{ m bar}$–$R_{ m 50, bar}$ plane, implying a nearly constant mean surface density, while HSD galaxies have a slope near 1, indicating increasing compactness with mass for less massive systems. The dichotomy is absent in the baryonic Tully–Fisher relation but moderately present in the angular momentum relation, suggesting the mass–size plane is more diagnostic of evolutionary state than angular momentum alone. The results point to two divergent evolutionary pathways for star-forming disks, with potential connections to MOND predictions and implications for distinguishing galaxy types and their evolution, complemented by ongoing expansion to larger, multi-wavelength samples and to passive systems in future work.

Abstract

The mass-size relations of galaxies are generally studied considering only stars or only gas separately. Here we study the baryonic mass-size relation of galaxies from the SPARC database, using the total baryonic mass ($M_{\rm bar}$) and the baryonic half-mass radius ($R_{\rm 50, bar}$). We find that SPARC galaxies define two distinct sequences in the $M_{\rm bar} - R_{\rm 50, bar}$ plane: one that formed by high-surface-density (HSD), star-dominated, Sa-to-Sc galaxies, and one by low-surface-density (LSD), gas-dominated, Sd-to-dI galaxies. The $M_{\rm bar} - R_{\rm 50, bar}$ relation of LSD galaxies has a slope close to 2, pointing to a constant average surface density, whereas that of HSD galaxies has a slope close to 1, indicating that less massive spirals are progressively more compact. Our results point to the existence of two types of star-forming galaxies that follow different evolutionary paths: HSD disks are very efficient in converting gas into stars, perhaps thanks to the efficient formation of non-axisymmetric structures (bars and spiral arms), whereas LSD disks are not. The HSD-LSD dichotomy is absent in the baryonic Tully-Fisher relation ($M_{\rm bar}$ versus flat circular velocity $V_{\rm f}$) but moderately seen in the angular-momentum relation (approximately $M_{\rm bar}$ versus $V_{\rm f}\times R_{\rm 50, bar}$), so it is driven by variations in $R_{\rm 50, bar}$ at fixed $M_{\rm bar}$. This fact suggests that the baryonic mass-size relation is the most effective empirical tool to distinguish different galaxy types and study their evolution.

Figures (5)

• Figure 1: Measurements of $R_{\rm 50, gas}$, $R_{\rm 50, \star}$, and $R_{\rm 50, bar}$ for three example galaxies: a gas-dominated one (UGC 731), a star-dominated one (NGC 2903), and an intermediate case (NGC 5585). The upper panels show the surface density profiles for gas (blue points), stars (red points), and total baryons (black line); the stellar profile was extrapolated at large radii with an exponential function (dashed red line). The lower panel shows the curve of growth of gas (blue line), stars (red line), and total baryons (black line); the dotted lines show the location of the corresponding half-mass radii.
• Figure 2: Gaseous (left panel), stellar (middle panel), and baryonic (right panel) mass-size relations of SPARC galaxies. Data points are colour-coded by the gas fraction $f_{\rm gas}$; the error bars denote the $1\sigma$ errors. The three panels span the same dynamic range on both axes. Two sequences are evident in the stellar and baryonic mass-size relations.
• Figure 3: Distributions of $f_{\rm gas}$, $\Sigma_{\rm 50, bar}$, $T$, and $\mu_{\rm 0, disk}$ for the two groups identified by DBSCAN (red and blue histograms). The dashed black lines show the separation values reported in Table \ref{['tab_roxy_baryons']}. Panels (a)-(c) show the distributions clustered by $\log\Sigma_{\rm 50, bar}$, while panel (d) shows the distributions clustered by $\mu_{\rm 0, disk}$ (after correction to face-on view).
• Figure 4: Baryonic mass-size relations considering all baryons (left panel) and the baryonic disks only (excluding stellar bulges, right panel). Galaxies are colour-coded by the gas fraction $f_{\rm gas}$. Crosses represent S0-to-Sc galaxies (Hubble type $T \leq 5$), while circles represent Sd-to-dI galaxies ($T > 5$). The red and the blue solid lines show the best-fit relations to the HSD and LSD sequences, respectively; the red and blue shaded areas indicate the best-fit intrinsic scatters. Dashed lines correspond to constant surface densities for multiples of $\Sigma_{\rm c}=125$ M$_\odot$ pc$^{-2}$ (see Sect. \ref{['sec_MR']} for details).
• Figure 5: Scaling relations of galaxy disks: the baryonic Tully-Fisher relation (left panel), the baryonic mass-size relation (middle panel), and the baryonic angular-momentum relation (right panel). The three panels cover the same dynamic range on the y-axis. The circles and squares are the same as those in Fig. \ref{['fig_main']}, colour-coded by the effective baryonic surface density $\Sigma_{\rm 50, bar}$. The blue and gold solid lines are the best-fit lines for LSD galaxies and HSD galaxies, respectively. The dashed black line in the right panel shows the best-fit line considering all 147 galaxies together.