Hidden Figures in the Sky: Evolution of low-surface brightness galaxies from a hydro-dynamical perspective

Abstract

Low-surface brightness galaxies (LSBGs) are defined as galaxies with central surface brightness levels fainter than the night sky, making them challenging to observe. A key open question is whether their faint appearance arises from intrinsic properties or stochastic events in their formation histories. We aim to trace the formation histories of LSBGs to assess whether their evolutionary paths differ from those of high-surface brightness galaxies (HSBGs), and to identify the key physical drivers behind these differences. We present a fast and efficient method to estimate stellar surface brightness densities in hydro-dynamical simulations and a statistically robust exploration of over 150 properties in the reference run \textsc{Ref-L0100N1504} of the \texttt{EAGLE} simulation. To minimise biases, we carefully match the stellar and halo mass distributions of the selected LSB and HSB samples. At $z=0$, LSBGs are typically extended, rotation-supported systems with lower stellar densities, older stellar populations, reduced star formation activity, and higher specific stellar angular momenta $j_*$ than their HSBG counterparts. They also exhibit larger radii of maximum circular velocity ($R_{\mathrm{Vmax}}$). We identify key transition redshifts that mark the divergence of LSBG and HSBG properties: $j_*$ diverges at $z\sim5-7$ and $R_{\mathrm{Vmax}}$ at $z\sim2-3$. Star formation activity and large-scale environment seem to play only a minimal role in the development of LSB features. LSBGs follow mass-dependent evolutionary pathways, where early rapid formation and later slowdowns, combined with their distinct structural properties, influence their response to external factors like mergers and gas accretion. Their LSB nature emerges from intrinsic dynamical and structural factors rather than environmental influences, with angular momentum as a key driver of divergence at high redshifts.

Figures (14)

• Figure 1: $M_*$ as a function of $M_{\mathrm{200c}}$ at $z=0$ for the in stellar and halo mass unbiased populations of LSBs (black solid contours and black crosses) and HSBs (filled contours in shades of orange and orange dots). Refer to \ref{['sec:sample_envr']} for statistical definitions.
• Figure 2: $\mu_{\mathrm{B}}$ as a function of $M_*$ (left panel) and $M_B$ (right panel) at $z=0$. We used \ref{['eq:SB']} to estimate the surface brightness density and corrected optical radii $R^{\dagger}_{\mathrm{opt}}$ as described in \ref{['sec:correct_ropt']}. The horizontal red dashed line marks the standard cut-off threshold of $\mu_{\mathrm{cut}}=25.6$$\mathrm{mag~arcsec}^{-2}$ to divide the dataset into LSB and HSB populations. Refer to \ref{['sec:sample_envr']} for statistical definitions.
• Figure 3: Statistically highest ranked properties: (a) $T_{\mathrm{SF}}$, (b) $\mathrm{age}_{{R_{\mathrm{1/2,*}}{$R_1/2,*$\xspace}}}$, (c) $\epsilon_{2{R_{\mathrm{1/2,*}}{$R_1/2,*$\xspace}}}$, (d) $j_{*}$, (e) sSFR, and (f) $R_{{v_{\mathrm{max}}{$v_max$\xspace}}}$, as a function of $M_*$ at $z=0$. See \ref{['fig:mstar2mhalo_unbiased']} for colour and symbol coding, and refer to \ref{['sec:sample_envr']} for statistical definitions.
• Figure 4: $\lambda_{2{R_{\mathrm{1/2,*}}{$R_1/2,*$\xspace}}}$ on the top and $({v_{\mathrm{rot}}{$v_rot$\xspace}}/{\sigma{$σ$\xspace}})_{2{R_{\mathrm{1/2,*}}{$R_1/2,*$\xspace}}}$ on the bottom as a function of $\epsilon_{2{R_{\mathrm{1/2,*}}{$R_1/2,*$\xspace}}}$ at $z=0$. See \ref{['fig:mstar2mhalo_unbiased']} for the colour and symbol coding, and refer to \ref{['sec:sample_envr']} for statistical definitions. The red dashed line represents the empirical boundary separating the rotation-supported sub-population from the dispersion-supported one.
• Figure 5: Median values for the $M_*$ assembly history within $25^{th}$ and $75^{th}$ percentiles for LSBs (dashed black line) and HSBs (solid orange line with with dots), respectively.