ODIN: Using multiplicity of Lyman-Alpha Emitters to assess star formation activity in dark matter halos

TL;DR

This paper investigates whether the multiplicity of LAEs within the same dark matter halo can serve as an observable proxy for halo mass. By combining ODIN narrowband LAE data with mock catalogs from IllustrisTNG100 and a Ly$ ext{Ly} ext{α}$ emission model, the authors quantify how multiplicity correlates with host halo mass, mean Ly$ ext{Ly} ext{α}$ and UV luminosities, and halo-wide surface brightness densities. They find that higher LAE multiplicity preferentially occurs in more massive halos and that group-scale Ly$ ext{Ly} ext{α}$ and UV densities increase with multiplicity, driven by more compact, actively star-forming environments; a subhalo-perturbation model can reproduce the minimum subhalo masses for LAEs at $z=2.4$, suggesting local perturbations trigger star formation in these systems. The results support LAE multiplicity as a practical tracer of halo mass and provide physical insight into star-formation triggering in high-density environments, with implications for interpreting LAE populations across cosmic time.

Abstract

We investigate if systems of multiple Lyman-alpha emitters (LAEs) can serve as a proxy for dark matter halo mass, assess how their radiative properties relate to the underlying halo conditions, and explore the physics of star formation activity in LAEs and its relation to possible physically related companions. We use data from the One-hundred-deg$^2$ DECam Imaging in Narrowbands (ODIN) survey, which targets LAEs in three narrow redshift slices. We identify physically associated LAE multiples in the COSMOS field at $z = 2.4$, $z = 3.1$, and $z=4.5$, and use a mock catalog from the IllustrisTNG100 simulation to assess the completeness and contamination affecting the resulting sample of LAE multiples. We then study their statistical and radiative properties as a function of multiplicity, where we adopt the term multiplicity to refer to the number of physically associated LAEs. We find a strong correlation between LAE multiplicity and host halo mass in the mocks, with higher multiplicity systems preferentially occupying more massive halos. In both ODIN and the mock sample, we find indications that the mean Ly$α$ luminosity and UV magnitude of LAEs in multiples increase with multiplicity. The halo-wide LAE surface brightness densities in Ly$α$ and UV increase with multiplicity, reflecting more compact and actively star-forming environments. The close agreement between the model and ODIN observations supports the validity of the Ly$α$ emission model in capturing key physical processes in LAE environments. Finally, a subhalo-based perturbation induced star formation model reproduces the minimum subhalo mass distribution in simulations at $z=2.4$, suggesting that local perturbations, rather than the presence of LAE companions, drive star formation in these systems. For the higher redshifts, neighbor perturbations do not seem to be the main driver that triggers star formation.

Figures (8)

• Figure 1: Ly$\alpha$ luminosity (left panels), rest-frame equivalent width (REW, middle panels) and UV absolute magnitude (M$_\text{UV}$, right panels) distributions per unit volume for redshifts $z=2.4$ (top panels), $z=3.1$ (middle panels) and $z=4.5$ (bottom panels). The green solid line shows the ODIN-COSMOS LAE sample (Ly$\alpha$ Lum $\geq$ 10$^{42}$ erg s$^{-1}$ and REW $\geq$ 20 Å), while the gray shaded area shows the same sample in the TNG100 simulation. The blue solid line shows a randomly selected subset representing 25$\%$, 33$\%$, and 40$\%$ of the LAEs in TNG100 at $z=2.4$, $3.1$, and $4.5$, respectively. The pink shaded areas show the latter samples applying a minimum luminosity cut of $10^{42.04}$, $10^{42.18}$, and $10^{42.37}$ erg s$^{-1}$ (vertical dashed line shown in the left panels), respectively. The orange shaded areas show the REW distributions noisified by photometric uncertainties, while the sky blue shaded regions show the corresponding noisified M$_\text{UV}$ distributions (see text).
• Figure 2: Mean halo mass as a function of the number of LAEs within the halo at $z=2.4$ (blue), $z=3.1$ (magenta) and $z=4.5$ (green). The TNG100 true multiples are shown with solid lines, while the TNG100 multiples identified using projected distances of $rp_{max} = 100$ pkpc at $z=2.4$, $70$ pkpc at $z=3.1$ and 50 pkpc at $z=4.5$ are displayed in dashed lines. Shaded regions and error bars show the error of the mean. As the number of LAEs in a halo increases, the true mean halo mass also increases, suggesting that the number of physically associated LAEs can serve as a proxy of halo mass. The identified multiples follow the underlying trend more closely at higher redshift.
• Figure 3: Mean Ly$\alpha$ luminosity (top panels) and UV magnitude (bottom panels) as a function of the LAE multiplicity, for redshifts $z = 2.4$ (left), $z = 3.1$ (middle) and $z=4.5$ (right). Solid black lines correspond to the TNG100-true multiples, dashed pink lines show the TNG-identified multiples (with $rp_{\text{max}} = 100$ pkpc at $z = 2.4$, 70 pkpc at $z = 3.1$, and 50 pkpc at $z = 4.5$), and dotted cyan lines correspond to the ODIN-COSMOS sample under the same selection criteria. The lighter black and pink lines indicate the mean UV magnitudes perturbed by photometric uncertainties for the mock samples. Error bars represent the standard error of the mean. Overall, both Ly$\alpha$ luminosity and UV magnitude tend to increase slightly with LAE multiplicity, supporting the idea that multiple LAEs trace more massive and actively star-forming halos.
• Figure 4: Virial surface brightness of Ly$\alpha$ (erg s$^{-1}$ kpc$^{-2}$) and UV (erg s$^{-1}$ Hz$^{-1}$ kpc$^{-2}$) luminosities as a function of LAE multiplicity. Left panels: Mean virial surface luminosities for Ly$\alpha$ (top) and UV (bottom) emissions in TNG100, defined as the total luminosity per projected area, at $z = 2.4$ (blue), $z = 3.1$ (magenta) and $z=4.5$ (green), plotted as a function of the fraction of LAEs relative to the total number of galaxies in the halo. Solid lines correspond to LAE-based densities, while dotted lines correspond to galaxy-based densities. Insets display the mean total luminosity before normalization. Error bars represent the standard error of the mean. Both Ly$\alpha$ and UV virial densities increase with the LAE fraction, reaching a peak at $\sim0.5$. This behavior is mainly driven by the Ly$\alpha$ model, which produces a more compact spatial distribution of LAE galaxies within halos. Right panels: Mean virial surface luminosities as a function of LAE multiplicity. Black lines show the TNG100-true values, pink lines the TNG100-identified sample, and cyan lines the ODIN-COSMOS data, with solid, dashed, and dotted lines denoting $z = 2.4$, $z = 3.1$, and $z=4.5$, respectively. Error bars represent the standard error of the mean in each bin. Higher LAE multiplicity appears to be associated with increased virial surface brightness densities.
• Figure 5: Distribution of minimum subhalos masses in the halos at $z=2.4$ (left panel), $z=3.1$ (middle panel) and $4.5$ (right panel). In the top panels, the minimum subhalo mass distributions from TNG100 are shown as shaded black histograms, while the model predictions described in Section \ref{['sec:model']} are overplotted as black dashed lines (with $f_\phi = 0.11$ and $f_M / f_r =2.43$ at $z=2.4$, $f_\phi = 0.39$ and $f_M / f_r =0.78$ at $z=3.1$, and $f_\phi = 0.50$ and $f_M / f_r =0.63$ at $z=4.5$). Using these factors, we test the model against all TNG100 LAEs (colored dashed lines in the top panels), TNG100 true 1 LAEs (orange dotted lines in the middle panels), and TNG100 true 2 LAEs (red dash-dotted lines in the bottom panels), comparing them with the corresponding minimum subhalo mass distributions of TNG100 LAES (shaded colored histograms). The model shows a good agreement with the minimum subhalo mass distribution for all galaxies, for all LAEs, and isolated LAEs, but struggles to reproduce as closely the distribution for LAE pairs.