PAC in DESI. II. Galaxy-halo connection into the $10^{6}{\rm M}_{\odot}$ frontier

Abstract

Understanding dwarf galaxy formation is crucial for testing dark matter models and reionization physics. However, constructing stellar-mass complete spectroscopic samples at low masses is increasingly difficult, and the potential existence of a local void complicates studies in an average environment. The Photometric object Around Cosmic webs (PAC) method, which combines deep photometric and spectroscopic data to measure the excess surface density $\bar{n}_2w_{\rm{p}}(r_{\rm{p}})$ of photometric objects around spectroscopic tracers, offers a promising path forward. We model 349 $\bar{n}_2w_{\rm{p}}(r_{\rm{p}})$ measurements from DESI Y1 BGS and DECaLS, reaching $M_*=10^{6.4}\,{\rm M}_{\odot}$, using a stellar mass-halo mass relation (SHMR)-based subhalo abundance matching framework applied to two high-resolution $N$-body simulations from the Jiutian suite. The resulting SHMR is constrained down to $M_{\rm h}\simeq10^{8.0}\,h^{-1}{\rm M}_{\odot}$, revealing a clear upturn at $\sim10^{10.0}\,h^{-1}{\rm M}_{\odot}$ toward lower masses, indicating rising star-formation efficiency (SFE) in small haloes. This feature persists under extensions of the model that allow mass-dependent scatter, reionization-induced suppression of the halo occupation fraction, galaxy assembly bias, and alternative cosmologies. Together with the finding from Paper I, we find that central red galaxies dominate the low-mass regime. Our results motivate a hypothesis in which SFE is significantly higher than previously thought prior to reionization, enabling relatively massive galaxies to form in small haloes. These systems are subsequently quenched by the UV background, producing the central red dwarf galaxies observed. Finally, we obtain $3σ$ and $5σ$ upper mass bounds of $10^{8.38}\,h^{-1}{\rm M}_{\odot}$ and $10^{8.71}\,h^{-1}{\rm M}_{\odot}$ on the smallest haloes required to exist by the data.

Figures (42)

• Figure 1: The 95% completeness stellar mass limits $M_{\rm c95}(z)$ for DECaLS and BGS samples. The quantized appearance of the results arises from the comparison of stellar mass functions in bins with a width of $\Delta\log_{10}(M_{*}/{\rm M}_{\sun})=0.2$.
• Figure 2: Total signal-to-noise ratio of each $\bar{n}_2w_{{\rm{p}}}(r_{\rm{p}})$ measurement in bins of $M_*^{\rm spec}$ and $M_*^{\rm photo}$.
• Figure 3: Normalized covariance matrices of $\bar{n}_2w_{{\rm{p}}}(r_{\rm{p}})$ measurements for samples with $M_*^{\rm photo}$ and $M_*^{\rm spec}$ fixed at $10^{10.6}\,{\rm M}_{\sun}$, with the other sample spanning 12 stellar mass bins in the range $[10^{9.4},\,10^{11.6}]\,{\rm M}_{\sun}$. Each small square represents the cross-covariance of $\bar{n}_2w_{{\rm{p}}}(r_{\rm{p}})$ measurements between two stellar mass bins, each containing 10 radial bins. Stellar mass increases from top to bottom and from left to right.
• Figure 4: Estimated covariance matrix of the normalized $\bar{n}_2w_{{\rm{p}}}(r_{\rm{p}})$ data vector $\tilde{\boldsymbol{\mathcal{A}}}$ in the PCA-truncated space (total dimension 1454), obtained from intra-group PCA and a low-rank approximation of inter-group covariances via truncated SVD. Each rectangle represents the cross-covariance between two Groups. In total, there are 17 Groups corresponding to stacked measurements in 17 $M_*^{\rm spec}\in[10^{8.3},10^{11.7}]\,{\rm M}_{\sun}$ bins. $M_*^{\rm spec}$ increases from top to bottom and from left to right.
• Figure 5: Mean stellar–halo mass relations (top) and stellar-to-halo mass ratios (bottom) for central (blue) and satellite (orange) galaxies constrained by the fiducial models. Results from both the non-parametric and parametric models are shown. Dots with error bars and curves with shaded regions denote the maximum a posterior (MAP) estimates and the corresponding $1\sigma$ intervals. The scatters of the relations, along with their uncertainties, are also shown in the top panel.