Joint Optical-HI mock catalogs and prospects for upcoming HI surveys

TL;DR

The paper develops a novel, data-driven approach to create joint optical HI mock catalogs for SKA precursor surveys by anchoring the HI–optical connection to the local ALFALFA–SDSS data. It uses a pixelized color–magnitude framework and a Schechter HI mass function to generate predictions for direct HI detections and HI stacking, incorporating survey sensitivity, inclination effects, and primary beam responses. A Bayesian inference framework combines Poisson counts for direct detections with mean stacked HI mass to constrain the HI mass function parameters across redshift, demonstrating that stacking significantly improves constraints when direct detections are scarce. The resulting joint mocks enable rapid, predictive planning for MIGHTEE-HI, LADUMA, and WALLABY, providing priors for full HIMF reconstruction and highlighting the importance of including inclination and optical pre-selection in interpreting upcoming HI surveys.

Abstract

Atomic hydrogen (HI) regulates star formation as cold gas fuels star formation. It represents a key phase of matter in the baryon cycle involving accretion, feedback, outflows, and gas recycling. Redshifted $21$ cm line emission originating from galaxies serves as a key tracer for investigating HI gas and its dynamics in the interstellar medium (ISM) and circumgalactic medium (CGM), and enables the study of galaxy evolution. Nonetheless, direct detections of HI are currently limited to $z \leq 0.4$ due to the inherently weak $21$ cm emission line. Ongoing and upcoming large radio surveys aim to detect $21$ cm emission from galaxies up to $z \gtrsim 1$ with unprecedented sensitivity. In current work, we present a novel approach for creating optical-HI joint mock catalogs for upcoming SKA precursor surveys: MIGHTEE-HI and LADUMA with MeerKAT and WALLABY with ASKAP. Incorporation of optical properties along with HI in our mock catalogs makes these a powerful tool for making predictions for upcoming surveys and provides a benchmark for exploring the HI science (e.g., conditional HIMF and optical-to-HI scaling relations) expected from these surveys. As a case study, we show the use of the joint catalogs for predicting the expected outcome of stacking detection for average HI mass in galaxies that are below the threshold for direct detection. We show that combining stacking observations with the number of direct detections puts a strong constraint on the HI mass function, especially in the regime where the number of direct detections is small, as often happens near the farther edge of HI surveys. This intermediate step may be used to set priors for the full determination of the HI mass function.

Figures (11)

• Figure 1: Distribution of galaxies in the colour-magnitude plane: In the left panel, we show sources in A100–SDSS catalog, while the right panel presents a volume-limited sample that displays the expected bimodality in colour and magnitude which we reproduced following the method discussed in Baldry. A black dashed line marks the optimal divider that separates two populations of galaxies. The number of sources differs in the two panels as they represent flux-limited and volume-limited samples, respectively.
• Figure 2: Characteristics of sources in the ALFALFA-Hi mock catalog. The redshift and Hi mass are plotted for 24000 sources. This illustration is for validation purpose with the number counts within the volume simulated in Giovanelli_2005.
• Figure 3: Comparison of mock catalog with the input A100-SDSS catalog (at $z\thicksim 0$): Left panel shows the optical properties of galaxies in A100-SDSS catalog, where r-band magnitude is displayed on the x-axis and the (u-r) colour on the y-axis. Sources are colour-coded according to their Hi mass. Right panel illustrates the corresponding properties for galaxies in our mock catalog, obtained from the ALFALFA joint survey described in Sec. \ref{['ssec:alfalfa_mock']}.
• Figure 4: The comparison of the mock HiMF in the A100-SDSS catalog. Red open squares show the HiMF estimated from our mock catalog for the joint survey. Blue open circles represent the HiMF estimated from the A100-SDSS catalog. Both estimates are obtained using the $1/V_{\rm max}$ method. Displayed Schechter fitting parameters correspond to the mock catalog. Bottom panel presents the distribution of Hi masses for both the mock catalog and the observational catalog.
• Figure 5: The inclination effect on our model of generating mock catalog: The scatter points in left-panel represent the number of sources detected blindly in the MeerKAT mock survey. The solid orange circles indicate the number of galaxies assuming an edge-on orientation, while the open teal circles show the detection when random inclinations are incorporated into our simulation. Accounting for random inclinations leads to a significant increase in the expected number of detected sources. Numbers are for the $5\sigma$ flux limit in $20$ deg$^2$ sky area in $25$ hrs of integration per MeerKAT pointing. The solid grey lines mark these flux limits for each case. The right panel shows the direct detections from a 6-pointing survey, assuming 8-hrs of ASKAP integration per pointing, with each pointing covering a survey area of $30$ deg$^2$.