Dark matter in ALFALFA galaxies: Investigating galaxy-halo connection

This paper aims to investigate the galaxy-halo connection using a large sample of individual galaxies with $\mathrm{H,I}$ integrated spectra. We determine their dark matter content by applying a dynamical method based on $\mathrm{H,I}$ line widths measured with the curve-of-growth technique, together with inclination corrections inferred from optical images. We build a sample of 2453 gas-rich predominantly late-type galaxies spanning a stellar mass range of $10^{8.7}M_\odot$ to $10^{11.4}M_\odot$ by matching them one-to-one with their counterparts from the ALFALFA survey and the TNG100 simulation, ensuring a direct match of stellar mass and $\mathrm{H,I}$ radius. We generate mock images and mock $\mathrm{H,I}$ integrated spectra for TNG100 galaxies, and apply the same dynamical method to both ALFALFA and TNG100 mock galaxies to infer their dark matter masses. Across all stellar mass bins, ALFALFA galaxies exhibit lower median dark matter masses than the mock TNG100 simulation results. In each bin, this offset is driven by a tail of galaxies with comparatively low dark matter content, which becomes more prominent toward higher stellar masses. In the highest mass bin ($M_* > 10^{11} M_\odot$), late-type ALFALFA galaxies show a median dark matter mass that is 23% lower than that of their counterparts in the TNG100 dark-matter-only simulation, with 32% of ALFALFA galaxies having $M_\mathrm{DM}(<R_\mathrm{HI})<10^{11.5} M_\odot$, compared to 17% in the mock TNG100 sample. These results suggest that a larger fraction of massive late-type galaxies reside in relatively less massive dark matter haloes than predicted by the TNG100 simulation.

💡 Research Summary

This paper investigates the galaxy–halo connection by directly measuring the dark‑matter mass enclosed within the H I radius of individual galaxies. The authors assemble a sample of 2 453 gas‑rich, late‑type galaxies from the ALFALFA H I survey, cross‑matched with SDSS DR16 imaging and the GSWLC‑X2 catalog for stellar masses and star‑formation rates. For each galaxy they determine the inclination from the optical axis ratio (using a mass‑dependent intrinsic disc thickness), measure the H I line width that contains 85 % of the total flux (V85) from the integrated spectrum, correct V85 for inclination, and convert it to a circular velocity Vc via an empirically calibrated relation. The H I radius RHI is not measured directly; instead it is inferred from the tight observed correlation between H I mass and RHI (log RHI = 0.51 log MHI − 3.59). With Vc and RHI they compute a dynamical mass Mdyn = Vc² RHI / G. The baryonic mass within RHI is estimated as the sum of the stellar mass (assumed to lie inside RHI for gas‑rich disks), the atomic gas mass (1.33 MHI to include helium, using a model surface‑density profile to allocate the total H I mass within RHI), and a molecular gas mass derived from the star‑formation rate via a calibrated SFR–Mmol relation. The dark‑matter mass is then MDM = Mdyn − Mbary.

To interpret these measurements, the authors construct a mock counterpart sample from the IllustrisTNG suite, specifically the TNG100‑1 hydrodynamic run and its dark‑matter‑only analogue TNG100‑1‑Dark. For each simulated galaxy they generate synthetic r‑band images and mock H I spectra that mimic the ALFALFA observational setup (Arecibo beam, channel width, noise, Hanning smoothing). The H I mass is defined as the mass within 2.2 × the true H I radius, a choice that reproduces the observed MHI–RHI relation for the simulated galaxies. Using the same pipeline as for the observations, they recover “observed‑style” dark‑matter masses for the mock galaxies and compare them with the true particle‑based masses to quantify methodological biases. The validation shows that the method recovers the true enclosed dark‑matter mass with a systematic offset of less than 5 % and scatter dominated by uncertainties in RHI and V85.

The core result is that, across all stellar‑mass bins, the ALFALFA galaxies have lower median dark‑matter masses within RHI than the TNG100 mock galaxies. The discrepancy grows with stellar mass; in the highest bin (M* > 10¹¹ M⊙) the median MDM is 23 % lower than in the simulation. Moreover, a pronounced low‑MDM tail is present in the observations: 32 % of massive ALFALFA galaxies have MDM < 10¹¹·⁵ M⊙, compared with only 17 % of the mock sample. This suggests that a substantial fraction of massive late‑type galaxies reside in less massive haloes than predicted by the current TNG100 model.

The authors discuss two possible interpretations. First, the sub‑grid physics in TNG100—particularly AGN feedback and baryonic processes—may over‑inflate the halo masses of massive disks, leading to an over‑prediction of enclosed dark matter. Second, the observational sample is biased toward H I‑rich, rotation‑supported disks, which may preferentially occupy lower‑mass haloes at fixed stellar mass, a selection effect not captured in abundance‑matching or clustering studies that average over all galaxy types. They also acknowledge methodological limitations: the reliance on an empirical V85‑to‑Vc conversion, the use of a global MHI–RHI relation (which may vary with environment or morphology), and the inclination cut that excludes face‑on systems.

In conclusion, the study demonstrates that large‑scale H I integrated‑spectrum surveys can provide robust, individual‑galaxy dark‑matter measurements out to the H I radius, offering a complementary probe to traditional statistical techniques. The observed tension with TNG100 highlights the need for refined feedback models and for future high‑resolution H I observations (e.g., with the SKA) that can directly measure RHI and rotation curves, thereby tightening constraints on the galaxy‑halo connection.