Unveiling the small-scale web around galaxies with miniJPAS and DESI

TL;DR

This work tackles how galaxies access the cold gas needed for star formation by probing the local, small-scale filamentary web around galaxies. The authors combine dense photometric data from miniJPAS with DESI spectroscopy and apply a probabilistic Monte Carlo filament reconstruction to detect filaments within $3$ pMpc of target galaxies in the range $0.2<z<0.8$, introducing the connectivity metric $K$. Mock catalogues built from the Henriques lightcone validate the method against ideal 3D connectivities, showing good agreement at low redshift and a consistent increase of $K$ with stellar mass. The results indicate that the detected filaments are predominantly local rather than large-scale structures, with tentative evidence that higher connectivity may modestly boost specific star-formation rate in certain mass bins, emphasizing connectivity as a physically motivated environmental descriptor for galaxy evolution and guiding future large-area surveys.

Abstract

We present the first statistical observational study detecting filaments in the immediate surroundings of galaxies, i.e. the local web of galaxies. Simulations predict that cold gas, the fuel for star formation, is channeled through filamentary structures into galaxies. Yet, direct observational evidence for this process has been limited by the challenge of mapping the cosmic web at small scales. Using miniJPAS spectro-photometric data combined with spectroscopic DESI redshifts when available, we construct a high-density observational galaxy sample spanning 0.2<z<0.8. Local filaments are detected within a 3 Mpc physical radius of each galaxy with stellar mass M* >10^(10) Msun using all nearby galaxies as tracers, combined with a probabilistic adaptation of the DisPerSE algorithm designed to overcome limitations due to photometric redshift uncertainties. Our methodology is tested and validated using mock catalogues built with random forest models applied to a simulated lightcone. Besides recovering the expected increase in galaxy connectivity (defined as the number of filaments attached to a galaxy) with stellar mass, we show that our connectivity measurements agree with 3D reference estimates from the mock galaxies. Thanks to these filament reconstructions, we explore the relation between small-scale connectivity and galaxy star formation rate, finding a mild positive trend which needs to be confirmed by follow up studies with larger sample sizes. We propose galaxy connectivity to local filaments as a powerful and physically motivated metric of environment, offering new insights into the role of cosmic structure in galaxy evolution.

Figures (13)

• Figure 1: Main properties of the JPAS+DESI catalogue (4328 galaxies) as a function of redshift. From top to bottom, we show the number of galaxies, the $r_\mathrm{SDSS}$ band magnitudes, stellar masses, and redshift precision distributions. The contours enclose 68, 95, and 99.7% of the data points, respectively, from the inner to the outer lines. Black solid lines show the properties of the observed JPAS+DESI catalogue (Sect. \ref{['SubSubSect:final_catalogue']}) while purple dashed ones correspond to the mock catalogue introduced in Sect. \ref{['Sect:mocks']}. For illustration, pink dots show a random subsample of the raw lightcone galaxies from which the mocks were derived. Orange and blue colours correspond respectively to galaxies from the matched (common) catalogue and from the J-PAS only dataset. The scattered points illustrate a random subsample of 1500 galaxies from the total JPAS+DESI catalogue.
• Figure 2: Distribution of the density of tracers $n_\mathrm{tracer}$ around observed and mock target galaxies as a function of redshift (Sect. \ref{['SubSubSect:mock_validation']}). The colormap presents the results for the observed JPAS+DESI target galaxies, while purple dashed contours show the mock target galaxy distribution. Contours enclose 68, 95, and 99.7% of the points. The black dotted horizontal line marks the 0.0117 $\mathrm{pMpc}^{-3}$ value (Sect. \ref{['SubSubSect:target_galaxies']}). Target galaxies above this line have at least 10 tracers in their local environment.
• Figure 3: Examples of five different observed target galaxies (red central squares). Points in shades of blue correspond to their associated Monte Carlo (MC) galaxy distributions. The gray outer circles show the aperture of radius 3 pMpc at the redshift of the target. The gray dots in the background show the positions of all the JPAS+DESI galaxies (no selection made) for illustration purposes. In the bottom panels, the black thin lines, over-plotted on top of the same galaxy distributions, show the positions of the resulting filaments. Orange dashed circles mark the aperture of $20 \times R_1$ used for connectivity measurements (Sect. \ref{['SubSect:MeasuringK_method']}).
• Figure 4: Filament frequency maps measured in our JPAS+DESI galaxy catalogue. Each panel corresponds to the projection of our data in a different redshift slice, each of equivalent thickness 12 pMpc (arbitrary choice, just for visualisation). Red circles show the target galaxies with sizes proportional to their stellar masses (Sect. \ref{['SubSubSect:target_galaxies']}). Orange points correspond to tracer galaxies from a single realisation. Pixel colours indicate the number of filament detections (or hits) normalised by the maximum number of hits within the slice (Sect. \ref{['Subsect:filament_detection']}). The dotted blue circle in the bottom left corner of the plots shows a scale of radius 3 pMpc measured at the mean redshift of the slice. Blue points in the background show the $\sim 1 , \mathrm{deg}^2$ footprint of the miniJPAS survey and gray points denote the artificial buffer region added at the survey edges, following Sect. \ref{['SubSubSect:miniJPAS']}.
• Figure 5: Connectivity distributions of the Monte Carlo realisations for 300 random galaxies (different colours in the mock and observational datasets, shown respectively in the top and bottom panels. Each distribution is rescaled by its median value and therefore centred at $x=0$ (thick vertical red line). The thin vertical lines with different line styles indicate the corresponding $K_\mathrm{2D,noMC}$ values, i.e. the results from a single measurement in the fiducial catalogue, without Monte Carlo sampling. These lines are often superimposed due to a quantisation effect: $K_\mathrm{2D,noMC}$ is an integer by definition; half-integer positions appear after rescaling when the median of the distribution is itself a half-integer.