Virgo Filaments VI: H$α$ clumps in the filaments around the Virgo galaxy cluster

TL;DR

This work tackles how large-scale filaments influence resolved star formation around the Virgo cluster by analyzing Hα maps of 685 galaxies and decomposing emission into clumps with a Scarlet-based wavelet pipeline combined with Photutils deblending. After stringent quality control and a careful FWHM-based matching scheme, 290 galaxies (157 filament, 133 non-filament) are compared to isolate environmental effects. The clump distribution reveals a fractal, self-similar structure with fractal dimension $D\sim 1.3$–$1.4$, and filament galaxies show a modest excess of peripheral clumps but no conclusive differences in clump sizes. The study provides a rigorous methodology to control observational biases in environmental comparisons and highlights the need for higher-resolution data to sharpen conclusions about the role of filaments in shaping star formation.

Abstract

It is still not clear which environmental processes operate in filaments. Given the ubiquity of filaments and their importance in feeding clusters, a proper understanding of these mechanisms is crucial to a more complete picture of galaxy evolution. To investigate them, we need large galaxy samples with resolved imaging. For this study, we analyse resolved H$α$ maps of 685 galaxies inside and outside the filaments around the Virgo cluster in addition to extensive measurements of integrated physical properties. We create a pipeline to decompose the H$α$ images into individual clumps. We find that the number and average size of clumps in a galaxy are well-defined functions of distance and angular resolution. In particular, the power-law relation between the number of clumps and the distance of a galaxy is consistent with a fractal structure of star forming regions. We formulate an algorithm to compare filament and non-filament galaxies after removing observational differences. Although we do not have any conclusive evidence for a difference in clump size distributions between filament and non-filament galaxies, we do find that filament galaxies have slightly more peripheral clumps than their non-filament counterparts.

Figures (10)

• Figure 1: Positions of all 685 galaxies in the H$\alpha$ sample. Galaxies represented by star symbols are not in galaxy groups, while square symbols are in poor groups and triangle symbols are in rich groups. We draw the 13 filaments identified in CastignaniII2022 and use the same colours to mark galaxies as being nearest to a given filament. Galaxies considered inside filaments (under $2.70$ Mpc from the spine of a given filament) have darker symbols than those outside the filament.
• Figure 2: Illustration of the analysis pipeline for galaxy VFID 1035 (NGC 3982). In the top panel, we show the decomposition of the H$\alpha$ image into four Scarlet scales. In the bottom panels, the H$\alpha$ image is shown in grayscale (in arcsinh scaling). In the bottom left, we also show the Scarlet clump mask in burgundy contours, set to the 99.7th percentile of ($\sim 3\sigma$ above) the noise. Given the seeing FWHM of $1.39\arcsec$, this mask consists of both Scarlet scale two and three, visible as small contours within slightly larger contours. The galaxy boundary from the fourth Scarlet scale is marked in the fuchsia curve. In the bottom right panel, we show the final clump map after source detection and deblending by Photutils. In the lower left corner, we show the image PSF for size comparison; 95% of the clumps are larger than the PSF. See the text for an extensive description of our analysis pipeline.
• Figure 3: Clumps detected by our analysis pipeline (coloured circles) in SINGS galaxy NGC 2976 at its original distance (first panel), and with the galaxy artificially moved to twice (second panel), thrice (third panel), and 4 times the distance (last panel, see text for details). We can see that while the clumps are always placed in the same general regions of the galaxy, the number of clumps decreases strongly with the distance.
• Figure 4: Dependence of the number of clumps on distance for SINGS galaxies NGC 2976, NGC 3938, NGC 0024, NGC 0337, and NGC 1512, with lines coloured by the original seeing (FWHM) of the galaxy. The change in the number of clumps has a strong negative correlation (median $R^2=0.983$) with increased distance. When considering all five galaxies simultaneously, we find a slope of $-1.35 \pm 0.07$ (black dashed line). This suggests that H$\alpha$ clumps are hierarchical (fractal) in nature, with $D\sim 1.3-1.4$ (see Section \ref{['sec:fractal']}).
• Figure 5: Number of clumps as a function of angular resolution (FWHM) for the same five SINGS galaxies (NGC 2976, NGC 3938, NGC 0024, NGC 0337, and NGC 1512), with curves once again coloured by the original seeing (FWHM). The curves are well fitted with quadratic forms (median $R^2=0.995$) and have a large scatter that is significantly correlated with the original angular resolution, suggesting a more complex relationship with clump properties than distance, likely related to the angular scales associated with Scarlet scales two and three.