Starburst galaxies in the Hydra I cluster

TL;DR

This study provides a multiwavelength census of Hydra I galaxies out to $\,sim 1.75\,r_{200}$ by combining new DECam optical imaging, MeerKAT HI maps, and WISE infrared data to quantify star formation histories on different timescales. By separating galaxies into Starburst, Main Sequence, and Quenching populations via their offsets from the optical/IR main sequence, the authors link recent ($\sim 10^{7}$ yr) and historical ($\sim 10^{9}$ yr) star formation to gas content and environment. They identify environment-driven processes, notably ram pressure stripping and tidal interactions, that reshape gas disks and trigger brief starbursts in a subset of galaxies, especially those infalling along filaments connected to Antlia and other large-scale structures. The analysis reveals a dual Hydra I: an evolved core with quenched/transitioning systems and an active infalling population whose star formation and gas properties reflect interactions with the ICM and the cosmic web, underscoring the complex pathways of galaxy evolution in cluster environments.

Abstract

We present a new catalog of 196 galaxies of the nearby Hydra I cluster out to $\sim$1.75$\rm r_{200}$, consisting of broad u,g,r,i,z along with narrowband H$α$ measurements. These deep optical images were obtained with the DECam camera (CTIO) and reach down to a surface brightness limit of $μ( 3σ;10''\times10'')$=26.9 mag $\rm arcsec^2$ in the g band. We also report the HI properties for 89 cluster members detected with MeerKAT. A color magnitude diagram (CMD) shows a bimodal distribution typical of a cluster population, more evolved than those found in isolation. We combine optical H$α$ and WISE infrared data to compare the star formation history at two distinct timescales. Differences in the star forming activity depicted by both populations manifest as starburst in 24 found members. Of these, 18 starburst galaxies have neutral gas measurements, and show disturbed HI disks that suggest an environmentally-triggered boost in star formation within the last 10$^7$ yrs. Processes such as ram pressure stripping or tidal interactions may underlie their enhanced star-forming activity and asymmetric disks. Since Hydra's dynamical history is unclear, we examine the spatial and velocity distribution of the sample. We reveal a possible link between the large scale structure feeding the Hydra I cluster and the heightened star-forming activity of the starburst galaxies. This feeding pattern matches the few substructure that has been identified in Hydra in previous works, and may explain its origin. Our results portray a picture of a cluster with an evolved nature, plus a population of new infalling galaxies that manifest the impact of their first contact with the cluster environment through star formation, color, morphology and gas content transformations.

Figures (19)

• Figure 1: Galaxies from our parent sample in equatorial coordinates. The black dashed circle is the virial radius $r_{\rm 200}$. The fuchsia and yellow squared regions show the coverage of our DECam and HI measurements, respectively. Gray squares represent the galaxies that do not belong to the H$\alpha$IR sample, but are members of Hydra I according to either 6dFGS or NED (see sec. \ref{['sec:Data']}). Colorized markers are discussed in sections \ref{['sec:SF']} and \ref{['subsec:ppsd']}, where we separate our sample in three populations according to their star formation activity relative to the main sequence (see section \ref{['sec:DMS']}): starburst galaxies (SBGs), main sequence galaxies (MSGs), and quenching galaxies (QINGs). Yellow filled markers show those galaxies that were detected in HI.
• Figure 2: Diagram of the selection process for the 196 Hydra cluster members from the final sample. Center-up: The selection of the 196 galaxies for this study was based on both optical and infrared data availability. Redshifts come, in order of preference: from radio HI estimations for the 89 galaxies with an HI detection, from 6dFGS for another 90 galaxies without an HI detection, and finally from NED for 17 galaxies without HI and spectroscopic detections. Center-down: Star formation rates (SFR) were calculated from 186 H$\alpha$ and 150 mIR confident detections. (*) 10 estimations of the SFR$_{\rm H\alpha}$ are lower limits due to high background signal from nearby galaxies, lower S/N, uncertain continuum subtraction, or the presence of very bright nearby sources. We include 46 estimations of SFR$_{\rm IR}$ based on upper limits measurements due to low S/N.
• Figure 3: False rgb color images (red - r band, green - g band, blue - u band) overlaid with H$\alpha$ map (fuchsia) for 4 starburst galaxies in the H$\alpha$IR sample (see sec. \ref{['sec:DMS']}): ESO 437-G003, ESO 501-G075, NGC 3312, and ESO 501-G065. In yellow, MeerKAT HI contours for each galaxy, and the column density they represent. The beam size is displayed at the top right. The compass at the top left points in the direction towards the center of the cluster. The distance indicated below the compass is the projected distance of the galaxy to the center of the cluster. A scalebar of 10kpc can be found at the bottom left.
• Figure 4: Star formation rate versus stellar mass of the H$\alpha$IR sample in a logarithmic scale, estimated from the mIR (orange) and H$\alpha$ (blue) emission respectively. The optical and infrared SFR of galaxies with low confidence measurements are plotted as empty triangles. The green region is what we have defined as the Main Sequence (MS) of star-forming galaxies. We used Jarrett+ in prep fit of the MS and the $\sigma$ that bok2020h employed to define their MS from WISE mIR observations of AMIGA galaxies. For reference, AMIGA isolated galaxies from bok2020h are also shown in the background (gray). Average uncertainties for detections in H$\alpha$ and mIR have been provided at the top of the diagram for clarity.
• Figure 5: Center: DMS as defined in Eq. \ref{['eq:DMS']}. That is, vertical distance between each SFR$_{\rm H\alpha}$ (blue) and SFR$_{\rm IR}$ (orange) marker, and the Main Sequence (MS), against the clustercentric projected distance. Empty triangles show the population of galaxies with lower (upper) limits in their SFR$_{\rm H\alpha}$ (SFR$_{\rm IR}$) estimations. All galaxies have been color-coded according to their stellar mass content. In addition, galaxies have been outlined in different colors in terms of their population: SBGs are marked in blue, MSGs in green, and QINGs in red. Left: Histogram of the DMS for the H$\alpha$ and mIR detections (blue and orange, respectively), and the H$\alpha$ and mIR lower/upper limits (black and gray, respectively). Top: Fraction of QINGs, MSGs, and SBGs (red, green and blue respectively) in distance bins of 0.96$\rm r/r_{200}$ out to 1.25$\rm r/r_{200}$.