Exploring the Origin of Rejuvenating Gas from MaNGA Nearby Galaxies

TL;DR

This study tackles the origin of gas fueling rejuvenation in nearby galaxies by identifying current rejuvenation events with two stellar absorption features, Dn4000 and EW(Hδ_A), in MaNGA IFU data and comparing RJGs to matched star-forming and quiescent samples across metallicity, gradients, environment, and HI content. It finds that the majority of rejuvenating gas is internal to the galaxy, as RJGs typically show metallicities on par with SF galaxies, standard metallicity gradients, similar gas kinematics, and HI richness, with only a minority suggesting external inflow. A detailed case, MaNGA 12080-12705, demonstrates that external gas accretion or a minor merger can trigger rejuvenation in an otherwise old galaxy, providing a concrete external-origin example. The results establish a practical, scalable method to identify current rejuvenation in large spectroscopic surveys, enabling statistical studies of rejuvenation from the nearby universe to higher redshift.

Abstract

This study investigates the origin of gas fueling secondary star formation, i.e., rejuvenation in nearby galaxies. From the MaNGA IFU survey, we use stellar absorption features D$_n$4000 and H$δ_A$ to identify regions that started the rejuvenation within the last $\sim$200~Myr. We compare the gas-phase metallicity, metallicity gradients, environments, and H\Romannum{1} gas fractions of the rejuvenating galaxies (RJGs) to controlled star-forming and quiescent galaxy samples. We demonstrate that, for the majority of RJGs, the rejuvenating gas is originally in the galaxy rather than accreted gas. The evidence includes: (1) gas metallicities consistent with the mass-metallicity relation of SF galaxies; (2) metallicity gradients that are not flattened, arguing against radial inflows; (3) gas velocities in rejuvenating regions consistent with their surroundings, and (4) high H\Romannum{1} gas fractions comparable to SF galaxies, indicating a pre-existing reservoir. Furthermore, we find no evidence that the rejuvenating events are triggered by tidal interactions with neighbors. While internal processes appear to dominate, we also present a clear example of rejuvenation triggered by gas accretion. The galaxy MaNGA 12080-12705 hosts a low-metallicity, kinematically distinct star-forming region in an overall old, massive galaxy, providing unambiguous evidence of an external origin, such as accretion or a minor merger. Our analysis demonstrates that using D$_n$4000 and EW(H$δ_A$) provides a reliable way to identify current rejuvenation events in large spectroscopic surveys. The method will enable statistical studies to understand rejuvenation across cosmic time.

Figures (11)

• Figure 1: Distribution of all MaNGA spaxels ($\sim 3$ millions) on D$_n$4000-EW(H$\delta_A$) plane, visualized using hexagonal binning. The color bar on the right indicates the number of data points in each hexagonal bin. The majority of spaxels distributes tightly on a diagonal sequence, and only a small fraction fall within our selection criteria (black dashed line).
• Figure 2: The visibility of the rejuvenation event based on our criteria ($\Delta t$). Each panel is for different stellar metallicities of the old stars. Colored lines indicate different intervals between the star-formation events ($\Delta T$). Our criteria tend to select rejuvenation events that happen in the past $\lesssim 200$ Myrs and have formed $\sim1\%$ of masses on top of a stellar population that is at least a few Gyr old.
• Figure 3: Distribution of stellar mass across different numerical morphological types. The associating distributions of stellar mass and morphology are shown as probability density distribution in upper sub-panel and bar chart in right sub-panel. Main panel consists of box plots where, for each type, the blue box spans the 25th percentile to the 75th percentile of the stellar mass distribution, with the vertical line inside the box indicating the median. Left and right bars represent the minimum and maximum value of stellar mass in each morphological type. Our RJGs are generally massive and mostly late-type.
• Figure 4: Stellar mass density-metallicity ($\Sigma_*$-Z) relation of (a) rejuvenating spaxels and (b) star-forming spaxels visualized by hexagonal binning. A color bar in each panel represents the number of data points in each hexagonal bin. Errorbars are medians of metallicity with the corresponding 16th and 84th percentiles in each stellar mass density bin. Black solid line represents the correlation derived from 507,000 star-forming spaxels belonging to 653 star-forming disk galaxies found in 2016BB, while the gray dashed line is for massive galaxies ($\mathrm{M_*>10^{10.5}M_\odot}$).
• Figure 5: The distribution of the difference in gas velocity between the rejuvenation regions and their surroundings. The errorbar on the top represents the 16th, 50th, and 84th percentiles of the distribution.