Strangulation as the primary mechanism for shutting down star formation in galaxies

Abstract: Local galaxies are broadly divided into two main classes, star-forming (gas-rich) and quiescent (passive and gas-poor). The primary mechanism responsible for quenching star formation in galaxies and transforming them into quiescent and passive systems is still unclear. Sudden removal of gas through outflows or stripping is one of the mechanisms often proposed. An alternative mechanism is so-called "strangulation", in which the supply of cold gas to the galaxy is halted. Here we report that the difference between quiescent and star forming galaxies in terms of stellar metallicity (i.e. the fraction of metals heavier than helium in stellar atmospheres) can be used to discriminate efficiently between the two mechanisms. The analysis of the stellar metallicity in local galaxies, from 26,000 spectra, clearly reveals that strangulation is the primary mechanism responsible for quenching star formation, with a typical timescale of 4 billion years, at least for local galaxies with a stellar mass less than 10^11 solar masses. This result is further supported independently by the stellar age difference between quiescent and star-forming galaxies, which indicates that quiescent galaxies of less than 10^11 solar masses are on average observed four billion years after quenching due to strangulation.

• The paper demonstrates that strangulation is the primary mechanism halting star formation in galaxies with stellar masses below 10^11 M☉.
• The analysis of ~26,000 SDSS galaxies reveals increased stellar metallicity in quiescent galaxies, indicating a prolonged 4 Gyr enrichment phase during strangulation.
• The study rules out rapid gas expulsion and emphasizes the need to investigate environmental factors and feedback mechanisms affecting galaxy evolution.

Strangulation as the Primary Mechanism for Shutting Down Star Formation in Galaxies

The cessation of star formation in galaxies presents a considerable enigma within astrophysics, centering on the processes that transition gas-rich, star-forming galaxies into quiescent, gas-poor systems. The paper by Peng et al. offers a compelling argument that strangulation, rather than sudden gas removal, is the leading mechanism responsible for this transformation in local galaxies with stellar masses below $10^{11} M_\odot$.

The authors leverage stellar metallicity as a diagnostic tool to discern between the two predominant quenching processes: strangulation and gas expulsion through outflows or stripping. By analyzing stellar metallicities in approximately 26,000 galaxies from the Sloan Digital Sky Survey (SDSS), they establish that strangulation is the primary quenching mechanism. This assertion is supported by the increased stellar metallicity observed in quiescent galaxies compared to their star-forming progenitors, a signature inconsistent with instantaneous gas removal which would maintain the progenitor's metallicity level.

The temporal dimension of quenching is crucial; the typical timescale for strangulation is identified as approximately 4 Gyr. During this period, galaxies continue to form stars from existing gas reserves without further gas inflow, leading to an enrichment in stellar metallicity. This 4 Gyr timescale is corroborated by age differences observed in quiescent and star-forming galaxies, computed independently from spectral absorption features.

The paper thoroughly investigates potential outflow scenarios, concluding that substantial outflows following strangulation do not align with the observed metallicity data, which supports a closed-system model post-strangulation. Statistical analyses suggest that while strangulation dominates the quenching process, alternative mechanisms such as rapid gas removal might play a supporting role in certain conditions, especially for high mass galaxies where the metallicity signature is less discriminative due to inherently lower gas fractions.

Implications of these findings extend to understanding the evolutionary pathways of galaxy populations, especially in the context of environmental influences that might affect gas inflow— an aspect briefly addressed regarding central and satellite galaxy dynamics. Moreover, this work encourages further exploration into the environmental and feedback conditions conducive to strangulation, with an emphasis on high-redshift analyses to encompass massive galaxies.

This study is a pivotal piece in galaxy evolution discourse, highlighting strangulation as a significant, yet complex, process in shaping the star-formation activity of galaxies. Future work must dissect the specific conditions under which strangulation prevails, potentially offering insights into the underlying feedback mechanisms from hot halos or other environmental factors.

In conclusion, the predominant quenching mechanism—particularly for galaxies with stellar masses $<10^{11} M_\odot$—appears to be strangulation, an insight that reshapes our understanding of galaxy quenching dynamics. This emphasizes the importance of leveraging metallicity differentials as a diagnostic framework in unraveling the intricate processes governing galaxy morphologies and transitions.