Put a ring on it: signposting the formation path of today’s S0 galaxies

José Luis Tous ^1,2,3 , Jaime Perea ⁴ , Helena Domínguez-Sánchez ⁵ , José Maria Solanes ^1,2,3

Abstract

S0 galaxies are the only Hubble type that is relatively abundant in both low- and high-density environments, suggesting that systems with this morphology may have followed more than one evolutionary path. This idea is supported by a wealth of new evidence indicating that the S0 designation actually accommodates a heterogeneous population with a wide range of properties. Recent high resolution numerical simulations also show that S0s formed through merger events may develop a transient star forming ring on their departure from the main sequence as they evolve to become quiescent objects. In an attempt to shed more light on the physical processes that drive their evolution, we have analyzed spatially resolved spectral data from a sample of ~500 S0s identified in the MaNGA survey. Inspection of the spectral maps has revealed the existence of a good number of lenticular systems with significant radial gradients in the star formation activity. A substantial fraction of the S0 for which star formation decreases inside-out harbors rings visible in the nebular (Hα) component and sometimes also in the stellar (D4000) one, especially in galaxies with a passive global spectrum. Objects with this type of quenching are preferentially located in low-mass galaxy aggregations. We speculate that the inside-out configuration may result from an evolutionary pathway involving a gas-rich merger, with a captured satellite in globally passive systems and with a similarly large companion for systems still holding a relatively important star formation activity. In contrast, S0s in which star formation is depressed from the outside in are found in large groups. This latter quenching is consistent with the suppression of star formation due to the removal of the least gravitationally bound cold gas in their outer disks as they infall towards high-density cluster regions.

Method

**I) PCA of spectral profiles of ~500 S0 MaNGA galaxies:** Radial binning and stacking of spaxels to generate the spectral profile of each galaxy (mean spectrum as a function of the galactocentric distance). Projection of the profiles on their first 2 principal components (~90% of the sample variance [4]). The closer to the PS in the PC1 - PC2 space, the weaker the activity (SF/AGN)[2].

**II) Vectorization and quenching classification:** Profiles are converted into vectors. Then, galaxies are classified according to the orientation of these vectors wrt the PS ridge. This orientation establishes how quenching proceeds in galaxies.

More details on the method

The set of principal components obtained for the MaNGA spectral profiles is equivalent to that derived in [4] for a sample of SDSS single-fiber spectra of ~70,000 S0 galaxies.

**Figure 1.** Black lines: Mean spectrum and first three principal components (ES) of the mean flux of the radial bins of the ~500 S0 galaxies. Red lines: residuals from subtracting to each component represented here the corresponding component derived from the SDSS single fiber spectra of S0 galaxies from [4].

In [4], S0 galaxies were classified in the PC1 - PC2 diagram into Passive (PS), Transition (TR) and Active (AC). Each class correspds to 70%, 5% and 25% of the population. The properties of galaxies of each class differ from each other, hence it is convenient to study them appart.

**Figure 2.** Projections of the SDSS single fiber spectra of ~70,000 S0 galaxies on their first 2 principal components derived in [4].

**Figure 3.** Distribution of probabilities of belonging to the PS class obtained from a logistic regression employed for classifying galaxies in the PC1 - PC2 diagram. Classes are deffined at the 95% of the interclass variance.

From [2] we know that the level of activity (SF/AGN) of a spectrum strongly correlates with its distance to the PS.

Results

Star-forming rings are common in S0s with inside-out profiles

**Figure 4.** Optical image and maps of an inside-out PS (top) and an AC (bottom) S0.

Fraction of ringed S0s: 0.22 - 0.37.
More frequent among the passive population.
〈EW(Hα)〉of rings is 4 times larger in AC than in PS galaxies.
Central peak in the gas density (kL(Hα)) deviating from the spatial distribution of SSFR (~EW(Hα)) and stellar ages (~D4000). Attributed to post-AGB stars in PS galaxies.

Activity in S0 galaxies with outside-in profiles concentrates in the inner regions

**Figure 5.** Like fig. 4 but for an outside-in AC S0.

Rare among the passive population.
Small stellar masses.
Dominate in low mass-groups (~10¹² to 5×10¹³M_☉).
Consistent with a scenario involving minor mergers [3].

Physical properties of ringed S0s

**Figure 5.** (Top) Fractions and (bottom) cumulative frequencies of present-day S0s harboring (r ≥ 6) or likely harboring (r ≥ 3) EW(H α) rings as a function of their total stellar mass (left) and environment (right). Results compatible with [1].

References

References:

[1] Díaz-García, S., Díaz-Suárez, S., Knapen, J. H., & Salo, H. 2019, A&A, 625, A146
[2] Jiménez-Palau, C., Solanes, J. M., Perea, J. D., del Olmo, A., & Tous, J. L. 2022, arXiv e-prints, arXiv:2206.06860
[3] Rathore, H., Kumar, K., Mishra, P. K., Wadadekar, Y., & Bait, O. 2022, MNRAS, 513, 389
[4] Tous J. L., Solanes J. M., Perea J. D., 2020, MNRAS, 495, 4135

Contact me! jtous@fqa.ub.edu

Acknowledgements

We acknowledge financial support from the Spanish state agency MCIN/AEI/10.13039/501100011033 and by 'ERDF A way of making Europe' funds through research grants PID2019--106027GB--C41 and PID2019-106027GB-C43. MCIN/AEI/10.13039/501100011033 has also provided additional support through the Centre of Excellence Severo Ochoa's award for the Instituto de Astrofísica de Andalucía under contract SEV-2017-0709 and the Centre of Excellence María de Maeztu's award for the Institut de Ciències del Cosmos at the Universitat de Barcelona under contract CEX2019-000918-M. JLT acknowledges support by the PRE2020-091838 grant from MCIN/AEI/10.13039/501100011033 and by 'FSE Invests in your future'.