When the Milky Way turned off the lights: APOGEE provides evidence of star formation quenching in our Galaxy

Abstract

Quenching, the cessation of star formation, is one of the most significant events in the life cycle of galaxies. While quenching is generally thought to be linked to their central regions, the mechanism responsible for it is not known and may not even be unique. We show here the first evidence that the Milky Way experienced a generalised quenching of its star formation at the end of its thick-disk formation ∼9 Gyr ago. The fossil record imprinted on the elemental abundances of stars studied in the solar vicinity and as part of the APOGEE survey (APOGEE is part of the Sloan Digital Sky Survey III) reveals indeed that in less than ∼2 Gyr (from 10 to 8 Gyr ago) the star formation rate in our Galaxy dropped by an order of magnitude. Because of the tight correlation that exists between age and α abundance, the general cessation of the star formation activity reflects in the dearth of stars along the inner-disk sequence in the [Fe/H]-[α/Fe] plane. Before this phase, which lasted about 1.5 Gyr, the Milky Way was actively forming stars. Afterwards, the star formation resumed at a much lower level to form the thin disk. These events observed in our Galaxy are very well matched by the latest observation of MW-type progenitors at high redshifts. In late-type galaxies, the quenching mechanism is believed to be related to a long and secular exhaustion of gas. Our results show that in the Milky Way, the shut-down occurred on a much shorter timescale, while the chemical continuity between the stellar populations formed before and after the quenching indicates that it is not the exhaustion of the gas that was responsible for the cessation of the star formation. While quenching is generally associated with spheroids in the literature, our results show that it also occurs in galaxies like the Milky Way, where the classical bulge is thought to be small or non-existent, possibly when they are undergoing a morphological transition from thick to thin disks. Given the demographics of late-type galaxies in the local Universe, in which classical bulges are rare, we suggest further that this may hold true generally in galaxies with mass lower than or approximately of M * , where quenching could directly be a consequence of thick-disk formation, while quenching may be related to development of spheroids in higher mass galaxies. We emphasize that the quenching phase in the Milky Way could be contemporaneous with, and related to, the formation of the bar, at the end of the thick-disk phase. We sketch a scenario on how a strong bar may inhibit star formation.

Spatially-resolved star formation histories of CALIFA galaxies

“…Lehnert et al (2014Lehnert et al ( , 2015 and Haywood et al (2016) extended this analysis, showing that this scenario is useful to explain the formation of MW-like galaxies at high z. They propose that there is a drop of the SFI in the thick disk phase to a more quiescent phase at z < 1, a drop that is more significant than the decrease expected from the exhaustion of gas given by a Schmidt-Kennicutt relation.…”

“…9 shows the results of this comparison. The left panel compares the evolution of the SFR of the inner disk of the MW as predicted by Haywood et al (2016), drawn as black solid lines, with the SFR(t) obtained with our analysis for Sbc (circles) and Sc (stars) galaxies in three M intervals covering stellar masses similar to that of the Galaxy. Our SFRs are computed for regions within R < 2 HLR, corresponding to ∼ 10 kpc for Sbc galaxies, and thus matching the size of the MW inner disk (Haywood et al 2016).…”

TheGaia-ESO Survey: Galactic evolution of sulphur and zinc

“…Wojno et al 2016;Haywood et al 2016;Mikolaitis et al 2014;Recio-Blanco et al 2014, and references therein), one expects (kinematically selected) thin disc stars to have lower α-to-iron ratios than thick disc stars of the same metallicities. If our dwarf stars sample is dominated by the thick disc stars, as suggested by Fig.…”

Bar quenching in gas-rich galaxies

“…Since a large fraction of galaxies have a bar at the present epoch and many galaxies (including the Milky Way, see Haywood et al 2016) passed through a stage of quenching, we want to study the possible relation between the bar formation and quenching phase for Milky Way-type galaxies. This link is intriguing, in particular in the Milky Way, where it has been established that a drastic (about a factor 10) and rapid (in about 1 Gyr) decrease in the star-formation rate occurred approximatively 9 − 10 Gyr ago, at the transition between the formation of the thick and thin disks of our Galaxy.…”

“…It is thus important to understand and quantify if there exists a causal link between these two phenomena. Haywood et al (2016), for example, proposed that in the Milky Way the action of a stellar bar could have increased the gas turbulence, stabilizing the disk against star formation, quenching its star formation. In this scenario, the suppression of the star formation is not associated to a substantial consumption of the available gas, as in other proposed mechanisms.…”