The SAURON project - XV. Modes of star formation in early-type galaxies and the evolution of the red sequence

Abstract

We combine SAURON integral field data of a representative sample of local early-type, red sequence galaxies with Spitzer/Infrared Array Camera imaging in order to investigate the presence of trace star formation in these systems. With the Spitzer data, we identify galaxies hosting low-level star formation, as traced by polycyclic aromatic hydrocarbon emission, with measured star formation rates that compare well to those estimated from other tracers. This star formation proceeds according to established scaling relations with molecular gas content, in surface density regimes characteristic of disc galaxies and circumnuclear starbursts. We find that star formation in early-type galaxies happens exclusively in fast-rotating systems and occurs in two distinct modes. In the first, star formation is a diffuse process, corresponding to widespread young stellar populations and high molecular gas content. The equal presence of co-and counter-rotating components in these systems strongly implies an external origin for the star-forming gas, and we argue that these star formation events may be the final stages of (mostly minor) mergers that build up the bulges of red sequence lenticulars. In the second mode of star formation, the process is concentrated into well-defined disc or ring morphologies, outside of which the host galaxies exhibit uniformly evolved stellar populations. This implies that these star formation events represent rejuvenations within previously quiescent stellar systems. Evidence for earlier star formation events similar to these in all fast-rotating earlytype galaxies suggests that this mode of star formation may be common to all such galaxies, with a duty cycle of roughly 1/10, and likely contributes to the embedded, corotating inner stellar discs ubiquitous in this population.

A star formation study of the ATLAS3Dearly-type galaxies with the AKARI all-sky survey

“…The best-fit parameters are listed in Table 4. A similar figure for the SAURON sample of 48 ETGs was presented by Shapiro et al (2010), but the much larger ATLAS 3D sample of 260 ETGs studied here (231 with relevant measurements) offers much clearer insight into the relationship between current SFR, stellar mass and stellar population age. Figure 9 shows that, at fixed stellar mass, all our sample ETGs have a lower current SFR than that of local star-forming galaxies, confirming that local ETGs are quiescent galaxies.…”

“…The luminosities thus derived are listed in Table A.2, and the total IR (TIR) luminosities (L TIR ) were estimated by simply taking the sums of those three components. To check the reliability of our PAH luminosities, we compared them to the PAH luminosities derived by Shapiro et al (2010) using the non-stellar Spitzer 8 µm emission (as done in Sect. 2.2 for our 90 and 140 µm flux densities).…”

“…Utilizing PAH emission observed with Spitzer, Shapiro et al (2010) investigated the star formation properties of the 48 ETGs in the SAURON sample (de Zeeuw et al 2002), and argued that ETGs are forming stars with surface densities and efficiencies Article number, page 1 of 30 similar to those of spiral galaxies. They also suggested that star formation in ETGs proceeds in at least two different modes (widespread and circumnuclear), that may reflect different evolutionary paths.…”

An optical/NIR survey of globular clusters in early-type galaxies

“…Evidence of recent star formation in later-type galaxies is also found in integrated light studies. Shapiro et al (2010) combine SAURON integrated field spectroscopy of early-type galaxies with Spitzer/IRAC imaging to investigate the presence of trace star formation in such systems. Connecting a detailed kinematic analysis given by the first dataset with direct probes of star formation from Spitzer, these authors find that star formation in early-type galaxies happens exclusively in fast rotating systems.…”

Star formation in the outer regions of the early-type galaxy NGC 4203

“…The 8µm image includes a contribution from stars in the galaxy, which we remove by subtracting the 3.6 µm image scaled by a factor X 3.6 as in Shapiro et al (2010). These authors find that X 3.6 has very low scatter about a median value of 0.264 for ETGs, and we adopt this value for NGC 4203.…”