The Odd Meanderings of the IMF Across Cosmic Time

Understand

It is difficult to reconcile the observed evolution of the star formation rate versus stellar mass (SFR-M*) relation with expectations from current hierarchical galaxy formation models.

• The observed SFR-M* relation shows a rapid rise in SFR(M*) from z=0-2, and then a surprisingly lack of amplitude evolution out to z~6+.
• Hierarchical models of galaxy formation match this trend qualitatively but not quantitatively, with a maximum discrepancy of ~x3 in SFR at z~2.
• One explanation, albeit radical, is that the IMF becomes modestly weighted towards massive stars out to z~2, and then evolves back towards its present-day form by z~4 or so.