HIGH-RESOLUTION STUDY OF THE CLUSTER COMPLEXES IN A LENSED SPIRAL AT REDSHIFT 1.5: CONSTRAINTS ON THE BULGE FORMATION AND DISK EVOLUTION

Abstract

We analyse the clump population of the spiral galaxy Sp 1149 at redshift 1.5.\nLocated behind the galaxy cluster MACS J1149.5+2223, Sp 1149 has been\nsignificantly magnified allowing us to study the galaxy on physical scales down\nto ~100 pc. We have used the publicly available multi-band imaging dataset\n(CLASH) to reconstruct the spectral energy distributions (SEDs) of the clumps\nin Sp 1149, and derive, by means of stellar evolutionary models, their physical\nproperties. We found that 40% of the clumps observed in Sp 1149 are older than\n30 Myr and can be as old as 300 Myr. These are also the more massive (luminous)\nclumps in the galaxy. Among the complexes in the local reference sample, the\nstar-forming knots in luminous blue compact galaxies could be considered\nprogenitor analogs of these long-lived clumps. The remaining 60% of clumps have\ncolors comparable to local cluster complexes, suggesting a similar young age.\nWe observe that the Sp 1149 clumps follow the M ~ R^2 relation similar to local\ncluster complexes, suggesting similar formation mechanisms although they may\nhave different initial conditions (e.g. higher gas surface densities). We\nsuggest that the galaxy is experiencing a slow decline in star formation rate\nand a likely transitional phase toward a more quiescent star-formation mode.\nThe older clumps have survived between 6 and 20 dynamical times and are all\nlocated at projected distances smaller than 4 kpc from the centre. Their\ncurrent location suggests migration toward the centre and the possibility to be\nthe building blocks of the bulge. On the other hand, the dynamical timescale of\nthe younger clumps are significantly shorter, meaning that they are quite close\nto their birthplace. We show that the clumps of Sp 1149 may account for the\nexpected metal-rich globular cluster population usually associated with the\nbulge and thick disk components of local spirals.\n