Observations of Lyα and O VI: signatures of cooling and star formation in a massive central cluster galaxy.
DONAHUE M., CONNOR T., VOIT G.M. and POSTMAN M.
Abstract (from CDS):
We report new Hubble Space Telescope COS and Space Telescope Imaging Spectrograph spectroscopy of a star-forming region (∼100 M☉ yr–1) in the center of the X-ray cluster RX J1532.9+3021 (z = 0.362), to follow-up the CLASH team discovery of luminous UV filaments and knots in the central massive galaxy. We detect broad (∼500 km s–1) Lyα emission lines with extraordinarily high equivalent widths (EQW ∼ 200 Å) and somewhat less broadened Hα (∼220 km s–1). Ultraviolet emission lines of N v and O VI are not detected, which constrains the rate at which gas cools through temperatures of 106 K to be <=10 M☉ yr–1. The COS spectra also show a flat rest-frame UV continuum with weak stellar photospheric features, consistent with the presence of recently formed hot stars forming at a rate of ∼10 M☉ yr–1, uncorrected for dust extinction. The slope and absorption lines in these UV spectra are similar to those of Lyman Break Galaxies at z~3, albeit those with the highest Lyα equivalent widths and star formation rates. This high-EQW Lyα source is a high-metallicity galaxy rapidly forming stars in structures that look nothing like disks. This mode of star formation could significantly contribute to the spheroidal population of galaxies. The constraint on the luminosity of any O VI line emission is stringent enough to rule out steady and simultaneous gas cooling and star formation, unlike similar systems in the Phoenix Cluster and Abell 1795. The fact that the current star formation rate differs from the local mass cooling rate is consistent with recent simulations of episodic active galactic nucleus feedback and star formation in a cluster atmosphere.