SIMBAD references

The optical-IR images of the northern and southern Hubble Deep Fields are used to measure the spectral and density evolution of early-type galaxies. The mean spectral energy distribution is found to evolve passively toward a mid-F star-dominated spectrum by z∼2, becoming more sharply peaked around the 4000 Å break. We demonstrate with realistic simulations that hotter elliptical galaxies would be readily visible if evolution progressed blueward and brightward at z>2, following a standard initial mass function (IMF). The color distributions are best fitted by a ``red'' IMF, deficient above ∼2 M_☉ and with a spread of formation in the range 1.5<z_f<2.5. Traditional age dating is spurious in this context; a distant elliptical can be young but appear red, with an apparent age greater than 3 Gyr independent of its formation redshift. Regarding density evolution, we demonstrate that the sharp decline in numbers claimed at z>1 results from a selection bias against distant red galaxies in the optical, where the flux is too weak for morphological classification, but is remedied with relatively modest IR exposures that reveal a roughly constant space density to z∼2, with 32 and 16 elliptical galaxies detected above and below z=1, respectively. We point out that the lack of high-mass star formation inferred here and the requirement of metals implicates cooling flows of preenriched gas in the creation of the stellar content of spheroidal galaxies. Deep-field X-ray images will be very helpful in examining this possibility.