High-redshift galaxies in the Hubble Deep Field: colour selection and star formation history to z   4

Abstract

The Lyman decrement associated with the cumulative effect of H I in QSO absorption systems along the line of sight provides a distinctive feature for identifying galaxies at z ∼ > 2.5. Color criteria, which are sensitive to the presence of a Lyman-continuum break superposed on an otherwise flat UV spectrum, have been shown, through Keck spectroscopy, to successfully identify a substantial population of star-forming galaxies at 3 ∼ < z ∼ < 3.5 (Steidel et al. 1996a). Such objects have proven surprisingly elusive in fieldgalaxy redshift surveys; quantifying their surface density and morphology is crucial for determining how and when galaxies formed. The Hubble Deep Field (HDF) observations offer the opportunity to exploit the ubiquitous effect of intergalactic absorption and obtain useful statistical constraints on the redshift distribution of galaxies considerably fainter than current spectroscopic limits. We model the H I cosmic opacity as a function of redshift, including scattering in resonant lines of the Lyman series and Lymancontinuum absorption, and use stellar population synthesis models with a wide variety of ages, metallicities, dust contents, and redshifts, to derive color selection criteria that provide a robust separation between high redshift and low redshift galaxies. From the HDF images we construct a sample of star-forming galaxies at 2 ∼ < z ∼ < 4.5. While none of the ∼ 60 objects in the HDF having known Keck/LRIS spectroscopic redshifts in the range 0 ∼ < z ∼ < 1.4 is found to contaminate our high-redshift sample, our color criteria are able to efficiently select the 2.6 ∼ < z ∼ < 3.2 galaxies identified by Steidel et al. (1996b).The ultraviolet (and blue) dropout technique opens up the possibility of investigating cosmic star and element formation in the early universe. We set a lower-limit to the ejection rate of heavy elements per unit comoving volume from Type II supernovae at z = 2.75 of ≈ 3.6 × 10 −4 M ⊙ yr −1 Mpc −3 (for q 0 = 0.5 and H 0 = 50 km s −1 Mpc −1 ), which is 3 times higher than the local value, but still 4 times lower than the rate observed at z ≈ 1. At z = 4, our lower limit to the cosmic metal ejection rate is ≈ 3 times lower than the z = 2.75 value. We discuss the implications of these results on models of galaxy formation, and on the chemical enrichment and ionization history of the intergalactic medium.

Dusty star-forming galaxies at high redshift

“…Splitting the measurement into redshift bins then gives more detailed information on sample evolution. Plots of SFRD against redshift or look-back time are often referred to as a Lilly-Madau diagram, first discussed in Lilly et al (1995) and Madau et al (1996). Figure 23 illustrates infrared-based estimates to the SFRD contribution from a variety of surveys and Figure 24 shows their contributions broken down by luminosity class.…”

Star formation sustained by gas accretion

“…The star-formation rate over the history of the universe has been studied since the mid90s (e.g., Lilly et al 1996;Madau et al 1996;Steidel et al 1999;Ouchi et al 2004;Madau and Dickinson 2014). It requires observing faint objects at high redshift and is thus one of the primary drivers for building larger telescopes with adaptive optics and infrared sensitivity.…”

Large-scale variations of the cosmic X-ray background and the X-ray emissivity of the local Universe

“…In particular, the cumulative X-ray volume emissivity of AGNs was much higher at redshifts of z ∼ 1−2 than it is at the present time (e.g. Maccacaro et al 1983;Barger et al 2005), the star-formation rate (and the associated rate of X-ray production) was similarly higher (Madau et al 1996), and so on. It is therefore clear that the integrated X-ray emissivity of a unit volume or of a unit mass of Universe has evolved with time, so knowing its local (z = 0) value is essential for understanding the evolution of different populations of X-ray sources, in particular supermassive black holes in the centers of galaxies.…”