Research

One of the main challenges in observational astronomy is to understand how the first luminous objects in the Universe formed and evolved through cosmic time. The earliest observable structure in the Universe is the Cosmic Microwave Background (CMB), which dates back to the epoch of recombination at a redshift of 𝓏≈1100, about 400,000 years after the Big Bang. The next most distant observable represents the population of galaxies and quasars at 𝓏≈6−12, when the Universe was less than a billion years old (see Figure 1). The interval between 𝓏∼1100 and 𝓏=6 contains many landmark events, such as the formation of the first stars, the assembly of the first galaxies, the growth of the first massive black holes and the reionisation of neutral Hydrogen in the intergalactic medium. The most recent measurements of the CMB from the Planck satellite indicate that reionisation occurred at 𝓏∼8 (600 million years after the Big Bang) but do not pin down when reionisation began or ended.

My research aims to help provide answers to outstanding questions in observational cosmology, such as: when, how and over what period was the Universe reionised? When did the first galaxies form and what were the properties of these first objects? Where and how did the earliest supermassive black holes form? Below I will describe my research in more detail: the search for the most distant quasars

Figure 1: Schematic history of the Universe with age on the bottom axis and redshift at the top. I am studying the ‘First Light’ period of the Universe.
Image credit: ESO.

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Distant quasars: probes of the early Universe

Quasars, powered by accretion onto supermassive, billion solar mass black holes, are the most luminous, non-transient sources known and hence have been found out to 𝓏>7. The high luminosity makes quasars the only sources in the early Universe whose physical properties can be studied in detail with current facilities and are therefore crucial to our understanding of this transformational epoch in the history of the Universe.

However, such luminous accreting black holes are rare, with an estimated density of 1 Gpc3, in other words at the largest distances there is only one accreting black hole per >100 deg2 on the sky. As a result, luminous quasars are not found in the deep, pencil beam observations with, e.g., the Hubble Space Telescope. Indeed, surveys covering a much larger area of the sky are needed to uncover a large number of quasars. About two decades ago, various optical surveys such as the Sloan Digital Sky Survey discovered the first luminous quasars at 𝓏~6, less than 1 Gyr after the Big Bang (Figure 2). These surveys could find quasars up to a redshift of 6.4. Beyond 𝓏=6.4, quasars become extremely faint in the optical and near-infrared photometry is required to locate these distant objects.

Figure 2: Redshift distribution of all 𝓏>5.6 quasars known to date. Over the last 15 years, various groups discovered ~120 quasars (hashed histogram). In the last few years, our group has discovered an additional 125 quasars in various optical and near-infrared surveys (red histogram, see, for example, Bañados, Venemans et al. 2014, 2015, 2016, 2017; Mazzucchelli et al. 2017; Venemans et al. 2013, 2015a, 2015b) , more than doubling the number of quasars at 𝓏>5.6. Especially at the highest redshifts, 𝓏>6.5, these projects provided a breakthrough in the field of distant quasars, including the discovery of the most distant quasar at 𝓏=7.5 (Bañados, Venemans et al. 2017).

Over the last 10 years, with the advent of wide field, near-infrared cameras, large near-infrared surveys started to image the sky to sufficient depth to uncover distant quasars. Our group at MPIA, led by Fabian Walter and me, has successfully been searching for the earliest quasars in these new surveys. The surveys include the Pan-STARRS1 survey (covering 3/4 of the sky in 5 filters up to 1 micron) and the VISTA Kilo-Degree Infrared Galaxy (VIKING) survey. The efforts resulted in a doubling of the number of quasars known at 𝓏~6 (see, for example, Bañados, Venemans et al. 2016) and pushed the redshift barrier to 𝓏~7.5 (e.g., Venemans et al. 2013; Bañados, Venemans et al. 2017, see Figure 2).