1. Research Home
  2. High-redshift Quasars
  3. High-redshift Galaxies
  4. Low-redshift Universe
  5. Others

  1. Research Home

    I have a range of research interests in extragalactic astronomy and observational cosmology, including high-redshift quasars and supermassive black holes, high-redshift galaxies and galaxy (proto)clusters, cosmic reionization, etc. The epoch of cosmic reionization marks the latest major phase transition of the universe. During this epoch the intergalactic medium was ionized by early astrophysical objects. Reionization started at redshift higher than ten and ended at redshift about six. This period is the time when the first galaxies started to form and shine, and the time when the first massive black holes started to accrete and grow. My research mainly focuses on astrophysical objects at the epoch of reionization.

    I use large ground-based telescopes to carry out surveys of high-redshift quasars at redshifts close to or greater than six, selected from massive imaging data such as the Sloan Digital Sky Survey (SDSS). I carry out multi-wavelength observations using ground-based and space telescopes to study the physical properties of high-redshift quasars, their host galaxies, their central massive black holes, and their cosmic environments. I also use the observations of these quasars to probe cosmic reionization.

    I use large ground-based telescopes to carry out spectroscopic surveys of high-redshift galaxies at redshifts around six or higher. I conduct multi-wavelength observations to study the physical properties of high-redshift galaxies, with ground-based and space telescopes such as Hubble Space Telescope (HST) and Spitzer Space Telescope. I use statistical properties of high-redshift galaxies to probe cosmic reionization.

    My research interests also include relatively lower redshift (still billions of light-years away) quasars/AGN and galaxies. I carry out spectroscopic surveys of low-redshift quasars and galaxies, understand their formation and evolution, and study their physical properties. In addition, I am also interested in other research fields. Details are coming soon.

    Finally, I am involved in several large projects, including CSST, DESI, Subaru PFS, and so on. For details, see the 'Projects' link.

  2. High-redshift Quasars

    One of my main research interests is to search for very high-redshift (z ≥ 6) quasars and study their physical properties.

    (I) We carry out surveys of quasars at z ≥ 6. For example, we have used the SDSS imaging data to search for high-redshift quasars. We have found quasars in the SDSS main survey, in the SDSS Stripe 82 (Jiang et al. 2008, AJ, 135; 2009, AJ, 138), and in the SDSS overlap regions (Jiang et al. 2015, AJ, 149). The final SDSS high-redshift sample of 52 quasars at z > 5.7 was published in Jiang et al. 2016 (ApJ, 833). This sample has been widely used to study high-redshift quasars, their host galaxies, central black holes, and cosmic reionization. Click here for the figure of the 52 quasar spectra. I was also involved in many other surveys of high-redshift quasars, including quasars at redshift ~ 7.5 (e.g., Yang et al. 2020, ApJL, 897, 14; Wang et al. 2021, ApJ, 907, 1).

    Figure: Optical spectra of 52 SDSS quasars at z ~ 6 (Jiang et al. 2016, ApJ, 833).

    (II) We carry out multi-wavelength observations to study the properties of high-redshift quasars. For example, we have used Spitzer Space Telescope to study mid-IR properties of quasars (e.g., Jiang et al. 2006, AJ, 132; 2010, Nature, 464). We have used Gemini near-IR spectrograph GNIRS to study their chemical abundances and black hole masses (e.g., Jiang et al. 2007, AJ, 134; Shen, Wu, Jiang, et al. 2019; Wang, Jiang, et al. 2022). We have also studies the demographics of z ~ 6 quasars in the black hole mass-luminosity plane (e.g., Wu, Shen, Jiang, et al. 2022).

    Figure: Demographics of z ~ 6 quasars. From left to right, we show the best-fit results for the black-hole mass function, Eddington ratio distribution function, and the quasar luminosity, based on a forward modeling of parameterized intrinsic distributions of masses and Eddington ratios (Wu, Shen, Jiang, et al. 2022).

    (III) We also use the observations of these quasars to probe cosmic reionization. An important question in the field is what objects provided ionizing photons for reionization. Recently we have shown direct observational evidence that the quasar/AGN contribution is negligible, suggesting that galaxies, presumably low-luminosity star-forming systems, are the major sources of hydrogen reionization (Jiang et al. 2022, Nature Astronomy, 6, 850).

    Figure: Quasar luminosity function and quasar contribution to reionization (Jiang, et al. 2022, Nature Astronomy, 6, 850). (a) Luminosity function and model fits. (b) Quasar contribution is less than 7% (95% confidence level).

  3. High-redshift Galaxies

    Another of my main research interests is to search for very high-redshift (z ≥ 6) galaxies and study their physical properties.

    (I) We carry out spectroscopic surveys of galaxies at z ≥ 6. For example, recently we have carried out a massive spectroscopic survey of luminous high-redshift galaxies using the multi-fiber spectrograph M2FS on the Magellan Clay telescope (Jiang et al. 2017, ApJ, 846). Based on this sample, we have discovered a giant protocluster of galaxies at z = 5.7 (Jiang et al. 2018, Nature Astronomy, 2). We have further built a large sample of >200 Lyman-alpha emitters (LAEs) at z = 5.7 and 6.6 over two square degrees on the sky (Ning, Jiang et al. 2020; 2022). We have also detected diffuse Lyman-alpha halos by stacking these LAEs (Wu, Jiang et al. 2020).

    Figure: Sample spectra of z ~ 5.7 galaxies. We have built one of the largest samples of LAEs at z ~ 5.7 and 6.6 (Ning, Jiang et al. 2020; 2022).

    (II) We carry out multi-wavelength observations to study the properties of high-redshift galaxies, using ground-based and space telescopes. For example, we are using imaging data obtained from HST and Spitzer to study the physical properties (Lyman-alpha line and UV continuum properties, size and morphology, stellar populations, etc.) of a large sample of spectroscopically confirmed LAEs and Lyman break galaxies (LBGs) at z > 6 (e.g., Jiang et al. 2013, ApJ, 772; 2013, ApJ, 773; 2016, ApJ, 816; 2020, ApJ, 889). In particular, we confirmed the existence of extremely blue galaxies at z > 5.7 (Jiang et al. 2020, ApJ, 889).

    Figure: Extremely blue, spectroscopically confirmed galaxies at z ≥ 6 (Jiang et al. 2020, ApJ, 889).

    (III) We also use statistical properties of high-redshift galaxies to probe cosmic reionization. For example, recently we calculated LAE luminosity function at z ~ 5.7 and 6.6 based on the large sample mentioned above (Ning, Jiang et al. 2022). We find a rapid evolution at the faint end of the luminosity function, but detect a density bump at the bright end, suggesting that very luminous galaxies tend to reside in overdense regions that have formed large ionized bubbles around them. Using the observational result, we put strong constraints on the fraction of neutral hydrogen in the IGM at z > 6.

    Figure: Luminosity function of LAEs at z ~ 5.7 and 6.6, and constraints on the fraction of neutral hydrogen in the IGM (Ning, Jiang et al. 2022).

  4. Low-redshift Universe

    My research interests also include lower-redshift (z < 6) quasars and galaxies, including spectroscopic surveys of these objects and their physical properties. My research on low-redshift quasars covers a variety of quasar/AGN aspects, including AGN reverberation mapping (e.g., Jiang et al. 2016, ApJ, 818; Wang, Shen, Jiang et al. 2019, 2020), quasar host galaxies (Yue, Jiang et al. 2018), surveys of low-redshift quasars (Jiang et al. 2006, AJ, 131; McGreer, Jiang et al. 2013; Pan, Jiang et al. 2022), quasar radio properties (Jiang et al. 2007, ApJ, 656), and other properties (e.g., Gibson, Jiang et al. 2008; Jiang et al. 2009, ApJ, 679; Guo, Roberto, Jiang et al. 2020).

    In the field of low-redshift quasars, we recently completed a program on quasar luminosity function at 3.5 < z < 5.0 (Pan, Jiang et al. 2022). Our quasar sample bridges previous quasar samples from brighter surveys and deeper surveys (see figure below). We found that the cumulative density evolution of quasars from z ~ 5 to 3.5 can be described by a pure density evolution model, suggesting a nearly uniform evolution of the quasar density at z = 3.5 - 7.

    Figure: Quasar luminosity function at 3.5 < z < 5.0 and cumulative density evolution of quasars from z = 0 to 7. (Pan, Jiang et al. 2022).

    In the field of low-redshift galaxies, we are carrying out a comprehensive study of spectroscopically confirmed LAEs from z = 2.2 to 4.8. Currently most studies of z > 2 galaxies are based on photometrically selected samples. Our plan is to build a large sample of spectroscopically confirmed LAEs over more than one square degree at each of the redshift slices z ≈ 2.2, 3.1, 3.7, 4.5/4.8, etc. The targets are all located in well studied deep fields with a large amount of archival data from X-ray to radio that will enable many sciences. When we combine these samples with our samples at z ≈ 5.7 and 6.5, we will be able to study LAEs from z = 2.2 to 6.5 systematically. we have obtained a sample of ~180 LAEs at z = 3.1 over 1.2 square degrees (Guo, Jiang et al. 2020). Observations of LAEs at other redshifts are going on.

    Figure: Luminosity function of LAEs at z ~ 3.1 based on our spectroscopically confirmed sample (Guo, Jiang et al. 2020).

  5. Others

    Coming soon ...