About

Prof. Brant Robertson leads the Computational Astrophysics Research Group in the Department of Astronomy and Astrophysics at UC Santa Cruz. His group focuses on exploring the most challenging problems in astrophysics through theory and computation. Their research addresses fundamental questions such as how galaxies and cosmological structures form and evolve, the nature of dark matter, the reionization of the intergalactic medium, and how deep learning can enhance our understanding of astrophysical phenomena. The group leverages sophisticated numerical methodologies and hardware technologies to investigate these outstanding questions, contributing to the advancement of computational astrophysics and our understanding of the universe.

Research topics

• Astronomy
• Astrophysics
• Physics

Selected publications

• Identification and properties of intense star-forming galaxies at redshifts z > 10

  Nature Astronomy · 2023 · 258 citations

  1st authorCorresponding
  • Physics
  • Astrophysics
  • Astronomy
  DOI

• JADES NIRSpec Spectroscopy of GN-z11: Lyman-<i>α</i> emission and possible enhanced nitrogen abundance in a <i>z</i> = 10.60 luminous galaxy

  Astronomy and Astrophysics · 2023 · 357 citations

  • Physics
  • Astrophysics
  • Astronomy

  We present JADES JWST/NIRSpec spectroscopy of GN-z11, the most luminous candidate z &gt; 10 Lyman break galaxy in the GOODS-North field with M UV = −21.5. We derive a redshift of z = 10.603 (lower than previous determinations) based on multiple emission lines in our low and medium resolution spectra over 0.7 − 5.3 μm. We significantly detect the continuum and measure a blue rest-UV spectral slope of β = −2.4. Remarkably, we see spatially extended Lyman- α in emission (despite the highly neutral intergalactic medium expected at this early epoch), offset 555 km s −1 redwards of the systemic redshift. From our measurements of collisionally excited lines of both low and high ionisation (including [O II ] λ 3727, [Ne III ] λ 3869, and C III ] λ 1909), we infer a high ionisation parameter (log U ∼ −2). We detect the rarely seen N IV ] λ 1486 and N III ] λ 1748 lines in both our low and medium resolution spectra, with other high ionisation lines seen in the low resolution spectrum, such as He II (blended with O III ]) and C IV (with a possible P-Cygni profile). Based on the observed rest-UV line ratios, we cannot conclusively rule out photoionisation from an active galactic nucleus (AGN), although the high C III ]/He II and N III ]/He II ratios are compatible with a star formation explanation. If the observed emission lines are powered by star formation, then the strong N III ] λ 1748 observed may imply an unusually high N / O abundance. Balmer emission lines (H γ , H δ ) are also detected, and if powered by star formation rather than an AGN, we infer a star formation rate of ∼20 − 30 M ⊙ yr −1 (depending on the initial mass function) and low dust attenuation. Our NIRSpec spectroscopy confirms that GN-z11 is a remarkable galaxy with extreme properties seen 430 Myr after the Big Bang.

  DOI

• JADES: Discovery of extremely high equivalent width Lyman-alpha emission from a faint galaxy within an ionized bubble at z=7.3

  arXiv (Cornell University) · 2023 · 13 citations

  • Physics
  • Astrophysics
  • Astronomy

  We report the discovery of a remarkable Ly$α$ emitting galaxy at z = 7.2782, JADES-GS-z7-LA, with EW$_0$(Ly$α$) $= 388.0 \pm 88.8$Åand UV magnitude -17.0. The spectroscopic redshift is confirmed via rest-frame optical lines [O II], H$β$ and [O III] in its JWST/NIRSpec Micro-Shutter Assembly (MSA) spectrum. The Ly$α$ line is detected in both lower resolution PRISM as well as medium resolution G140M grating spectra. The LSF-deconvolved Ly$α$ FWHM in the grating is $383.9 \pm 56.2$ km/s and the Ly$α$ velocity offset compared to the systemic redshift is $113.3 \pm 80.0$ km/s, indicative of very little neutral gas or dust within the galaxy. We estimate the Ly$α$ escape fraction to be &gt;70%. JADES-GS-z7-LA has a O32 ratio of $11.1 \pm 2.2$ and a R23 ratio of $11.2 \pm 2.6$, consistent with low metallicity and high ionization parameters. Deep NIRCam imaging also revealed a close companion source (separated by 0.23"), which exhibits similar photometry to that of JADES-GS-z7-LA, with a photometric excess in the F410M NIRCam image consistent with [O III]+H$β$ emission at the same redshift. The spectral energy distribution of JADES-GS-z7-LA indicates a "bursty" star formation history, with a low stellar mass of $\approx 10^7$ $M_\odot$. Assuming that the Ly$α$ transmission through the intergalactic medium is the same as its measured escape fraction, an ionized region of size &gt; 1.5 pMpc is needed to explain the high Ly$α$ EW and low velocity offset compared to systemic seen in JADES-GS-z7-LA. Owing to its UV-faintness, we show that it is incapable of single-handedly ionizing a region large enough to explain its Ly$α$ emission. Therefore, we suggest that JADES-GS-z7-LA (and possibly the companion source) may be a part of a larger overdensity, presenting direct evidence of overlapping ionized bubbles at $z&gt;7$.

  DOI

• Spectroscopic confirmation of four metal-poor galaxies at z = 10.3–13.2

  Nature Astronomy · 2023 · 356 citations

  • Physics
  • Astrophysics
  • Astronomy

  Finding and characterizing the first galaxies that illuminated the early universe at cosmic dawn is pivotal to understand the physical conditions and the processes that led to the formation of the first stars. In the first few months of operations, imaging from the James Webb Space Telescope (JWST) has been used to identify tens of candidates of galaxies at redshift (z) greater than 10, less than 450 million years after the Big Bang. However, none of such candidates has yet been confirmed spectroscopically, leaving open the possibility that they are actually low-redshift interlopers. Here we present spectroscopic confirmation and analysis of four galaxies unambiguously detected at redshift 10.3 ≤ z ≤ 13.2, previously selected from JWST Near Infrared Camera imaging. The spectra reveal that these primeval galaxies are metal poor, have masses on the order of about 107–108 solar masses and young ages. The damping wings that shape the continuum close to the Lyman edge provide constraints on the neutral hydrogen fraction of the intergalactic medium from normal star-forming galaxies. These findings demonstrate the rapid emergence of the first generations of galaxies at cosmic dawn. As part of the JWST Advanced Deep Extragalactic Survey (JADES), NIRSpec has spectroscopically confirmed four young and metal-poor galaxies at redshift 10.3–13.2, from an early epoch of galaxy formation.

  DOI

• JADES NIRSpec Spectroscopy of GN-z11: Lyman-$α$ emission and possible enhanced nitrogen abundance in a $z=10.60$ luminous galaxy

  arXiv (Cornell University) · 2023 · 43 citations

  • Physics
  • Astrophysics
  • Astronomy

  We present JADES JWST/NIRSpec spectroscopy of GN-z11, the most luminous candidate $z&gt;10$ Lyman break galaxy in the GOODS-North field with $M_{UV}=-21.5$. We derive a redshift of $z=10.603$ (lower than previous determinations) based on multiple emission lines in our low and medium resolution spectra over $0.8-5.3 μ$m. We significantly detect the continuum and measure a blue rest-UV spectral slope of $β=-2.4$. Remarkably, we see spatially-extended Lyman-$α$ in emission (despite the highly-neutral IGM expected at this early epoch), offset 555 km s$^{-1}$ redward of the systemic redshift. From our measurements of collisionally-excited lines of both low- and high-ionization (including [O II]$\lambda3727$, [Ne III]$λ3869$ and C III]$\lambda1909$) we infer a high ionization parameter ($\log U\sim -2$). We detect the rarely-seen N IV]$\lambda1486$ and N III]$\lambda1748$ lines in both our low and medium resolution spectra, with other high ionization lines seen in the low resolution spectrum such as He II (blended with O III]) and C IV (with a possible P-Cygni profile). Based on the observed rest-UV line ratios, we cannot conclusively rule out photoionization from AGN, although the high C III]/He II and N III]/He II ratios are compatible with a star-formation explanation. If the observed emission lines are powered by star formation, then the strong N III]$\lambda1748$ observed may imply an unusually high $N/O$ abundance. Balmer emission lines (H$γ$, H$δ$) are also detected, and if powered by star formation rather than an AGN we infer a star formation rate of $\sim 20-30 M_{\odot} yr^{-1}$ (depending on the IMF) and low dust attenuation. Our NIRSpec spectroscopy confirms that GN-z11 is a remarkable galaxy with extreme properties seen 430 Myr after the Big Bang.

  DOI

Recent grants

• MRI: Acquisition of a High Performance Computer for Computational Science at UC Santa Cruz

  NSF · $1.5M · 2018–2021

• MRI: Acquisition of a Graphics Processor Unit-Accelerated High Performance Computer for Astrophysics, Computer Science, and Broad Numerical Research at the University of Arizona

  NSF · $1.3M · 2012–2015

Frequent coauthors

• S. Charlot

  Sorbonne Université

  234 shared

• R. Maiolino

  171 shared

• Sandro Tacchella

  University of Cambridge

  163 shared

• Christina C. Williams

  NSF’s NOIRLab

  153 shared

• Andrew J. Bunker

  145 shared

• Kristan Boyett

  ASTRO-3D

  144 shared

• Emma Curtis-Lake

  140 shared

• Zhiyuan Ji

  135 shared

Education

• Ph.D., Astronomy

  University of California, Santa Cruz

  1996

• M.S., Astronomy

  University of California, Santa Cruz

  1993

• B.S., Astronomy

  University of California, Santa Cruz

  1991

Similar researchers at University of California, Santa Cruz

• Jonathan J Fortneyh-125
• Jason Xavier Prochaskah-125
• Piero Madauh-108
• Jin Zhong Zhangh-106
• Carol Greiderh-100