JWST’s First Deep Field

On Monday, the first full-color image from the James Webb Space Telescope (further JWST, read our post here on its naming controversy) was released.  The image, shown in the figure to the bottom left, is titled “Webb’s First Deep Field,” and it is of the field of SMACS J0723.3-7327, a galaxy cluster in the constellation of Volans. It is so massive its gravitational pull distorts the light from galaxies behind it in a mechanism called “gravitational lensing.”  This field has been imaged before with WFC3 and ACS on the Hubble Space Telescope (HST), shown in the figure below and to the right, and was imaged this past June with JWST.  A draggable comparison of these two images is at the link here.  You’ll notice that there are 8 spikes in the JWST image, but only 4 spikes on every star in the HST image.  The 8 spikes come from JWST’s non-circular primary mirror (each segment is a hexagon) and support arms for its secondary mirror, while the 4 spikes in the HST images come from the four support arms holding the secondary mirror in place, see here. You can learn more about diffraction spikes here. Every speck of light, save for the diffraction spiked stars in our own Milky Way, is from a galaxy far, far away.  The name, “Deep Field,” is not just illustrative – it refers to the several “Deep Fields” taken with HST!  In today’s bite, we investigate a few notable Hubble Deep Fields.  The full list can be found here.

In Christmas of 1995, over the span of 10 days HST was pointed at a dark spot in the northern constellation, Ursa Major (which has a famous asterism, “The Big Dipper.”).  This spot was chosen because it had very few foreground stars in the field, and so would be filled with only light from distant galaxies.  The resulting image, shown below, was titled “Hubble Deep Field,” commonly shortened to HDF or HDF-N, and it is composed of optical observations from the Wide Field and Planetary Camera 2 (WFPC2).  The image was foundational in studies of our early universe.

A southern complement to this image, “Hubble Deep Field – South (HDF-S)” was taken in 1998 with the telescope pointed at a dark patch of sky in the constellation of Tucana.  This image was created with deep optical observations from WFPC2, and infrared images taken simultaneously with the Space Telescope Imaging Spectrograph (STIS) and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS). The main science purpose of this image was to determine if the galaxy distribution crafted from HDF-N was representative of the sky as a whole (it was!). Spectroscopy was also taken simultaneously with STIS for all galaxies in this image.

The next Hubble Deep Field, “Hubble Ultra Deep Field” (HUDF) came in early 2004.  To take this image, the Advanced Camera for Surveys (ACS) was used on a dark spot in the southern constellation of Fornax. The field-of-view of this image is relatively very small – just 1/10 the angular diameter of the Moon (viewed from Earth), and it includes nearly 10,000 galaxies!  The Red-Green-Blue three color image below is made up of observations at 775 nm (R), 606 nm (G), & 435 nm (B).  Like with most three color images, these filters are not the wavelengths associated with the red, green, and blue visible light that we see. Instead three filters of high scientific interest (here, two visible light filters [B & G] and one infrared [R]) are used and are artificially dyed blue, green, and red.

The next Hubble Deep Field, the Hubble eXtreme Deep Field (HXDF), was released in the fall of 2012, and didn’t include any new observations. Instead it focuses on a smaller portion (~80%) of the HUDF and is a compilation of 10 years of optical and infrared HST imaging taken by ACS and Wide Field Camera 3 (WFC3). This image includes ~5,500 galaxies.

An update to HUDF was offered again in 2014, with the “Hubble Ultra Deep Field 2014” which is very similar to HXDF, but it includes UV imaging from WFC3.  Since the Earth’s atmosphere blocks most UV light, an image such as this is only made possible by space-based telescopes and observatories.  There are a number of UV space telescopes still in operation today, including HST, Hisaki, & the Lunar-based Ultraviolet Telescope (LUT).

The images above are the deepest images we’ve ever taken and show the farthest galaxies we’ve ever seen.  Every single speck of light in each of these images, save for that coming from the few foreground, diffraction-spiked stars, comes from a distant galaxy.  These images have allowed several scientific surveys, such as GOODS, CANDELS, & the Legacy Survey, and have inspired instrumental/choral albums and films!   There are over 10,000 galaxies in just one of these images alone.  Each of those galaxies has its own millions of stars and thousands of planets.  It is so humbling that such a small piece of our dark sky can hold such vibrancy and diversity.  How beautiful it is to live in such a Universe.

Edited by: Suchitra Narayanan, Macy Huston

Featured Image Credit: NASA, ESA, CSA, and STScI

About Huei Sears

Huei Sears is a fifth-year PhD candidate at Northwestern University studying astrophysics! Her research is focused on long Gamma-Ray Burst host galaxies. In addition to research, she cares a lot about science communication, and is always looking for ways to make science more accessible. In her free time, she enjoys walking along the lake, listening to Taylor Swift, & watching the X-Files.

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