Webb’s First Deep Field: A Grain of Sand Held at Arm’s Length

Hold a grain of sand at arm’s length and look at it. That tiny patch of sky, pointed at by the James Webb Space Telescope, held thousands of galaxies. The first deep field picture, released on 12 July 2022, is among the most reproduced images in astronomy ^[1].

• Webb’s First Deep Field shows galaxy cluster SMACS 0723 as it appeared 4.6 billion years ago ^[1].
• The image combines 12.5 hours of exposures from the Near-Infrared Camera at different wavelengths ^[1].
• It covers an area about the size of a grain of sand held at arm’s length ^[1].
• The cluster’s mass acts as a gravitational lens, magnifying far more distant galaxies behind it ^[1].

The target

The picture shows the galaxy cluster SMACS 0723. We see the cluster as it was 4.6 billion years ago, because light takes time to travel ^[1]. The image was made with Webb’s Near-Infrared Camera, combining exposures at different wavelengths totalling 12.5 hours ^[1].

A lens made of gravity

Massive objects bend space, and the light passing through it. The combined mass of the cluster acts as a gravitational lens, magnifying much more distant galaxies behind it ^[1]. Imagine looking through the bottom of a wine glass: shapes stretch into arcs. The streaks and curves in the picture are distant galaxies stretched by this effect.

What was new

The European Space Agency’s Webb page says thousands of galaxies, including the faintest objects ever observed in the infrared, appeared in Webb’s view for the first time, along with star clusters and diffuse features in distant galaxies ^[1].

The comparison with Hubble

The source notes that this image reached depths at infrared wavelengths beyond the Hubble Space Telescope’s deepest fields, which took weeks of observation, while Webb needed 12.5 hours ^[1]. The comparison is of infrared depth specifically, since the two telescopes see different parts of the spectrum.

The other instruments, and the oldest light

NASA’s description of the release notes that light from the youngest galaxies in the field traveled for 13.1 billion years before reaching Webb’s mirrors, while the cluster itself is seen as it was 4.6 billion years ago ^[2]. Other instruments contributed on the same target: the Mid-Infrared Instrument captured mid-infrared observations that highlight the distribution of dust, the near-infrared spectrograph NIRSpec observed 48 individual galaxies simultaneously using its microshutter array, and NIRISS used wide-field slitless spectroscopy to capture spectra across the field ^[2].

The point of the other instruments is that a picture is only the first step. The image shows shapes and colors, while spectra reveal each galaxy’s distance, composition and motion. NASA says the image revealed thousands of galaxies of varying shapes, sizes, colors and brightness, allowing researchers to study galaxy formation, evolution and composition across cosmic history ^[2]. That is the sense in which the First Deep Field was the start of a program of observations, not a finished result.

How to read the picture

• Every dot is a galaxy or star. The sharp diffraction spikes belong to foreground stars.
• The arcs are lensed galaxies. They are farther away than the cluster.
• Colour is assigned. Infrared light is invisible, so colours represent different wavelengths.

Questions readers ask

What is Webb's First Deep Field?

An image of galaxy cluster SMACS 0723 released on 12 July 2022, made from 12.5 hours of Near-Infrared Camera exposures ^[1].

How does gravitational lensing help?

The cluster’s combined mass magnifies much more distant galaxies behind it, letting Webb see objects that would otherwise be too faint ^[1].