Hubble’s Faint New Galaxy Image Is Part of a Much Bigger Mission to Map the Cosmic Flow

What the image reveals

Galaxies like ESO 490-017 are classified as dwarf irregulars because they lack the bright central bulges and sweeping spiral arms of more familiar galactic structures. Its stellar population is spread out in a loose, disorganized arrangement . Because the galaxy's surface brightness is extremely low, it does not shine with a bright central glow. Instead, it appears as a gentle scattering of faint lights, requiring Hubble's high sensitivity to resolve against the blackness of space .

In the deep background of the image, numerous red, orange, and beige dots are visible. Many of these are distant galaxies, some with clearly distinct spiral structures, providing a stunning sense of cosmic depth .

Why astronomers are observing this dim galaxy

This image is not just a pretty picture; the data was acquired as part of a significant Hubble observing program focused on understanding the "cosmic flow" . While the universe is expanding, the distribution of matter within it is uneven. The gravitational pull of massive structures, such as clusters of galaxies and dark matter overdensities, causes deviations from uniform expansion. This large-scale gravitational movement is known as the cosmic flow .

To map these three-dimensional currents, astronomers need accurate distances to galaxies. Hubble provides those distances using red giant stars as "standard candles"—objects of a known intrinsic luminosity. By measuring how bright these stars appear, scientists can calculate their distance, a crucial step for turning a flat map of the sky into a 3D map of cosmic structure and motion .

The role of red giant stars

Red giant stars in a specific, brief phase of their evolution are particularly useful standard candles. When a low-mass star exhausts the hydrogen in its core and ignites helium burning, it reaches a peak in brightness known as the tip of the red giant branch (TRGB). This peak luminosity is remarkably consistent across different stars, making TRGB stars reliable distance markers .

This same technique has recently been used by the Carnegie-Chicago Hubble Program to make an entirely new measurement of the universe's expansion rate—the Hubble constant—which fell squarely between two previous, competing values, deepening a significant debate in cosmology known as the Hubble tension . The program that captured ESO 490-017 builds a critical foundation for this work by measuring distances to nearby galaxies, helping to calibrate the first rung of the cosmic distance ladder .

The scientific value of dark-matter-dominated galaxies

Beyond mapping cosmic flows, dwarf irregular galaxies like ESO 490-017 are scientifically valuable in their own right. They are thought to be dominated by dark matter, making them excellent natural laboratories for studying galaxy formation and evolution at the smallest scales .

Observing such galaxies at multiple wavelengths helps astronomers understand how stars form in low-metallicity, low-mass environments. These conditions are thought to be similar to those that existed for the first galaxies in the early universe. Furthermore, the observing program creates a lasting legacy archive of the types of stellar populations present in local galaxies, which can be used for studies well beyond the original cosmic flow science goals .