Hubble Reveals the Two Forces That Warped Spiral Galaxy NGC 4654 Into a Lopsided Shape

On July 24, 2026, NASA and ESA released a Hubble Space Telescope image of NGC 4654 that shows a spiral galaxy so asymmetrical it looks like half its arms are missing. The lopsided structure is the visible result of two cosmic forces acting simultaneously: ram-pressure stripping from the hot gas of the Virgo Cluster and a gravitational fly-by with a neighboring galaxy. Researchers are using a combination of Hubble ultraviolet-optical data, VLA radio observations, and ALMA submillimeter maps to connect these processes directly to where stars form and where star formation is quenched.

Why NGC 4654 Looks Like a One-Sided Spiral

NGC 4654 is an intermediate spiral galaxy located about 72 million light-years away in the constellation Virgo, part of the dense Virgo Cluster . An "intermediate" spiral has characteristics of both unbarred and barred spirals, with a weak bar structure at its center .

The galaxy's most striking feature in the 2026 image is its asymmetry: a rounded, clearly defined edge on one side and a long tail of gas stretching out from the opposite side — the tail extends beyond Hubble's field of view . This "one-sided spiral" appearance is the hallmark visual feature described in NASA's July 24, 2026 release .

The Two Processes Shaping the Galaxy

1. Ram-pressure stripping — As NGC 4654 moves through the hot intracluster medium of the Virgo Cluster at high speed, gas is pushed and stripped from the galaxy, creating the long gaseous tail on one side . This is the same process that affects many galaxies in the crowded Virgo Cluster environment .

2. Gravitational tidal interaction — NGC 4654 had a rapid close encounter with the massive neighboring galaxy NGC 4639 . This fly-by gravitational interaction distorted the galaxy's stellar and gas distribution, pulling material asymmetrically .

Crucially, neither process alone can explain NGC 4654's full asymmetry. Numerical simulations have shown that only a model combining both a tidal interaction and ram-pressure stripping reproduces the observed data . A 2006 study comparing deep VLA H I observations with dynamical models concluded that "only a model involving a combination of a tidal interaction and ram pressure can reproduce the data" .

What Hubble and Ground-Based Data Reveal About Star Formation

The data used to construct the July 2026 image were collected as part of a Hubble observing program that investigates the star formation and stellar feedback cycle in nearby galaxies . This program uses Hubble's ultraviolet and optical capabilities to trace young stellar populations and ionized gas .

Complementary multi-wavelength studies of NGC 4654 have used radio continuum and H I observations from the VLA, as well as far-infrared and submillimeter data from ALMA and Herschel, to map molecular gas, atomic hydrogen, and the compressed gas regions near the outer edge of the optical disk .

A 2021 study in Astronomy & Astrophysics specifically analyzed NGC 4654 as a galaxy undergoing nearly edge-on ram-pressure stripping and a fly-by gravitational interaction with NGC 4639 . That research used IRAM 30 m HERA CO(2−1) data to study the physical conditions of the interstellar medium and found a strongly compressed gas region near the outer edge of the optical disk, with H I surface densities significantly exceeding the canonical value of 10−15 M⊙ pc−2 . The study concluded that a Toomre parameter of Q ∼0.8 combined with an increase in the velocity dispersion of ∆vdisp ∼5 km s−1 are necessary conditions to simultaneously reproduce the observed star formation .

Additional archival data from Hubble's Wide Field Planetary Camera 2 (WFPC2) and Advanced Camera for Surveys (ACS), stored in the Hubble Legacy Archive, have been used in studies of the galaxy's star clusters and double stellar nucleus .

Together, these datasets allow researchers to trace a direct line from the galaxy's asymmetric gas distribution and dynamics to the locations and rates of star formation — showing where gas compression from the two processes triggers new star birth and where gas stripping quenches it . The northwest region of NGC 4654, where gas is compressed, shows a higher star formation efficiency even though its molecular gas ratio is lower, suggesting that the inflow of low-metallicity gas due to intracluster medium pressure can enhance star formation .