Hubble Space Telescope Observes NGC 4625

The NASA/ESA Hubble Space Telescope has taken a picture of a dwarf galaxy called NGC 4625.

This image of the dwarf galaxy NGC 4625 is a composite of separate exposures acquired by the Advanced Camera for Surveys onboard the NASA/ESA Hubble Space Telescope. Two filters were used to sample various wavelengths. The color results from assigning different hues to each monochromatic image associated with an individual filter. Image credit: NASA / ESA / Hubble.

NGC 4625, also known as LEDA 42607 and SDSS J124152.71+411626.3, is located approximately 31 million light-years away in the constellation Canes Venatici.

The newly released image, acquired with Hubble’s Advanced Camera for Surveys (ACS), reveals the single spiral arm of NGC 4625, which gives it an asymmetric appearance.

But why is there only one spiral arm, when spiral galaxies normally have at least two?

Astronomers looked at NGC 4625 in different wavelengths in the hope of solving this mystery.

Observations from NASA’s Galaxy Evolution Explorer (GALEX) in the ultraviolet (UV) provided the first hint: in UV light the disc of the galaxy appears 4 times larger than on the image depicted here.

An indication that there are a large number of very young and hot — hence mainly visible in UV light — stars forming in the outer regions of the galaxy.

These young stars are only around one billion years old, about 10 times younger than the stars seen in NGC 4625’s optical center.

At first scientists assumed that this high star formation rate was being triggered by the interaction with another, nearby dwarf galaxy called NGC 4618.

They speculated that NGC 4618 may be the culprit ‘harassing’ NGC 4625, causing it to lose all but one spiral arm.

In 2004 astronomers found proof for this claim — the gas in NGC 4618’s outermost regions has been strongly affected by NGC 4625.

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