Peanut butter and jelly. Ham and eggs. Good things happen by combination. The two pictured galaxies make a lovely couple, don’t you think? But while they might appear to be on a collision course, it’s only illusion – they’re in the same line of sight and aligned by chance.
Interacting or colliding galaxies often have shapes deformed by the gravitational tug of one upon the other. While the foreground galaxy, called NGC 3314A, is a bit out of whack – notice the inflated spiral arms dotted with hot blue patches of star formation on the right – its out-of-round shape is intrinsic and not due to the pull of NGC 3314B in the background.
Scientists studying the pair with the Hubble Space Telescope discovered each galaxy is moving independently through space as oblivious of the other as two teens texting in a dorm room.
We happen to see them now in near perfect alignment, a situation perfect for highlighting the dark clouds of interstellar dust in the arms of the foreground galaxy against the billions of stars in the nucleus of NGC 3314B.
Astronomers are also using the overlapping galaxies to look for new planets, black holes and other massive compact halo objects or MACHOs that may be composed of enigmatic dark matter. One way to find these objects is through gravitational microlensing, a phenomenon predicted long ago by Albert Einstein.
Massive objects bend light. Light from more distant stars passing close the sun’s edge will be bent by the sun’s mass in a slightly different direction. If an object like a exoplanet passes directly between you and a distant background star, the planet’s gravity will temporarily intensify the star’s light, causing it to brighten. It does this by focusing the distant star’s light just like a lens into in a tiny, bright halo.
The mass of the foreground object determines how much time it takes for the star to brighten and fade. The more massive, the longer the event. Jupiter-mass planets make for flares lasting one to several days. Planets can either be found via microlensing either orbiting stars or whizzing about the galaxy alone. Recent studies have shown there are probably as many unbound planets as those bound to parent stars.
There’s only one problem with lensing: precise lineups are extremely rare. You have to watch millions of stars repeatedly to catch just a few microlensing events. That’s why our peanut butter and jelly galaxies make good hunting grounds for undiscovered stars, planets and oddball massive objects we otherwise can’t see – the billions of objects in the foreground galaxy have to potential to overlap any of the billions of stars in the background galaxy.
Scientists probing what makes up the mass of a galaxy are interested in the NGC 3314 pair for more than just their good looks.