Streams of stars abound in the Milky Way halo, as discussed in the April 2017 issue of Sky & Telescope. They are the remains of dwarf galaxies that once orbited the Milky Way, passed by too closely, and tore apart in our galaxy's gravitational well. "Spirits of Our Galaxy's Past" examines these galactic ghosts and what they tell us about the Milky Way's formation. We showcase here additional stunning images to illustrate the streams' elusive nature.
This map of the sky shows the stars that are part of the Sagittarius streams, as imaged by the SDSS, against a backdrop of the galactic plane, which the SDSS largely avoids. The stars' colors indicate their distances — blue marks nearby stars while red marks stars farther away. The dotted red lines trace out the Sagittarius streams as they disappear behind the galaxy from our viewpoint. The blue ellipses in the center show the current location of what remains of the Sagittarius Dwarf Galaxy on the far side of the galaxy from us.
S. Koposov / SDSS-III collaboration
This artist's conception illustrates the geometry of the tidal tails of stars (white) streaming from the Sagittarius Dwarf Galaxy (orange) that orbits the Milky Way. The bright yellow circle to the right of the galaxy's center is our Sun (not to scale). Sagittarius is on the other side of the galaxy from us, but we can see its tidal tails of stars stretching across the sky as they wrap around our galaxy.
Amanda Smith / University of Cambridge
Such looping stellar streams, the results of "collisions" with minor galaxies, are common in other nearby spirals too, such as these loops around NGC 5907 (seen edge-on).
R. J. Gabany in collaboration with Martínez-Delgado et al. (2010)
Simulations can provide insight into the origin of observed stellar streams, such as this stream observed around the Whale Galaxy, also known as NGC 4631 (left). The simulation of a satellite galaxy disrupted by its larger companion's gravity 3.5 billion years ago, reproduces the observed stream (right).
Martinez-Delgado et al. 2015
Visit R. Jay GaBany's website for more beautiful images of stellar streams.
Another team that has begun tackling the immense job of imaging these huge, faint structures around spiral galaxies is led by Pieter van Dokkum, Roberto Abraham, and Charlie Conroy. They have built the Dragonfly telescope to mimic its namesake's compound eye, coupling multiple Canon lenses to CCD cameras to achieve these results:
Nearby galaxies surveyed by the Dragonfly telescope, from left to right, top to bottom: NGC 1042, NGC 1084, NGC 2903, NGC 3351, NGC 3368, NGC 4220, NGC 4258, and M101. The last square shows a composite image of M31, composed of both Dragonfly and PANDAS data. (It has also been modified to show the galaxy as it would appear from 20 million light-years, which is characteristic of distances to nearby galaxies.) While most of these galaxies host stellar streams of some kind, three galaxies (M101, NGC 3351, and NGC 1042) show no signs of a stellar halo at all. Astronomers are still trying to understand the sheer variety of features (or lack thereof) around these galaxies.
Allison Merritt et al.
Just for fun, you can also check out the Victorian-era spirit photography, provided by the National Media Museum in the UK, that served as inspiration for this ghost-ridden article.