Credit: V. Belokurov, D. Erkal, S.E. Koposov (IoA, Cambridge). Photo: Color image of M31 from Adam Evans. Dark matter clumps from Aquarius, Volker Springel (HITS)
Caption: Artist's impression of dark matter clumps around a Milky Way-like galaxy. These clumps are invisible and can only be detected through their gravitational effect on visible matter. The clumps, also known as subhaloes, come in a range of sizes with the smallest one set by the mass of the yet to be discovered dark matter particle. The more massive the dark matter particle, the slower the dark matter moves, and the easier it is for regions in the early universe to collapse and form small subhaloes. In this work, a tidal stream from a disrupting globular cluster is used to probe their presence.
Image of the Pal 5 tidal stellar stream on sky from SDSS
Credit: S.E. Koposov, D. Erkal, V. Belokurov (IoA, Cambridge) with data from the Sloan Digital Sky Survey
Caption: Composite color image of the stars near the Pal 5 cluster. Pal 5 (short for Palomar 5) is the globular cluster in the center and the stellar stream is the faint overdensity of stars going from bottom left to top right. The number of background stars from the Milky Way has been reduced to make the stream more prominent. Note that this image has been made using stellar catalogues from the Sloan Digital Sky Survey with the location, brightness, and color of each point set by the location, brightness, and color of the actual stars.
Observations of Pal 5 compared with simulations
Credit: V. Belokurov, D. Erkal, S.E. Koposov (IoA, Cambridge)
Caption: Comparison between the observed stream and two simulated streams. The blue points show the observed stream which has been colored blue to distinguish it from the other streams. In reality, the color of its stars look more like the previous figure. Note the underdense regions on the left and right. The green points show a simulated stream evolved in a smooth potential without dark matter clumps. In contrast to the observed stream, this stream appears smooth and does not have any gaps. The red points show a simulated stream which has been struck by two clumps of dark matter with masses of one million Suns (left) and fifty million Suns (right). These perturbations produce the same gaps as what is seen in the data. Although the dark matter clumps themsevlves are invisible, they create gaps in the stream which can be detected. If confirmed, these two dark subhaloes would represent the lowest mass clumps detected to date.
Observations of Pal 5 compared with simulations (no text)
Credit: V. Belokurov, D. Erkal, S.E. Koposov (IoA, Cambridge)
Caption: Comparison between the observed stream and two simulated streams. This is the same as the previous figure but with no labels.
On sky evolution of Pal 5
Credit: D. Erkal, S.E. Koposov, V. Belokurov (IoA, Cambridge)
Caption: Movie showing the evolution of two simulations of a Pal 5-like stream over the past 2 billion years. The top panel shows the stream evolved in a smooth potential with no dark matter clumps. The resulting stream is smooth with no obvious underdensities. The bottom panel shows the same stream including two impacts by dark matter subhaloes. Impact 1 is by a fifty million solar mass dark matter clump while Impact 2 is by a one million solar mass clump. The resulting stream is a good match to what is observed in Pal 5. If this interpretation is correct, these would represent the lowest mass dark matter clumps detected to date and have important implications for the mass of the dark matter particle. Note that this movie shows the stream as viewed by an observer at the center of the Milky Way in coordinates which are aligned with the stream. Thus at the final time the streams do not look exactly like the previous images despite being same simulations.
Dark matter around a Milky Way-like galaxy with a perturbed stream
Credit: D. Erkal, S.E. Koposov, V. Belokurov (IoA, Cambridge)
Caption: Simulated globular cluster stream evolved in a Milky Way-like galaxy for 8 billion years. The stream is shown in green while the galaxy and dark matter are shown in orange. When a dark matter clump passes near the stream, it pulls on the stars and creates gaps and wiggles in the stream. These features can be used to infer the presence of the invisible dark matter clumps. Note that the stream used in this simulation is not meant to match Pal 5. The side of this image is 650,000 light years across.
Simulated stream evolving in the presence of dark matter around a Milky Way-like galaxy
Credit: D. Erkal, S.E. Koposov, V. Belokurov (IoA, Cambridge)
Caption: Movie showing the evolution of a stream in the presence of a Milky Way-like galaxy. The galaxy and dark matter are shown in red while the stream is shown in green. The stream is evolved for 8 billion years until the present. During that time, the stream grows in length and suffers two impacts: 3.8 and 1.5 billion years ago (a=0.75 and a=0.9 respectively). The second, smaller impact creates a gap like what we see in Pal 5. Since most of the dark matter clumps are invisible, the only way to infer their presence is through their imprint on the stream. The bar on the top shows the length scale and is roughly 1 million light years across at the present. In the top right the time is given in terms of scale-factor with a=0.5 being 8 billion years ago and a=1 being today. Note that this stream is not meant to match Pal 5.
