December | 2014 | Frontier Fields

The Frontier Fields project’s examination of galaxy cluster MACS J0416.1-2403 has led to a precise map that shows both the amount and distribution of matter in the cluster. MACS J0416.1-2403 has 160 trillion times the mass of the Sun in an area over 650,000 light-years across.

The mass maps have a two-fold purpose: they identify the location of mass in the galaxy clusters, and by doing so make it easier to characterize lensed background galaxies.

Mass map of galaxy cluster MCS J0416.1–2403

The galaxy clusters under observation in Frontier Fields are so dense in mass that their gravity distorts and bends the light from the more-distant galaxies behind them, creating the magnifying effect known as gravitational lensing. Astronomers use the lensing effect to determine the location of concentrations of mass in the cluster, depicted here as a blue haze. Credit: ESA/Hubble, NASA, HST Frontier Fields

Astronomers use the distortions of light caused by mass concentrations to pinpoint the distribution of mass within the cluster, including invisible dark matter. Weakly lensed background galaxies, visible in the outskirts of the cluster where less mass accumulates, may be stretched into slightly more elliptical shapes or transformed into smears of light. Strongly lensed galaxies, visible in the inner core of the cluster where greater concentrations of mass occur, can appear as sweeping arcs or rings, or even appear multiple times throughout the image. And as a dual benefit, as the clusters’ mass maps improve, it becomes easier to identify which galaxies are strongly lensed, and which galaxies are farther away.

Stronger lensing produces greater distortions. Astronomers can work backwards from the distortions to pinpoint the greater concentrations of mass responsible for producing such altered images. Credit: A. Feild (STScI)

The depth of the Frontier Fields images allows astronomers to see extremely faint objects, including many more strongly lensed galaxies than seen in previous observations of the cluster. Hubble identified 51 new multiply imaged galaxies around this cluster, for instance, quadrupling the number found in previous surveys. Because the galaxies are multiples, that means almost 200 strongly lensed images appear in the new observations, allowing astronomers to produce a highly constrained map of the cluster’s mass, inclusive of both visible and dark matter.

The dark matter aspect is particularly intriguing. Because these types of Frontier Fields analyses create extremely precise maps of the locations of dark matter, they provide the potential for testing the nature of dark matter. Learning where dark matter concentrates in massive galaxy clusters can give clues to how it behaves and changes. And as the mass maps become more precise, astronomers are better able to determine the distance of the lensed galaxies.

In order to obtain a complete picture of MACS J0416.1-2403’s mass, astronomers will also need to include weak lensing measurements. Follow up observations will include further Frontier Fields imaging, as well as X-ray measurements of hot gas and spectroscopic redshifts to break down the total mass distribution into dark matter, gas, and stars.

The Frontier Fields’ project has detected the glow of about 200 billion freely drifting stars within the massive galaxy cluster Abell 2744. The stars were dragged from their home galaxies by gravitational tides during collisions and interactions over the course of 6 billion years.

As many as six Milky Way-sized galaxies were torn apart in the cluster. The light of the outcast stars is believed to contribute to 10 percent of the cluster’s brightness, though that light is quite faint because the density of the stars is low. The combination of depth and multiwavelength observations provided by the Frontier Fields program makes this study of such dim stars possible.

The total starlight of galaxy cluster Abell 2744 is depicted here in blue in this Frontier Fields image. Not all the starlight is contained within the galaxies, which appear as blue-white objects. A portion of the light comes from stars that have been pulled from their galaxies and now drift untethered within the cluster. Credit: NASA, ESA, M. Montes (IAC), and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)

The stars are rich in heavy elements such as oxygen, carbon, and nitrogen, which means they formed from material released by earlier generations of stars. The presence of these elements indicates that the stars likely came from galaxies with similar mass and metallicity to our own Milky Way galaxy, which have the ability to sustain ongoing star formation and thus build populations of such chemically enriched stars. Elliptical galaxies are low in star formation while dwarf galaxies lack the kind of constant star formation that would be essential.

This discovery indicates that a significant fraction of the stars that would otherwise end up in these galaxies is being stripped out in the merger process. Astronomers intend to look for the light of such estranged stars in the remainder of the Frontier Fields galaxy clusters.