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The Science

Arcus will usher in a new era of X-ray spectroscopy that will effect many fields of astronomy by archieving top Decadal science goals while enabling a broad range of community science. Our main goals, however, are to answer the following questions.

Galaxies

(1) How does matter cycle in and out of galaxies and clusters of galaxies?

Bright background sources act like flashlights, illuminating material at and beyond the edges of galaxies and galaxy clusters. By observing several lines of sight through galaxies and clusters, Arcus will allow us to map the locations, motions, and temperatures of the hot gases there. This will lead to a deeper understanding of the formation and cycling of metals in and out of these structures, and provide clues about the origin and fate of the hot gases.

The image at left demonstrates the high quality spectra we expect to achieve in only a 500 ks exposure for a QSO at redshift z = 0.2, and the very large galactocenteric distances we will able to get measurements for.

Galaxies

(2) What powers supermassive black hole winds that can affect entire galaxies and clusters?

X-ray spectra are necessary to investigate how physics behaves at the extremes of space and time near a black hole and learning how supermassive black holes have formed and evolved. In order to be able to make these observations in a reasonable amount of time -- a few kiloseconds -- a large effective area is required. High spectral resolution over a broad bandpass is also needed, in order to measure spectral changes that reveal the ionization state, density, and total mass flux in outflowing material near the black hole. Arcus detects nearly every iron ion, allowing far more sensitive studies of absorption than other observatories.

The image at right shows that Arcus will be able to resolve features in these black hole winds to a level comparable to those seen in optical and UV bands, thus for the first time permitting comparisons of these phenomena across wavelength regimes. No other currently existing or planned X-ray spectrometer can match this capability, regardless of exposure time.

Galaxies

(3) How do stars and protoplanetary disks form and evolve?

The final stage of star formation is driven by the accretion of a protoplanetary disk onto the nascent star. During this phase, planets are also formed. But a long-standing mystery surrounds young stars: they are rotating far too slowly, given that they are still adding mass and contracting. Do young stars lose angular momentum from an accretion-driven wind? And how do the streams of accreting material interact with the stellar magnetic fields, producing levels of magnetic activity far higher than that seen in their main sequence counterparts? Only high-resoltuion X-ray spectra can shed light on these processes.

The image at left shows the effect of an accretion shock on a young star. Arcus will be able to detect potential turbulent broadening of the O VII line in the post-shock plasma, thus providing clues to exactly how stars form.