1.

INTRODUCTION: CONTEXT

In the 2020’s, current and future wide-deep telescopes will be surveying the sky at wavelengths ranging from gamma rays to radio waves. E-ROSITA (launch 2018) will perform an all-sky X-ray survey with unprecedented sensitivity and resolution; Subaru’s Hyper Suprime Cam (HSC; operating now) and Prime Focus Spectrograph (PFS; 2020), and the VLT’s LEGA-C spectral survey will concentrate on understanding the evolution of galaxies at redshifts z~1-2 through optical spectroscopy; the Large Synoptic Survey Telescope (LSST, 2021) will map the southern sky discovering billions of new galaxies and stars and detecting transient objects; the Wide-Field Infrared Survey Telescope (WFIRST, launch 2025) will make an imaging and slitless spectroscopic survey of the sky at near-IR wavelengths; and the Square Kilometer Array (SKA; 2021+) and other radio telescopes will map a billion galaxies using the 21-cm hydrogen line. These surveys will be highly synergistic leading to new, important discoveries.

But there is a glaring hole in this vision: it lacks a UV-sensitive telescope. This will hinder all astronomy, because the UV spectral region is so rich in diagnostics that it has become a natural and necessary companion to space and ground-based telescopes observing at all wavelengths. We are therefore developing the CETUS (“Cosmic Evolution Through Ultraviolet Spectroscopy”) mission concept to fill that hole. NASA selected CETUS in March 2017 for study as a Probe-class mission (<$1B). Our objective is to determine how CETUS can best our understanding of our cosmic origins (How did we get here?) and the physics of the cosmos (How does the universe work?) in collaboration with other survey telescopes on the ground and in space.

Fig. 1:

CETUS will collaborate with other survey telescopes of the 2020’s to solve major problems in astrophysics.

Some examples of potential collaboration are the following.

1.1

Properties of the circumgalactic medium and its role in promoting/suppressing star formation

Figure 2:

Sample far-UV spectroscopy of the circumgalactic medium (CGM) of galaxies G1 and G2 backlit by a QSO (upper left image). Spectral absorption lines from are at the redshift of CGM of G1 and G2. Figure credit: Tumlinson et al. (2011).

1.2

Properties of Tidal Disruption Events (TDE’s) as first detected by LSST with follow-up FUV spectroscopy by CETUS.

Figure 3.

Schematic of a tidal disruption event. CETUS will monitor the far-UV spectra of TDE’s to faint magnitudes. The shredding of a star and the (re)-birth of an accretion disk around a massive, inactive black hole at the nucleus of galaxy as seen as a flare in brightness lasting over a month. Its far-UV spectrum resembles that of Wolf-Rayet stars. Image credit: M. Weiss, CXO.

1.3

Survey of redshift, z~1-2 galaxies observed in the optical-NIR by Subaru’s Prime Focus Spectrograph (PFS) and in the UV by CETUS

Fig. 4:

Sample rest-frame spectra of a star-forming z~1 galaxy as would be obtained by Subaru PFS (top) and by CETUS (bottom). The observed PFS spectrum is dominated by nebular emission lines, while the CETUS spectrum is dominated by the stellar continuum and stellar wind lines.

1.4

Follow-up spectroscopy and photometry of the aftermath of a gravitational-wave event

Fig. 5:

The source of GW 170817 is thought to be the merger of two neutron stars. CETUS UV observations would focus on region close to the merger. Image credit: CILAB/GSFC.

1.5

Multi-wavelength properties of galaxies

Fig. 6:

Based on observations of the elliptical galaxy UGC 1382 by SDSS, GALEX, and the VLA, we expect the combination of CETUS far-UV or near-UV images with those from Euclid and SKA will bring big surprises and useful insights.

2.

CETUS PAYLOAD

CETUS implementation includes a wide-field-of-view optical telescope assembly OTA yielding a product of area x solid angle over 40x that of Hubble. The design is for minimum reflections, and through advanced aluminum coatings with LiF and Atomic Layer Deposition overcoats, and highly efficient new detectors, allows for operations as short in wavelength as 100nm.

Figure 7:

Schematic of OTA and CETUS instrument compliment: Multi Object Spectrograph, Camera and Single Objece Spectrograph. The OTA is actually a Three Mirror Anastigmat, not properly represented in this schematic.

A true representatin of the OTA ray trace is given in figure 8.

Figure 8:

Three Mirror Anastigmatic form of CETUS OTA. The primary mirror has a clear aperture of 1.5m.

Figures 9, 10 and 11 represent the scientific instruments fed by the OTA.

Figure 2-3:

Multi Object Spectrometer, including a micro shutter array MSA at the right, and an Offner relay, with the secondary mirror being a spherical convex grating.

Figure 2-4

is the Camera, similar in scale and field to the MOS, except beam-split into two wavelength regions, and does not include a MSA.

Figure 2-5

represents the Far Ultraviolet channel FUV or the Point Source Spectrograph PSS or Single Object Spectrograph. A very broad slit will span entire galaxies.

3.0

TECHNOLOGY CONTENT IN CETUS

CETUS relies heavily on international technology. Table 3-1 below examines principal technologies needed for the CETUS Payload. In some cases, technologies reside in either North America, and in other cases, technologies reside only in Europe. In some cases, there is suitable proficiency both in Europe and North America to produce the part, sub-assembly or even assembly.

Table 1:

CETUS critical technologies come from both Europe and USA.

Some technology is approximately equivalent from both. A bold box indicates a gap.

For the OTA and each of the three instruments, there is roughly the same capability in Europe and in the US. Some components come only from one continent, but as in the case of JWST and Herschel/Planck instruments, this should be able to be navigated. While this is planned as a NASA project, the PI and team would welcome international participation both in implementation and in science.

4.0

CONCLUSION

CETUS is envisioned in the context of science of great interest both in The US and in Europe. CETUS architecture has favored what we believe to be the best-suited technology, without regard to country of origin. While still in a study phase, the CETUS team welcomes international discussions on both the scientific and technical level.