The Science of SDSS-V
The scientific goals of SDSS-V span the inner workings of our Sun's nearest stellar neighbors to the growth of black holes from the earliest days of the Universe. See below for more details about many of the exciting topics SDSS-V will address.
Supermassive Black Holes
Quasars are among the most luminous objects in the Universe. Powered by accretion onto supermassive black holes (SMBHs), quasars/AGN are beacons marking and tracing the growth of SMBHs across cosmic distance and time. The tight correlation between the mass of the central SMBH and the properties of its host galaxy demonstrates a clear connection between the formation of the galaxy’s stellar component and the growth of its central black hole. Thus, SMBH studies are critical not only for understanding the objects themselves and their accretion physics, but also address closely linked to galaxy formation and evolution questions being explored by SDSS-V.
Observational tests of SMBH theories require three primary measurements: precise mass constraints, multi-wavelength SEDs, and a detailed characterization of variability. SDSS-V will provide at least some of these measurements for tens of thousands of quasars, adding wide-area, multi-epoch optical spectroscopy to the era of time-domain imaging and the next-generation X-ray surveys (e.g., ZTF, LSST, and eROSITA). In addition to the reverberation mapping data (below), SDSS-V will definitively characterize the spectral variability of more than 20,000 other quasars, sampling light-travel, dynamical, and thermal timescales of days to decades; revealing emission line profile variations; SMBH binarity; and rare "changing-look" quasars that shut off/on in just a few years, challenging our fundamental assumptions of SMBH accretion astrophysics.
A prime example of the power of time-domain variability to constrain quasar models is reverberation mapping (RM) of the broad emission line region (BLR). RM delays measure typical sizes of the BLR and, combined with the velocity width of the broad emission lines, allow a virial estimate of mass, that most fundamental of all black hole parameters. Sufficiently large, representative samples of quasars with spectral variability measured over long baselines do not yet exist. SDSS-V will measure black hole masses for ∼1000 quasars across a broad range of luminosity and redshift, dwarfing the historical RM sample of <100 nearby and low-luminosity active galactic nuclei (AGN).
X-ray Luminous Sources
The recently launched SRG/eROSITA instrument, with both its high sensitivity and large field of view, will discover as many new X-ray sources in its first 12 months as are known today, after more than 50 years of X-ray astronomy. Incorporating this dataset into models of energetic processes throughout the Universe requires spectroscopic follow-up to characterize the source type and determine distance. This sample will be comprised especially of both obscured and unobscured quasars/AGN, along with X-ray emitting galaxy clusters, X-ray-bright stars (e.g., compact binaries and flaring late-type stars) in the Milky Way and nearby galaxies, extreme/rare objects, transients, and other peculiar variables. Combining X-ray discovery and optical spectral characterization is necessary for thoroughly describing the X-ray sky and revealing the connections between large, statistical populations of X-ray sources and the cosmic structures in which they are embedded.
SDSS-V will provide optical spectroscopic measurements (to about iAB<21.5), including identifications and redshifts, of several hundred thousand eROSITA sources detected in the first 1.5 years of the eROSITA mission's all-sky survey.
The goal of Galactic Archaeology is to infer the Milky Way’s formation history and evolution from the current stellar fossil record. Through panoptic all-sky coverage and infrared spectroscopy of millions of stars, SDSS-V will produce the first spectroscopic stellar map that is contiguous, densely sampled, and contains detailed information for much of the Galaxy’s stellar distribution. When enhanced with Gaia data, this high-dimensional star map (numerous chemical elements, ages, and 6D phase space) will reveal the chemical and dynamical meso-structure of the Milky Way and answer the question of how the correlations and gradients of chemical abundance and kinematics change on physical scales ranging from molecular clouds (100 pc) to the entire Galactic disk (10 kpc). The GGS targets luminous, red giant stars to explore an unprecedented volume of the Galaxy precisely where its mass is concentrated. Approximately one million bulge stars will be observed, and hundreds of thousands of low-latitude stars will be used to produce a high-resolution dust map of the dust-enshrouded Galactic plane. The combination of SDSS-V and optical surveys such as TESS and Gaia will truly set the standard for “precision” Galactic Archaeology.
The Young Milky Way
Studying parts of the Galaxy where stars may soon form, are currently forming, or recently formed is critical for understanding the physics of the ISM and of stars, and how the interaction of the two influences the evolution of the galaxy as a whole. SDSS-V seeks to understand the spatial and temporal structure of star-forming regions over time, including clustering and co-evolution in star-forming clouds, along with the mechanisms by which stars build up their mass and angular momentum. By studying gas and stars in a range of environment, SDSS-V will trace how global patterns like spiral arms influence star formation and how the properties of star-forming regions at difference scales affect the mass, multiplicity, and other properties of the newly formed stars.
Gas in the Galaxy
The energy exchange between interstellar gas and stars that occurs as stars form reflects the detailed physics of gas accretion and stellar feedback. SDSS-V will map thousands of square degrees of interstellar ionized gas emission in the Milky Way midplane and the Magellanic Clouds with sufficient spatial resolution (tenths to tens of parsecs) to distinguish individual star formation knots and the filamentary structures and shock networks between them. With these data, we will be able to connect gas physics studies from the parsec to kiloparsec scales. This connection is essential to understanding the physics governing star formation, the structure of the interstellar medium, and their impact on the baryon cycle of galaxies.
SDSS-V will conduct an extensive survey to classify and characterize roughly 100,000 young stars in the Galaxy across the entire stellar mass range. These will include the luminous but short-lived massive stars that forge the elements and drive the cycling of matter from the ISM into stars and then back into the ISM. At the other end, the sample will also include the faint, long-lived, low-mass stars still embedded in molecular clouds, which provide the stable planetary environments where life might evolve.
SDSS-V will survey virtually all of the known star-forming regions within 3 kpc of the solar neighborhood with the APOGEE and BOSS spectrographs, with complementary coverage of the Southern plane regions from the new optical integral-field unit. It will also survey all of the optically-visible star-forming regions in the Magellanic Cloud galaxies with the integral-field unit, providing a unique perspective of young stars of all types and how they exchange energy with the ISM in a wide range of environment.
SDSS-V will collect multi-epoch observations of hot, OBAF stars in the TESS CVZ. Known eclipsing binary systems will be observed at least epochs, to identify stars for complete orbit determination and to compare dynamical masse and radii with seismic-derived values. Non-eclipsing systems will also be monitored for RV variability. Outside of the CVZ footprints, we expect to survey virtually all of the OB stars within 8 kpc of the Sun.
Understanding galaxy evolution---whether of the Milky Way or in the Universe more broadly---ultimately relies on understanding the complete life-cycle of stars, from birth to death, including heavy element enrichment and the effects of multiplicity. Seismic measurements of stellar interiors have highlighted shortcomings in our stellar models that we use to describe these life-cycles. Combining these seismic measurements with spectroscopy of stellar surfaces is crucial for constraining the physics of stars.
Observations of several hundred pulsating eclipsing binaries in the TESS CVZs and several thousand pulsating single stars will form an unprecedented data set for calibrating and improving models of high-mass stellar structure and evolution. In the binary systems, the physical questions that will be addressed include interior mixing in stars and its impact on stellar evolution, angular moment redistribution inside stars and between stellar components and their orbits, and the two-way interplay between tidal forces and interior mixing processes on the evolution of massive stars. Multi-epoch observations of several thousand single OB stars will provide a census of early-type multiplicity as a function of mass, along with atmospheric parameters to complement the asteroseismic analyses. This in turn will enable studies into the influence of binarity and metallicity on stellar pulsations.
Placing the Galaxy in an evolutionary context and distinguishing amongst competing formation models and evolutionary mechanisms requires precise stellar ages. Efforts such as APOKASC, CoRoTGEE, and other dedicated programs have demonstrated the huge rewards to be gained in this area by combining high-cadence photometric (seismic) measurements and high-quality photospheric spectroscopy. These datasets literally let us peer past the limit of the stellar photosphere deep into the hearts of stars, connecting stellar masses and surface abundances to constrain ages.
To date, these programs have focused on specific subclasses of stars or small samples in particular regions of the Galaxy. SDSS-V will provide the spectroscopic observations necessary for precise age measurements for hundreds of thousands of bright stars with seismic information across the Milky Way, primarily from TESS. In particular, SDSS-V will observe >95% of the bright (H<11) red giants in the TESS CVZs.
All stars born with masses less than 8-10 solar masses end their lives as white dwarfs: Earth-sized stellar embers, in which the pressure of degenerate electrons balances gravity, and which are the most common outcome of stellar evolution. The short main-sequence lifetime of stars with initial masses greater than about 1.5 solar masses implies that the majority of A/F-type stars formed throughout the history of the Galaxy are already white dwarfs. As such, white dwarfs play a central role across a variety of areas in astrophysics. Their study provides essential constraints on various aspects of stellar evolution theory, including mass- and angular momentum-loss in intermediate-mass stars and fundamental nuclear reaction rates. Because of their well-constrained ages, white dwarfs provide an insight into the age of the Galactic disk and open and globular clusters, and they can even trace variations in the Galactic star-formation rate.
White dwarfs also provide important insight into the formation and evolution of planets. The host stars of virtually all planetary systems---including the Sun---will evolve into white dwarfs, and a sizeable fraction of white dwarfs retain the remnants of their planetary systems. White dwarfs that accrete tidally disrupted planetesimals display photospheric trace metals, which provides a unique opportunity to measure the bulk composition of extra-solar planets.
Finally, white dwarfs are laboratories of extreme physics that are unachievable on Earth, including atomic and molecular physics in the presence of strong magnetic fields and in high-density plasmas.
SDSS-V will target ~200,000 white dwarfs that have been identified with the Gaia mission, providing the spectroscopic information that is necessary to accurately measure the masses, ages, atmospheric compositions, and magnetic field strengths to address the above scientific questions.
Stellar System Architecture
Most stars in the Milky Way have companions --- other stars, brown dwarfs, planets, or combinations of these. Our understanding of stellar lifecycles relies on understanding the causes and effects of stellar systems, not just individual stars. What determines the multiplicity and mass ratios of these systems? How do companions affect a star's evolution?
Multi-Star and Planetary Systems
The "Binaries Across the Galaxy" sample within SDSS-V will measure the environmental dependence of the multi-star fraction in the disk, bulge, halo, and stellar clusters; probe the brown dwarf desert beyond the realm of solar-type stars; and detect long-period planets that may survive their host's evolution. It will add numerous high-precision RV measurements to tens of thousands of red giant stars with companions, yielding in the end up to 40 epochs per system, with baselines spanning more than a decade. This sample includes systems with stars spanning orders of magnitude in mass, orbital periods of less than an hour to more than a decade, and heliocentric distances of a few parsecs to more than 15 kiloparsecs. Large samples like this are essential to understand the interplay of multiplicity, age, environment, chemistry, internal structure, and rotation across the HR Diagram.
TESS Planet Hosts
SDSS-V will follow up TESS planetary host candidates with high-resolution, high-SNR infrared observations, in order to constrain the stellar properties necessary to fully interpret the planetary signatures. The APOGEE instrument has proven highly effective at characterizing M dwarfs, a main target class of the TESS planet detection program. This sample will comprise a few hundred thousand short-cadence and planet-host TESS CVZ targets, with the primary goal of connecting host star composition and properties to exoplanet frequency and habitability.
White Dwarf Binaries
White dwarfs in close binary systems can end in powerful thermonuclear explosions, and they are strong sources of the low-frequency gravitational waves that will serve as calibrators for LISA. They also produce the most common Galactic transients---cataclysmic variables and classical novae---as well as more exotic systems like white dwarf pulsars, and they are the most versatile laboratories for the study of accretion physics.
More generally, binaries containing white dwarfs represent the simplest case of the evolution of compact binary systems. As such, they are an ideal population to develop, test, and calibrate our understanding of the complex physics involved in the pathways to thermonuclear supernovae (Type Ia supernovae and related phenomena, such as SN Iax and the calcium-rich transients), and in the formation and evolution of the more extreme (and much rarer) binaries harboring neutron stars and black holes.
SDSS-V will identify and characterize an unbiased sample of several 10,000 white dwarf binaries, i.e., a sample sufficiently large enough to span the entire range of system properties and evolutionary parameters. The statistical analysis of this sample will provide the much-needed observational input to develop an overarching and complete understanding of compact binary evolution.
Synergy with TESS
As detailed above, the combination of data from SDSS-V and NASA's TESS mission will enable a truly transformative perspective on stellar physics and stellar systems. These synergies include -- but are not limited to --
- High resolution spectroscopic characterization of planet host candidates, including chemical abundances
- Spectroscopic characterization of nearly all bright TESS targets in the CVZ for unbiased population studies
- Spectroscopic characterization of bright red giants with seismic information, especially in the CVZs
- Multi-epoch radial velocities of companion host candidates
- Multi-epoch radial velocities of massive stars (especially eclipsing binaries) to measure dynamical masses/radii for comparison with seismic values