Science Topic 2: Stars and Galaxies

Chair: Michael Merrifield, University of Nottingham, UK

This session looked at some of the scientific questions about the relationship between stars and galaxies that might be addressed using an Extremely Large Telescope. Specifically, we have considered: Following on from these science-driven issues, we also asked: As with the other summaries to the Workshop, this document is not intended to provide a definitive description of the astounding opportunities for innovative science that will become possible with an Extremely Large Telescope; rather, it seeks to provide a summary of the issues discussed at the Workshop, and a starting point for discussion.

A link to a quite comprehensive paper on this area of ELT work by Rosie Wyse is available HERE (or will be when Rosie is happy with her document).

When and How Were Stars Formed?

Nearby Galaxies

In nearby galaxies, we will be studying resolved stars, allowing us to determine: From such data, we will be able to quantify the ages and metallicity distributions of stars in different environments, and their initial mass functions. Amongst the issues that could be addressed with such data is quantifying the heavy element abundances of the most metal-poor stars in the local Universe.

To make any definitive statement about the stellar populations of galaxies, it is vital to sample galaxies of all Hubble types. In order to investigate elliptical galaxies satisfactorily, it is necessary to carry out the above observations to at least the distance of the Virgo or Fornax clusters. This constraint imposes magnitude limits of:
mV~36diffraction-limited imaging(optical)
~32intermediate-dispersion spectroscopy (R ~ 5000)(infrared OK?)
~29high-dispersion spectroscopy (R ~ 30000)(infrared OK?)

In order to resolve stars at small radii in Local Group galaxies (to test for spatial variations in their properties), we require diffraction-limited imaging from a 70-metre ELT.

Intermediate-Distance Galaxies

With diffraction-limited imaging to mV ~ 36, we will be able to study intermediate-mass stars (clump and AGB) out to the distance of the Coma cluster. Calibrating such data using observations of nearby galaxies (from 8-metre telescopes), it will be possible to obtain definitive ages and metallicities for galaxies in this rich environment.

Distant Galaxies

Similar image quality will also resolve high-mass stars out to a redshift of z ~ 1, providing a direct probe of the variation in star formation rate and initial mass function with radius. Issues of confusion, with many O/B stars forming in each star formation region, may compromise such studies, even with a 70-metre telescope. However, we can certainly resolve typical integrated properties of star formation regions (e.g. their luminosities) as a function of redshift.

When and How Were Galaxies Assembled?

Nearby Galaxies

With diffraction-limited spectra, it will be possible to measure the kinematics of individual stars in nearby galaxies. These data will also provide the >1000 objects per system that one requires in order to model both the global distribution function of the galaxy and the form of the gravitational potential that contains it. Each of these factors provides important evidence for any study of galaxy evolution: the distribution of stellar orbits reflects the process by which the baryonic component formed, while the potential provides information on the arrangement of dark matter in the system.

Any substructure in the phase-space distribution of stars, such as moving groups, star streams, etc, will also be apparent in these data. This fine structure provides "archeological" access to more recent events in these systems' evolution such as minor mergers.

Where confusion becomes an issue (in the centres of nearby systems and the outer parts of more distant galaxies), it will still be possible to "kinematically resolve" individual stars: high dispersion spectra will resolve the absorption lines of separate stars with different Doppler shifts, allowing their kinematics to be disentangled.

Distant Galaxies

Spatially-resolved galaxy kinematics will be observable to z ~ 1, in both emission lines (gas) or absorption lines (stars). Studies of the changes in the Tully-Fisher relation and fundamental plane with redshift will provide direct observations of galaxy evolution, and the evolving relationship between star formation (luminosity) and dark matter (mass inferred from kinematics).

Such spectral studies will also provide spatially-resolved chemical abundances out to this redshift, measuring the star formation history of galaxies, their recycling and feedback efficiencies, etc.

Out to a redshift z ~ 1, investigation of the galaxy luminosity function down to dwarf spheroidal luminosities (mV ~ 32) will show directly whether these systems form the basic "building blocks" from which galaxies are constructed. The variation in luminosity function with redshift offers a direct picture of the hierarchical merging of systems. Spectra of these building blocks will reveal their masses and chemical properties.

How Do Stars Interact with Other Galaxy Components?

Central Black Holes

Diffraction-limited imaging and spectroscopy will resolve the environments of the central black holes in nearby galaxies, allowing us to study their impact on star formation and the mass flows in these extreme regions.

Studies of the central regions of dwarf galaxies will allow us to explore the bottom end of the central black hole mass function: is there a cut-off, or do the central black holes just continue to scale down in mass as one might extrapolate from larger galaxies?

Black hole demographics can also be studies as a function of environment, to see whether their masses and environments differ between clusters and the field. To resolve the dynamically-interesting scale of ~1 pc around the central black holes in the nearest cluster environment (Fornax/Virgo) requires a 70-metre telescope.

Dark Halos

Dynamical studies of dwarf ellipticals in different environments (field, cluster) will address such basic issues as whether the smallest galaxies are embedded in dark halos.

Microlensing studies with an Extremely Large Telescope will produce spatially-resolved events, and even allow direct observation of the lensing object (in the Local Group). At larger distances (out to Virgo/Fornax), the "pixel lensing" technique will offer a direct probe of any dark matter component consisting of massive objects.

Big Bang Nucleosynthesis

At present, the low primordial deuterium abundance derived from quasar absorption spectra is inconsistent with Li7 abundances seen in Galactic halo stars. To see if this inconsistency arises from a localized enhancement in Li7, we need to determine its abundance in external galaxies using observations of low-mass stars (where the element will not have been heavily processed).

What Are the Implications for an ELT?

In order to carry out most of the above science programme, we would place the following design constraints on an Extremely Large Telescope: