Draft 2/8/00

The Origin of Structure in the Universe:

From the Big Bang...to Galaxies, Stars, and Planets

Joan Najita & Arjun Dey (NOAO)

1. Overview

The 20th century witnessed a revolution in our knowledge of the Universe. We now know

that the Sun is a star, composed primarily of hydrogen, and only one among more than 10 billion

stars in our Galaxy, the Milky Way. Our Galactic home itself is but one among billions of galaxies

spread out over the vast expanse of space and time in a distribution whose complexity we are

only beginning to appreciate. We have also learned that these galaxies are expanding away from

each other as the result of the explosive birth of the Universe more than 10 billion years ago.

Closer to home, we have found compelling evidence for planets around nearby stars which raises

the possibility of life and intelligence elsewhere in the Galaxy.

As a result of these discoveries, our questions about the Universe have become progressively

more sophisticated. What is the origin of structure in the Universe? From the cosmic soup of the Big

Bang, how did clusters, galaxies, stars, and planets arise? How did the chemical evolutionary history

of the Universe result in the genesis of life on Earth? And how common is life in the Universe?

The keys to answering these questions lie in deciphering the past: ancient events observable in the

distant Universe, the historical record preserved in the stellar populations of the Milky Way and

nearby galaxies, and present-day analogues of the birth of the sun and solar system.

While current 8-m and 10-m telescopes will greatly advance our understanding of these ques-

tions, we already know that many answers lie beyond their reach. As we describe below, answering

the questions above requires a novel combination of greater sensitivity, larger �elds-of-view, and

higher angular resolution than is currently available. Moreover, the discoveries that will be made

by planned space-based or multiwavelength ground-based facilities will only be fully realized with

a new large-aperture optical-infrared telescope. For example, the stars that �rst illuminated the

Universe may be detected by NASA's Next Generation Space Telescope, but investigating their

astrophysical properties (e.g., age, metallicity) is well beyond the capabilities of any current or

planned astronomical instrument. Similarly, the Atacama Large Millimeter Array will probe the

current birthplaces of stars in the Galaxy, but understanding the formation of planets in our so-

lar system will be a joint e�ort, relying heavily on the next generation of large optical-infrared

telescopes.

In the following pages, we brie y discuss some of the major questions that drive the need for a

new large-aperture optical-infrared telescope and outline the trade studies that need to be carried

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out over the next year in order to specify the instrumental and operational capabilities of this new

facility.

1.1. Astronomy in 2015

Astronomy in 2015 will be powered by a diverse set of observing capabilities, covering a broad

range of wavelengths, on the ground and in space. By 2015, NGST will have been launched,

and perhaps already completed its (nominal 5-year) mission to explore the early universe and the

creation of the �rst stars and galaxies. It will undoubtedly have revolutionized our view of the

MIR universe. ALMA will be operational (target completion date 2011) and examining an equally

broad range of astronomical problems, from the study of dusty galaxies at high redshift to the

initial conditions and physical processes that produce stars, and the search for extra-solar planets.

The SKA will begin probing the structural evolution of primordial hydrogen gas and the formation

of the �rst (gaseous) structures in the Universe. Chandra will have long completed its inventory of

the X-ray background, and Constellation-X will be on the verge of probing the formation of galaxy

clusters and the distribution of hot gas in the Universe.

By 2015, 8/10-m class ground-based O/IR telescopes will have been operating for nearly 2

decades. Experience with AO will have produced huge gains in sensitivity and perhaps brought us

to the point of replicating the excellent imaging quality available in space. We will by then have

glimpsed the high-redshift universe, perhaps to z > 8; as well as the large-scale structure of galaxies

at z � 4 based on studies of the brightest (> L�) galaxies. However, the evolution of structure

associated with the formation of the �rst collapsed objects (z > 8) and the clustering properties of

sub-L� (i.e., typical) galaxies (which provide the critical test of hierarchical formation models) will

be beyond the reach of these facilities. Closer to home, we will have studied planet forming systems

within � 200 pc using 8/10-m ground-based telescopes and thereby obtained, from samples of a

few tens of systems, tantalizing hints to the questions of when and how planets form. However,

de�nitive answers to these questions and the central question of how frequently planets form, as

well as an understanding of the role of dynamical evolution and its impact on the habitability of

planetary systems, would await a future, more powerful facility.

In this climate of intense activity, in which numerous discoveries will be made and fundamental

astrophysical problems solved, the availability of forefront ground-based optical and infrared tele-

scopes and instrumentation will remain critical to expanding the frontiers of astrophysical knowl-

edge. They will not only drive unique, ground-breaking science on their own, but will also remain

critical to the scienti�c success of capabilities at other wavelengths and in space.

The reasons for this are several. Firstly, the observational diagnostics on which our under-

standing of stars and galaxies are based (chemical composition, gravity, stellar mass, age etc.) lie

in the optical and infrared. The depth of understanding of these diagnostics are the basis for both

future progress and the interpretation of observations made at other wavelengths. Consequently,

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there are many opportunities for synergy between ground and space, and between multiple wave-

length regions that will lead to tremendous astrophysical progress. For example, while the SKA

will probe the formation of the �rst gaseous structures, and NGST will determine the morpholo-

gies of the �rst stars and galaxies, spectroscopy with a 30-m class ground-based telescope will be

needed to determine the physical properties (age, stellar content, mass, etc.) that are needed to

understand the formation and evolution of these objects. A 30-m ground-based telescope is the

natural spectroscopic complement to NGST in much the same way that the Keck telescopes were

for the Hubble Space Telescope. The scienti�c legacy of the Hubble Deep Field is a compelling

example of how space-based imaging plus ground-based spectroscopy is a powerful combination

for astrophysical discovery. Similarly, while space missions such as MAP will probe the primor-

dial density uctuation spectrum, and Constellation-X will probe the structure of hot gas in the

Universe, ground-based optical/IR spectroscopy will be needed to obtain the critical complemen-

tary information on the large-scale structure of galaxies. As another example, while ALMA will

determine the range of initial conditions and physical processes that give rise to stars, optical/IR

spectroscopy will be needed to determine the properties of the ultimate products of star formation

process, the stars themselves.

Secondly, compared with space-based missions, ground-based facilities can deploy much more

complex instrumentation that is not subject to the mass and energy limitations of space-based

platforms. These instruments can therefore better use all of the photons reaching the telescope to

maximize scienti�c understanding. For example, large FOVs can be used to maximize scienti�c

gain in situations where observations of large samples of objects are critical. The instruments

needed to accept large FOVs are typically prohibitively large for space-based facilities. As a result,

NGST will have a relatively small FOV (50), which largely precludes studies of, for example, the

large-scale structure of galaxies, the structure of the Galactic Halo, and the dynamical structure

and merger history of nearby clusters. As another example, high spectral resolution also typically

requires larger instruments. As a result, NGST will not explore spectral resolutions R > 10000,

which excludes, for example, the possibility of detailed studies of planet formation environments,

the identi�cation of merger remnants in nearby galaxies, and the chemical enrichment histories of

those galaxies.

1.2. Benchmarks: Why a � 30-m Ground-based Telescope?

Sensitivity and FOV for Studies of Large-Scale Structure: A large aperture, ground-based

telescope can provide the critical combination of sensitivity and FOV that is needed for de�nitive

studies of the evolution of large-scale structure. For example, a seeing-limited spectroscopic survey

of the large-scale structure of galaxies on � 100 Mpc scales (27 AB magnitude galaxies; e.g., L�

protogalaxy at z = 9) carried with a 30-m telescope with a 300 FOV would take 1.5 years (1.6

hrs per setup; 50,000 objects per �eld; 10,000 objects observed at a time; total survey area of 25

sq. deg.; R = 5000, s=n = 2 per resolution element; cf. number counts from Yan et al. 1998).

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Fig. 1.| Left: Comparison of the per ob-

ject, seeing-limited sensitivity of the 8-m

Gemini and a new 30-m telescope. Right:

Comparison of the time required to study

structure formation with the same tele-

scopes. The impressive e�ciency of the

30-m is due to its aperture, �eld-of-view,

and slit density. With this superior e�-

ciency, it is feasible to study multiple 100

Mpc patches of sky in order to build up

the \fair sample" of the Universe that is

needed for a de�nitive study.

In comparison, a seeing-limited 8-m ground-based telescope with the same FOV (currently non-

existent) would require �50 years of observing time to complete the same project. NGST with a

50 FOV would require 140 years to complete the project. The same combination of sensitivity and

FOV would be critical to other studies, such as the formation and evolution of our own Galaxy.

Sensitivity and Angular Resolution for Studies of Resolved Stellar Populations: A

large aperture, ground-based telescope will have the critical combination of sensitivity and angular

resolution needed to trace the merger and star formation history of nearby galaxies from studies

of individual stars. For this purpose, the main sequence turno� population is the \gold standard"

by which stellar ages and age spreads can be established. With the sensitivity and resolution

of a 30-m telescope (8 mas at J , di�raction limited), it is possible not only to resolve the main

sequence population in the bulge of M31, but also to obtain spectra of these objects and thereby

measure radial velocities and metallicities. Similar studies using brighter stellar populations would

be possible out to the distance of Virgo (18 Mpc; red giants as tracers) and Coma (115 Mpc;

supergiants as tracers). In comparison, high s/n, moderate resolution spectroscopy of these faint

stars (J > 28) is out of the question for both NGST and ground-based 8-m telescopes. In addition,

the crowding-limited stellar density is an order of magnitude larger for the 30-m compared to 8-m

telescopes.

Sensitivity for Low Resolution Spectroscopy: A large aperture, ground-based telescope

will have the sensitivity to study the physical properties of the �rst collapsed objects. With a

di�raction-limited 30-m telescope, we would be able to obtain spectra of the �rst star clusters in

8 nights (H ' 31:5; R = 5000; s=n = 3 per resolution element). While NGST would be able to

detect these objects, obtaining spectra of them would be prohibitively time-consuming (100 times

the observing time).

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Limiting Magnitudes for J-band Spectroscopy with Large Telescopes

R=1,000 R=5,000 R=10,000

Telescope Di�raction Seeing Di�raction Seeing Di�raction Seeing

Limit Limit Limit Limit Limit Limit

NGST 28.0 26.3 25.5

Gemini 27.9 26.1 26.3 25.2 25.5 24.7

30-m 30.8 27.5 29.1 26.7 28.4 26.3

50-m 31.9 29.0 30.2 27.3 29.5 26.9

Notes to Table 1: Limiting magnitudes are in AB units and correspond to a J-band signal-to-noise ratio

of 1.0 per resolution element in a 1 hour exposure. R is the spectral resolution. The read noise and dark

current for the detector are assumed to be 4e� and 0.02e�/s respectively. The J{band sky is assumed to be

18.8 AB mag/sq.arcsec. (between the OH lines) for the ground-based telescopes and 21.6 AB mag/sq.arcsec.

for NGST. Seeing of 0.5 arcseconds is assumed for the ground-based telescopes.

Sensitivity for High Resolution IR Spectroscopy: A large aperture, ground-based telescope

will have the sensitivity to carry out detailed studies of planet formation environments. With a

di�raction-limited 30-m aperture, we would be able to study star forming regions up to 1.5 kpc away

(e.g., s=n = 300 on a T Tauri star at 5�m). The few hundreds of potentially planet-forming systems

systems that we could, thereby, study is the minimal sample size needed to determine when, where,

and how frequently planets form. In contrast, the current generation of 8-m telescopes will be

limited to systems within � 200 pc, i.e., to samples of a few tens of systems, which are completely

inadequate for this purpose. With a 50-m aperture, we would be able to study potentially planet-

forming systems up to 2.5 kpc away, giving us access to > 1000 systems. This line of investigation is

beyond the capabilities of NGST since it will not implement spectroscopy at resolutions R > 5; 000.

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2. Large-Scale Structure

Recent maps of the sky have brought us two very di�erent pictures of the Universe we live

in: the remarkably smooth, nearly-featureless Cosmic Microwave Background Radiation (CMBR)

that reveals the structure of the very early Universe (z � 1200), and the frothy distribution of

hierarchically clustered luminous galaxies at the present epoch (Fig. 2). In studies of the galaxy

population, structure is found on a wide range of scales, from dynamically-organized, kpc-sized

galaxies to sheets of galaxies extending over 100 Mpc or more. How did the coherent structures we

see today evolve from the tiny density uctuations of the cosmic microwave background radiation?

Fig. 2.| How did structure in the Universe evolve from the tiny temperature uctuations observed

in the all-sky COBE map of the CMBR (left; REF) to the present{day hierarchically clustered

distribution of stars, galaxies and clusters, as exempli�ed by this UVK image of the Coma cluster

(right; Eisenhardt et al. 2000)?

Current theory predicts that structure develops quite di�erently under di�erent cosmological

models (Fig. 3). Since these are best di�erentiated at high redshift, the relative merits of di�erent

models can be determined from observational studies at z > 1. While existing (e.g., CfA) and

ongoing (e.g., Sloan) galaxy surveys will map out structure at low redshifts (z < 0:5), current

work over small angular �elds already indicates the existence of signi�cant structure at z > 3 (e.g.,

Steidel et al. 1998).

In the coming decades, the challenge will be to carry out de�nitive tests of theories of structure

formation, by comparing available theoretical predictions with future observations of the CMBR

uctuation spectrum and the evolving galaxy and intergalactic gas distributions at z > 1. While

space missions such as MAP will measure the CMBR uctuation spectrum, extensive ground-

based programs will be needed to make the critical measurements of the galaxy and intergalactic

gas distributions. To probe e�ectively the galaxy distribution from z=0.5 to the epoch of formation

of the �rst galaxies (zf � 10) over angular scales � 100 Mpc, we will need to obtain spectra of

hundreds of thousands of faint galaxies spread over a range of mass and over large areas (> 5��5�).

Faint galaxies must be studied in order to probe both the high redshift population and to obtain the

dense sampling and the consequent high dynamic range that will reveal the complex, �lamentary

structures predicted. Galaxies at z = 5�10 are typically very faint (> 25 AB magnitude; R = 5000

at s=n = 1 27 AB mag requires 50 hrs with a seeing-limited Gemini telescope), and beyond the

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ΛCDM

SCDM

z=3 z=1 z=0

240 h-1Mpc6.7o at z=1

NH at z=2

15’

Fig. 3.| Di�erences in structure formation models are more apparent at higher redshifts (left;

Virgo collaboration, e.g., White 1997). Similarly intricate structure is expected for the intervening

neutral hydrogen column density (right; Katz et al. 1996).

spectroscopic reach of current large telescopes.

To carry out such studies, we will need

� A large aperture (� 30-m) telescope capable of moderate resolution spectroscopic studies of

faint, distant galaxies (� 27 AB mag).

� A large FOV, capable of e�ciently sampling structure on scales � 10�.

� Signi�cant O/IR MOS capability at R � 5000 in order to resolve bright OH sky emission.

The intergalactic gas distribution, as typically observed in absorption, is shaped by the un-

derlying dark matter distribution, the depletion of gas as a result of galaxy formation, and the

ionizing capability of luminous matter. Current theories envision the gas distributed in a complex

�lamentary structure tracing the dark matter distribution (see �gure 2) with clusters of galaxies

forming at the nexus of �laments. Moreover, the bulk of the baryonic matter may be in a hot

phase (106 K) and di�cult to detect except in absorption against background sources. To probe

e�ectively the 3-dimensional structure of the intergalactic gas distribution, we will require dense

sampling of the gas distribution on 100 Mpc size scales and over a large, well-sampled redshift range

(i.e., 106 absorbers over a 5� � 5� degree �eld per �z = 1). The object density and the relevant

redshift ranges to be probed require the use of faint, distant objects (� 25 AB mag) as background

beacons. Thus, we will require

� A large aperture (� 30-m) telescope capable of high resolution (R � 10; 000) spectroscopy of

faint (> 25 mag) sources at high signal-to-noise ratio.

� A large FOV, capable of e�ciently probing the gas distribution on angular scales of a few

degrees.

� E�cient O/IR MOS capability at R � 10; 000.

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� Since it reduces the background contribution, excellent angular resolution is a plus.

3. Galaxy Formation

Complementary to the evolution of large-scale structure is the formation and evolution of

galaxies themselves. The formation of galaxies is a complex tale, spun from the intertwined story

lines of how galaxies obtained their current masses (i.e., galaxy assembly), and the origin of luminous

galactic matter (i.e., star formation history). Closely related to the latter theme is the chemical

enrichment history of the Universe, where we seek to understand the conditions under which the

�rst stars formed, the subsequent history of star formation in the Universe, and its relation to the

history of galaxy assembly.

Fig. 4.| Galaxy formation involves both galaxy assembly, e.g., the formation of massive galaxies

from many smaller fragments such as those highlighted here (left; Pascarelle et al. 1998), and star

formation, the global history of which is currently under study (right; Steidel et al. 1999).

Recent work has led to a provocative �rst look at the star formation history of the Universe

(e.g., Steidel et al. 1999; Fig. 4). The results imply a relatively constant star formation rate over

the �rst half of Universal history, followed by a gradual tapering o� until the present era. There are

signi�cant limitations to our current understanding (e.g., the statistics are poor at all redshifts) and

many signi�cant issues remain (e.g., When, and under what conditions, did the �rst stars form?).

Moreover, since we currently use global measures of star formation integrated over all galaxies at

each redshift, our current understanding can say little about the evolution of individual systems

such as the Milky Way.

Consequently, a challenge of the future will be to improve our understanding of galaxy forma-

tion and evolution by carrying out a large census of galaxy properties. By measuring star formation

rates, stellar population ages, metallicities, and gas kinematics for individual galaxies as a function

of galaxy mass, morphological type, and environment over a large range of redshift (z > 0:5 to

z > 10), we will reconstruct the assembly and star formation history of the galaxy population. In

order to carry out such a census, we will need:

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� A large aperture (� 30-m) telescope capable of R > 5000 spectroscopic studies of faint,

distant galaxies (> 25 AB mag). The large aperture is needed both to study extremely

distant galaxies and to probe the evolutionary histories of faint galaxies as well as their more

luminous counterparts in each redshift interval.

� A large FOV, capable of studying galaxy properties in the context of large-scale structure on

scales � 10�.

� Signi�cant O/IR MOS capability.

To complement the large census, we will require observations of galaxy structure in order to

measure dynamical galaxy masses (e.g., rotation curves) and galaxy substructure due to merging

and star formation. For this purpose, we will need:

� A large aperture (� 30-m) telescope capable of spectroscopic studies of galaxy fragments

(� 28 AB mag)

� Integral �eld spectroscopy at high angular resolution (10 mas � 50h�1 pc at z = 3).

� A signi�cant FOV (> 10) in order to target all fragments that will eventually comprise the

galaxy (cf. Pascarelle et al. 1998; Fig. 4).

4. Resolved Stellar Populations

To unravel the history of galaxy formation and evolution, a critical complement to the studies

described above is the study of the stellar populations of the Milky Way and its neighbors. In

comparison with distant galaxies (d > 100 Mpc), which must be studied as spatially unresolved

systems, and where information on stellar kinematics, age, and metallicity are obtained from the

integrated light of billions of stars, studies of spatially resolved stellar populations, such as can

be performed for the Milky Way and its neighbors, provide essential detailed information. In

resolved systems, stellar kinematics, ages, and metallicities can be measured for individual stars.

Correlations between these quantities and the existence of clustered subpopulations reveal the star

formation, merging, and chemical enrichment history of the system.

For example, in the case of the Milky Way, recent studies have uncovered the existence of a

dwarf galaxy that is currently merging with our Galaxy (the Sagittarius Dwarf; Ibata et al. 1994),

thereby providing de�nitive evidence that mergers have contributed to the formation of the Milky

Way. The possible existence of moving groups in the Galactic halo (Majewski et al. 1996) indicates

that multiple mergers may have contributed to the formation of the Galaxy. Correlations between

stellar abundance ratios, such as alpha/Fe vs. Fe/H, are being used to understand the interplay

between star formation and Galactic gas dynamics (infall, ejection, and mixing) that has driven

the chemical enrichment history of the Galaxy.

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~ 30’

z=0.5 z=0.3 z=0.1

10 h-1

Mpc

22’

Fig. 5.| Resolved stellar systems such as M31 (left) and intracluster stars (right; simulation

by Moore et al. 1999) will provide critical clues to the star formation, merging, and chemical

enrichment history of galaxies.

While, in the coming decade, we expect to use these techniques to obtain a more complete

understanding of the formation history of the Milky Way, the challenges of future decades will be to

understand whether the formation history of the Milky Way thus obtained is representative of the

histories of other galaxies, of similar, larger, and much lower masses. We already have tantalizing

hints of diversity in Galaxy formation histories that suggest the need for studies of galaxies beyond

the Milky Way. For example, the metal-rich M31 halo (Rich et al. 1996) suggests a di�erent

evolutionary history for M31, a galaxy that appears otherwise quite similar to our own. With the

telescope proposed here, we will be able to reconstruct the star formation, merging, and chemical

enrichment histories of galaxies out to the Virgo cluster and beyond.

We will also extend our studies beyond the study of individual nearby galaxies, to intracluster

stars (e.g., in the Virgo Cluster). In analogy to the streaming motions of stars in our Galactic halo

that record the merger history of the Milky Way, the kinematics of the intracluster stars in galaxy

clusters preserve the dynamical interaction history of the cluster. Chemical and kinematic studies

of these stellar streams provide a direct test of cluster formation theories and the chemical and

morphological evolution of the cluster.

To address this challenge, we will need

� A large aperture (� 30-m) telescope capable of probing external galaxies at a level of detail

comparable to that of our studies of the Milky Way, and at distances that encompass a

su�ciently large and diverse sample of galaxies (d < 100 Mpc).

� High angular resolution (� 10 mas) over moderate �elds (2� 30) in order to spatially resolve

individual stars in these galaxies.

� Large (200�1�) seeing-limited �eld of view with signi�cant O/IR MOS (R > 2000) capability

in order to densely map out the merger/stripping streams of intracluster stars in nearby

galaxy clusters.

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5. Star Formation

One of the most important processes governing galaxy formation and the chemical enrichment

of the Universe is the formation of stars. Recent work reveals star formation to be a complex process

that relies on the interplay between numerous physical processes. Because of this complexity, the

general trends that have emerged from the detailed study of large numbers of star forming systems

have proved to be critical in addressing fundamental star formation issues (e.g., the existence of a

universal IMF, the physical origin of young stellar out ows).

From recent multiwavelength studies of the ISM and stars in nearby star forming regions,

we have come to understand the range of possible outcomes of the star formation process at the

current epoch (distributions of initial stellar masses, stellar rotation, accretion rates, star formation

e�ciencies, etc.) and the initial conditions (molecular cloud kinematics, temperatures, density

distribution, magnetic �eld strengths) from which they originated. Theoretical attempts to connect

the outcomes to the initial conditions have led to remarkable progress in both the identi�cation of

the physical processes that govern star formation at the current epoch (e.g., ambipolar di�usion,

infall, star-disk coupling, magnetocentrifugal winds) and the construction of detailed theories that

explain how these physical processes determine fundamental quantities such as stellar masses (e.g.,

reviews by Shu et al. 199x; Najita 1999).

Fig. 6.| Studies of high density regions such as the Quintuplet cluster at the Galactic center (left;

Figer et al. 1999) and low metallicity regions such as high velocity clouds (right; Wakker et al.

1999) are needed to develop a predictive theory of star formation.

In the coming decades, the challenge will be to extend these ideas beyond their ability to

explain observed phenomena into the regime where they can predict how star formation occurs

under conditions that cannot be directly observed (e.g., in the early universe), i.e., the development

of a predictive theory of star formation. To develop such a theory, we will need to study star

formation under a wider range of conditions, e.g., at very high initial densities and at higher and

lower metallicities. This requires the ability to probe in detail ongoing star formation in distant

regions of the Galaxy (the Galactic center and outer Galaxy) and in nearby galaxies (the LMC,

M31; Fig. 6). Thus, we will need:

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� A large aperture (� 30-m) telescope with a moderate FOV (< 100).

� Signi�cant O/IR MOS capability at R > 2000.

� High angular resolution to minimize crowding and enable resolved stellar population studies

of stellar clusters in nearby galaxies.

6. Planet Formation and Evolution

The discovery that giant planets are relatively frequent companions to nearby solar-type stars

implies that planet formation is a relatively common outcome of the star formation process. The

idea that centrifugally-supported circumstellar disks, which perform the angular momentum redis-

tribution that is necessary for the formation of stars, are also potential sites for planet formation

has, in fact, propelled the search for disks forward over the last few decades. We now have highly

compelling evidence for attened rotating disks based on imaging (e.g., McCaughrean & O'Dell;

Fig.7) and high resolution spectroscopic data (e.g., Carr et al. 1993). Over the same time period,

we have developed fairly sophisticated theories of planet formation and evolution aimed primarily

at explaining the origin of the planets in our solar system. This understanding has been challenged

by the discovery of extra-solar planets and their surprisingly diverse properties. The existence of

other planetary systems with properties di�erent from those of our solar system implies a variety

of possible outcomes to the planet formation process and raises the question, \How typical is our

solar system?"

Fig. 7.| Recent results provide strong evidence for the existence of protoplanetary disks, the

potential sites of planet formation (left; McCaughrean & O'Dell 1996). Will these systems produce

planets similar to those in our solar system (right)?

In the coming decades, the challenges will be several. We will want to study the dynamics of

the planet formation process in order to answer questions such as, \When, where, how frequently

do planets form?" \How important is dynamical evolution in planetary evolution and consequent

habitability?" To do this, we will need to study planet formation, as currently in progress, in star

forming regions > 100 pc away. Since we will be unable to resolve typical planet-star separations

at the distance of the nearest star forming regions (1 AU = 2 mas at Orion), we will have to obtain

the answers to these questions using high signal-to-noise, high spectral resolution studies in order

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to recover the equivalent spatial resolution (cf. Carr & Najita 1997). To do this, we will need

� A large aperture (� 30-m), di�raction-limited telescope. The large aperture is needed to

reach distances (& 500 pc) that encompass a statistically signi�cant sample of planet-forming

systems and to minimize the thermal background at high angular resolution.

� R = 100; 000 spectroscopy in the thermal infrared (> 4�m).

We will also want to determine the physical (masses, radii) and atmospheric properties of

mature planets around nearby stars (d � 10 pc) in order to determine their composition, (which

may constrain the formation history of the planet) and their habitability. For these studies, we will

need:

� A large aperture (� 30-m) , di�raction limited telescope capable of resolving the planet from

the star at a level that allows spectroscopy of the planet.

� Imaging and spectroscopy at optical through thermal infrared wavelengths.

7. Trade Studies

The new scienti�c frontiers described above can only be attained with a telescope more capable

than any built thus far. The set of fundamental astrophysical problems described above, while broad

and diverse, in all likelihood represent only a small fraction of the eventual scienti�c contribution

of such a pioneering facility. If the examples of HST and Keck are a guide, the impact of such

a facility will be beyond all expectation, particularly when working synergistically with future

multiwavelength ground-based facilities such as ALMA and space-based telescopes such as NGST.

Based on the preliminary studies conducted by science working groups, we believe that a large

fraction of the science goals described here could be achieved by a 30-m telescope, with a seeing

limited �eld of view of 200 � 1�, and a di�raction limited �eld of view of 2� 50.

Some of the more challenging scienti�c programs that require extremely high angular resolution

and spectroscopy of extremely faint objects would clearly bene�t from a still larger (> 30-m)

telescope. The technological and scienti�c studies that will be required to construct a facility

as large as the one proposed would clearly lay the foundation for the construction of the next

generation facility. Here, we outline the trade studies we propose to carry out during the next year

in order to re�ne the scienti�c case and determine the design speci�cations for the telescope and

its instrument complement.

General:

� Coordinate with instrument, telescope, and AO working groups to estimate signal-to-noise

performance with and without AO for exposure time estimates.

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� Determine the optimal \scienti�c discovery space" (aperture, FOV, wavelength coverage, etc.)

in the context of existing 8- and 10-m telescopes and other facilities on the ground and in

space (NGST, ALMA, SKA, etc.)

Large-scale Structure:

� Determine the optimal sampling rate, area coverage, and depth in order to discriminate

between structure formation theories. Numerical simulations of structure formation are a

likely starting point.

Galaxy Formation:

� Determine the optimal sample size (range of galaxy masses, etc.), spectral resolution, wave-

length coverage and signal-to-noise to reconstruct a robust empirical (star formation, merging,

and chemical enrichment) history of galaxy formation.

� Determine the optimal FOV and angular resolution for integral �eld spectroscopy.

Resolved Stellar Populations:

� Identify optimal tracer populations, wavelength regions, and spectral resolution for the mea-

surement of stellar ages, metallicities and kinematics.

� Study science trades against AO performance (e.g., simulations of the recoverable stellar

properties in the presence of the likely crowding due to composite stellar populations).

Star Formation:

� Investigate star forming cluster sample selection given the limitations of depth vs. crowding.

� Study science trades against AO performance.

Planet Formation and Characterization:

� Determine AO requirements for spatially resolving planetary companions from central stars;

study science trades against AO performance.

� Study performance as a function of thermal IR optimization and site characteristics.

Synergy with Multi-wavelength Space- and Ground-based Telescopes:

� Determine system requirements for providing observations complementary to ALMA, NGST,

and Constellation-X

{ 15 {

8. Process and Management

In our current plan, the science case would be developed and re�ned through a series of trade

studies carried out in collaboration with other project teams (e.g., AO, Optical design, Systems)

and the community. A steering committee of � 10 people from the community would direct and

oversee this process. Over the 2-3 year funding period, we would hold 2 community workshops, fund

trade studies to be carried out in the community, and host at NOAO visiting science consultants

from the community.

The activities might unfold according to the following schedule:

Month 1: Steering committee meeting in Tucson to plan a community workshop of � 40 people.

Goal: Discuss science goals; identify participants (science and technical) in accordance with the

trade studies that are likely to be needed, and the likely working groups.

Month 3: Kicko� � 40 person community workshop in Tucson. Presentations from science and

other project teams (e.g., AO, Optical design, Systems). Goal: cross-cultural exchange in under-

standing science goals and technical challenges; identi�cation of necessary trade studies; formation

of (NOAO+community; science+technical) working groups to carry out trade studies.

Months 4-20: � 10 visitors invited to Tucson to carry out trade studies here; grants (to fund

graduate students) distributed in community for further trade studies. Periodic steering committee

meetings to review and direct progress.

Month 20: Steering committee meeting in Tucson to plan the �nal community workshop of � 40

people. Goal: Discuss the results of the trade studies and revisions to the science goals and

priorities.

Month 22: Final � 40 person community workshop in Tucson. Presentations from science and

other project teams (e.g., AO, Optical design, Systems) reviewing the results of the trade studies.

Goal: cross-cultural exchange in understanding revised science goals and technical challenges; input

for preparation of �nal report.

Month 24: Final report submitted.

To carry out these activities, we would request the following funds:

4 Steering committee meetings (10 people x 4 meetings): 40K

2 Community workshops (40 people x 2 meetings): 80K

Visiting consultants (10 people x 1 week visits): 20K

Community Grants (10 grad students; 10K/semester): 100K

-----

240K

{ 16 {

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This preprint was prepared with the AAS LATEX macros v5.0.