4Stars and Galaxies




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23/04/2016




4Stars and Galaxies

A very large optical-infrared telescope will allow us to derive the important processes of galaxy formation and evolution in the full range of environments. Several complementary lines of attack are possible, directly connecting present-day Universe with the high redshift Universe, where the old stars near the Sun formed. Both the star formation histories of galaxies and the mass assembly histories of galaxies will be elucidated. We know that interactions and mergers between galaxies occur and play a (probably crucial) role in determining the morphological type of a given galaxy, but we do not know what merged, or when it merged -- how do the merging histories of non-baryonic dark matter and of baryons compare? Similarly, stars clearly form(ed) at some rate from gas, but at what rate, where, with what stellar Initial Mass Function, and what were the effects of this star formation on the remaining gas? What is the connection between galaxies and the supermassive black holes at their centres? The unprecedented spatial resolution of a 50-100m telescope, comparable to those achieved by VLBI in the radio, will provide unique insight. Thus E-ELT will provide the data that are required to underpin an analysis of the physical properties of galaxies over the age of the Universe.


A European ELT as envisaged is a critical component of a multi-faceted approach to understanding our Universe. GAIA will provide an astrometric capability that is impossible from the ground, supplying proper motions and distances. An ELT complements and thus strengthens the capabilities of ALMA, which focuses on analyses of the dust and gas content of galaxies.
In this Chapter we describe the impact that a 50-100m ELT will have on our understanding of the formation of galaxies in relation to their constituents, including gas, stars, star clusters and black holes. The following sections are arranged in order of increasing distance, starting with the interstellar medium (ISM) in our own Galaxy, on to resolved stars in galaxies beyond our own, and finally to star clusters and black holes at cosmological distances.

4.1The Interstellar Medium1

In the following sections we show how a 50-100m ELT could be used to study the physical properties of the interstellar medium (ISM), such as the density, temperature, structure and chemical content of the ISM, and even the physical properties of dust grains in neighbouring galaxies.


These studies would make use of photon-starved modes of observing, i.e. those which extract the most information from the incoming light: high and ultrahigh resolution spectroscopy; polarimetry and spectropolarimetry; ultrahigh signal-to-noise spectroscopy.

4.1.1Temperature and density probes in the thermal infrared

The ro-vibrational transitions of a range of molecular species occur in the 4 to 25 micron region of the infrared spectrum accessible from the ground (Table 4 .1). These include species such as HCN, SiO and NH3 which are also detectible at millimetre wavelengths through their rotational transitions, but also a number of species such as SiH4, C2H2, and CH4 which, because of their symmetry, can not be detected at radio wavelengths. The transitions of these species can probe a range of physical conditions: the symmetric top species can be used for temperature diagnostics while molecules with large permanent dipole moments, such as HCN and CS, can serve as density probes. Using these species we can build results from the current generation of 4 and 8 metre telescopes by pushing to higher extinctions, higher resolutions and shorter wavelengths.


Table 4.1 A selection of simple molecules detectable in the near-and mid-infrared ro-vibrational transitions, in the circumstellar and interstellar environments.

CO

4.67m 1-0 fundamental

HCN

3m 3 fundamental

SiO

8.3m 1-0 fundamental

NH3

10.3m 3

C2H2

2.44m, 3m, 13.7m (not observable from ground)

SiH4

11m 4

CH4

3.3m 3



Notes on Design Requirements

Observation Type: spectroscopy

Spectral Resolution: R=104 up to 106. Many gaps in the high atmospheric extinction exist in the 16-25 micron region; some of these are very narrow and a resolving power of 1 million is needed to make significant progress in these regions.

Wavelength Range: Mid-IR: 7-25 microns

Other comments: Mid-infrared observations are heavily affected by telluric absorption (for example the detection of C2H2 lines around 7 and 13 microns (Lacy et al. 1989) undertaken at a resolving power of 104 required a substantial effort to flat field using the Moon; mid-infrared observations generally benefit from the higher dispersion which an ELT, with its greater light gathering, would make possible.


4.1.2Fine structure in the ISM from Ultrahigh signal-to-noise spectroscopy


One current limit to high signal to noise spectroscopy is the ability to flat field the detection at the same time as being limited by the background photon flux. For example, the venerable CGS4 InSb detector on UKIRT cannot be pushed much further than a flat fielding accuracy of 1 in 1000, as it is essential to keep the light from the source on as small a number of rows as possible. An ELT's light gathering power could be used to disperse a spectrum laterally across thousands of rows of a CCD or infrared detector, producing an immense gain in flat-fielding accuracy, roughly proportional to the square root of the number of rows used. At present, S/N of 1,000,000 is very hard to achieve in the presence of systematics; with an ELT it should be commonplace, for sources which are currently studied to S/N of a few hundred. With suitable choice of targets, this will bring within reach ultrahigh S/N studies of the fine structure in the interstellar extinction, and because each visitation will be very quick there is clear potential for monitoring this structure for changes and modelling those changes in the light of small-scale cloud structure in the local ISM. At present this work is very painstaking and slow, but the rewards may be a complete understanding of the structure and chemical content of the local interstellar medium.
Notes on Design Requirements

Observation Type: Ultra-high signal to noise spectroscopy

Date constraint: Monitoring needed to observe changes in small-scale cloud structure.

4.1.3High-redshift


The 217.5nm interstellar extinction bump, at redshift 1, is located in the blue part of the spectrum. At redshift 2 it is in the yellow-red. This feature has been well studied in the local ISM and when combined with measurements of the visible extinction curve can be related to the metallicity of the galactic environment of the dust. The possibility to directly detect the 217.5nm bump in samples of galaxies at redshift greater than 1 will give an independent measure of the metallicity of those early environments. Using background QSOs as silhouette sources, it should be possible to map out the carbonaceous dust content of the Lyman alpha forest.
Quasars behind dusty galaxies may permit studies to distinguish between massive and low/intermediate mass stars as sources of the carrier carbon grains; if these grains come from lower mass stars then the presence of significant production would be possible only if the age of the host galaxy is above a lower limit.
High redshift may bring currently inaccessible dust features into reach, for a price which even with the ELT would be modest compared to satellites. It may be possible to find a system with high enough redshift that the EUV is redshifted into the optical window. A second resonance in graphitic grains (a prediction of models) beyond the Lyman limit would then be detectable. This might, of course, fall to hydrogen opacity in the Lyman alpha region.
UV lines in the regime between 1200A and 2000A can be used to measure both photospheric and interstellar abundances in galaxies. Large telescopes are beginning to open up the possibility of tracing metallicity and star formation at very long look-back times. Through optical spectroscopy in the visual and red, this is beginning to be exploited to redshift of order 2 using 8-m class telescopes to observe UV-luminous galaxies (e.g. de Mello et al. 2004), and to z>3 in lensed systems (Villar-Martin et al. 2004). Systematic work on representative samples at lookback times longer than 10.5Gyr requires will require larger apertures and equivalent spectroscopy capabilities in the near-infrared (see also Section 5.2).
Notes on Design Requirements

Observation Type: spectroscopy

Field of View: single sources

Wavelength Range: Near-IR

4.1.4Measuring Dust properties via polarimetry


Multi-wavelength polarimetry of interstellar dust provides a key indicator of mean grain size. To just detect a linear polarization of 1% (with a signal-to-noise of 3) requires a signal-to-noise on the received flux in excess of 400. This limitation has restricted polarimetry to the brightest objects and regions, but the technique is immensely powerful. ELTs rectify the photon starvation by providing the required photon flux. Possible projects include (i) dust properties and alignment in neighbouring galaxies, at the level of individual ISM clouds; (ii) the interstellar polarization curve at high resolution (suitable for the definitive study of the relationship between dust and molecular carriers of interstellar features).
Notes on Design Requirements

Observation Type: Multi-wavelength Polarimetric measurements

4.1.5Optical studies in heavily extinguished regions


The E-ELT's light grasp will open up the dark clouds to scrutiny at visible wavelengths for the first time. In many ways, the visible regime is better for studies of molecular abundances in these regions, with many well-understood diagnostics relevant to the sort of abundances modelled in the big codes. However, lines of sight through these regions are currently completely inaccessible. In our galaxy, optical studies of highly reddened stars in or behind dense clouds is certainly important, to determine properties which are currently estimated indirectly via observations at longer wavelengths; namely spectral types, ratios of total-to-selective extinction, molecular column densities, etc. What's possible with existing telescopes has yet to be fully explored though.


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