Future Instruments

The next twenty years promises to be an exciting time for studying the ISM. Each of the following telescopes is described in detail on a separate wordpress page (click on the acronym) and on their official websites (click the full name).

Atacama Large Millimeter Array: ALMA

ALMA will begin early science observations with Cycle 0 in September, 2011 and should be completed in 2013. The high spatial resolution of ALMA will allow astronomers to image young planets embedded in disks around nearby

The James Webb Space Telescope: JWST

JWST is an exciting space-based infrared observatory that promises to acquire a wealth of photometric and spectroscopic information. For studies of the ISM, JWST will be particularly useful for mapping the distribution of dust and for observing obscured systems such as young stellar objects and circumstellar disks (see Gardner et al. 2006).

Thirty Meter Telescope: TMT

TMT will conduct near-UV, optical, and near-infrared observations of young stellar objects, protoplanetary disks, and hot, young Jovian planets. The large primary mirror of the telescope and the adaptive optics system will allow TMT to produce high-resolution images of star and planet formation that include small-scale details that are unobservable with current telescopes.

Giant Magellan Telescope: GMT

GMT has the same strong science case as TMT, but will be a ~25m telescope in the southern hemisphere. The main differences between GMT and TMT are shown in the table below.

Comparison of GMT and TMT. GMT information from http://www.gmto.org/tech_overview. TMT information from http://www.tmt.org/observatory/telescope.

The Astro2010 Decadel Survey identified U.S. participation in a Giant Segmented Mirror Telescope (either GMT, TMT, or E-ELT) as Priority 3 for large, ground-based missions (after the Large Synoptic Survey Telescope and a Mid-Scale Innovations Program). As part of the process, the National Academy of Sciences conducted an independent cost estimates for the telescopes optics and instruments for GMT and TMT. The resulting estimates at 70% confidence are $1.1 billion for GMT construction and $1.4 billion for TMT construction. These cost estimates assume that the telescopes will begin science operations with adaptive optics and three instruments in spring 2024 for GMT and between 2025 and 2030 for TMT. Although both the TMT website and the GMT website indicate science observation start dates in 2018, the Decadal Survey estimates are probably more realistic.

The Giant Magellan Telescope

An artist's conception of the Giant Magellan Telescope at twilight. Note the truck at the lower right for scale. Image copyright Giant Magellan Telescope - GMTO Corporation.

The Giant Magellan Telescope is a collaboration between the Carnegie Institution for Science, Harvard University, the Smithsonian Astrophysical Observatory, Texas A&M University, the Korea Astronomy and Space Science Institute, the University of Texas at Austin, the Australian National University, the University of Arizona, Astronomy Australia Ltd. and the University of Chicago. GMT should be completed around 2018.

The Telescope

The primary mirror of GMT will be composed of seven circular segments 8.4m in diameter arranged as shown in the figure below. In order to properly focus the light, the outer six segments are shaped asymmetrically like potato chips. The resolving power of GMT will be equivalent to the resolving power of a 24.5 meter telescope. The secondary mirror (also pictured below) consists of an adaptive shell for each of the primary mirror segments and will be controlled by the adaptive optics system to correct for atmospheric turbulence over a field of view 10′-20′ in diameter.

An artist's conception of the primary and secondary mirrors of GMT. Image copyright Giant Magellan Telescope - GMTO Corporation.

The Site

The chosen site for GMT is Cerro Las Campanas in Chile. Cerro Las Campanas, pictured below, is located at an altitude of >2550 meters and has dry weather, dark skies, and good seeing. For more information about the site, see the GMT site selection page.

An artist's conception of GMT on the peak of Cerro Las Campanas in Chile. Image from GMTO Corporation.

Instruments

GMT’s instruments will be placed behind the central primary mirror. There will be a large (6m x 5m) space directly behind the mirror for large instruments and a rotating platform for smaller instruments. See the technical overview page for more information about instrument mounting.

The proposed first generation instruments for GMT are shown in the table below from the GMT Progress Report SPIE Conf. 7012-46. According to the report, three instruments will be selected for first light.

GMT Instrument Concepts. (Table 6 from SPIE 7012-46, Progress on the GMT by Matt Johns at Carnegie Observatories)

Science Goals

The science goals that will be addressed by GMT include:

• Detection and characterization of exoplanets
• Study of dark matter and dark energy
• Observations of stellar populations and the origin of elements
• Observations of black hole growth
• Study of galaxy formation
• Observations of the epoch of reionization

The Thirty Meter Telescope

Artist's Impression of the Thirty Meter Telescope from NASA.

The Thirty Meter Telescope (TMT) is a collaboration between the Association of Canadian Universities for Research in Astronomy, the California Institute of Technology, the University of California, the National Astronomical Observatory of Japan, the National Astronomical Observatories of the Chinese Academy of Sciences, and the Department of Science and Technology of India. According to the TMT Timeline, First Light should occur in October 2017 and the first science should be conducted in June 2018.

The Telescope

The thirty meter primary mirror of TMT will be segmented into 492 1.44m hexagonal segments as shown in the image below. After hitting the primary mirror, the light will be reflected onto a tiltable 3.1m secondary mirror and then onto a 3.5m x 2.5m elliptical tertiary mirror that will send the light into the instruments on the Nasmyth platforms. TMT will have two Nasmyth platforms with space for eight instruments total.

An artist's conception of the segmented primary mirror of TMT. The 1.44m hexagonal segments will be placed only 2.5mm apart. The elliptical tertiary mirror is shown at the center of the primary mirror. Note the tiny person in the upper left for scale. (TMT Collaboration)

The Site

TMT design operations are based in Pasadena, CA, but the selected telescope site is within the 36-acre “Area E” on the summit of Mauna Kea in Hawaii as shown on the map below. Mauna Kea is a well-established site for observatories due to the high-quality seeing, dry conditions, and typical lack of cloud cover. Once constructed, the TMT complex would consist of a dome 56m in height and 66m wide, 5 acres of roads, and 1.44 acres of buildings.

Proposed Site for TMT in Area E on the summit of Mauna Kea. For reference, the locations of existing telescopes are indicated by the numbered yellow circles. Map produced by UH and USGS.

Instruments

In addition to the Narrow Field Infrared Adaptive Optics System (NFIRAOS), TMT will have three first light instruments:

1. Wide Field Optical Spectrometer (WFOS): Spectroscopy and imaging without AO at near-ultraviolet and optical wavelengths (0.3-1.0 microns) over a >40 square arcminute FOV.
2. InfraRed Imaging Spectrometer (IRIS): Integral-field spectroscopy and diffraction-limited imagaing at near-infrared wavelengths (0.8-2.5 microns).
3. InfraRed Multi-object Spectrometer (IRMS): Slit spectroscopy and diffraction-limited imaging at near-infrared wavelengths (0.8-2.5 microns) over a 2′ diameter FOV.

Science Goals

As explained in the TMT Science Case, the science goals for TMT are:

• Spectroscopy of the first galaxies
• Observations of the formation of large-scale structure
• Detection and investigation of central black holes
• Observations of star and planet formation
• Characterization of exoplanet atmospheres
• Direct detection of exoplanets

The Atacama Large Millimeter Array

Eight of the 12m ALMA Antennas. Image from ALMA (ESO/NAOJ/NRAO)

The Array

The construction of ALMA began in 2003 and should be finished in 2013. Although the array is still under construction, ALMA is currently accepting proposals for Fall 2011 using 16 antennas and four of the ten receiver bands. More information on the Early Science Cycle 0 Call for Proposals is available on the ALMA website. The deadline for submission of Notices of Intent is April 29, so get writing!

When completed, ALMA will consist of 50 12-m antennas. Like the antennas in the Very Large Array and the Submillimeter Array, the ALMA antennas will be mobile to allow for different observing configurations and consequently different spatial resolutions. In addition to the 50 12-m antennas, ALMA will also consist of 12 7-m antennas. Those 7-m antennas and four of the 12-m antennas will make up the Atacama Compact Array (ACA) and will remain in roughly the same position for all observations to increase ALMA’s ability to map large scale structures.

The Site

The Atacama Large Millimeter Array is currently under construction on the Chajnantor plain in the Atacama desert in Chile. Since the site is at an altitude of 5000 meters and quite dry (precipitable water vapor ~ 1 mm), the atmospheric transparency should be excellent for submillimeter observations. The figure below displays a plot of the atmospheric transmission at Chajnantor and the ALMA Observation Bands. Logically, the ALMA Observation Bands were chosen to fit between the major absorption features of water and oxygen.

Atmospheric Transmission at Chajnantor. The colored bands indicate the ALMA Observing Bands. The red bands (3, 6, 7, and 9) will be available first. The primary sources of absorption are H2O (22.2, 183, 325, 380, 448, 475, 557, 621, 752, 988, and 1097 GHz) and oxygen (50-70 GHz and 118 GHz),

Science Goals

ALMA will achieve three main goals:

1. Detect line emission from CO or CII in under 24 hours from galaxies at a redshift of z=3.
2. Observe gas kinematics in protostars and protoplanetary disks within 150 pc.
3. Acquire high dynamic range images at high angular resolution (0.1″).

Extrasolar Planets and Protoplanetary Disks

ALMA will be particularly useful for detecting extrasolar planets and stars during the early stages of formation. The figure below shows a simulation by Wolf & D’Angelo 2005 of possible ALMA observations of embedded Jovian planets. The 1 Jupiter mass and 5 Jupiter mass planets are clearly visible at both 50 pc and 100 pc!

Simulation of ALMA observations of an embedded planet. The dot in the lower left represents the combined beam size. Left: 1 Jupiter mass planet around a 0.5 Solar mass star in a 0.01 Solar mass disk. Right: 5 Jupiter mass planet around a 2.5 Solar mass star, Top: Distance of 50 pc, Bottom: Distance of 100 pc. Figure 2 from Wolf & D'Angelo 2005.

AGB Stars and Interstellar Dust Grains

ALMA will also advance studies of interstellar dust grains by allowing scientists to create high resolution (<0.1") images of the dust condensation zones around AGB stars at distances of a few hundred parsecs. By comparing the angular sizes of CO envelopes around evolved stars to the known distances of those stars, astronomers will be able to determine the physical size of CO emitting regions. The distances to other evolved stars could then be estimated by comparing the observed angular size of the CO emitting regions around stars of unknown distances to the newly discovered characteristic physical size of CO emitting regions. Once the distances to a large number of evolved stars have been determined, astronomers could then map out the distribution of AGBs.

Other Research Areas

Since many molecular transitions occur at submm wavelengths, ALMA will be sensitive to the presence of a wide range of molecules. Additionally, ALMA will be able to measure the radii and rotation rates of stars and monitor the activity of the Sun.

Observations

ALMA will conduct observations in ten different bands from 84 GHz to 720 GHz at resolutions between 6 mas and 0.7″. The resolution is frequency- and baseline-dependent; the resolution decreases with decreasing baselines and lower frequencies. Within a given band, ALMA will produce a data cube containing up to 8192 frequency channels with widths between 3.8 kHz and 2 GHz. In the most compact configuration, ALMA will have baselines between ~18m and ~125m. In the extended configuration, the baselines will be between ~36m and ~400m. See the ALMA Capabilities page for more detailed observation about planning observations with ALMA.

The James Webb Space Telescope

Schematic of JWST. The large sunshield (blue) blocks radiation from the Sun, Earth, and Moon from reaching the science instruments in the ISIM (Integrated Science Instrument Module). Image from NASA.

An artist's conception of JWST folded and ready for launch aboard an Ariane 5 rocket. Image from Arianespace/ESA/NASA.

The 6.5 meter James Webb Space Telescope (named for former Apollo-era NASA Administrator James Webb) is scheduled to be launched from the Ariane 5 launch site in French Guiana in 2014. Unlike Hubble, which is in Earth orbit, JWST will orbit around the Earth-Sun L2 Lagrange point. The decision to send JWST to L2 was motivated by the need to cool the spacecraft in order to conduct observations in the infrared. Although parts of JWST are actively cooled, the remainder of the spacecraft will be passively cooled by placing the spacecraft in deep space and deploying a tennis court sized shield to block light from the Sun, Earth, and Moon.

Size comparison of JWST's 6.5 m diameter primary mirror to Hubble's 2.4 m diameter mirror. Graphic from NASA.

Due to their large size, the sunshield and the primary mirror of the telescope will be folded to fit inside the payload compartment of the Ariane 5 ECA launch vehicle. The figure on the right compares the JWST’s large primary mirror to Hubble’s 2.4 m mirror and the figure on the far right displays JWST in launch configuration. After launch, the telescope mirror will magically unfold and the solar panels will be deployed as shown in the deployment animation below.

Once the telescope has unfolded, JWST will conduct observations to address four key science goals:

1. “The End of the Dark Ages: First Light and Reionization”
2. “Assembly of Galaxies”
3. “The Birth of Stars and Protoplanetary Systems”
4. “Planetary Systems and the Origin of Life”

JWST will address those goals using four instruments attached to the Integrated Science Instrument Module(ISIM):

1. Mid-InfraRed Instrument (MIRI)
2. Near-InfraRed Camera (NIRCam)
3. Near-InfraRed Spectrograph (NIRSpec)
4. Fine Guidance Sensor Tunable Filter Imager(FGS-TFI)

As the name suggests, MIRI is sensitive to mid-infrared wavelengths between 5 and 27 micrometers (or 29 micrometers for spectroscopy). MIRI will be actively cooled to 7K and used for wide-field broadband imagery and medium-resolution spectroscopy. More information about MIRI is available from the University of Arizona and the Space Telescope Science Institute.

NIRCam serves the dual role of acquiring high angular resolution images at 0.6-5 microns over a 2.2’x2.2′ field of view and conducting wavefront sensing using the Optical Telescope Element wavefront sensor. Although JWST will not have to contend with the Earth’s atmosphere, minor differences in the shape and position of the primary mirror segments could introduce disortions and phase variations in the wavefronts received by each segment. The Optical Telescope Element wavefront sensor will be used to monitor such distortions and reshape and realign the mirror segments to correct the wavefront errors.

For science observations, NIRCam will be operated in one of three imaging modes (survey, small source, or coronagraphic) or in medium-resolution spectroscopy mode. More information about the NIRCam imaging modes and filters is available from the Space Telescope Science Institute.

NIRSpec will use a “microshutter array” to acquire simultaneous 0.6-5 micron spectra of 100 objects over a 3’x3′ field of view. In addition to the novel “Micro-Shutter Assembly” (MSA) mode, NIRSpec may also be operated in Fixed Slit mode or Integral Field Unit mode with the spectral resolutions shown the following table from STSCI.

Description of NIRSpec Instrument Modes from the Space Telescope Science Institute.

The Fine Guidance Sensor component of FGS-TFI consists of a 1-5 micron broadband guide camera capable of finding a guide star at 95% probability anywhere in the sky. FGS will be used to monitor JWST’s pointing throughout the mission and to properly deploy the primary mirror during the unfolding phase of the mission. The second half of FGS-TFI, the Tunable Filter Imager is a science instrument that will be used to acquire narrow-band images between 1.6-4.9 micrometers at R~100 resolution over a wide 2.2’x2.2′ field of view. TFI is being built by the Canadian Space Agency.

ARTICLE: HII Regions and the Abundance Properties of Spiral Galaxies

Read the paper by D. Zaritsky, R.C. Kennicutt Jr, & J.P. Huchra (1994)

Summary by: Courtney Dressing

Abstract

We investigate the relationships between the characteristic oxygen abundance, the radial abundance gradient, and the macroscopic properties of spiral galaxies by examining the properties of individual H II regions within those galaxies. Our observations of the line flux ratio (O II) lambda lambda 3726, 3729 + (O III) lambda lambda 4959, 5007)/H beta for 159 H II regions in 14 spiral galaxies are combined with published data to provide a sample of 39 disk galaxies for which (O II) + (O III)/H beta has been measured for at least five H II regions. We find that the characteristic gas-phase abundances and luminosities of spiral galaxies are strongly correlated. This relationship maps almost directly onto the luminosity-metallicity relationship of irregular galaxies and is also quite similar to that found for elliptical and dwarf spheroidal galaxies. Within our sample of spirals, a strong correlation between characteristic abundance and Hubble type also exists. The correlation between luminosity and Hubble type complicates the issue, but we discuss several interpretations of the correlations. The relationship between circular velocity and characteristic abundance is also discussed. We find that the slopes of the radial abundance gradients, when expressed in units of dex/isophotal radius, do not significantly correlate with either luminosity or Hubble type. However, the hypothesis that both early and very late type spirals have shallower gradients than intermediate spirals is consistent with the data. We find suggestive evidence that the presence of a bar induces a flatter gradient and also briefly discuss whether abundance gradients are exponential, as is usually assumed. We investigate the properties of individual H II regions in a subset of 42 regions for which we have spectra that cover almost the entire spectral range from 3500 to 9800 A. We use those data to estimate the densitites and ionizing spectra within the H II regions. We confirm that the ionizing spectrum hardens with increasing radius and decreasing abundance. We find no correlation between the ionization parameter and either radius or abundance, but this may be due to significant scatter introduced by the simple conversion of line ratios to ionization parameter.