Taurus Tunable Filter Research Articles

Hierarchical Bayesian approach for estimating physical properties in spiral galaxies: Age Maps for M74

One of the fundamental goals of modern Astronomy is to estimate the physical parameters of galaxies from images in different spectral bands. We present a hierarchical Bayesian model for obtaining age maps from images in the Ha line (taken with Taurus Tunable Filter (TTF)), ultraviolet band (far UV or FUV, from GALEX) and infrared bands (24, 70 and 160 microns (μm), from Spitzer). As shown in [1], we present the burst ages for young stellar populations in the nearby and nearly face on galaxy M74.As it is shown in the previous work, the Hα to FUV flux ratio gives a good relative indicator of very recent star formation history (SFH). As a nascent star-forming region evolves, the Ha line emission declines earlier than the UV continuum, leading to a decrease in the HαFUV ratio. Through a specific star-forming galaxy model (Starburst 99, SB99), we can obtain the corresponding theoretical ratio Hα / FUV to compare with our observed flux ratios, and thus to estimate the ages of the observed regions.Due to the nature of the problem, it is necessary to propose a model of high complexity to take into account the mean uncertainties, and the interrelationship between parameters when the Hα / FUV flux ratio mentioned above is obtained. To address the complexity of the model, we propose a Bayesian hierarchical model, where a joint probability distribution is defined to determine the parameters (age, metallicity, IMF), from the observed data, in this case the observed flux ratios Hα / FUV. The joint distribution of the parameters is described through an i.i.d. (independent and identically distributed random variables), generated through MCMC (Markov Chain Monte Carlo) techniques.

Tunable filter imaging of high-redshift quasar fields

We have used the Taurus Tunable Filter to search for Lyα emitters in the fields of three high-redshift quasars: two at z∼ 2.2 (MRC B1256−243 and MRC B2158−206) and one at z∼ 4.5 (BR B0019−1522). Our observations had a field of view of around 35 arcmin2, and reached AB magnitudes of ∼21 (MRC B1256−243), ∼22 (MRC B2158−206) and ∼22.6 (BR B0019−1522) depending on wavelength. We have identified candidate emission-line galaxies in all the three fields, with the higher redshift field being by far the richest. By combining our observations with simulations of the instrumental response, we estimate the total density of emission-line galaxies in each field. 17 candidate emission-line galaxies were found within 1.5 Mpc of BR B0019−1522, a number density of (4.9 ± 1.2) × 10−3 Mpc−3, suggesting a significant galaxy overdensity at z∼ 4.5.

A search for shock-excited optical emission from the outflows of massive young stellar objects

We have searched for optical shock-excited emission lines in the outer parts of the bipolar outflows from massive young stellar objects where the flow terminates and the extinction is expected to be low. The Taurus Tunable Filter (TTF) at the Anglo-Australian Telescope (AAT) was used to obtain narrow-band ( 12-15 angstrom) images of the H alpha, [NII] 6583, and [SII] 6717/6731 lines around Mon R2 IRS3, S255 IRS1/3, GL 961 and GL 989. No clear examples of shocked emission were found. A bow shock feature in the GL 989 region maybe related with the molecular outflow NGC2264 D. A weak optical bow- like feature is seen in GL 961, but which of the stars in the cluster is the driving source cannot be uniquely identified. Other emission line features were found in Mon R2 and S255 that are consistent with an H.. region origin. The observed fluxes and the upper limits were compared with predictions from radiative shock models available in the literature. Any head-on collisions of a jet with speeds of the order of 500 km s(-1) should have been detected if the extinction A(v) 75 degrees), which would argue against the presence of highly collimated jets from high mass young stellar objects.

A Search for Very Extended Ionized Gas in Nearby Starburst and Active Galaxies

We report the results from a pilot study of 10 nearby starburst and active galaxies conducted with the Taurus Tunable Filter (TTF) on the Anglo-Australian and William Herschel telescopes. The main purpose of this imaging survey is to search for warm emission-line gas on the outskirts (galactocentric distances R 10 kpc) of galaxies to provide direct constraints on the size and geometry of the zone of influence of these galaxies on their environment. Gaseous complexes or filaments larger than ~20 kpc are discovered or confirmed in six of the galaxies in the sample (NGC 1068, 1482, 4388, 6240 and 7213, and MR 2251-178). Slightly smaller structures are seen for the first time in the ionization cones and galactic winds of NGC 1365, NGC 1705, the Circinus galaxy, and ESO 484-G036. The TTF data are combined with new optical long-slit spectra, as well as published and archived radio and X-ray maps, to constrain the origin and source of ionization of these filaments. A broad range of phenomena is observed, including large-scale ionization cones and galactic winds, tidal interaction, and ram-pressure stripping by an intracluster medium. The source of ionization in this gas ranges from shock ionization to photoionization by the central AGN or in situ hot young stars. The sample is too small to draw statistically meaningful conclusions about the extent and properties of the warm ionized medium on a large scale and its relevance to galaxy formation and evolution. The next generation of tunable filters on large telescopes promises to improve the sensitivity to faint emission-line fluxes at least tenfold and allow systematic surveys of a large sample of emission-line galaxies.

Extraplanar Emission‐Line Gas in Edge‐On Spiral Galaxies. I. Deep Emission‐Line Imaging

The extraplanar diffuse ionized gas (eDIG) in 17 nearby, edge-on disk galaxies is studied using deep Taurus Tunable Filter H? and [N II] ?6583 images and conventional interference filter H?+[N II] ??6548, 6583 images that reach flux levels generally below ~1 ? 10-17 ergs s-1 cm-2 arcsec-2. [N II] ?6583/H? excitation maps are available for 10 of these objects. All but one galaxy in the sample exhibit eDIG. The contribution of the eDIG to the total H? luminosity is relatively constant, on the order of 12% ? 4%. The H? scale height of the eDIG derived from a two-exponential fit to the vertical emission profile ranges from 0.4 to 17.9 kpc, with an average of 4.3 kpc. This average value is noticeably larger than the eDIG scale height measured in our Galaxy and other galaxies. This difference in scale height is probably due in part to the lower flux limits of our observations. The ionized mass of the extraplanar component inferred by assuming a constant filling factor of 0.2 and a constant path length through the disk of 5 kpc ranges from 1.4 ? 107 to 2.4 ? 108 M?, with an average value of 1.2 ? 108 M?. Under these same assumptions, the recombination rate required to keep the eDIG ionized ranges from 0.44 ? 106 to 13 ? 106 s-1 cm-2 of the disk, or about 10%-325% of the Galactic value. A quantitative analysis of the topology of the eDIG confirms that several galaxies in the sample have a highly structured eDIG morphology. The distribution of the eDIG emission is often correlated with the locations of the H II regions in the disk, supporting the hypothesis that the predominant source of ionization of the eDIG is photoionization from OB stars located in the H II regions. A strong correlation is found between the IR (or far-IR) luminosities per unit disk area (basically a measure of the star formation rate per unit disk area) and the extraplanar ionized mass, further providing support for a strong connection between the disk and eDIG components in these galaxies. The excitation maps confirm that the [N II]/H? ratios are systematically higher in the eDIG than in the disk. Although photoionization by disk OB stars is generally able to explain these elevated [N II]/H? ratios, a secondary source of ionization appears to be needed when one also takes into account other line ratios; more detail is given in a companion paper (our Paper II).

Taurus Tunable Filter — Seven Years of Observing

AbstractThe Taurus Tunable Filter (TTF) has now been in regular use for seven years on the Anglo–Australian Telescope (AAT). The instrument was also used for three years (1996–1999) on the William Herschel Telescope (WHT). We present a brief review of the different applications in order to illustrate the versatility of tunable filters in optical/IR spectrophotometric imaging. Tunable filters are now either planned or under development for 6–10 m class telescopes which ensures their use for years to come.

Detection and measurement from narrow-band tunable filter scans

The past 5 years have seen a rapid rise in the use of tunable filters in many diverse fields of astronomy, through Taurus Tunable Filter (TTF) instruments at the Anglo-Australian and William Herschel Telescopes. Over this time we have continually refined aspects of operation and developed a collection of special techniques to handle the data produced by these novel imaging instruments. In this paper, we review calibration procedures and summarize the theoretical basis for Fabry–Perot photometry that is central to effective tunable imaging. Specific mention is made of object detection and classification from deep narrow-band surveys containing several hundred objects per field. We also discuss methods for recognizing and dealing with artefacts (scattered light, atmospheric effects, etc.), which can seriously compromise the photometric integrity of the data if left untreated. Attention is paid to the different families of ghost reflections encountered, and the strategies used to minimize their presence. In our closing remarks, future directions for tunable imaging are outlined and contrasted with the Fabry–Perot technology employed in the current generation of tunable imagers.

A Tunable Lyot Filter at Prime Focus: a Method for Tracing Supercluster Scales atz∼ 1

Tunable narrow-band, emission-line surveys have begun to show the ease with which star forming galaxies can be identified in restricted redshift intervals to z ~ 5 with a 4m class telescope. These surveys have been carried out with imaging systems at the Cassegrain or Nasmyth focus and are therefore restricted to fields smaller than 10 arcmin. We now show that tunable narrowband imaging is possible over a 30 arcmin field with a high-performance Lyot filter placed directly in front of a CCD mosaic at the prime focus. Our design is intended for the f/3.3 prime focus of the AAT 3.9m, although similar devices can be envisaged for the Subaru 8m (f/2), Palomar 5m (f/3.4), VISTA 4m (f/6), Mayall 4m (f/2.6) or CFHT 3.6m (f/4). A modified Wynne doublet ensures sub-arcsecond performance over the field. In combination with the new Wide-Field Imaging 8K x 8K mosaic (WFI) at the AAT, the overall throughput (35%) of the system to unpolarised light is expected to be comparable to the TAURUS Tunable Filter (TTF). Unlike the TTF, the field is fully monochromatic and the instrumental profile has much better wing suppression. For targetted surveys of emission-line sources at z ~ 1, a low-resolution (R ~ 150 at 550nm) Lyot filter on a 4m telescope is expected to be comparable or superior to current instruments on 8-10m class telescopes. We demonstrate that the 30 arcmin field is well matched to superclusters at these redshifts such that large-scale structure should be directly observable.

Tunable-Filter Imaging of Quasar Fields at [CLC]z[/CLC] ∼ 1. I. A Cluster around MRC B0450−221

Using a combination of multicolour broad- and narrow-band imaging techniques and follow-up spectroscopy, we have detected an overdensity of galaxies in the field of quasar MRC B0450-221, whose properties are consistent with a cluster at the quasar redshift z=0.9. An excess of red galaxies (V-I>2.2, I-K'>3.8) is evident within 1' of the quasar, with the colours expected for galaxies at z=0.9 that have evolved passively for 3 Gyr or more. A number of line-emitting galaxies (nine candidates with equivalent widths EW>70A) are also detected in the field using the TAURUS Tunable Filter (TTF). Three have been confirmed spectroscopically to indeed lie at z=0.9. The TTF candidates with the strongest [O II] line emission cluster in a group which lies 200-700 kpc away from the quasar and the red galaxy excess, and therefore most likely on the outskirts of the cluster. These observations are the first in a series probing quasar environments at z~1 with TTF.

The Taurus Tunable Filter Field Galaxy Survey: Sample Selection and Narrowband Number Counts

Recent evidence suggests a decline in the volume-averaged star formation rate (SFR) with the advance of cosmic time since z ~ 1. It is not clear, however, the extent to which the selection of such samples influences the measurement of this quantity. Using the Taurus Tunable Filter (TTF) we have obtained an emission-line sample of faint star-forming galaxies over comparable look-back times: the TTF Field Galaxy Survey. By selecting through emission lines, we are screening galaxies through a quantity that scales directly with star formation activity for a given choice of initial mass function. The scanning narrowband technique furnishes a galaxy sample that differs from traditional broadband-selected surveys in both its volume-limited nature and selection of galaxies through emission-line flux. Three discrete wavelength intervals are covered, centered at Hα redshifts z = 0.08, 0.24, and 0.39. Galaxy characteristics are presented and comparisons made with existing surveys of both broadband and emission-line selection. Little overlap is found in a direct comparison between the TTF Field Galaxy Survey and a traditional galaxy redshift survey, as a result of the respective volume and flux limitations of each. When the number counts of emission-line objects are compared with those expected on the basis of existing Hα surveys, we find an excess of ~3 times at the faintest limits. While these detections are yet to be confirmed independently, inspection of the stronger subsample of galaxies detected in both the line and continuum (line-on-continuum subsample; 13%) is sufficient to support an excess population. The faintest objects are galaxies with little or no continuum, rendering them undetectable by conventional redshift surveys. This increase in the emission-line field population implies higher star formation densities over z ≲ 0.4. However, further study in the form of multiobject spectroscopic follow-up is necessary to quantify this and confirm the faintest detections in the sample.

Tunable Filter Surveys of Star‐forming Galaxies

This thesis describes the development of a tunable filter and its use in a survey for star-forming galaxies in the field at different redshifts. The Taurus Tunable Filter (TTF) is a narrow-gap Fabry-Perot interferometer that has been implemented as a fully tunable narrowband filter on the AngloAustralian Telescope. Characteristics of the device are presented in the thesis, including the use of a CCD charge-shuffling technique. This charge shuffling is used in conjunction with TTF to obtain multiple narrowband images on a single CCD frame and has use in either science or calibration exposures. Alternative methods of tunable imaging are discussed and compared to TTF. A technique is presented for assessing the parallel nature of the glass plates central to TTF use. This is essential in ensuring correct use of the instrument. The plate spacings used by TTF are much smaller than those of conventional Fabry-Perot devices and, as such, cannot make use of standard techniques for obtaining parallel plates. A survey for star-forming galaxies making use of the tunable filter is described. This TTF Field Galaxy Survey is novel in its use of the scanning TTF narrow band in the search for faint Ha emission indicative of star formation in the field. Three discrete wavelength intervals are covered, spanning Ha galaxies with redshifts –0.4. The narrowband technique furnishes a z ∼ 0.1 galaxy sample that differs from traditional broadband-selected surveys in both its volume-limited nature and selection of galaxies through emission-line flux. Methods of candidate selection from a set of narrowband images are discussed. Particular attention is given to the selection of a homogeneous subsample from data such as these. Galaxy characteristics are presented and comparisons made with existing surveys of both broadband and emission-line origin. Small overlap is found in a direct comparison between the TTF Field Galaxy Survey and a traditional galaxy redshift survey (The Autofib Galaxy Redshift Survey; R. S. Ellis et al. 1996, MNRAS, 280, 235), due to the respective volume and flux constraints of each. The star formation rates from the galaxies inferred to be Ha are at least ∼2 times higher than previous measurements in all redshift intervals. This excess is primarily due to the significant numbers of TTF galaxies with little or no continuum, many of which are beyond the broadband selection limit of conventional redshift surveys. While the detections are yet to be independently confirmed, inspection of the stronger subsample of galaxies detected in both line and continuum (13%) also supports an excess population. Further study in the form of multiobject spectroscopic follow-up is suggested to test this hypothesis and to confirm the estimates of contamination by background emission-line galaxies. Closing remarks are made regarding the future directions of both the technical and astronomical aspects of the thesis.

The large-scale distribution of warm ionized gas around nearby radio galaxies with jet-cloud interactions

Deep, narrow-band Halpha observations taken with the TAURUS Tunable Filter (TTF) on the 4.2m WHT telescope are presented for two nearby radio galaxies with strong jet-cloud interactions. Although the brightest emission line components are closely aligned with the radio jets --- providing nearby examples of the ``alignment effect'' most commonly observed in high redshift (z > 0.5) radio galaxies --- lower surface brightness emission line structures are detected at large distances (10's of kpc) from the radio jet axis. These latter structures cannot be reconciled with anisotropic illumination of the ISM by obscured quasar-like sources, since parts of the structures lay outside any plausible quasar ionization cones. Rather, the distribution of the emission lines around the fringes of the extended radio lobes suggests that the gas is ionized either by direct interaction with the radio components, or by the diffuse photoionizing radiation fields produced in the shocks generated in such interactions. These observations serve to emphasise that the ionizing effects of the radio components can extend far from the radio jet axes, and that deep emission line imaging observations are required to reveal the true distribution of warm gas in the host galaxies. We expect future deep imaging observations to reveal similar structures perpendicular to the radio axes in the high-z radio galaxies.

Parallelism of a Fabry‐Perot Cavity at Micron Spacings

ABSTRACTWe describe a method to quantify the degree of parallelism between two transparent glass mirrors spaced a few microns apart. Our technique, which permits the measurement and correction of deviations as small as λ/10000 from parallelism, is fundamental to the successful operation of tunable narrowband interference filters for two reasons. First, the highest throughput is achieved when the plates are parallel at any plate spacing. Second, the lowest resolution (largest bandpass) imaging is achieved when the plates are only a few microns apart, but there is a real danger of the plates touching if parallelism is not maintained. The Taurus Tunable Filter (TTF) is a Fabry‐Perot cavity with an adjustable plate spacing of 2–13 μm. The parallelism measurement involves repeated imaging through a focal‐plane slit and a series of pupil‐plane masks. This approach is particularly efficient when the plate scanning is synchronized with the movement of the charge on the CCD. We assess the effects of wavelength‐dependent phase changes within the inner surface coatings of the plates. These become important as the plates approach a spacing comparable in size to the thickness of the coatings.

Taurus Tunable Filter: A Flexible Approach to Narrowband Imaging

AbstractThe Taurus Tunable Filter (TTF) is a tunable narrowband interference filter covering wavelengths from 6300 Å to the sensitivity drop-off of conventional CCDs (∼9600 Å), although a blue ‘arm’ (4000–6500 Å) is to be added by the end of 1997. The TTF offers monochromatic imaging at the Cassegrain foci of both the Anglo-Australian and William Herschel Telescopes, with an adjustable passband of between 6 and 60 Å. In addition, frequency switching with the TTF can be synchronised with movement of charge (charge shuffling) on the CCD, which has important applications to many astrophysical problems. Here we review the different modes of TTF and suggest their use for follow-up narrowband imaging to the AAO/UKST Galactic Plane Hα Survey.

TTF Survey of Galaxy Populations

AbstractThe TAURUS Tunable Filter (TTF) affords a new approach to observational cosmology, allowing a wide field (10 arcmin) to be imaged monochromatically in contiguous wavelength intervals (6–60 Å bandpass) over the R and I bands. In a 200 s exposure with the AAT, the TTF can detect Hα emission powered by star formation rates as low as 0·1 M⊙ yr−1 at z = 0·08 and 1 M⊙ yr−1 at z = 0·24 in 2 arcsec seeing (cf. 0·26 M⊙ yr−1 for the LMC). In this paper we describe an emission-line survey currently under way using the TTF on the AAT to detect redshifted Hα over the ranges z = 0·06–0·1 and z = 0·22–0·26. Such detections will be of timely interest to the Southern HI Sky Survey which is motivated along similar lines.