Science with OSIRIS
H. Castañeda,1 , J. Cepa,1,3 , E. Alfaro,2 , J.J. González,4 , J.I. González
Serrano,5 , M. Sánchez-Portal6 and the OSIRIS team
1
2
3
4
5
6
Instituto de Astrofı́sica de Canarias, La Laguna, Spain
hcastane@iac.es
Instituto de Astrofı́sica de Andalucı́a, Granada, Spain
Departamento de Astrofı́sica, ULL, La Laguna, Spain
Instituto de Astronomı́a, UNAM, México
IFCA(CSIC-U. de Cantabria), Santander, Spain
HSC,INSA/ESAC, Madrid, Spain
Summary. OSIRIS (Optical System for Imaging and low Resolution Integrated
Spectroscopy) is designed as an optical Day One instrument for the GTC. This
versatile instrument will be able to perform broad band and tunable filter imaging,
low resolution long slit multiple-object spectroscopy, and fast spectrophotometry. In
this paper we present some of the scientific programs that have been proposed in
anticipation of Day One operation.
1 The OSIRIS instrument
OSIRIS is an imaging system and low resolution long-slit and multi-object
spectrograph developed for the Gran Telescopio Canarias (GTC). It represents
a new generation of instrumental observing techniques, including the use of
tunable filters and charge shuffling on the CCD detectors.
The instrument was developed with the aim to attack a wide variety of
valuable scientific projects. Its main motivation is to be a Star Formation
Machine, unique to provide an homogeneous and consistent mapping of star
formation indicators in nearby and back to the furthest observable galaxies
with GTC.
The Science Group is composed of nearby 90 astronomers from different
countries, with the largest number of members belonging to Spain and Mexico.
Their members form teams that plan and carry out scientific projects for
the optimal scientific exploitation of the instrument in cutting edge areas of
astrophysics, to test all observing modes and instrumental capabilties, and to
consider the benefit of using other scientific utilities.
Here we present and overview of the main observational operation modes of
OSIRIS, and review several of the research programs that have been proposed
2
H. Castañeda et al.
for the first years of instrument operation for the Tunable Filter Imaging Mode
and for the Spectroscopic Mode.
2 Imaging with Tunable Filters
A key feature of OSIRIS is the use of Tunable Filters. A Tunable Filter (TF)
is a low resolution (R 100-1000) Fabry-Perot interferometer, where the gap
between the plates is small (the order of 2-12 µm). Their use largely remove
the need for buying arbitrary narrow and intermediate interference filters, as
it is possible to tune the bandpass and the centroid of the bandpass by selecting the spacing. Galactic and extragalactic astronomical studies will benefit
greatly from instruments with tunable filter technology at a large telescope as
GTC. OSIRIS will allow two-dimensional studies of very faint emission lines
objects (and relatively faint absorption systems) at a continuous selection of
wavelengths and redshifts.
There are two tunable filters available in the instrument, with a range
of resolution from R ∼ 300 to 1000. A blue optimized TF is prepared for
operation from 365 to 670 nm, with a reflectivity of 91%. The red optimized
TF has a reflectivity of 94%, for operation from 620 to 1000 nm. The tunable
filters were manufactured by IC Electronics (UK).
2.1 Examples of observational programs
Absorption line strength of elliptical galaxies represents a very relevant
astronomical observation to pursue, as thery are a record of the galaxies
formation history 2341 Measurements of tunable filter line strenghs allows to obtain line strength maps in galaxies with different kinematical,
dynamical, and photometric properties ([1]).
• Tunable filters make it possible to cover the strong emission lines of galaxies and their continuun regions. Different types of galaxies have each one
a different type of emission line spectra. Seyfert 2 galaxies are strong in
[OIII]λ5007. Liners have an strong [SII]λλ6716,6731 doublet. Star forming galaxies could be detected looking to the ratios [SII]λλ6716,6731/Hα.
[OI]λ6300/Hα, and [NII]λ6584/Hα ([2]).
• The use of an etalon tuned to the appropiated wavelength of selected emission emission lines let the astronomer study the emission lines of ionized
gas and the physical condition of the several components of the interstellar mediun, including HII regions, supernova remnants, planetary nebulae,
and diffuse ionized gas outside the HII regions.
• Selected edge-on galaxies show ionized gas emission, as revealed by observations with narrow band imaging. To know the physical conditions of the
gas implies to measured the ratio of selected emission lines in the high-z
areas, a task ideally suited for a Tunable Filter. These studies can provide
•
Science with OSIRIS
3
information about the source of ionization of the gas (either ”filtered”
diluted radiation from OB stars from the disk, or shocks) ([3]).
• Radio-loud AGNs are favoured as pointers to high-z structures because
they can been seen out to the highest redshifts and have been found that
they tend to inhabit environments of above average galaxy density. A powerful way of searching for galaxies around high-redshift AGN is narrowband
imaging using Tunable Filters to detect emission line galaxies ([4]).
• Tunable Filters are ideally suited for surveys of star forming galaxies in
different environments. The Star Formation Rate (SFR) is measured via
the Hα flux. Hα luminosity functions for z = 0.8, 0.24 and 0.39 were
measured with Tunable Filters ([5]). The star formation history up to z =
1 was studied with a TF on the Hubble Deep Field North ([6]).
3 Spectroscopic Modes
For the spectroscopic modes grisms are used as dispersive elements. Grisms
are a combination of transmission gratings and prisms, manufactured in a
way that the central wavelength of the first order spectrum is passed without
deviation. The maximum guaranteed resolution is R = 2500, and the minimum
resolution ir R = 250. The grisms were made by Spectra-Physics (Rochester,
NY, USA). A total of 6 grisms have been delivered at this date.
Traditional spectroscopy can be done selecting one of the available long
slits fixed width masks available in the slit mask loader. A slit subsystem
(mask cassette) selects, inserts and remove spectroscopic mask to and from
the telescope focal plane. In addition to user costumized mask for multi-object
spectroscopy, a number of fixed width long slit masks are available, as well as
one mask for point-like fast photometry and another one for charge shuffled
continuum subtraction. The mask cassette has a capacity of up to 13 masks.
3.1 Examples of observational programs
Massive O-type main sequence stars can be observed in Local Group star
forming galaxies and medium-high resolutio with 8-10 meter class telescopes. OSIRIS can be used to determine parameters of the massive stellar populations, such as the IMF, the ratio of red-blue supergiants, the
mass loss rates and wind momentun of the blue supergiants, the relative
numbers of Wolf-Rayet stars and their various subtypes ([7]).
• Dwarf early-type galaxies constitute a dominant galaxy population in
nearby clusters. Long-slit spectroscopic data in dwarf elliptical in the Virgo
Cluster will allow to measure the degree of which these spheroids are rotationally supported, and to analyse the stellar population gradients, in term
of mean age, metallicity, initial mass function and star formation histories.
• Only techniques based on the detection of stellar features can be successful
in the detection of starbursts in AGNs, that require spectra with a high
•
4
H. Castañeda et al.
signal-to-noise ratio in the continuun, to compare the observed stellar features with the predictions of evolutionary synthesis models and, in this
way, to constrain the starburst properties. High sensitivity instruments as
OSIRIS+GTC allow to obtain the adequate S/N spectra with reasonable
exposure times ([8]).
4 Other Programs
Multi-object spectroscopy open the door to a more efficient use of telescope
time, and the application of new observational methods. With nod-and-shuffle
excellent sky subtraction can be achieved. With the use of the technique and
nod-and-shuffle and multi-object spectroscopy in micro-slit (pinhole) mode, it
is possible to study the dynamics and star-formation properties of clusters of
galaxies ([9]).
The high resolution capabilities of the instrument allows for rapid variability studies. Fast spectrophotometry for selected Low Mass X-ray binaries
(LMXBs) provides information on the structure of the accretion disks. Tunable filters can be used with charge shuffling i a time series mode. With this
technique, brown dwarfs wre observed to search for weather patterns studying
the variability of the light curve ([10]).
Simultaneous use of imaging and spectroscopic modes will be useful in
the target of opportunity mode, for example observing gamma rays bursts
(GRBs). GRBs can be indentified in the imaging modes, and studied by afterglow follow-up spectroscopy, tracking the change of the spectra and brightness
with time.
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
J.L. Cervantes-Rodriguez, A. Vazdekis & J. Cepa : NewAstrRev, 49, 670 (2006)
S. Veilleux et al. : AJ, 126, 2185 (2003)
S.T. Miller & S. Veilleux : ApJSS, 148, 383 (2003)
J.C. Baker et al. : AJ, 121, 1821 (2001)
D.H. Jones & J. Bland-Hawthorn : ApJ, 550, 593 (2001)
K. Glazebrook et al. : AJ, 128, 2652 (2004)
F. Bresolin et al. : ApJ, 577, L107 (2002)
R.M. Gonzalez-Delgado : RMAA, 16, 189 (2003)
K. Glazebrook & J. Bland-Hawthorn : PASP, 113, 197(2001)
C.G. Tinney & A.J. Tolley : MNRAS, 304, 119 (1999)