OSIRIS - Science Program

The Science Programs

OSIRIS has been developed as a Day One instrument that provides the facilities to attack a wide variety of valuable scientific projects thanks to its design as a flexible optical imager and spectrograph. Here we discuss the science research that it is proposed for the first years of operation of OSIRIS. We present the structure of the Science Group, give an overview of the Guaranteed Time Program, and conclude with a summary of the different research lines that have been proposed by the astronomical community.

The Science Group

The Science Group is composed by nearly 90 astronomers from different countries, with the large numbers 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 instrument scientific capabilities, and to consider the benefit of using other scientific utilities. They have direct access to OSIRIS documents and facilities relevant to prepare scientific proposals, and also the advantage of a direct contact and relation with the instrument team for further or deeper information

General Structure

The Science Group is formed by the Principal Investigator (PI), the Project Scientist (PS), the Core Group, the Scientific Advisory Commitee (SAC) and the Scientific Team.

The PI and Core members are fixed, stable and permanent. SAC members are by invitation only and they will not be involved in the scientific projects. The PI is responsible for obtaining a high quality and competitive instrument and to exploit it with high quality and competitive scientific projects. The PS is in charge of the scientific aspects of the development of the instrument. The Core Group are the persons who helped to promote the instrument and that have a relevant role in the instrument definition and development. The SAC is intended to asses the PI and Core Group with the technical and/or scientific advise needed. It is composed by astronomers selected by the Core Group. The Scientific Team is composed of Group Leaders and Responsible investigators, formed by public calls to the spanish and mexican astronomical communities since May 2000. Only members of the Scientific Team have access to the Scientific Projects documentation, the raw and reduced data, the codes devised within the Team to analyse and exploit the data, the unpublished scientific results and, in general, to any other product resulting from the collaboration within the Team. Group Leaders are in charge of co-ordinate the activities within each Project. Responsible investigators are responsible for specific tasks of the scientific Project(s) to which they belong. They can participate in several scientific Projects and proposals.

The Scientific Program

So far, the OSIRIS Scientific Program is constituted by the following general Projects: i) Evolution of Galaxies; ii) The Local Universe; iii) Late Stages of Stellar Evolution; iv) Insterstellar Medium.

Each Project is developed by Responsible Investigators leaded by one or two Group Leaders that usually belong to the Core Group. The way to carry out the Projects is via scientific proposals. The templates, schedule and evaluation procedures for the proposals are provided via our Web Page (www.iac.es/project/OSIRIS). Each proposal must fit within one of the previous general Projects, or present a strong alternative. Each Project can have several proposals to develop it.

The Guaranteed Time

In order to accomplish the maximun scientific return in the use of the instrument, the Instituto de Astrofísica de Canarias decided to open its share of guaranteed time (GT) to the astronomical community, with its administration in charge of the OSIRIS PI. An Call for Proposals for the Science Group was made public, with a deadline by October 31, 2001. As a response, 19 proposals were received.

A SAC was formed to evaluate the proposals, giving them a rating, ranking, and an advice to improve the scientific cases. The Core Group and Project Scientist appraised them both scientifically and technically. A written report of evaluations was issued and send to the proposal PI individually. It is expected that the proposals shall undergo two evaluations, the first one done by October 2001 (deadline for proposal submission), and the second one, after implementing the correction and including technical details, one year later.

At that point, and after evaluation of their ratings and rankings, the Core Group will recommend the OSIRIS PI which proposals shall receive guaranteed time and with which amount. A Committee composed by the OSIRIS PI, the Head of the IAC Scientific Division and the Head of the IAC Technological Division will supervise the distribution of the observing time. The final decision will be taken by December 2002 and in any case before the GTC call for proposals in 2002.

The Science Programs

The main scientific motivation for OSIRIS 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. Objectives of highest priority for OSIRIS are two main areas: star formation rates in field and cluster galaxies at intermediate redshifts, and the UV emission spectra of large redshift galaxies. For nearby galaxies it will be possible to study the processes of star formation using either a full optical spectra of a few HII regions, or a few lines of all HII regions through narrow band images. Finally, the stellar absorption spectra provide an independent and differently weighted indicator of star formation history than emission nebular lines, using specially tuned spectral indicators (for age, abundances and initial mass function determinations) based on absorption lines and synthesis techniques. We detail in the following sections same of the areas on which the OSIRIS Science Group aims to make significant contributions.

Planets and Stars

In its standard imaging mode (with broad band filters) it will be feasible to perform deep imaging (I = 27 mag) of selected fields that can discover massive proto-Jupiters (1-10 MJup) (Zapatero-Osorio et al 2000 ).

Thanks to the high time resolution capabilities of the instrument, it will be viable to perform spectrophotometry for selected Low Mass X-ray Binaries (LMXBs), that together with multiwavelength studies will provide information about the structure of the accretion disks (Beskin et al. 1994 ). It will be possible to observe X-ray binaries, and obtain very fast spectrophometry (i.e. 10 sec time resolution) of the objects, with long follow-ups using charge shuffling or frame-transfer techniques. There are also programs using standard spectroscopic modes. For example, direct abundance measurements are the direct method to trace the radial metallicity distribution in nearby galaxies, and help to model their chemical evolution. This type of measurements are withing the realm of possibility with 8-10 meter class telescopes.

With intermediate resolution spectroscopy and large telescopes we can procure individual stellar spectra in nearby galaxies as M31 and M33 (Lennon et al. 1999 ), studying problems as rotational mixing, the temperature scale for O stars in a range of metallicity environments, and a better calibration of the Wind Momentun-Luminosity relation. For example it is possible to obtain metallicity gradients and compare with the gradients obtained by other methods as HII chemical abundances. The study of the proces that triggers and controls star formation in galaxies is a key problem in astrophysics. One group expect to study the origin of shells and shupershells in galaxies (i.e. explosion of stellar origin or collision with HVC) (Efremov 2001 ), as a road to understand the formation of stars in those complexes.

Normal Galaxies

Capitalizing in the Tunable Filter (TF) imaging capabilities of OSIRIS, together with the use of a 10 meter telescope, unprecedent studies of the ionized gas in galaxies will be performed.

In the case of normal galaxies, we know that massive stars ionize the medium. Emission lines originate in ionized gas and used for the study of the physical condition of the several components of the ISM, including HII regions, supernova remnants, planetary nebulae, and the H+ outside the HII regions called the Diffuse Ionized Gas (DIG). In a more traditional venue, the TF is ideally suited for the study of HII region populations, examining their physical conditions, electron densities and chemical abundances.

One of the most interesting problems is the existence of DIG (Rand et al. 1990 ), ionized gas at the kpc scale over the plane of spiral and even blue compact dwarf galaxies and starbusts. The use of the TF tuned to the appropiated wavelengths of selected emission lines will let the astronomers to attack the puzzle about the ionization mechanism of the DIG (Lyman continuum radiation emitted by the OB associations or shocks?). In this way it is possible to attack a variety of problems as the question of the disk-halo interaction, the star formation in the disk, and the connection with the gas outflows, that sometimes leads to the extended DIG.

The spectrum of a given galaxy contains the sum of the the spectral features of its stellar content. With the spectroscopic modes at intermediate resolution it is possible to use spectral indices of the galaxies together with theoretical models to model their stellar population. Spectroscopy provides information about the kinematics (rotation) and the line strenght (stellar population).

As an example, in elliptical galaxies, looking to line strenght gradients we can study of stellar population (Vazdekis & Arimoto 1999 ). Metalicity gradients should be different in the scenarios of monolithic collapse or hierarchical merging, and can be used to discriminate between these two different theories. In this case, the TF will produce two-dimensional maps of several spectral features, in substitution to the conventional long-slit approach.

In the study of blue compact dwarf galaxies a classical puzzle is if the galaxies are actually young or only old dwarfs. To solve the problem it is necessary to observe at very low surface brightness, looking for the subyacent stellar population and disentangle the characteristics of the stellar population (Cairos et al. 2001), an ideal program for a 10 meter telescope . OSIRIS allows for multiple analysis in all the modes of the instrument: broad band, narrow band, TF, and long slit spectroscopy

Active Galaxies

Active galaxies will be also studied, either with imaging, for example, looking to the extended emission (TF) of ionized gas or looking to the stellar population in AGNs.

With the spectroscopic capabilities of the instrument it will be possible to search for the stellar populations of AGN host galaxies, examining the link between starburst and stellar population (Nolan et al. 2001 ). A detailed study can be done with a subtraction of the nuclear light and making use of stellar population models.

Excitation mechanisms in extended emission line regions around AGNs (Robinson 1997 ), starbursts, and normal galaxies are photoionization by the ionizing radiation field (with several mechanisms proposed) or shock ionization. To clarify the situation it is necessary to study the physical conditions and spatial distribution of the extended emission line regions. So imaging at selected wavelengths, taking into account the redshift of the object, can help to understand the source of excitation, a method that can be applied to a variety of objects, from AGNs to starbursts.

At larger redshifts we can exam the conection between the starbursts and AGN (Gonzalez Delgado et al. 2001 ) in radio galaxies, selected at low and high redshift.

Distant galaxies and quasars

A very important problem associated with the study of Ly-alpha absorbers and their relation with galaxies is if high and low column density absorbers share a common origin (Fernandez-Soto et al 1996 ). A method to attack this question is to find galaxies in the same field and same redshifts, using the TF tuned to the [OII] emission at the appropiate redshift.

With a similar idea it is possible to look for galaxies around QSOs, using Ly-alpha imaging, at the redshift of the QSO. A program aims to study the environments of radio loud and radio quiet quasars, to check if the environment that they inhabit is different (Smith et al. 2000 ), and the relationship between the level of quasar activity and the host galaxy environment.

Cluster of galaxies

One key issue is the study of clusters to infer their evolution as a function of the environment. For this subject it will be possible to perform the identification of the population of emission line galaxies in clusters using TFs, looking for star forming galaxies (Butcher & Oemler 1984 ), and achieve a further characterization using MOS of the stellar population and chemical evolution. The star formation rate will be derived, as well as the global kinematics of the galaxies, morphological types and abundances, and AGN contents of clusters. In the very novel technique of nod-and-shuffle (Glazebrook & Bland-Hawthorn 2001) , it is possible to study the dynamics and star-formation properties of clusters, with MOS in microslit-mode (pinholes), determining the kinematics of the cluster in a more efficient manner

Targets of opportunity

It is expected that there will be certain type of objects (i.e. supernovae, comets) that will be observed in the Target of Opportunity --ToO-- Mode. Gamma Ray Burst (Gehrels 1999 ) are ideally suited for this type of programs. GRBs can be studied by afterglow follow-up spectroscpy, tracking the change of the spectra and brightness with time. This could allow deriving the identity of the GRB progenitors,their nature and origin.

Conclusions

Galactic and extragalactic astronomy studies will benefit greatly from instruments with tunable filter technology at a large telescope such as GTC. OSIRIS at the GTC will permit two dimensional studies of very faint emission line objects (and relatively faint absorption systems) at a continuous selection of wavelengths and redshifts. Together with its complementary spectroscopic modes, its large field of view, and the image quality provided by GTC, OSIRIS will be a very competitive tool of wide use for the GTC astronomical community, and a prime instrument with a potential to attack a wide range of classical and edge-front observational programs.

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Last update August 8, 2005, by Héctor Castañeda