SYNERGY WITH SPACE-BASED MISSIONS

The New Character of Ground-Based Support for NASA Missions:     By 2010, a host of NASA missions will be generating images of the sky at wavelengths from -Rays to the far-infrared. Some of these missions will provide all-sky surveys whereas others will provide deep images of small ( 1-100 sized) regions of the sky. The much greater sensitivity of these NASA missions (Chandra/AXAF, SIRTF, GALEX, FIRST, Constellation-X) compared to previous missions is expected to produce detections of typically 1000 - 10,000 objects/, orders of magnitude higher than previous missions (please see Table 4). The new character of NASA missions calls for much more efficient modes of spectroscopic followup, i.e., the highly multiplexed multi-object spectroscopy of SWIFT.

 

Table 4: Source Densities for Present and Future NASA Missions

Mission	Wavelength	FOV	texp	Flim /band	Nsources	Surveyed
(Launch)		(')	(sec)		per sq.deg.	Area (deg2)
SIRTF	MIR/FIR	5	12	88Jy/3.6micron	2000
(2001)			5x200	4Jy/3.6micron	72000
NGSS	MIR	34		~100microJy/3.6micron		42000
(2004)
GALEX	UV	75	100	21.7AB/(0.13-0.3m)	260	42000
(2003?)			20000	26AB/(0.13-0.3m)	6500	200
ROSAT	X-ray					42000
(1990)				10-18W/m2/(0.5-2keV)	970	0.3
Chandra	X-ray	17-30	typical	10-17 W/m2/(0.2-10keV)	300	500
(1999)			deep	10-19 W/m2/(0.2-10keV)	2000-10000	50
XMM	X-ray	30	10-	10-17 -	300-	100-200
(2000)			100 ksec	10-18 W/m2/(0.1-4.5keV)	-1000	(GTO)

 

These missions will require both ground-based optical and near-IR imaging (to identify the optical and near-IR counterparts of the objects detected in the space-based surveys) as well as optical and near-IR spectroscopy (in order to classify the object, determine its redshift, and understand its physical properties - composition, internal dynamics, bulk motion, kinematics, stellar content, formation history, mass, age, etc.).

Spectroscopic Follow-up of Many Faint Sources per Square Degree:     Typical source densities for objects detected by these surveys is of the order of 3x10-10⁵/sq.deg. (Table 4). The optical and near-infrared counterparts of the bulk of the sources detected by the NASA missions will range between 19-27 AB mag.

For example, at X-ray wavelengths, typical Chandra images to F_X(2-10keV) = 10^-14 erg/s/cm² will contain about 300 sources/sq.deg. The deepest Chandra pointings (to F_X(2-10keV) = 3x10^-16 erg/s/cm²) will contain about 2,000-10,000 sources/sq.deg. -- completely uncharted territory! Although ROSAT succeeded in resolving part of the X-Ray background, the nature of the objects responsible for the soft-X-Ray background, i.e., whether they are normal galaxies or a new population of low-luminosity or obscured AGN, remains a mystery. Understanding the nature of this faint population, and their relationship to normal galaxies, will require spectroscopy of a significant fraction of these high surface-density sources.

The source density limits are comparable at mid-/far-infrared wavelengths: deep SIRTF exposures (5200 seconds with IRAC) will reach Jy at 3.6microns, and should have source densities of about 72,000 sources/sq.deg. More typical exposures of 12 sec will reach 88Jy and have field source densities of 2,000 sources/sq.deg. The SIRTF surveys will uncover dusty galaxies at intermediate and high redshifts, and possibly identify sources which are missing from our optical catalogs due to their red colors. The dusty galaxy population may contribute significantly to the global star formation and metal production rate at early epochs, and little is known about this population at present. Similarly, GALEX, which operates at UV wavelengths, will obtain near- and far-UV imaging surveys in fields of 200 sq.deg. to 26 AB mag and over the entire sky down to 21.7 AB mag, with typical source densities (for extragalactic objects) at these two limits of 6500 and 260/sq.deg. respectively.

Even at optical and near-infrared wavelengths, space-based facilities have resulted in a vast quantity of exquisite imaging data. Hubble Space Telescope imaging programs (using the WFPC2 and NICMOS cameras) have resulted in exciting discoveries, but many of these projects have been critically reliant on spectroscopic observations obtained with ground-based telescopes.

Comprehensive Follow-up Maximizes Scientific Return:     It is imperative to have spectroscopic observations of very large numbers of the detected sources in order to maximize the scientific return from the NASA space missions. The reasons are twofold: first, the most interesting scientific return is usually in identifying the various populations that are detected at other wavelengths (i.e., X-Rays, UV, IR, Sub-mm), quantifying their contribution to the total source population and understanding their cosmological significance. At the faint end, where the source densities are high and follow-up requires a highly multiplexed capability, the resolved sources may contribute significantly to the extragalactic background and are of importance to cosmology and galaxy evolution. Second, the most unusual source, and one which often provides a critical astrophysical clue, is invariably hidden in the vast number of sources at the catalog limit. For example, IRAS FSC10214+4724, the most luminous infrared source in the Universe, was only barely detected by IRAS and found only by spectroscopically surveying a very large sample of faint IRAS sources. The potential for serendipity is enormous with a large-scale ground-based spectroscopic effort complementing the planned space missions.