This document quantifies the availability of guide stars for the ALTAIR facility adaptive optics system on Gemini North for specific observing programs that will be undertaken with the Gemini Near-infrared Integral-Field Spectrograph (NIFS).

2 Applicable Documents

Document ID	Source	Title
SND0002.00	RSAA	NIFS Science Drivers
SDN0004.01	RSAA	NIFS Performance Model

3 Introduction

The Gemini Near-infrared Integral-Field Spectrograph (NIFS) is an adaptive optics instrument. As such, its scientific potential is defined both by signal-to-noise ratio considerations and by the availability of suitable guide stars. Signal-to-noise ratio considerations are addressed in SDN0004.01 (NIFS Performance Model). Guide star availability was considered statistically in the NIFS Conceptual Design Review Documentation. However, the impact of guide star availability on science scope can only be assessed accurately by considering its consequences for observations of key science targets. The broad science cases for NIFS (SDN0002.00, NIFS Science Drivers) are further refined in this document into specific observing programs based on real objects. The availability of guide stars for these objects is assessed.

4 Guide Star Requirements

NIFS will be operated in three modes; with the ALTAIR natural guide star system, with the ALTAIR laser guide star upgrade, and on its own without ALTAIR. Observations that achieve the best image quality using NIFS with the ALTAIR natural guide star system will require an optical AOWFS guide star brighter than R ~ 15 mag as well as a near-infrared OIWFS guide star. The AOWFS guide star must be located within ~ 20² of the science object. The limiting magnitude for OIWFS guide stars is not known precisely. It is expected to be significantly fainter than the AOWFS limit because the OIWFS needs only to track slow flexure changes when used with the ALTAIR natural guide star system. The OIWFS guide star must be located within the 120² diameter circular field passed to NIFS by ALTAIR and outside the 25.4² diameter circular region vignetted by the NIFS pick-off probe. NIFS observations with the ALTAIR laser guide star upgrade will require a star within 30² of the science object for fast tip-tilt and focus correction. This star will either be sensed in the optical by the AOWFS or in the near-infrared by the OIWFS. If the OIWFS is used, the guide star will have to be significantly brighter than the flexure correction star because much shorter integration times will be required. NIFS observations that do not use ALTAIR at all can access OIWFS guide stars over the full 180² diameter field accepted by NIFS. The OIWFS will sense only slow flexure changes in this mode so fainter OIWFS guide stars will suffice.

5 Sky Coverage Estimates

Sky coverage issues for AO on Gemini have been discussed by Ellerbroek & Tyler (1998). They plot guide star density functions based on the Bahcall & Soneira (1980) model of the Galaxy (Figure 1). The AO corrected field of ~ 20² diameter corresponds to ~ 2.4´10^-5 deg², so guide star densities of order ~ 4´10⁴ deg^‑2 are required for complete sky coverage. Actual guide star densities to R ~ 15 mag are closer to 10² deg^-2 (Figure 1), so an AO natural guide star sky coverage of ~ 0.3% is expected. Sky coverage fractions for different Strehl ratios in the J, H, and K bands have been calculated in detail by Ellerbroek & Tyler (1998) and give a similar result for optimal performance (Figure 2).

The 120² diameter ALTAIR field over which OIWFS guide stars are accessible corresponds to 8.7´10^-4 deg², so guide star densities of ~ 10³ deg^-2 are required for complete sky coverage. Full sky coverage at 30° Galactic latitude would require an OIWFS guide star limit of R ~ 18 mag.

The OIWFS field for NIFS observations without ALTAIR corresponding to 3.5´10^-3 deg². Guide star densities of ~300 deg^-2 are required for complete sky coverage, so an OIWFS flux limit of R ~ 18 mag would provide nearly complete sky coverage to 90° Galactic latitude.

Figure 1: Guide star density functions for 30° Galactic latitude and 90° Galactic latitude (Ellerbroek & Tyler 1998).

Figure 2: Sky coverage probabilities at 90° Galactic latitude for natural guide stars (lower curves) and laser guide stars (upper curves) in median seeing, 0° zenith distance, and typical windshake (Ellerbroek & Tyler 1998).

6 Guide Star Selection

Selection of specific guide stars requires detailed knowledge of stars in the vicinity of a science target. The US Naval Observatory (USNO) Catalog is one of the largest star catalogs in existence. However, tests show that it is incomplete, at least in the vicinity of external galaxies that are prime NIFS targets. Digitized Sky Survey images show faint stars, but only in clear sky regions away from other objects and where the original photographic plates were not saturated. The 2MASS near-infrared sky survey is good for identifying near-infrared OIWFS stars. However, this is only ~50% complete at the time of writing and, as with all ground-based surveys, its relatively coarse spatial sampling (~ 1²) limits its ability to detect faint stars against extended objects. HST WFPC2 and NICMOS images, where available, provide the best data for selecting faint guide stars, especially near bright extended objects such as galaxies. In practice, a combination of all four images is required to confidently predict the subarcsecond structure of an object and select AOWFS and OIWFS guide stars.

A Perl script (gs_search.pl) has been written to automate the process of obtaining and inspecting second generation Digitized Sky Survey red images, 2MASS K band images, HST WFPC2 and NICMOS preview images, and overlaying USNO Catalog stars. A file containing a list of object names is input to gs_search.pl. The script then uses the name resolver function of the NASA Extragalactic Database (NED) to obtain object coordinates, retrieves the various images from on-line sources, and displays them along with overlays of USNO Catalog stars and the NIFS science field on a workstation (Figure 3). The display can be recentered at any position within the field, object coordinates and intensities can be printed, and suitable AOWFS and OIWFS guide stars can be marked.

Figure 3: Display of gs_search.pl output for the Seyfert galaxy NGC 1275. The displays show (clock-wise from top-left) the DSS-2 red image, the 2MASS K band image, a 1.6 mm NICMOS image, and a 0.70 mm WFPC2 image. USNO Catalog stars are marked with orange squares. The 3.0²´3.0² NIFS field-of-view is marked with a green square. The three red circles correspond to the 3¢ diameter NIFS window, the 2¢ diameter field passed by ALTAIR, and the 25.4² diameter region vignetted by the NIFS pick-off probe. The inner blue circle marks the 20² radius region within which a natural guide star should be located for the ALTAIR natural guide star system. The outer blue circle marks the 30² radius region within which a tip-tilt star should be located for the ALTAIR laser guide star system.

The gs_search.pl tool has been used to visually determine whether the objects discussed below are suitable for measurement with NIFS and whether they have the required guide stars for either the ALTAIR natural guide star system or the laser guide star upgrade to ALTAIR. Objects with insufficient data to make the assessment are classified as uncertain.

7 Massive Black Hole Galaxies

One of the main science drivers for NIFS is the detection of massive black holes in the cores of nearby spiral galaxies. The availability of guide stars for this program has been gauged by inspecting on-line images for 392 galaxies in Tully’s Nearby Galaxy Catalog that are closer than 20 Mpc, north of declination –30°, and have orderly spiral structure. The distance limit is required to resolve the sphere of influence of the black hole. The declination limit is required to achieve good image quality from Mauna Kea. Small irregular galaxies lacking spiral structure were deemed to be unlikely to contain detectable nuclear black holes. The results of this search are shown in Table 1. Very few nearby galaxies can be studied with the ALTAIR natural guide star system because the nuclei of most galaxies are too extended in WFPC2 images to be used as the ALTAIR guide object. Of the 24 galaxies that can be measured in this way (Appendix A, §20.1), nuclear mass limits or black hole detections are already known for at least five of these. Only 16 galaxies are reasonable massive black hole candidates. The larger sample of ALTAIR laser guide star system targets (Appendix A, §20.2) reflects the larger field-of-view over which tip-tilt stars can be accessed and their fainter limiting magnitude for tip-tilt-only correction.

Table 1: Guide Star Availability for Massive Black Hole Galaxies.

	ALTAIR NGS System			ALTAIR LGS System
	Y	?	N	Y	?	N
#	24	106	262	123	109	160
%	6%	27%	67%	31%	28%	41%

A central issue of this study is whether spiral and elliptical galaxies follow the same or different relations between black hole mass and bulge luminosity (the Kormendy-Richstone relation). If spiral galaxies follow the Kormendy-Richstone relations derived for elliptical galaxies, then we expect spiral galaxies with smaller bulge luminosities to have smaller black hole masses. The problem of distinguishing these smaller black hole masses from stellar mass contributions in nearby spiral galaxies was alluded to at the NIFS CoDR. We can quantify this better now that we have a specific target list.

The Kormendy-Richstone relation for known nuclear black hole galaxies (Ho 1998, Magorrian et al. 1998) is shown in Figure 4. Enclosed stellar mass limits in a 0.2″×0.2″ aperture are also shown for the subset of the ALTAIR natural and laser guide star samples that have sufficient data available. Nuclear stellar masses are estimated from central H band magnitudes measured from archival NICMOS images. The absolute calibration of these HST preview images remains slightly uncertain, but we proceed anyway. K band magnitudes have been inferred assuming H-K = 0.2, typical of late-type stars. The stellar luminosity was then converted to a mass using a maximal M/L_K = 5. As noted at the NIFS CoDR, this is a factor of two higher than the maximum value attained by an old stellar population; this inferred mass is the minimum mass that could unambiguously be ascribed to a nuclear black hole. Bulge luminosities are from Ho, Filippenko, & Sargent (1997).

The open symbols in Figure 4 show that NIFS will only be able to unambiguously establish consistency for spiral galaxies with the Kormendy-Richstone relation derived for elliptical galaxies.

Figure 4: Kormendy-Richstone relation for known nuclear black hole galaxies from Ho (1998; filled circles) and Magorrian et al. (1998; filled squares). Open symbols show the stellar mass in a 0.2″×0.2″ aperture inferred from archival NICMOS images assuming a maximal M/L_K = 5 for the ALTAIR natural guide star sample (open circles) and the ALTAIR laser guide star sample (open squares).

At face value, this is a disappointing result. However, there are two mitigating factors. The first is that the nuclear magnitudes used above are based on surface brightnesses integrated along the whole line-of-sight through the galaxy nucleus. Detailed dynamical models will deproject the light distribution so actual mass limits will be based on the slightly smaller luminosity of the central three-dimensional volume. Tests performed using a sky brightness taken immediately adjacent to the nucleus (to crudely subtract foreground and background light) show that this leads to a reduction in log M of only 0.1-0.3 for the centrally peaked light distributions typical of the ALTAIR natural guide star sample. A larger margin may arise by asking whether M/L_K = 5 is a realistic upper limit for an enclosed stellar population. The assumed value is a factor of two larger than the predicted value for a 10¹⁰ yr old solar abundance stellar population with a exponentially declining star formation rate having a time constant of 10⁹ yr (Thatte et al. 1997). In fact, Moriondo, Giovanardi, & Hunt (1998) measure a mean bulge M/L_K = 0.6±0.2 and a mean disk M/L_K = 1.0±0.4 for nine early-type spiral galaxies. This suggests that the actual enclosed stellar mass in any of our target galaxies may be up to one order of magnitude lower than the limits depicted in Figure 4. This would lead to an ambiguous black hole detection. To be sensitive to the implied enclosed masses of ~ 10⁶ M_ʘ, NIFS must be able to measure a velocity dispersion of ~ 30 km s^‑1 (FWHM ~ 70 km s^‑1) at a radius of 0.1″ from the nucleus. This should be possible with good signal-to-noise ratio spectra, given that the two-pixel velocity resolution will be ~ 60 km s^-1.

In summary, guide star availability for this core science project restricts the sample of good target objects with the ALTAIR natural guide star system to just 16 galaxies. The 0.1″ width of the NIFS slitlets means that unambiguous black hole detections in these galaxies are likely only if they possess black holes with masses in excess of the Kormendy-Richstone relation. It will not be possible to unambiguously separate black hole and stellar mass contributions at lower enclosed masses, making the interpretation of these results subjective. The ALTAIR laser guide system will be needed to significantly extend the sample size. However, the same interpretation ambiguities will apply to the laser guide star sample.

8 Seyfert Galaxies

Determining the structures of the inner Narrow-Line Regions (NLRs) in nearby Seyfert galaxies is the second main science driver for NIFS. We specifically address the study of nearby Seyfert galaxies having radio structure that is resolved on arcsecond scales so that details of the interaction of the radio jet with the host galaxy interstellar medium can be studied with NIFS. Radio surveys of nearby Seyfert galaxies have been performed by Ulvestad & Wilson (1984a,b, 1989), Kukula et al. (1995), Nagar et al. (1999), Thean et al. (2000), and Schmitt et al. (2000), among others. We have selected our target objects to be those Seyfert galaxies from these papers having linear double or triple radio structure on scales larger than ~ 0.3″. The results of this selection are listed in Table 3 (and in more detail in Appendix A, §20.3). It is noteworthy that some classic Seyfert galaxies, such as NGC 2110, Mrk 3, NGC 2992, NGC 3227, NGC 4051, NGC 4388, and NGC 5506, are not able to be measured with NIFS and ALTAIR. The prototype Seyfert 1 galaxy, NGC 4151, and the prototype Seyfert 2 galaxy, NGC 1068, will be difficult to measure with long integration times; NGC 4151 lacks an OIWFS guide star (Figure 5) and NGC 1068 only has one OIWFS guide star at the extreme edge of the unvignetted ALTAIR field (Figure 6). It may be possible to measure these bright objects with short exposures which do not require an OIWFS star. The classic “ionization cone” Seyfert galaxy, Mrk 573, is used as an example of this observation in the NIFS OCDD. However, it is unclear whether ALTAIR will be able to use the slightly resolved nucleus seen in the WFPC2 image (Figure 7) for high-order adaptive optics correction.

Table 3: Guide Star Availability for Radio-Resolved Seyfert Galaxies.

	ALTAIR NGS System			ALTAIR LGS System
	Y	?	N	Y	?	N
#	22	17	30	32	15	22
%	32%	25%	43%	46%	22%	32%

Figure 5: Display of gs_search.pl output for the prototype Seyfert 1 galaxy, NGC 4151. Note that there are no suitable OIWFS guide stars within the ALTAIR field (second large circle).

Figure 6: Display of gs_search.pl output for the prototype Seyfert 2 galaxy, NGC 1068. Note that the WFPC2 image is saturated so it is not possible to tell if the nucleus is unresolved (although we expect that it is) and that the only OIWFS guide star (marked by a small circle at lower left) is at the extreme edge of the ALTAIR field (second large circle).

Figure 7: Display of gs_search.pl output for the “ionization cone” Seyfert galaxy, Mrk 573. It is unclear whether ALTAIR will be able to use the resolved nucleus for high-order adaptive optics correction.

We can now use our sample of 22 radio-resolved Seyfert galaxies with suitable ALTAIR guide stars to ask whether these objects have detectable near-infrared spectra. We aim to define the morphology and kinematics of the [Fe II] 1.257 μm or [Fe II] 1.644 μm emission that are shock diagnostics in these objects. Seven radio-extended Seyfert galaxies with ALTAIR natural guide stars had their near-infrared spectra measured by Veilleux, Goodrich, & Hill (1997). Estimates of the average emission line surface brightnesses for these galaxies were presented at the CoDR and are repeated in Table 5. The [Fe II] 1.257 μm line is typically brighter than 2×10^-22 W cm^-2 arcsec^-2 and will be seen against a J band continuum of ~ 14.3 mag arcsec^-2. Performance estimates suggest that a signal-to-noise ratio of 10 per spectral pixel will be achieved in ~ 8 hr of on-source integration under these conditions on a 500 km s^-1 wide line.

Clearly, the inner Narrow Line Region will have to be significantly clumped (either spatially or in velocity) to be easily detected at full spectral resolution with NIFS.

Table 5: Average emission line surface brightnesses (from Veilleux, Goodrich, & Hill 1997).

Object	Aperture (″)	J Continuum (mag arcsec^-2)	K Continuum (mag arcsec^-2)	Pb 1.282 μm (W cm^‑2 arcsec^‑2)	[Fe II] 1.257 μm (W cm^‑2 arcsec^‑2)	H₂ 1-0 S(1) 2.122 μm (W cm^‑2 arcsec^‑2)	H I Brg 2.166 μm (W cm^‑2 arcsec^‑2)

Mrk 176	3×3	14.3	14.6	4.2×10^-23	1.8×10^-22	2.4×10^-23	6.2×10^-24
Mrk 477	3×3	15.5	14.5	1.2×10^-21	8.6×10^-22	1.0×10^-22	1.1×10^-22
Mrk 573	3×3(J) 1.5×1.5(K)	14.3	13.0	2.9×10^-22	2.7×10^-22	5.6×10^-23	1.2×10^-22
Mrk 1066	3×3(J) 1.5×1.5(K)	13.8	12.4	1.3×10^-21	1.3×10^-21	6.0×10^-22	6.0×10^-22
NGC 1068	1.5×1.5	…	8.2	…	…	1.3×10^-21	5.6×10^-21
NGC 5728	3×3	15.0	13.9	2.4×10^-22	3.3×10^-22	7.8×10^-23	2.5×10^-23
NGC 7212	1.5×1.5	…	14.3	…	…	8.9×10^-23	1.2×10^-22

In summary, 22 radio-extended Seyfert galaxies are expected to be measurable with the ALTAIR natural guide star system. Only the brighter ones of these are expected to be detectable with NIFS in less than one night per object unless the inner Narrow Line Region is significantly clumped.

9 Extrasolar Planets

NIFS may be used to search for planets around nearby stars. This is a long shot! All known extrasolar planet detected by Doppler techniques are too close to the star to be resolved. These systems may contain other gas giant planets at larger radii and it may be possible to resolve these planets, but current expectations are that they would be too faint to detect with NIFS. It is more realistic to consider measuring spectra of very low mass stellar and substellar binary companions. Nevertheless, we use the list of stars known to possess planets at http://exoplanets.org as a source list (Appendix A, §20.4) for assessing guide star availability for this class of observation (Table 6). The large fraction of objects accessible with the ALTAIR natural guide star system is obviously due to the presence of a bright host star at the field center.

Table 6: Guide Star Availability for Stars With Extrasolar Planets.

	ALTAIR NGS System			ALTAIR LGS System
	Y	?	N	Y	?	N
#	17	9	13	17	9	13
%	44%	23%	33%	44%	23%	33%

10 Young Stellar Objects

NIFS will be used to study jets from Young Stellar Objects (YSOs), to study their circumstellar environments, and to determine the nature of pre-main-sequence binary stars. The most extensive list of pre-main-sequence stars is the unpublished Herbig & Bell Catalog[1]. The availability of guide stars for all objects listed in the Herbig & Bell Catalog is shown in Table 7. These objects are listed in Appendix A (§20.5). The majority of stars in the Herbig & Bell Catalog can be measured with NIFS and the ALTAIR natural guide star system. This high fraction is due to the bright, stellar nature of the target objects and their low Galactic latitudes. However, this high fraction significantly over-represents the science scope of NIFS; what would be the scientific purpose of measuring predominantly single stars with an integral field spectrograph? Nebulous objects are clearly of interest. So too are unresolved T Tauri stars with associated stellar jets (e.g., DG Tau). We estimate that only ~ 20% of objects in the Herbig & Bell Catalog are useful NIFS targets in this sense.

Table 7: Guide Star Availability for Pre-Main Sequence Stars in the Herbig & Bell Catalog.

	ALTAIR NGS System			ALTAIR LGS System
	Y	?	N	Y	?	N
#	332	183	112	419	176	32
%	53%	29%	18%	67%	28%	5%

A more restricted sample can be defined by selecting those pre-main sequence stars which have been shown to posses nearly edge-on, optical thick, circumstellar disks (Padgett et al. 1999; Monin & Bouvier 2000); of these objects, Haro 6-5B, DG Tau B, and HV Tau C can be measured with the ALTAIR natural guide star system, the situation is unclear for HK Tau B, and CoKu Tau/1 cannot be measured at all.

The circumstellar environments of massive protostars will also be studied with NIFS, although many protostars themselves will be too bright to measure without using a neutral density filter or occulting disk. A selection of 53 well-known massive protostars has been checked for guide stars (Appendix A, §20.6). The result is shown in Table 9. Many fewer of these objects can be measured with NIFS and ALTAIR because embedded objects cannot be used as their own AOWFS guide stars.

Table 9: Guide Star Availability for High Mass Protostars.

	ALTAIR NGS System			ALTAIR LGS System
	Y	?	N	Y	?	N
#	7	22	24	23	28	2
%	13%	42%	45%	43%	53%	4%

11 Late Stages of Stellar Evolution

The processes by which asymptotic giant branch (AGB) stars evolve into planetary nebulae and shape the appearance of their planetary nebula are still poorly understood. At the end of the AGB phase, the star evolves to hotter temperatures as a Proto-Planetary Nebula (PPN). Eventually, the star becomes hot enough to photoionize its envelope. At this point, it is recognized as a young planetary nebula by its radio emission and recombination-line emission spectrum. The youngest planetary nebulae show definite point symmetry so the processes that shape planetary nebulae must already be occurring in the PPN phase. NIFS will be used to detect and map the earliest signs of extended near-infrared line emission in PPNs. Broad Hα emission is seen in some PPNs and is attributed to a stellar wind. (e.g., the F8 hypergiant IRC +10 420 developed Hα emission as recently as twenty years ago and now shows blueshifted near-infrared H I emission; Oudmaijer et al. 1994). Several PPN also show CO first-overtone emission in their near-infrared spectra (e.g., Oudmaijer et al. 1995). Densities of ~ 10¹⁰ cm^-3 and temperatures of ~ 6000 K are required to excite these bands. They are probably collisionally excited, possibly in a region where a recent fast wind compresses and heats a pre-existing slower wind (Hrivnak, Kwok, & Geballe 1994). Shock-excited near-infrared H₂ emission may also arise under these conditions.

Large numbers of PPNs have been identified from their far-infrared fluxes detected by IRAS. We have selected northern objects from van der Veen, Habing, & Geballe (1989), Van de Steene & Pottasch (1995), García-Lario et al. (1997), Sahai & Trauger (1998), Meixner et al. (1999), Ueta, Meixner, &, Bobrowsky (2000), and unpublished lists (Appendix A, §20.7). The guide star statistics for these objects are summarized in Table 10. The large fraction of objects that can be measured with the ALTAIR natural guide star system arises because many PPNs are unresolved bright stars. These stars will be suitable targets for NIFS only if extended emission (i.e., emission from the developing planetary nebula) is detected at the high spatial resolutions achieved with NIFS and ALTAIR.

Table 10: Guide Star Availability for PPNs.

	ALTAIR NGS System			ALTAIR LGS System
	Y	?	N	Y	?	N
#	51	20	70	120	20	1
%	36%