NIFS Science

The primary science drivers for NIFS are the study of the demographics of massive black holes in the nuclei of galaxies, and the study of the excitation of the inner narrow-line regions of Seyfert galaxies. A goal is to study the dynamical evolution of galaxies at high redshift. NIFS will also be used for a diverse range of other science.

Massive Black Holes in Galactic Nuclei

One of the most profound results from the Hubble Space Telescope (HST) is the evidence for the existence of massive (10⁷ to 10⁹ M_Sun) black holes in the nuclei of many nearby galaxies. The mass distribution and frequency of occurrence of these central black holes is still poorly known: understanding the demographics of these massive black holes is one of the most urgent problems astronomers face.

Spatially resolved, high resolution dynamical studies of the innermost nuclear stellar populations at near-infrared wavelengths are needed to progress these studies. Observations of both surface brightness distributions, mean rotation, and radial velocity dispersion profiles, with spatial resolution of a few parsecs and spectral resolution of 3000 to 5000, are required to model the stellar dynamics and infer properties of the central black hole.

The aperture of HST is too small to make a definitive study of central black holes: the aperture of an 8 m class telescope is needed, with adaptive optics resolution. The central regions of most galaxies contain obscuring dust which complicates the interpretation of optical data from HST. Near-infrared data are essential. A bright, compact nuclear emission-line core very often contaminates direct imaging and spectroscopy of the inner stellar population, particularly in spiral galaxies. An occulting disk is required to suppress contamination from the nucleus. In summary, a near-infrared integral-field spectrograph with an adaptive optics system and occulting disk capability is the best way to make a definitive study of central black holes in the decade before the Next Generation Space Telescope. The five essential elements for this work are the large aperture, the adaptive optics system, the integral-field spectroscopic capability, the occulting disk capability, and the near-infrared wavelength coverage.

Dynamical masses can be estimated from the virial theorem for isotropic, isothermal stellar cores. NIFS should be capable of detecting enclosed masses > 3 × 10⁷ M_Sun in galaxies at distances > 10 Mpc. The CO (2-0) absorption bandhead at 2.294 microns is ideal for measuring stellar velocity dispersions in low redshift galaxies. This band is strong in late-type stellar spectra and will be accessible with NIFS to a redshift of about 0.05. Typical central K band surface brightnesses have been estimated from aperture photometry and growth curves for about 50 bright early-type spiral galaxies. In a 3.2" diameter aperture, the average K band surface brightness is typically 13.7 mag arcsec^-2.

Excitation of the Inner Narrow-Line Regions of Seyfert Galaxies

Many nearby galaxies possess active nuclei which are characterized by broad (FWHM ~ 500 km s^-1) emission-lines originating in their central regions over size scales of 100 pc up to ~ 2 kpc. This is the so-called narrow-line region (NLR). The ultimate energy source is believed to be accretion onto a massive black hole in most objects, although intense starbursts in dense regions may be responsible for some LINER-like activity. Emission from the immediate vicinity of the accretion disk produces the broad-line region (BLR) which remains unresolved with existing telescopes. Understanding the nature of the central energy source, its interaction with the host galaxy, and the global implications for the evolution of galaxies are continuing themes in the study of active galactic nuclei (AGN).

High spatial resolution optical studies of AGN with HST have revealed a wealth of information about the structure and excitation of the inner NLR. While it has traditionally been believed that the NLR clouds are photoionized by the central source, these recent high spatial resolution imaging and dynamical studies have demonstrated that NLR clouds may instead be predominantly shock-excited by energetic thermal and non-thermal mass outflows from the central object. Strong dynamical interactions between the emission-line gas and radio-emitting ejecta can be explained if the NLR is formed from shells of ambient interstellar medium swept up and compressed by the supersonic expansion of hot gas heated by interactions with the advancing radio jet.

The nuclear regions of Seyfert galaxies are invariably obscured by dust clouds making near-infrared observations of the inner NLR desirable. The near-infrared region also offers the best ground-based spatial resolution using adaptive optics correction. [Fe II] 1.257 µm, [Fe II] 1.644 µm, H I Pbeta 1.282 µm, and H I Brgamma at 2.166 µm emission-lines are well-suited to excitation and dynamical studies of the high-excitation precursor zones associated with fully radiative shocked regions. Strong coronal emission-lines are the primary initial coolants of hot gas in partially radiative shocks. With NIFS, the [Si VI] 1.961 µm coronal line will become accessible at modest redshift. The mechanical energy flux from the jet can be estimated from the [Fe II] and H I Pbeta lines in less obscured regions, and from H I Brgamma in more obscured regions. H₂ 1-0 S(1) 2.122 µm emission in Seyfert galaxies is also collisionally-excited, but generally has a smaller velocity width of ~ 300 km s^-1 suggesting that it may arise in a different emission region. X-ray heating from the AGN core, shock-heating by the interaction of the radio jets with the interstellar medium, and shock-excitation in outflows from star formation regions may all contribute to the H₂ emission from Seyfert galaxies. High spatial resolution dynamical studies may provide a means of distinguishing between these alternatives.

Dynamical Evolution of Galaxies

Understanding the formation histories of galaxies will arguably be one of the most important astronomical legacies of our generation, and is certainly one of the main justifications for constructing the Gemini telescopes. Massive elliptical galaxies formed early in the Universe at redshifts z > 2. However, there is much theoretical and observational evidence that spiral galaxies formed over a long period of cosmic time extending to redshifts z < 1. How spiral galaxies accumulated their material in the redshift range 1 < z < 2 is one of the most important problems of observational cosmology, and one that will be tractable with NIFS.

Integral-field spectroscopy of high redshift galaxies will provide fundamentally new and more precise information about the evolution of star-forming disk galaxies with look-back time. Measurements of disk rotation velocity with resolutions of about 50 km s^-1 at galactocentric radii of order one disk scale-length will provide kinematic estimates of the total galaxy mass, and hence probe the mass assembly history of disk galaxies. Galaxy luminosities combined with assumptions about stellar mass-to-light ratios will allow crude separation of the luminous and dark matter components, and so provide information on the separate mass accumulation histories of these components and on the importance of biasing. H alpha spectroscopy produces the best emission-line rotation curves, but H alpha passes into the near-infrared region at redshifts z > 0.5. With near-infrared spectroscopy, H alpha will be accessible from z = 0.5 to z = 1.0 in the J band, z = 1.3 to z = 1.7 in the H band, and beyond z = 2 in the K band, well beyond z = 1 where current galaxy formation models predict present day galactic disks were still forming. H beta can be measured from z = 1.0 to z = 1.7 in the J band at redshifts where H alpha is not accessible from the ground. Integrated H alpha and H beta luminosities of disk galaxies provide instantaneous total star formation rates which can be compared with the inferred mass accumulation rate, and integrated over cosmic time to constrain possible star formation histories.

For Canada-France Redshift Survey (CFRS) galaxies with z < 0.3, L^*_H alpha = 10^42.13±0.13 erg s^-1, and H alpha luminosity is related to absolute blue magnitude by M(B_AB) = 46.7 - 1.6log L(H alpha). The B luminosity function for blue CFRS galaxies brightens by about 1 mag between z = 0.3 and z = 1. If M(B_AB) is related to L(H alpha) at z = 1 as it is at z < 0.3, then L^*_H alpha = 10^42.73 erg s^-1 at z = 1. The median L(H alpha) at 0.75 < z < 1.9 is 10^42.43 erg s^-1 from slitless grism spectroscopy with NICMOS. We adopt this value. With H₀ = 50 km s^-1 Mpc^-1 and q₀ = 0.5, a typical galaxy at z = 1 would then have an integrated H alpha flux of 4.6 × 10^-23 W cm^-2. Typical disk scale lengths are about 4 kpc, corresponding to a half-light radius of about 0.77" on the sky. If the H alpha emission is uniformly distributed across the galaxy disk, the H alpha surface brightness will be about 2.5 × 10^-23 W cm^-2arcsec^-2.

Secondary Science Opportunities

The instrumental capabilities of NIFS for its primary science goals are common to a wide range of other science. Indeed NIFS will be able to perform much of the science planned for GNIRS on Gemini North and for the deployable IFUs proposed for the Gemini near-infrared multi-object spectrographs, albeit by measuring these objects sequentially. These secondary science opportunities include: