Near-Infrared Integral-Field Spectrograph (NIFS):
An Instrument Proposed for Gemini

Peter J. McGregor , Peter Conroy , Gabe Bloxham , Jan van Harmelen, PASA, 16 (3), 273.
Next Section: The NIRI Legacy
Title/Abstract Page: Near-Infrared Integral-Field Spectrograph (NIFS):
Previous Section: NIFS Overview
Contents Page: Volume 16, Number 3

Subsections


NIFS Science Drivers

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 dynamical evolution of galaxies at high redshift.


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

$10^9 \> M_\odot$) black holes in the nuclei of many nearby galaxies (Table 1 derived from Franceschini, Vercellone, & Fabian 1998). The mass distribution and frequency of occurrence of these central black holes is still poorly known (Kormendy & Richstone 1995): understanding the demographics of these massive black holes is one of the most significant astrophysical problems we currently face.



Table 1: Massive Black Hole Galaxies
Galaxy D MB MBH Ref.
  (Mpc) (mag) ($M_\odot$)  
M 32 0.7 -15.51 $3\times10^6$ 1,2
NGC 4486B 20 -17.15 $1\times10^7$ 3,4
Milky Way - -17.65 $2.4\times10^6$ 5
M 31 0.7 -18.82 $3\times10^7$ 6
NGC 3377 14 -19.74 $1.8\times10^8$ 7
NGC 3115 10 -20.46 $2\times10^9$ 8
NGC 4258 9 -20.46 $7\times10^7$ 9
NGC 4374 27 -21.42 $1.5\times10^9$ 10
M 87 20 -21.86 $3\times10^9$ 11
NGC 4261 30 -21.87 $9\times10^8$ 12
NGC 4594 16 -23.14 $1\times10^9$ 13
References: 1. Bender et al. (1996). 2. van der Marel et al. (1997). 3. Kormendy et al. (1996b). 4. Lauer et al. (1996). 5. Eckart & Genzel (1997). 6. Kormendy & Richstone (1995). 7. Kormendy, et al. (1998). 8. Kormendy et al. (1996a). 9. Miyoshi et al. (1995). 10. Bower et al. (1998). 11. Ford et al. (1994). 12. Ferrarese, Ford, & Jaffe, (1996). 13. Kormendy et al. (1996b).

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 due to its limited light gathering power: the aperture of an 8 m class telescope is needed, with AO 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 may be required to suppress contamination from these nuclei. In summary, a near-infrared integral-field spectrograph with an AO 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 AO system, the integral-field spectroscopic capability, the occulting disk capability, and the near-infrared wavelength coverage.

The dynamical mass, Mdyn, enclosed within a radius, R, can be estimated from the first moment of the collisionless Boltzman equation (Sargent et al. 1978). For isotropic, isothermal stellar cores

\begin{displaymath}M_{dyn} = \frac{2 \sigma_*^2 R}{G} \approx 3\times10^7 \left[... ...\right] \left[\frac{\sigma_*}{50 km~s^{-1}}\right]^2 \> M_\odot\end{displaymath}


where $\sigma$ is the radial component of the stellar velocity dispersion, G is the gravitational constant, and we assume that the stellar radial velocity dispersion is constant over the region of interest and that the mass density distribution

$\rho(R) \propto r^{-2}$ as found for 13 spiral galaxies by Devereux, Becklin, & Scoville (1987). With a velocity resolution of 55 km s-1, NIFS should be able to measure velocity dispersions of this order, and so be capable of detecting mass concentrations of

$\sim 3\times10^7 \> M_\odot$ within 0.5'' of the centers of galaxies at distances of $\sim 10$ Mpc.

The CO (2-0) absorption bandhead at 2.294 $\mu $m 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 $\sim $ 0.05 (Fig. 2). The photometric H band contains the second overtone CO absorption bands ((3-0) at 1.56 $\mu $m, (4-1) at 1.58 $\mu $m, (5-2) at 1.60 $\mu $m, (6-3) at 1.62 $\mu $m) and the Si I 1.589 $\mu $m atomic absorption line. These features will be accessible with NIFS to a redshift of $\sim $ 0.1 (Fig. 2). Typical central K band surface brightnesses have been estimated from aperture photometry and growth curves for $\sim $ 50 bright early-type spiral galaxies (Griersmith, Hyland, & Jones 1982). In a 3.2'' diameter aperture, the average K band surface brightness is typically 13.7 mag arcsec-2 (Fig. 3).

Figure 2: Wavelengths of strong absorption and emission features as functions of redshift. Features plotted are [O II] 0.3727 $\mu $m, H$\beta $ 0.4861 $\mu $m, [O III] 0.4959,0.5007 $\mu $m, H$\alpha $ 0.6563 $\mu $m, Ca II triplet 0.8498,0.8542,0.8662 $\mu $m in the rest frame optical, CO (3-0), (4-1), (5-2), (6-3) and Si I 1.589 $\mu $m in the rest frame H band, and CO (2-0), (3-1), (4-2) in the rest frame K band. Regions measured by the four NIFS gratings are indicated by dashed lines.
\begin{figure} \centering\leavevmode \epsfysize=100mm \epsfbox{nifs_redshift.ps}\end{figure}

Figure 3: Histogram of central K band surface brightness inferred from aperture photometry for $\sim $ 50 bright early-type spiral galaxies (Griersmith et al.). The average value of 13.7 mag arcsec-2 is indicated by a dotted line.
\begin{figure} \centering\leavevmode \epsfysize=100mm \epsfbox{nifs_griersmith.ps}\end{figure}


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 seemed to have 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

$1 \leq z \leq 2$ 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 $\sim $ 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 $z \sim 0.5$ to $z \sim 1.0$ in the J band, $z \sim 1.3$ to $z \sim 1.7$ in the H band, and beyond $z \sim 2$ in the K band (Fig. 2), well beyond $z \sim 1$ where current galaxy formation models predict present day galactic disks were still forming. H$\beta $ can be measured from $z \sim 1.0$ to $z \sim 1.7$ 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 $z \leq 0.3$,

$L^*_{H\alpha} = 10^{42.13\pm0.13}$ erg s-1, and H$\alpha $ luminosity is related to absolute blue magnitude by

$M(B_{AB}) = 46.7 - 1.6\log L(H\alpha )$ (Tresse & Maddox 1998). The B luminosity function for blue CFRS galaxies brightens by $\sim $ 1 mag between $z \sim 0.3$ and $z \sim 1$ (Lilly et al. 1995). If M(BAB) is related to $L(H\alpha )$ at $z \sim 1$ as it is at $z \leq 0.3$, then

$L^*_{H\alpha} \sim 10^{42.73}$ erg s-1 at $z \sim 1$. McCarthy et al. (1999) find a median $L(H\alpha )$ at

0.75 < z < 1.9 of 1042.43 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 $z \sim 1$ would then have an integrated H$\alpha $ flux of

$\sim 4.6 \times 10^{-23}$  W cm-2. Typical disk scale lengths are $\sim $ 4 kpc (Schade et al. 1995), corresponding to a half-light radius of $\sim 0.77''$ on the sky. If the H$\alpha $ emission is uniformly distributed across the galaxy disk, the H$\alpha $ surface brightness will be

$\sim 2.5 \times 10^{-23}$ W cm-2 arcsec-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:

  • High resolution near-infrared spectroscopy of individual stars and substellar objects.
  • Studies of the dynamics and excitation of pre-main-sequence star disks.
  • Studies of the inner regions of pre-main sequence star jets.
  • Dynamical and abundance investigations of stellar populations in galaxies, including the center of our galaxy.
  • Investigation of the dynamics, excitation, and abundances of gas in the inner narrow-line regions of nearby active galactic nuclei.
  • Dynamical, excitation, and abundance studies of the cores of starburst galaxies and ultra-luminous IRAS galaxies.
  • Observational confirmation of the existence of massive central black holes in the host galaxies of QSOs through direct measurement of high stellar velocity dispersions.
  • Study of stellar and gaseous dynamics and excitation in radio galaxy hosts, and the origin of radio jet/host galaxy alignment at high redshift.

Next Section: The NIRI Legacy
Title/Abstract Page: Near-Infrared Integral-Field Spectrograph (NIFS):
Previous Section: NIFS Overview
Contents Page: Volume 16, Number 3

Welcome... About Electronic PASA... Instructions to Authors
ASA Home Page... CSIRO Publishing PASA
Browse Articles HOME Search Articles
© Copyright Astronomical Society of Australia 1997
ASKAP
Public