Science with NIFS, Australia's First Gemini Instrument

Peter J. McGregor , Michael Dopita , Peter Wood , Michael G. Burton, PASA, 18 (1), in press.
Next Section: Gemini Core Science
Title/Abstract Page: Science with NIFS, Australia's
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Subsections


NIFS Core Science


Massive Black Holes in Nearby 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 $M_\odot$) black holes in the nuclei of many nearby early-type galaxies (e.g., Kormendy & Richstone 1995; Lauer et al. 1995; Faber et al. 1997). An apparent relationship exists between the black hole mass and bulge mass which suggests that either central black holes grew by accreting inner bulge stars, or else that the central black hole and the bulge formed coevally in major merger events. However, this correlation suffers from strong observational selection effects (Ford 1997), and it is yet to be determined whether elliptical galaxies and spiral galaxies follow the same or different correlations. Given the potential close link between black hole formation and galaxy evolution, it is important to define the mass distribution and frequency of occurrence of central black holes in both classes of galaxies. These are still poorly known; most particularly in late-type spiral galaxies in which the nuclear regions are obscured by dust and the bulge masses are smaller so the black hole masses may also be smaller.

Observations with NIFS will help determine the demographics of massive black holes in galactic nuclei. Spatially resolved high resolution dynamical studies of the innermost nuclear stellar populations and LINER-like gaseous accretion disks at near-infrared wavelengths are necessary to do this. Observations of surface brightness distributions, mean rotation, and radial velocity dispersion profiles with spatial resolutions of a few parsecs and spectral resolutions of 3000-5000 are required to model the stellar dynamics or gaseous accretion disk dynamics and infer properties of the central black hole. The high spatial resolution required dictates the use of AO correction. This, and the presence of obscuring dust in the central regions of many galaxies which complicate the interpretation of optical data from HST, dictates the use of near-infrared observations. The CO (2-0) absorption bandhead at 2.3 $\mu $m is ideal for measuring velocity dispersions of cool stellar populations in the nuclei of low redshift galaxies (e.g., Gaffney, Lester, & Doppmann 1995; Shier, Rieke, & Rieke 1996; Böker, van der Marel, & Vacca 1999). The presence of a mass concentration is indicated by a rising stellar velocity dispersion profile near the nucleus. The emission lines of H I P$\beta$ 1.282 $\mu $m, H I Br$\gamma$ 2.166 $\mu $m, [Fe II] 1.257 $\mu $m, and [Fe II] 1.644 $\mu $m are expected from shock-excited gas in circumnuclear LINER-like accretion disks. The enclosed mass is inferred from the rotational velocity, assuming that the gas follows Keplerian orbits about the mass concentration.

The velocity dispersion profile of M32 rises from

$\sigma_V \sim 60$ km s-1 at 1.0'' radius to

$\sigma_V \sim 95$ km s-1 at 0.1'' radius (Bender, Kormendy, & Dehnen 1996). Stellar velocity dispersions are therefore expected to be $\sim 50$ km s-1 in the outer parts and $\sim 100$ km s-1 at the centers of galaxies containing low mass black holes. These velocity dispersions correspond to Gaussian FWHMs of $\sim $ 118 km s-1 and $\sim $ 235 km s-1, respectively. A FWHM velocity resolution of $\sim $ 100 km s-1 will therefore suffice to measure stellar velocity dispersions from the CO (2-0) absorption bandheads in the K band. The LINER gas disk in the elliptical galaxy M84 has peak rotational velocities of $\pm$400 km s-1 (Bower et al. 1998), and the gas disk in NGC 4261 has peak rotational velocities of $\pm$200 km s-1 (Ferrarese, Ford, & Jaffe 1996). A velocity resolution of < 100 km s-1 will be required to measure disk rotational velocities in a range of lower mass objects. Similarly high spectral resolving powers of $R \sim$ 4000-5000 are required to significantly separate individual OH airglow lines in the J and H bands in order to perform sensitive measurements of the emission lines from circumnuclear LINER-like disks.

High Strehl ratios are required for these observations. Galaxy nuclei are in general too faint and diffuse to be efficiently used as guide objects for ALTAIR. The laser guide star upgrade to ALTAIR will be required to observe these objects. A natural guide star within $\sim 30''$ of the nucleus is still required for tip-tilt correction. The required brightness of this star will depend on observing conditions. However, the limiting magnitude in median conditions is expected to be $R \sim 18$ mag (§3). Approximately 30 galaxies closer than 20 Mpc, north of declination -30$^\circ$, and having central K band surface brightnesses $\mu_K < 14.0$ mag arcsec-2 at 1'' resolution have been identified with suitable guide stars. This sample of mainly S0 galaxies will form the initial target list. It will be necessary to accurately determine the point spread function (PSF) either by frequent measurements of a nearby PSF star, by reconstructing the PSF from OIWFS frames, or by modeling based on the AO control loop output (Véran et al. 1997).

It is expected that integration times of several hours will be required to detect massive black holes from stellar velocity dispersions in spiral galaxies closer than $\sim $ 20 Mpc. Figure 1 shows a simulated 3600 s K grating exposure on the nucleus of a spiral galaxy modeled using the light distribution of the Galactic center. The model galaxy has been shifted to 10 Mpc and no interstellar extinction has been applied. The peak K band surface brightness in the central

0.1'' x 0.1'' region is $\sim $ 10.5 mag arcsec-2 and is $\sim $ 12.8 mag arcsec-2 at a fiducial radius of 1.0'' ($\sim $ 50 pc at 10 Mpc). Figure 2 shows the spectrum extracted from the central

0.1'' x 0.1'' region after subtraction of a 3600 s sky exposure, transformation for 2D wavelength calibration, and division by a smooth spectrum star. The simulated spectrum has a signal-to-noise ratio of $\sim $ 100 in this central region (excluding systematic effects due to incomplete sky subtraction and correction for terrestrial atmospheric absorption). Continuum signal-to-noise ratios of $\sim $ 30 are generally required to measure stellar velocity dispersions for dynamical studies (Kormendy, priv. comm.). When the model galaxy is shifted to 20 Mpc, the K band surface brightness (with no extinction) at the fiducial radius of 1'' ($\sim $ 100 pc at 20 Mpc) is $\sim $ 13.4 mag arcsec-2. A signal-to-noise ratio of 30 is expected to be reached in $\sim $ 4.5 hr of on-source integration with NIFS at this surface brightness (§2).

Figure 1: Simulated 3600 s K grating exposure of the nucleus of a spiral galaxy at 10 Mpc in 0.4'' seeing with a Strehl ratio of 0.6. Each NIFS slitlet is stacked vertically and dispersed horizontally with wavelength increasing from $\sim $ 2.0 $\mu $m to $\sim $ 2.4 $\mu $m towards the right. The CO $\Delta $v=2 absorption bands in the galaxy spectrum are apparent at right. The vertical lines are sky emission lines.
\begin{figure} \centering\leavevmode \epsfysize=100mm \epsfbox{nifs0416.ps}\end{figure}

Figure 2: Extracted, sky-subtracted spectrum of the central

0.1'' x 0.1'' region of the simulated observation of a spiral galaxy at 10 Mpc shown in Figure 1.
\begin{figure} \centering\leavevmode \epsfysize=100mm \epsfbox{sp0416a.ps}\end{figure}

Measurements of black hole masses in nearby galaxies will be limited by spatial resolution as much as by signal-to-noise ratio. The enclosed mass at any radius is derived from the measured stellar velocity dispersion profile and the light distribution using the collisionless Boltzmann equation. Under the reasonable assumptions that the velocity dispersion is approximately constant in the inner region of the galaxy and that the volume mass density is proportional to $\sim r^{-2}$, the mass enclosed within radius r is given very approximately by

\begin{displaymath} M_{enc} \approx 3 \times 10^7\left[\frac{r}{0.5''}\right]\le... ...0~{\rm Mpc}}\right]\left[\frac{\sigma}{50~{\rm km/s}}\right]^2 \end{displaymath} (1)

where D is the distance to the galaxy, $\sigma $ is the constant velocity dispersion, and Menc is in units of $M_\odot$. Our model galaxy at 10 Mpc has a velocity dispersion of $\sim $ 70 km s-1 at 0.1'' ($\sim $ 5 pc) so an enclosed mass of

$\sim 1.2 \times 10^7 \> M_\odot$ would be inferred. However, our ability to discern the presence of a significant dark enclosed mass will depend on the derived value of the enclosed mass-to-light ratio, Menc/LK. Normal stellar populations can have Menc/LK in the range 0.0-2.5 (e.g., Thatte et al. 1997), so we will require mass-to-light ratios in excess of a least 5.0 to unambiguously indicate the presence of a massive black hole. Based on the light distribution of the Galactic center, observations with a resolution of $\sim 0.1''$ will therefore be sensitive to dark enclosed masses

$\ge 2 \times 10^8 \> M_\odot$ at a distance of 10 Mpc. This is comparable to the limit reached by HST with similar spatial sampling.


Nearby Active Galactic Nuclei

Many nearby galaxies possess active galactic nuclei which are characterized by broad (FWHM $\sim $ 500 km s-1) emission lines originating in their central regions over size scales of 100 pc up to $\sim $ 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 (Terlevich & Melnick 1985). 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 (e.g., Capetti et al. 1996; Winge et al. 1997; Axon et al. 1998) 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 (Ferland & Netzer 1983; Wilson & Tsvetanov 1994), 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 interaction 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 (Pedlar, Unger, & Dyson 1985; Taylor, Dyson, & Axon 1992; Steffen et al. 1997). 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 AO correction. [Fe II] 1.257 $\mu $m, [Fe II] 1.644 $\mu $m, H I P$\beta$ 1.282 $\mu $m, and H I Br$\gamma$ 2.166 $\mu $m 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 $\mu $m coronal line will become accessible at modest redshift. The mechanical energy flux from the jet can be estimated from the [Fe II] and H I P$\beta$ lines in less obscured regions, and from H I Br$\gamma$ in more obscured regions. H2 1-0 S(1) 2.122 $\mu $m emission in Seyfert galaxies is also collisionally-excited, but generally has a smaller velocity width of $\sim $ 300 km s-1 suggesting that it may arise in a different emission region (Veilleux, Goodrich, & Hill 1997). 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.

Seyfert activity is frequently associated with circumnuclear starbursts, often in rings, but the role these play in fueling or refueling the active nucleus is still unclear. The presence of circumnuclear starburst rings demonstrates that large quantities of gas have been channeled into the region close to the nucleus. This gas may accrete directly onto the black hole, but it must lose its remaining angular momentum to do this. It may be the stars, or their remnants, formed in the starburst ring or the central star cluster that feed the central black hole. High angular resolution spectral imaging of Seyfert galaxy cores will reveal structure interior to the starburst ring. The morphology and dynamics of the emission line regions will permit new insight into how gas is funneled into the core and the role possibly played by stellar bars in driving such gas flows. Measurement of stellar mass-to-light ratios, M/LK, will probe the star formation histories of the regions. Detailed comparison of spatially-resolved spectra with starburst models (e.g., Leitherer et al. 1999) will provide estimates of the starburst ages, masses, and star formation histories. These can then be compared to particular models for AGN fueling (e.g., Norman & Scoville 1988). The potential of AO corrected imaging of Seyfert galaxy cores in addressing these issues is beginning to be explored (Marco, Alloin, & Beuzit 1997; Chapman, Walker, & Morris 1999; Rouan et al. 1998; Marco & Alloin 1998, 2000).

Measurement of the central black hole masses in Seyfert galaxies is also highly desirable. Although these can be estimated using reverberation-mapping techniques for a few objects (e.g., Wandel, Peterson, & Malkan 1999), stellar velocity dispersions provide a more direct determination. Stellar velocity dispersions can be measured using the CO (2-0) absorption bandheads and interpreted in the same way as for normal galaxies (§4.1) to place limits on the enclosed mass and hence detect or constrain black hole masses. The analysis is complicated in the case of common Seyfert galaxies by the intense Seyfert core emission. Minimizing this contamination will depend on achieving high Strehl ratios in the telescope, ALTAIR, and NIFS. However, approximately 43% of nearby galaxies show a detectable level of nuclear activity (Ho, Filippenko, & Sargent 1997). These galaxies may contain either lower mass black holes or massive black holes that are currently accreting at well below their Eddington limit. These galaxies will be prime candidates for observation with NIFS. The core of the nearest Seyfert galaxy, Circinus, has a radius of < 1.5 pc at K (Maiolino et al. 1998), corresponding to < 0.08''. The stellar velocity dispersion within $\sim $ 40 pc ($\sim 2''$) of the nucleus is $\sim $ 75 km s-1, corresponding to a Gaussian FWHM of $\sim $ 180 km s-1. We require a velocity resolution of $\sim $ 100 km s-1 to confidently measure such velocity dispersions.

The AO requirements on Seyfert galaxy programs are less severe than for normal galaxies. Many Seyfert nuclei are bright enough and sufficiently compact to use as ALTAIR guide stars. A large number of nearby Seyfert galaxies in the Shapley-Ames catalog were checked for nearby stars. Approximately 20% have suitably bright guide stars within $\sim 30''$ of the nucleus. High Strehl ratios are needed to measure black hole masses, and are desirable when studying emission from NLR clouds.

Next Section: Gemini Core Science
Title/Abstract Page: Science with NIFS, Australia's
Previous Section: Guide Star Requirements
Contents Page: Volume 18, Number 1

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