Largest SMBH

Author: Mr. Ryan Houghton, Astrophysics, University of Oxford, U.K.Collaborators: N. Thatte (Oxford, UK), R. Davies (Oxford, UK), M. Sarzi (Oxford, UK), J. Magorrian (Oxford, UK), J. Binney

(Oxford, UK), T. Statler (Ohio, USA), T. de Zeeuw (Leiden, NL).

Super Massive Black Holes (SMBHs) are the only viable candidates for the massive dark objects inferred at the centres of galaxies. Crucially, the BH mass appears to be strongly correlated to the properties of the host galaxy bulge (Magorrian et al. 1998; Ferrarese & Merritt 2000; Gebhardt et al. 2000; Marconi & Hunt 2003), which indicates that their mass accretion history is linked to the evolution of their hosts (de Zeeuw 2003)

The so called M – σ relation links the mass of the SMBH to the velocity dispersion of the host galaxy bulge (Ferrarese & Merritt 2000; Gebhardt et al. 2000). This relation is remarkably low in scatter (< 0.3 dex; Tremaine et al. 2002, hereafter T02) considering it relates quantities on vastly different scales (SMBH ~10-5pc vs. Bulges ~103pc) and it provides an important constraint on models of galaxy assembly (Haehnelt & Kauffmann 2000). However, the current best sample used to derive this relationship (T02) is limited and biased!

The Current Best Sample:

Our Targets:

Of the 31 galaxies used to determine the relation of T02 (shown as thumbnails along the left and right borders): 18 are elliptical, 9 are lenticular and 4 are spiral (only 2 of the ellipticals have ‘cores’): the distribution is strongly biased towards galaxies with cuspy surface brightness profiles amenable to HST spectroscopy. This restricts the sample to intermediate velocity dispersions and BH masses.

Analysis of NGC 1399The giant elliptical NGC 1399 was one of the first targets in this program and we completed 8h of AO assisted long-slit NIR observations with NACO (ESO VLT). After developing advanced data reduction techniques and applying extensive analysis, the latest spherical orbit superposition models of the data indicate that the nucleus harbors a (4 1) x 109 Msun black hole. The M – σ relation of T02 predicts a mass of 1.7 x 109 Msun.

However, the data also reveals non-spherical structure: counter rotation in the central 0.”6 and an elongation of the nucleus to the SE on a scale of 0.”2. Furthermore, the non-parametric LOSVDs show strong non-Gaussian features within the central 0.”2 which cannot be represented with the modified Gaussian LOSVD parameterization. These complexities are not included in our current spherical modeling.

Smallest SMBH

The sample is not evenly distributed in log(σ): it lacks galaxies at very low and very high σ, where the M – σrelation is defined.

NGC1399: Giant Elliptical E1

NGC2865: Giant Elliptical E3/4

NGC4486A: Dwarf Elliptical E2

NGC3705: Spiral SAB

Low mass bulges (e.g. in spirals) tend to be dusty which causes significant extinction at shorter wavelengths (the extinction at 2.2μm is only 10% of that at V). HST does not have an IR spectrograph and so is unable to probe the nuclei of such galaxies

Left: the M – σ plane of T02; morphological types are shown with different symbols; the bulge dispersion measurements for NGC1399, NGC 2865, NGC 3705 and NGC 4486A are shown in grey. Right: The central 30”x30” V-band HST-WFPC2 images of our targets, showing the AO reference stars. All images are exposure time divided and shown with the same greyscale cut.

The more massive ellipticals are more distant and have low surface brightness cores. Consequently, they are not suited to the 2.4m primary of HST and are under-represented.

Above Left: The results of the latest spherical orbit superposition model, constrained with AO assisted long-slit NIR spectroscopy from NACO. The 2 contours (separated by 1 ) are shown as a function of the mass-to-light ratio and central BH mass. The minimum in 2 occurs for a BH mass of (4 1) x 109 Msun. Above Middle: the parameters of the modified Gaussian line-of-sight velocity distribution (LOSVD) fits to the AO assisted long-slit NIR NACO data for NGC 1399. Previously published data is shown as blue triangles (Longo et al. 1994) and yellow squares (Graham et al. 1998). The PA of the slit was 5.06º so that the positive radius is approximately northward. Above Right: The LOSVDs for the central 0.”5 of NGC 1399 as derived from the NACO data. The non-parametric LOSVDs are shown in yellow (dot-dash line with error bars); the parameterised modified Gaussian LOSVDs are shown in black (solid line). Note the peculiar wings on the non-parametric LOSVDs at -0.”08, 0.”08 and 0.”14 from the nucleus indicating large high velocity components.

Above: NACO image of the nucleus of the galaxy NGC 1399. Isophote ellipses are over-plotted in white. Note the elongation of the nucleus to the SE. Each pixel represents 0.”027.Summary

AO assisted long-slit NIR spectroscopy together with a spherical orbit superposition model indicates that the giant elliptical galaxy NGC 1399 hold a (4 1) x 109 Msun black hole at it’s center, larger that predicted by the canonical relationship. However, non-spherical structure revealed by the same observations requires this estimate to be refined: AO assisted integral field spectroscopy together with more general non-spherical models would provide a robust black hole mass estimate for NGC 1399. In addition, it would be possible to resolve the extended nucleus and reveal its velocity structure and nature.

This work marks the beginning of a long-term effort to populate the sparse region of the M – σ plane with AO assisted NIR observations.

Large ground based telescopes with adaptive optics (AO) facilities and IR spectrographs can match spatial resolution on HST and probe the M – σ relation on larger mass scales. NACO (ESO VLT) is an AO assisted IR imager/spectrograph (1 – 5 μm) coupled to an 8m primary (diffraction limit of 0.057” at 2.2μm). A catalogue of targets suitable for AO assisted ground observations was assembled by trawling the HST image archive for galaxies with nearby foreground stars: we found 4 galaxies eligible for AO assisted NIR observations: NGC 1399, NGC 2865, NGC 3705 and NGC 4486A. Observations of NGC 3705 and NGC 4486A have begun while observations of NGC 1399 with NACO are complete: the analysis is shown below.