GeminiFocus July 2019 | Page 11

Figure 1. GMOS spectrum of the AGN in the low- mass spiral NGC 4395, showing the narrow [NII] and Hα lines superposed on the broad Hα emission used for the reverberation measurement (left). The narrow [SII] lines at longer wavelength were used as proxies for the central stellar dispersion (right). [Figure reproduced from Woo et al., Nature Astronomy, 2019, in press (arXiv 1905.00145).] tween variations in the intensity of the con- tinuum light from the AGN, which excites the gas within the BLR, and the line emission itself: R = c τ , where c is the speed of light. Be- cause lines of different ionization show dif- ferent delays, the same line should be used for determining both σ and τ . Typical AGNs powered by supermassive black holes of mil- lions of solar masses (M B ) have delay times measured from Balmer lines ranging from a few days to many months. A new study published in Nature Astronomy has measured the mass of the black hole as- sociated with one of the lowest luminosity AGNs known. The AGN resides within a nu- clear star cluster at the center of the nearby dwarf spiral NGC 4395, and the study was led by Jong-Hak Woo of Seoul National Uni- versity. Using spectroscopic data from the Gemini Multi-Object Spectrograph (GMOS) at Gemini North, Woo’s team measured a line-of-sight velocity dispersion of 426 kilo- meters per second (km/s) from the width of July 2019 the broad Hα line (Figure 1). Combined with a reverberation time delay of 83 minutes based on a combination of broad- and nar- row-band imaging collected at several small telescopes, the implied black hole mass is about 9,100 M B . Previous estimates ranged from 5 to 40 times higher, but were much more poorly constrained. The new result is securely within the realm of the elusive “intermediate-mass” black holes, which may be the seeds from which supermassive black holes grow. There are well established relations for mas- sive galaxies between central black hole mass and the properties of the stellar bulge; it is interesting to ask how NGC 4395, a pure disk galaxy without any bulge, fits into these. The new study estimated the central stellar velocity dispersion σ ★ from the width of the narrow [SII] emission line, finding σ ★  ≈ 18 km/s, consistent with a previous upper limit. Using this value, they place NGC 4395 on the diagram of M BH versus velocity disper- GeminiFocus 9