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On the M-Σ Relationship and SMBH Mass Estimates of Selected Nearby Galaxies

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On the M-Σ Relationship and SMBH Mass Estimates of Selected Nearby Galaxies International Journal of Astronomy and Astrophysics, 2013, 3, 1-9 http://dx.doi.org/10.4236/ijaa.2013.33A001 Published Online July 2013 (http://www.scirp.org/journal/ijaa) On the M-σ Relationship and SMBH Mass Estimates of Selected Nearby Galaxies Alper K. Ateş, Can Battal Kılınç, Cafer İbanoğlu Astronomy and Space Sciences Department, Ege University, İzmir, Turkey Email: [email protected] Received March 11, 2013; revised April 13, 2013; accepted April 20, 2013 Copyright © 2013 Alper K. Ateş et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ABSTRACT Super massive black holes are believed to influence galactic evolution and dynamics. A histogram of SMBH masses for different redshift regimes may reveal clues on how the SMBH evolve in time. A prominent method for SMBH mass estimation is based on the linear correlation between the bulge velocity dispersion and the SMBH mass. Known as M-σ relationship, this method is known to provide reasonable but not very accurate mass estimates due to considerable scat- ter in data. In order to increase the precision, we surveyed the literature and gathered SMBH and velocity dispersion data for low redshift (z < 0.02) spiral galaxies. We report the M-σ relationship for low redshift spiral galaxies as, M BH log 8.10 0.16 2.67 0.66 log 1 M Sun 200 km s By using this refined M-σ relationship we measured 32 SMBH masses and determined upper and lower mass bounda- ries and the mass histogram for spiral galaxies in a narrow redshift regime (0.016 < z < 0.017). The spectroscopic data are obtained from The SLOAN Digital Survey and The National Observatory of Turkey (TUG). The targets are selected within a low redshift range for discernible [OIII] lines. TUG observations are carried out on the RTT150 1.5 m tele- scope using TUG Faint Object Spectrographic Camera and the SLOAN data are obtained from the 7th data release of the survey. We measured the bandwidths of narrow [OIII] lines, which are shown to be indicative in estimating stellar bulge velocity dispersion and estimated the central black hole masses from the refined version of the empirical M-σ relationship. The estimated masses vary between 9.51 × 106 - 2.36 × 108 solar masses. Keywords: Galaxies; Active Galaxies; AGN; SMBH; Stellar Velocity Dispersion; Bulge Velocity Dispersion 1. Introduction gine” residing at the host galaxy nucleus. The source of the energy for the engine is directly attributed to gravita- In the past decades existence of extraordinary compact tional attraction of a central object with a mass in the objects known as super massive black hole (SMBH) at 6 the center of spiral and elliptical galaxies have been con- order of 10 or more solar masses. The massive central firmed by numerous observational studies, most notably objects have no optical signal and thus proven to be in the Milky Way (Ghez, et al., 1996 [1]) and in M31 SMBH. Therefore the active galactic nuclei have been (Dressler and Richstone, 1988 [2]; Kormendy, 1988 [3]). thought to be a SMBH surrounded by an accretion disk. The existence of SMBH has first been suspected in active The disk is enveloped by a dust torus. The materiel inside galaxies. In many cases these galaxies had star-like cores the torus displays a wide emission band due to high ve- emitting up to 1046 ergs/sec (in the case of luminous locity of the gas surrounding the SMBH, which is known Seyfert galaxies) and displaying significant optical varia- as a Broad Line Region (BLR). Outside the torus the gas tions pointing out fluctuations in generations of vast orbits slowly, displaying a narrow emission line (NLR). amounts of energy. The physics of the active galactic Compression of the gas surrounding SMBH triggers en- nuclei has been debated over the years; creative scenarios ergy generation in a way far more efficient than stars. such as synchronized pulsations of supergiant stars have SMBH turns out to be ubiquitous. Many normal gal- been suggested and fell short of explaining this complex axies are observed to have a SMBH. It is presently be- structure. The most plausible explanation is a single “en- lieved that active or not all galaxies have SMBH at their Copyright © 2013 SciRes. IJAA 2 A. K. ATEŞ ET AL. centers. The difference between an active galaxy and a 2000. Based on approximately 50 BH masses obtained normal one is either the existence or lack of fuel; SMBH from various galaxy types Gültekin et al. (2009) [16] re- without accretion disks is believed to be dormant en- ports the current best fit as gines. So far estimated SMBH masses are in the range of 6 9 log M 1.4 × 10 (Greenhill et al., 1997 [4]) - 21 × 10 (McCon- BH nell et al., 2012 [5]) solar masses. (1) 8.12 0.08 4.24 0.41 log It is believed that the AGN and the host galaxy are 1 200 km s closely related; entire system shares an evolutionary his- tory (Rees, 1984 [6]; Dressler, 1989 [7]; Magorrian, et al. This linear relationship permits estimation of the 1998 [8], Richstone, 1998 [9]). In the last decade, it has SMBH masses from a single epoch spectroscopic data. been observationally proven that several properties in- cluding the velocity dispersion in the bulge region are 2.2. Reliability of the M-σ Relation: Fine Tuning directly proportional with the SMBH mass (Ferrarese and of the Relationship Merritt, 2000 [10]; Gebhardt et al., 2000 [11]; Häring Despite its promise, M-σ relation is not a first order mass and Rix, 2004 [12]). Such an influence can only be at- estimation method. Two sources of uncertainty can play tributed to a co-evolution of the SMBH and the host gal- hampering roles in the precision of the mass measure- axy. In order to attain a better understanding of this phe- ments. Equation (1) expresses that there is considerable nomenon, SMBH masses from all kinds of galaxies need to be known. scatter in M-σ plot. It has also been reported that there In order to measure SMBH masses various methods are some potential sources of error in velocity measure- based on dynamic studies of gas and stars have been de- ments such as line asymmetries, blending or stellar tem- veloped. In particular, “reverberation mapping” and kine- perature mismatch (Nelson et al., 2004) [17]. matic studies are known to provide the most reliable We also believe that the M-σ relation can be improved mass measurements (Kaspi et al., 2000 [13]). Studies us- by taking different galaxy morphologies into account. ing these methods enabled a census of well-known SMBH For instance, the linear relation given in Equation (1) are masses, which provide a base for further treatments of obtained from a wide array of galaxies: 12 ellipticals, 2 photometric and spectroscopic data. Various features such probable ellipticals, 13 barred spirals, 12 spirals, an ir- as line intensities, Doppler broadening of the prominent regular and some cross-type galaxies. Every one of these lines and line strength ratios of the host galaxies’ spectra types must have followed a different evolutionary track have been investigated for potential and mostly success- and end-up with varying morphologies. SMBH formation ful empirical relationships towards accurate measurement and growth histories must be linked to host galaxy prop- of the SMBH masses. erties including morphological differences and the host galaxy bulge type. In order to increase the precision of 2. M-σ Relation the mass estimation, new fits based on galaxy types would be worth enthused. For instance, in an investiga- 2.1. M-σ Relation and Its Background tion of the scatter in M-σ relationship by Gültekin (2009) All galaxies display composite spectra that can be used in [18], a slightly different fit for elliptical galaxies was estimating SMBH masses. Since the galactic light is a observed. It is also important to take into account cosmic conglomerate of stars and luminous gas, the spectrum distance and time scales. Since redshift is a measure of will inescapably show all ingredients superimposed onto distance and time, the M-σ relationship has to be ob- each other. Some of the spectral lines are more promi- tained for objects that have close redshift values thus nent and easily discernible. One of the most prominent belonging to the same epoch. features in a galactic spectrum is the [OIII] emission line To understand potential variations for the M-σ rela- (λ[OIII]rest= 5007 Å). The Doppler broadening of this line tionship for different morphologies, we took into account shows velocity dispersion (σ) in the bulge region of the two different types: elliptic and spiral galaxies. We sur- galaxy (Gebhardt et al., 2000 [11]; Nelson, 2000 [14]; veyed the literature for the known masses and bulge ve- Boroson, 2003 [15]). A logarithmic plot of known SMBH locities by paying attention to the following points: masses versus σ reveals a linear relation known as an All masses are measured by primary methods such as M-σ relationship (Ferrarese and Merritt 2000 [10]). Ac- reverberation mapping, stellar dynamics and gas dy- cording to the M-σ relationship the velocity of the gas in namics. motion in the bulge region is directly proportional to the All objects have low redshifts (z < 0.1); this ensures mass of the SMBH residing at the galactic center. As that they belong to the same epoch. It is also easy to more mass measurements have became available the M-σ identify morphological type and orientation of nearby relationship is been refined since its first conception in galaxies.
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