DOVE TROVARE GLI ARTICOLI: Preprint service (per articoli non ancora pubblicati, o conference proceedings):(o fate una ricerca su Google su Astrophysics preprints per trovare un mirror site) Articoli pubblicati: ADS astronomy and Astrophysics Abstract Service SIGRAV Graduate School in Contemporary Relativity and Gravitational Physics Laura Ferrarese Rutgers University Lecture 3: Gas Dynamics Lecture Outline Historical Perspective: The Milky Way (again!) Water Maser Disks: A detailed look at NGC 4258 Incidence of water masers in galactic nuclei HST studies of dust and gas disks A detailed look at NGC 4261 Biases and Systematics of the Dynamical Models Incidence of nuclear dust/gas Disks Historical Perspective Historically, the kinematics of ionized and neutral gas near (< 8pc) the galactic center presented the first indication for the existence of a central mass concentration (early review and references in Genzel & Townes 1985 ARA&A 25, 377). VLA 6cm image of the inner few pcs Historical Perspective Mass distribution from the 2 m stellar brightness profile Models with a 3.0 10 6 and 2.5 10 6 central mass concentration. Mass estimate from the stellar velocity dispersion Neutral circumstellar ring (1.7 < R < 9pc) - dominated predominantly by circular motion Ionized gas streamers(< 1.7) Historical Perspective Gas kinematics, however, has traditionally been dismissed in fear that forces other than gravity might push the gas around, and therefore that gas motion might not be a good tracer of mass: ..as usual it is not certain that gas velocities measure mass (Kormendy & Richstone 1995, ARA&A, 33, 581, referring to the maser disk in NGC no less!). Gas dynamical mass estimates started to draw serious attention with the discovery of regular, small nuclear disks of gas and dust in a significant fraction of early type galaxies (Jaffe et al. 1993, Nature, 364, 213; Ford et al. 1994; Ferrarese, Ford & Jaffe). A Very Special Case: NGC 4258 NGC 4258 was one of the 12 galaxies originally identified by Seyfert. Based on its spectral line profiles, NGC 4258 is a Seyfert 2 galaxy, i.e. the active nucleus is hidden, because of projection effects, by the surrounding dust torus. Palomar 0.9inch telescope, BVR composite image NGC 4258 Optical, radio and X-ray images of NGC 4258 NGC 4258 Water maser emission at 22 GHz was discovered in NGC 4258 by Claussen, Heiligman & Lo (1984) Historically, water maser emission was first detected in 1968 (Cheung et al. 1969, Nature, 221, 626) and subsequently identified in a large number of interstellar star forming regions and late-type stellar envelopes. It is believed to result from collisional excitation of warm (1000 K) interstellar gas (Neufeld & Melnick 1991, ApJ, 368, 215). However, the maser emission in NGC 4258 (as well as in the other 4 AGNs in which a detection had been made at the time) exceeds the luminosity of galactic sources by five orders of magnitudes. Based on estimates of the size of the emitting region, Clausen & Lo (1986) concluded that the emission did not originate from the superposition of galactic type masers powered by massive stars. Given that the excitation energy needed to produce the masers exceeded the IR luminosity of the galaxy by a factor 300, they concluded that the excitation was produced by the active nucleus, and suggested that the masing might arise from the obscuring torus which is the key ingredient in AGN unification schemes. NGC 4258 Nakai et al (1993) discovered high velocity H 2 0 maser emission at v~1000 km/s relative to the galaxys systemic velocity. Data taken with the Effelsberg 100m telescope; from Greenhill et al. 1995, A&A, 304, 21 The Importance of H 2 0 Masers Whats the relevance of H 2 O emission in the SBH context? At 1.35cm, water maser observations can be carried out at exceptional spatial and velocity resolution with the VLBI: = , v= 0.2 km/s VLBI spatially resolved observations of water maser emission in NGC 4258 have allowed to measure the mass of the central SBH with unparalleled precision measure the distance to the galaxy with unparalleled precision, thus providing a potentially very important test of the extragalactic distance scale. Myioshi et al. 1995,Nature, 373, 127 Properties of the H 2 0 Emission in NGC 4258 The high velocity feature trace a nearly perfect Keplerian rotation curve. This is naturally explained by assuming that the masers are located in a disk in Keplerian motion around a central mass. Myioshi et al. 1995,Nature, 373, pc 0.13 pc M ~ 3.7 10 7 M r h = 1 M pc -3 D=7.3 0.3 Mpc Properties of the H 2 0 Emission in NGC Features at the galaxys systemic velocity show a secular drift of 9 km/s/year, and a constant spatial secular frequency change of 280 km/s/mas, while the high velocity features remain essentially fixed in velocity. This is consistent with centripetal acceleration in a rotating disk (Greenhill et al. 1995, A&A 304, 21, Greenhill et al. 1995, ApJ, 440, 619) A Model for NGC 4258 Information about the velocity gradient among the systemic features, their acceleration in the line-of-sight velocity, and the existence of the high velocity features can be incorporated to give a comprehensive model for the disk kinematics. For the systemic clumps, the line of sight velocity varies linearly with projected distance from the center of the disk. Since v los =v(r)x/r, either the disk is in solid body rotation (i.e. v(r) r ), or all of the emission comes from essentially the same radius. From the spread in the v los vs. r plot: r / r = 2 v/v ~ 0.3. To Observer r x v(r) A Model for NGC 4258 The line-of-sight-velocity is simply equal to: where v(r) is the circular velocity at radius r of the annulus (equal to its measured angular size, times the distance D to the galaxy), and theta is the angular diameter between the maser cloud and the observer, given the distance D to the galaxy. The temporal change in the line-of-sight velocity of the systemic component is equal to the centripetal acceleration at the far edges of the disk: The angular change in the line-of-sight velocity of the systemic velocity component is This is a set of two equations with two unknowns (M,D). From Greenhill et al. 1995, ApJ, 440, 619 Properties of the H 2 0 Emission in NGC The preceding discussion assumes that the disk is flat and seen perfectly edge on. This assumption can also be relaxed because the spatial distribution of the maser disk is resolved: The declination spread of the systemic features is unresolved at 3 pc (Moran et al Proc. of the Nat. Ac. of Sci. 92, 11427) The declination of the high velocity features shows systematic trends that are antysimmetric with respect to the disk center, indicating that the disk is warped. A (More Complex) Model NGC The degree of warping can be constrained: a warped disk can be modeled using nine parameters, namely : 1-2. the (x,y) positions of the center of mass, 3.the galaxy systemic velocity, 4-5. the inclination as a function of radius (2 parameters) 6-8. the position angle as a function of radius 9.the central mass. The observables are: 1. relative position of the clouds in the sky, 2. line of sight velocity and 3. acceleration for each of the maser clouds. Therefore the problem is fully constrained. From top to bottom: Position angle changes with radius; Both position angle and inclination change with radius best fitting flat disk (can be excluded because it predicts a systemic velocity in significant disagreement with the observed value). From Herrnstein, Greenhill & Moran 1996, ApJ, 446, L17 Other SBH Detections from H 2 O Masers Circinus (Greenhill et al. 2003, astro-ph/ ): M BH =(1.7 0.3) 10 6 M The edge-on disk extends from 0.1 to 0.4 pc. The rotation curve is nearly Keplerian, although the disk is probably fairly massive (up to 25% the central mass) and therefore self-gravity is not negligible. A second population of masers traces a wide angle outflow up to 1pc from the central engine. Other SBH Detections from H 2 O Masers NGC 1068 (Greenhill et al. 1996, ApJ, 472, L21): M BH ~10 7 M The rotation curve is sub-Keplerian, the disk might be self-gravitating and there might be a significant turbulent component. Additional Detections (no VLBI) Mrk 1419 (LINER) (right, Henkel et al. 2002, ApJ, 394, L23) NGC 3079 (top, Trotter et al. 1998, ApJ, 495, 740) The velocities of the masers are consistent with their lying in the inner parsec of a molecular disk rotating in the same sense as the rest of the galaxy. However, the velocity field has a significant non-rotational component, which may indicate supersonic turbulence. NGC4945 (right, Greenhill et al. 1997, ApJ, 481, L23) Water Maser Surveys How common are water masers? Braatz, Wilson & Henke 1996, ApJS, 106, 51 354 galaxies, including a distance and magnitude limited sample of Seyfert and LINER galaxies with cz< 7000 km /s, plus some active galaxies, including radio galaxies, at higher redshift. Detection rate is 7% among 216 Seyfert 2 nuclei and LINERs, with no masers occurring in Seyfert 1 nuclei (Braatz, Wilson, & Henkel 1997, ApJS, 110, 321). Greenhill et al. 1997, ApJ, 486, L15 26 AGNs observed with the 70m antenna of the NASA Deep Space Network. One detection (NGC3735), with emission at systemic velocity only (4% detection efficiency). Greenhill et al. 2002, ApJ, 565, 836 131 AGNs observed at the Parkes Observatory. One detection, with emission at systemic velocity only (1% detection efficiency). Water Maser Surveys Greenhill et al. 2003, ApJ, 582, L11: survey of 160 nearby (cz< 8100 km/s) AGNs with the 70m antenna of the NASA Deep Space Network in Australia. Larger sensitivity and wider wavelength coverage than previous surveys. 7 new sources detected (4% detection rate), with two sources exhibiting high velocity masers (figure at right). Besides the fact that water maser emission is not detected in Seyfert 1 galaxies, no strong correlations have yet been found between maser emission and the global properties of the host galaxies, although where X-ray measurements are available, all known H 2 O masers lie in galaxies with large X-ray obscuring columns, cm -2 (Braatz et al. 1997, ApJS, 110, 321). A Complete Census of H20 Maser Detections GALAXYREFERENCEAGN TYPEDISK?VLBI? M51Hagiwara et al. 2001Seyfert 2perhaps no NGC253Nakai et al. 1995Starburstno NGC1052Braatz et al. 1996LINERno NGC 1068Greenhill et al. 1996Seyfert 2yesyes, disk is self gravitating. NGC1386Braatz et al. 1996Seyfert 2no NGC2639Braatz et al. 1996Seyfert 2no NGC2824Greenhill et al. 2003?no NGC2979Greenhill et al. 2003Seyfert 2no NGC 3079Trotter et al. 1998Seyfert 2no NGC3735Greenhill et al. 1997Seyfert 2 no NGC4258Greenhill et al. 1995Seyfert 2yesyes, best case for a SBH NGC4945Greenhill et al Seyfert 2yesno NGC5347Braatz et al. 1996Seyfert 2no NGC5506Braatz et al. 1996Seyfert 2no NGC5643Greenhill et al. 2003Seyfert 2no NGC6300Greenhill et al. 2003Seyfert 2no NGC6929Greenhill et al. 2003Seyfert 2yesno IC1481Braatz et al. 1996LINERno IC2560Braatz et al. 1996Seyfert 2no Mrk1Braatz et al. 1996Seyfert 2no Mrk1210Braatz et al Seyfert 2no Mrk1419Henkel et al. 2002Seyfert 2yesno CircinusGreenhill et al. 2003Seyfert 2yesyes, good SBH mass estimate ESO269+G012Greenhill et al. 2003Seyfert 2yes no ESO103-G35Braatz et al Seyfert 2no IRASF Greenhill et al. 2003Seyfert 2no IRASF Greenhill et al. 2002Seyfert 2no Larger Scale Gas/Dust Disks A small (10 2 pc), nuclear dust/gas disk was first discovered in the E2 galaxy NGC 4261, using the Hubble Space Telescope (Jaffe et al. 1993, Nature, 364, 213) Why are the disks intriguing? They are very regular, suggesting a simple dynamical structure. They are very thin, suggesting that the kinematics of the dust and gas are dominated by rotation. They contain ionized gas, which produces easily detectable emission lines, which can be used to study the disks kinematics They are always found in low-luminosity AGNs (radio galaxies and LINERS). In all cases, the minor axis of the disk is roughly aligned with the radio jets, suggesting a causal connection between the disks and the central engines. The origin and dynamical evolution of the disks are not known, but hold clues to the evolution of their host galaxies. Disks and Radio Jets Emission Lines from the Disk The line profiles are symmetric, excluding a one-direction outflow The largest velocities are measured along the major axis of the disk, excluding a bi- directional outflow (which would produce the largest velocities along the minor axis, unless the outflow is misaligned with the radio structure) The forbidden lines are broad, implying that the lines are broadened by rotation. NGC4261 (Ferrarese, Ford & Jaffe 1996) Gas Motion in the M87 Nucleus From Macchetto et al. 1997, ApJ, 489, 579 Gas Motion in the M84 Nucleus Analysis of Dust Disk Kinematics Procedure: 1) given the observed surface brightness profile, build an axisymmetric mass model for the stellar population, under the assumption of a (constant) mass to light ratio. Dust obscuration needs to be taken into account. 2) Construct the central potential, as the sum of the stellar potential, disk potential and the potential of a central point mass. 3) Derive the circular velocity corresponding to the potential. Notice that this step is much simplified compared to the case in which stellar kinematics is involved: the assumption here is that the system under study is 2-dimensional and dominated by rotation. 4) Project the circular velocity for a grid of disk inclination and position angles. 5) Compare to the observables, and iterate until the potential and geometrical parameters of the disk than minimizes the 2 of the fit are found. Analysis of Gas Disk Kinematics Brightness profile: the brightness profile can be deprojected uniquely (once an inclination angle is assumed for the galaxy) to give a luminosity density and, under the assumption of a stellar mass-to-light ratio, a stellar mass density profile. A minor complication here is that in the (critical) central region the brightness profile is affected by dust obscuration, also the dust itself might add some mass to the system. The dust obscuration, and disk mass can be calculated if continuum images in more than one band are available. The extinction law, and an average gas to dust mass ratio are assumed (it is also assumed that the dust lies in the galaxys midplane). NGC4261 (Ferrarese, Ford & Jaffe 1996) Analysis of Gas Disk Kinematics The observed (projected) velocity is a function of location within the disk, and of the inclination angle of the disk relative to the line of sight. We can also allow for the possibility that the kinematical axis is not aligned with the major axis of the large scale dust disk. i r Kinematical Axis Dust Disk M(r) is the total mass within radius r, including: 1) a central mass M 2) the stellar mass (M/L) (r, , )drd d 3) the disk mass (known from the optical depth analysis) Unknowns are: I. the central mass M II. the stellar mass to light ratio M/L III. the kinematical position angle IV. the disk inclination angle i Analysis of Gas Disk Kinematics: NGC 4261 The total mass to light ratio within 0.1 arcsec is 2100 M /L Analysis of Gas Disk Kinematics: NGC 4261 Dust Disk Stellar Isophotes Inner Disk (from the dynamical models) Gas Disks: Potential Problems There are instrumental effects which need to be accounted for in the preceding analysis, or biases can arise. In particular, smearing due to the finite width of the slit, and PSF blurring can be important (Maciejewski & Binney 2001) but, thankfully, easily quantifiable. There are, however, several other issues which are difficult to quantify given the quality of the available data. Is the disk structure really as simple as it appears? Probably not! In particular, we need to account for: Presence of a significant intrinsic velocity dispersion in all of the disks, which may not be gravitational in nature (Harms et al. 1994, Ferrarese, Ford & Jaffe 1996, Ferrarese & Ford 1999, Cappellari et al. 2002, Verdoes Kleijn et al. 2002, etc..) Presence of warps in the disk (Ferrarese, Ford & Jaffe 1996, Ferrarese & Ford 1999, Cappellari et al. 2002). Stellar and gas dynamical estimates of M BH have been carried out in only one galaxy, IC1459 (Verdoes Kleijn et al. 2000, Cappellari et al. 2002) M BH (gas) = (0.4 1.0) 10 9 M (depending on the assumptions made for the gas velocity field) M BH (stars) = (4.0 6.0) 10 9 M (using 2I axisymmetric modeling of ground based data) M BH (stars) = (2.6 1.1) 10 9 M (using 3I modeling of HST/STIS data with N 0 /N c =2.0) Incidence of Dust Disks How common are dust disks? Van Dokkum & Franx (1995, AJ, 110, 2027): 64 E-type galaxies from HST archive Incidence of dust 49%Incidence of dust disks 13% Rest et al. (2001, AJ, 121, 2431): 67 E-type galaxies drawn from a volume a magnitude limited sample Incidence of dust 43%Incidence of dust disks 15% Laine et al. (2003, AJ, 125, 428): 81 BCGs from HST snapshot program Incidence of dust 38%Incidence of dust disks 14% A Census of SBH Detection From Gas Disks GalaxyType DistanceM BH + - Reference (Mpc) (10 8 solar masses) N4261E Ferrarese et al. 1996, ApJ, 470, 444 N4342S Cretton & v.d. Bosch 1999, ApJ, 514, 704 N4374 E Bower et al. 1998, ApJ, 492, L111 N4486 E0pec Macchetto et al. 1997, ApJ, 489, 579 N6251 E Ferrarese & Ford 1999, ApJ, 515, 58 N7052 E v.d. Marel & v.d. Bosch 1998, AJ, 116, 2220 M81 SA(s)ab Devereux et al. 2003, AJ, 125, 1226 N2787 SB(r) Sarzi et al. 2001, ApJ, 550, 65 N3245 SB(s)b Barth et al. 2001, ApJ, 555, 685 N5128 S0pec Marconi et al. 2001, ApJ, 549, 915 CygA E Tadhunter et al. 2003, astro-ph/ Points to Bring Home Gas dynamics present a powerful alternative to stellar dynamical studies, at least in low luminosity AGNs residing in early type galaxies (optical nuclear dust disks) and Seyfert 2 galaxies (water maser disks). About 15% of all early type galaxies host nuclear dust disks, while perhaps 4% to 7% of Seyfert 2 (and some LINERS) host water maser disks. Gas dynamics is subject to systematic biases which are completely independent from those afflicting stellar dynamical studies (or, as we will see, reverberation mapping studies). Comparing mass estimates for the same galaxy using different methods can yield useful insights onto the nature of such systematics. Gas dynamics and stellar dynamics are somewhat complementary. For instance, gas dynamics allow to probe large, spherical, pressure supported ellipticals, or late type spirals and AGNs which are problematic for stellar dynamical studies. The study of nuclear dust disks is interesting beyond the SBH mass issue. The disks can tell us about the history of their host galaxies and the feeding habits of the central monster. Suggested Readings Water masers (review, although a little dated) Moran et al. 1999, in the Journal of Astronomy and Astrophysics (India), proceedings of the Meeting on the Physics of Black Holes, astro-ph/ , Nuclear Dust Disks: M87, the Saga Continues Ford et al. 1994, ApJ, 435, L27 Harms et al. 1994, ApJ, 435, L35 Macchetto et al. 1997, ApJ, 489, 579