This week at Astro 3303

Transcription

1 This week at Astro 3303 Lecture 13, Oct 11, 2017 Pick up PE#13 I will hand back HW#4 HW#5 is posted Today: AGN (final) part 3 The Galactic center: overview Young massive star clusters: e.g., The Arches Cluster Infrared observations of stars near Sgr A* Reading: Chapter 4 and 8.6/8.7 of textbook

2 Rest-frame à Hubble s law à Dis. Modulus à HW #4, Part III

3 HW #4, Part III

4 HW #4, Part III

5 Reprocessed Radiation AGN produce a lot of ionizing radiation <= Accretion Disk Radiation is intercepted by gas & dust and re-processed qdust Torus: IR radiation qgas: Emission lines qnarrow Lines => NLR qbroad Lines => BLR qx-ray fluorescence Sanders et al. 1989

6 X-ray Reflection and Fluorescence A SMBH is surrounded by an accretion disk. Suppose that X-rays are generated above the disk and illuminate it: l We observe some photons directly. l Others hit the accretion disk. l Some are reflected. l Some eject an inner shell electron from an atom to give fluorescent line emission. NGC 4945 direct fluorescence reflected Madejski et al. 2000

7 MCG : Kα Fe line X-ray spectroscopy in Seyferts has revealed highly broadened 6.4 kev iron Kα lines on the order of 10 4 km/s Greene et al. derived a mass of about 5 x 10 6 M sun

8 MCG : Kα Fe line The line is intrinsically narrow, but relativistically broadened to ~2 kev (~0.3c). The profile is skewed with an extended red wing due to gravitational redshift, and a prominent blue wing which is relativistically boosted due to the high orbital velocities of the disk. rotating disk profile Rapidly-rotating accretion disk Fabian et al. 2000

9 Reprocessed Radiation What is the origin of the BLR and NLR?

10 The Broad Line Region Is the BLR just simply a collection of gas clouds in the gravitational field of the SMBH, or a smoother filamentary structure with high velocity gradients?

11 BLR: Some Simple Inferences Temperature of gas is ~10 4 K: Thermal width ~10 km s 1 (<< line width) Density is high, by nebular standards (n e ³ 10 9 cm 3 ) (forbidden lines are suppressed) Efficient emitters, may be low mass Line widths FWHM ,000 km/sec Þ Gas moves supersonically

12 BLR size: Quasar/AGN variability Quasar: L ~ L Quasars are sometimes observed to vary on timescales of a few days to months A SMBH s variability is an indication of its size. intensity t 1 t 2 Time Ä Flux begins to increase when the light from the near side reaches the observer, at t 1 =d/c. Flux returns to quiescent level when the light from the far side arrives, at t 2 =(d+2r)/c So, the size of the emission region is 2r = (t 2 - t 1 )/c d 2r à PE#12

13 Broad-Line Flux and Profile Variability BLR emission-line fluxes vary with the continuum, but with a short time delay. Inferences: Gas is photo-ionized (since it responds to continuum variation with a time delay) Gas is optically thick (based on line-ratios, EW, line strengths) Line-emitting region is fairly small (variability).

14 Reverberation mapping NGC 5548, Peterson (2001) Measure time-lag Measuring the size of the BLR: Any variation in the ionizing continuum flux of the central compact source will cause a flux variation of the BLR emission lines in response. The light travel time delay between the continuum and line variations is proportional to the size r of the broad line region. r ~ c Dt Changes in the appearance and relative strength of different spectral lines can give insight into the nature of the SMBH. Mapping the time delay as a function of radial velocity/line width gives kinematic information on the SMBH mass. For Keplerian rotation, the virial theorem gives (w/ line velocity dispersion s v ): M BH = f x rs v2 / G where f~1 is a scale factor due to geometry and kinematics. Note: this method is independent of distance, and can probe regions as small as 1000 R S!

15 Reverberation Mapping: SMBH Mass Measurement The central mass is then given by: 5 æ ct öæ v rms ö M» ( M ) ç ç 3 1 (Wandel, Peterson & Malkan 1999) - è lt - day øè10 km s ø 2 b= 1/2 Different lines give you the same answer, even if the r BLR measured is different. logv FWHM = a + b log ct The masses derived by this method range from M BH = 10 7 M sun for Sy-1s (i.e., in the range of the LINER NGC 4258) to M BH = 10 9 M sun for QSOs (Peterson & Wandel 2000)

16 What is the BLR? Simple Cloud Model Crab Nebula First notions based on Galactic nebulae, especially the Crab system of clouds or filaments. Number of clouds N c of radius R c : Covering factor N c R c 2 Line luminosity N c R c 3 Combine these to find large number (N c > 10 8 ) of small (R c cm) clouds. Combine size and density (n H ~ cm -3 from lines), to get column density (N H ~ cm -2 ), compatible with X-ray absorption. Þ Total mass of line-emitting material: ~1 to few M sun.

17 Large Number of Clouds? If clouds emit at thermal width (10 km/s), then there must be a very large number of them to account for the lack of small-scale structure in line profiles. or: winds originating at acc. disk? (similar to accreting binaries/ysos) NGC 4151 Arav et al. (1998) Laor (2004)

18 Disk Winds: Evidence for Outflows in AGNs Clear blueward asymmetries in higher ionization lines in narrow-line Seyfert 1 galaxies Leighly (2001) Peaks of high ionization lines are blueshifted relative to systemic. Maximum blueshift increases with luminosity. Espey (1997)

19 Large Scale Molecular Outflows in Mrk 231 Mrk 231 QSO/ULIRG: CO J=1-0 broad (~700 km/s) CO line velocity wings narrow component: starburst disk P-Cygni profiles with blueshifted absorption in far-ir OH doublets Þ rapid, massive kpc-scale outflows outflow rate: 700 M sun yr -1 (~3.5x SFR) E kin ~ 1.2 x erg/s (~2.5% L bol,agn ) OH 119 µm Þ AGN feedback on host galaxy OH 79 µm Þ Quenching of star formation & black hole growth? [M BH -s v relation] Feruglio et al. 2010; Fischer et al. 2010

20 A Plausible Disk-Wind Concept BAL: Small opening angle Large optical depth Þ warm, highly ionized wind Þ deep, highest velocity blueshifted absorption outflow

21 BLR properties l Emission line widths up to 1000s km/s or even 10,000s km/s l Gas temperatures of several K (~10 km/s thermal cloud width) l Doppler broadening through bulk motion of the gas in the gravitational field l High velocities up to 0.3c imply distances of ~100 R s (v~r -1/2 ) l Only ~10% of continuum emission are absorbed by BLR l Volume filling factor is low, only 10-6 occupied by BLR clouds l Mass in BLR is only a few M sun l Broad lines are very smooth: either they are made of many clouds (10 9 with R~R sun ) or it is a coherent structure (wind?) l Suppression of forbidden lines indicates n>10 9 cm -3 l Size of BLR is few up to 100 light days across (reverberation)

22 The Narrow Line Region [OIII]/Ha Falcke et al. The [OIII]/Ha maps outline the areas of high excitation by a central source. The ionization cone suggests that the NLR is simply hot gas in the host galaxy illuminated by beamed radiation from the AGN through the opening angle of the dust-torus.

23 l Assumed clouds l Density cm -3 (permitted, semi-forbidden, and forbidden lines are present) l Large & small column densities l Size ~300 pc (typ. resolved in Sy) l Radial distribution l Confinement l Covering factor >0.02 l total mass ~ 10 6 M sun The Narrow Line Region l Evidence for asymmetric line profiles => outflows/inflows? l Spectrum: coronal lines in the IR Bound system? FWHM ~ km/s Small EW lines

24 Optical vs. Infrared View of the Galactic Center visible light 2MASS (2 micron all-sky) survey image made from star counts (not a direct image) total of 250 million stars in 2MASS

25 The Galactic Center The Galactic Center is heavily extincted in the optical (~28 mag in the V band). à need to observe in the infrared! GC: in Sagittarius At 2.2 µm: direct stellar emission from cooler stars (types K & M) that coexist in clusters with the hotter ones that heat the HII regions. Time-lapse imaging + spectroscopy allows reconstruction of orbital motions At 10 µm: emission from dust which is heated by higher energy (optical) photons emitted by stars and then re-emits in the IR. At 100 µm, emission due to cooler dust, more extended, heated by energetic photons from hot stars over 10 s of parsecs distant. 1 = 0.04 pc assuming a distance of 8 kpc to the Galactic Center.

26 Near-IR (Gemini) image of star cluster MIRLIN image of our Galaxy's center at 9, 13, and 21 microns. Cornell PhD student Ryan Lau 10 µm image of the Galactic Center: Giant stars and hot dust (Gemini: Abu)

27 Sgr A* Radio image (90cm) shows: HII regions Arcs that follow magnetic fields Supernova remnants The mini-spiral The radio point source, Sgr A*

28 Radio continuum Mini-Spiral Core source: inner 3 pc Sgr A Thermal emission from HII regions around young stars Sgr A* Sgr A*: compact nonthermal radio source Unusual time-variable 511 kev e - - e + annihilation radiation

29 The Galactic Center: X ray view

30 X-rays from Sgr complex The CXO image reveals complex structures around the GC, including several discrete clumps of emission. The red structure near the center is X-ray emission associated with Sgr A*, the compact non- thermal radio source. GC is very complex environment CXO image of 40x40 pc region Baganoff et al 2003 ApJ 591, 91

31 X-rays from Sgr A Sgr A West is believed to be an HII region. Sgr A East is believed to be a SNR. A compact X-ray source coincident with Sgr A* has been seen to flare by 3x over a period of 1 hour. CXO image of 20x20 pc region Baganoff et al 2003 ApJ 591, 91

32 The Galactic Center: Composite 1 = 2.4 pc Image ~ 60 pc Combination of X-ray (blue) 25 µm (green) and 20 cm (red) observations Both thermal and non-thermal components seen

33 The Galactic Center: Phenomena 1E à XRB/microquasar

34 Galactic Center: Near-Infrared Image credit: Andrea Ghez The stellar density in the Galactic Center is 300,000x that of the solar neighborhood. => need adaptive optics to resolve

35 NGS: natural guide star LGS: laser guide star (create artifical star through fluorescence of sodium (Na) atoms in atm. at ~100km)

36 Extreme Environment High density ~ 3 x 10 5 M pc -3 Strong tidal forces Clusters disrupted in < 10 to 50 Myr

37 Young stellar clusters in the Galactic Center Young, massive, high density The young stellar clusters in the GC are truly remarkable for their population of high mass stars Central cluster: located within the central pc. Contains over 30 massive stars; age ~ 3 to 5 Myr; Moves with Sgr A* to within 70 km/s Arches cluster: within the central 30 pc. Contains > 150 O stars within 0.6 pc radius; very high density; Age ~2.5 ±0.5 Myr (younger). Quintuplet cluster: also within 30 pc of center. > 30 massive stars; age ~ 3 to 5 Myr.

38 Case Study: The Arches Cluster HRD TOPCAT plot of Espinoza+09 data Densest known cluster Subject to strong tidal forces Extinction varies from * to * PE #13 How is the process of star formation different in such an environment (compared to solar neighborhood)? Does IMF vary?

39 Case Study: The Arches Cluster IMF: What is it? Near-IR CMDs High fraction of binary stars in clusters like Pleiades; what about Arches? Fewer disks around stars? => quick destruction of disks? Isochrone at 2.5Myr Contamination or reddened members? Extinction in the visual range: tens of magnitudes Þ Study in the IR! Espinoza, Selman & Melnick 2009 A&A 501, 563

40 Initial mass function: PE#13 & the Arches stars per mass interval per unit volume that are created: x(m) = Const M -(1+a) What is the Salpeter IMF? What if the slope flattens towards the center of the Arches cluster?

41 Is the IMF everywhere/time the same? In the solar neighborhood, we have the Salpeter IMF, a = 1.35 (Salpeter 1955)

42 Case Study: The Arches Cluster Stellar mass density within 2 pc diameter region of the Sun: 0.2 M sun pc -3 à 1,000,000 lower!

43 Sgr A*: the compact radio source Sgr A* is a compact radio source projected toward the Galactic center. Its motion can be measured with respect to background extragalactic radio sources. Motion: < 20 km/s. High resolution observations of the compact radio source show that is ~ 0.4 A.U. in size (37 => no dense star cluster!). While Sgr A* is a bright radio continuum source, it is a very dim infrared source. Position determined by Reid et al. (2003) using VLA/VLBA observations of SiO maser stars with accuracy of 10 mas Þ coincides with center of acceleration of stars to within 13 ± 10 mas.

44 Sgr A*: X-Ray source X-ray source coincident within 0.27 ± 0.18 with radio position and with dynamical center (Baganoff et al. 2003). X-ray source has two components: - steady state, stable within 10% over 4 years size ~ 1 - unresolved variable component; flux density rises by ~10x for 1 to a few hours once per day Overall, Sgr A* is a very weak AGN, with L bol ~10 36 erg/s Also: infrared variability:

45 Motions of Rapidly Orbiting Stars Near Sgr A* Observations now on-going for 15+ years Now include >20 stars with proper motions within 0.4 (0.016 pc) of Sgr A* 7 have orbital solutions (not just observed motions) Stars are on Keplerian orbits à the central mass is much larger than the mass of the stars Accelerations pinpoint the location of the responsible mass to within 1.5 mas based on proper motion measurements, stars move at typically 1400 km/s! SO-16 came within 45 AU ( pc = 600 R S at a velocity of km/s) Central dark mass of 4.1(±0.6)x10 6 M within a volume of < 45 A.U.» 600 R S (and improving!) à Supermassive Black Hole

46 The Schwarzschild Radius We can also write the equation in more convenient units as: R S = ~3 km per solar mass Schwarzschild radius: 1 solar mass: ~3 km 1 earth mass: ~9 mm The Schwarzschild radius of a 4 million solar mass black hole is ~ 0.08 A.U. ~ 18 solar radii ~ 40 light seconds

47 The Galactic Center: Near-Infrared Mosaic Andrea Ghez UCLA web site Mosaic of J,H,K (1.2 to 2.2µm) images of the Galactic Center region. blue= hot ; red= cool

48 Stellar Orbits Determining orbits: Proper Motions & Radial Velocities à Due to the high orbital velocities and small scales, the proper motions are high despite the large distance of 8.5 kpc S0-1, S0-2 and S0-16 orbit clockwise S0-4, S0-5, S0-19 and S0-20 orbit counterclockwise Orbits: simultaneous fit to orbit around central point mass.

49 The Milky Way s Supermassive Black Hole Star S0-2 Orbital period is T = 15.6 years Closest approach at 17 light hours(!) = 120 AU Speed reaches 5000 km/s Semimajor axis: 0.12 arcsec At 8 kpc, 1 arcsec = 0.04 pc tan (angle in radian) ~ angle (in radian) = r/8 kpc angle = 0.12 arcsec/ arcsec/rad = 5.8 x 10-7 radians r = 5.8 x 10-7 x 8 x 1000 x AU = 960 AU Using Kepler s 3 rd law: M BH = 4p 2 /G r 3 /T 2 ~ 4 x 10 6 M sun

50 The Next Generation: 30-m class Telescopes + Wide-Field AO

51 Consider If the Earth orbited around a SMBH of 4 million solar masses in a circular orbit of 1 A.U., what would the length of an Earth-year then be? P 2 = 1./4 million = 2.5 x 10-7 P =.0005 year X 365¼day/yr X 24hr/day = hrs Circumference = 2πa For Earth around Sun P = 1 30 km/s For Earth around SMBH =.0005 year Same distance traveled Around SMBH, Earth would travel 1/.0005 times faster That s 2000x30km/s = 60,000 km/s or 0.2c! The Schwarzschild radius of a 4 x 10 6 solar mass black hole ~ 0.08 A.U. ~ 18 solar radii ~ 0.7 light minutes ~ 40 light seconds 1 A.U. = light minutes = 499 light seconds

52 Galactic Center: Rotation curve Stars seen to orbit with speeds up to >10000 km/s. The stellar density is tremendous; strong tidal forces must be present. How does star formation occur there, and how is it different from star formation elsewhere? Limits on enclosed mass minimum mass density The dynamic center of the MW appears to be coincident with the compact, non-thermal radio source known as Sgr A*. The motions of IR stars see within the inner few pc implies the presence of a very compact, dark, massive object of 3.7(±0.02)x10 6 M sun à SMBH Schoedel et al. (2003)

53 Hypervelocity stars (HVS) Hills (1988) predicts stars ejected at ~1000 km/s from 3-body interaction with SMBH Yu & Tremaine (2003) predict that SgrA* ejects one HVS every ~10 5 years => 10 3 HVS within 100 kpc of MW. Brown et al. (2005) report discovery of first HVS: 3 M MS star with V GSR ~ +709 ± 12 km/s at 110 kpc. Roughly 20 unbound HVS now known Cannot be explained by binary disruption Further study of population can give clues to stellar population in GC Brown et al. (2005)