Nonaxisymmetric and Compact Structures in the Milky Way

Nonaxisymmetric and Compact Structures in the Milky Way 27 March 2018 University of Rochester

Nonaxisymmetric and compact structures in the Milky Way Spiral structure in the Galaxy The 3-5 kpc molecular ring Noncircular motions and the Galaxy s bar The Galactic center (r < 3 pc): Keplerian motion around a black hole Reading: Kutner Ch. 16.5 16.6, Ryden Ch. 19, and Shu Ch. 12 NGC 3351, a spiral galaxy resembling the shape of the Milky Way. Image from Adam Block, Mt. Lemmon Sky Center, U. Az. 27 March 2018 (UR) Astronomy 142 Spring 2018 2 / 36

Structure of the Galactic ISM Sun Last time: using radial velocity measurements to derive the Galactic rotation curve, v(r) = v r,max + r Ω(r ) sin l Once the rotation curve is known, the positions (subject to distance ambiguity) can be determined: v sin l r = r v r + v sin l l θ θ + l vr r [Ω(r) Ω(r )] v t θ + l 27 March 2018 (UR) Astronomy 142 Spring 2018 3 / 36

Molecular & atomic line surveys of the Galaxy Molecular and atomic clouds are our main probes of the global structure of the Milky Way. HI and CO emission are bright and negligibly extinguished, so they offer a view of the whole Galaxy. As previously discussed, the clouds have small internal random velocities, so they probe the disk s rotation a bit more clearly than the stars. However, such studies are supplemented by IR stellar surveys to measure stellar densities in regions like the bulge and spiral-arm tangents. Ultimately, Galaxy-wide IR surveys of stellar radial velocity and parallax are the best tools. The Gaia mission is providing such data for the brightest billion stars in the Milky Way (Lindegren et al. 2016). 27 March 2018 (UR) Astronomy 142 Spring 2018 4 / 36

Molecular cloud complexes in the Galactic plane Velocity-integrated CO map of the Milky Way (top) and sampling region (bottom). From Dame et al. (2001). 27 March 2018 (UR) Astronomy 142 Spring 2018 5 / 36

Molecular cloud complexes in the Galactic plane Longitude-velocity map of CO emission (top) and survey mask (bottom) (Dame et al. 2001). 27 March 2018 (UR) Astronomy 142 Spring 2018 6 / 36

Molecular cloud complexes in the Galactic plane A striking feature of these surveys is the prominence in the inner Galaxy of systematic noncircular motions that violate the trends of prograde circular orbits, and non-axisymmetric spiral structure in the outer Galaxy. 27 March 2018 (UR) Astronomy 142 Spring 2018 7 / 36

Understanding the noncircular motions Consider a few simple situations and the resulting v r (l). Uniformly rotating ring observer v r l l 27 March 2018 (UR) Astronomy 142 Spring 2018 8 / 36

Understanding the noncircular motions Nonrotating, expanding ring observer v r l l 27 March 2018 (UR) Astronomy 142 Spring 2018 9 / 36

Understanding the noncircular motions Rotating, expanding ring observer v r l l 27 March 2018 (UR) Astronomy 142 Spring 2018 10 / 36

Understanding the noncircular motions Bar structure at Galactic center: The noncircular motions are best described as the superposition of a set of expanding/contracting orbits, similar to those shown. The origin of the orbits is best described by a bar-shaped inner stellar distribution. This would explain the 3-5 kpc molecular ring as a resonant orbit ( Lindblad resonance ) in the rotating barred gravitational potential (Weiner & Sellwood 1999). 27 March 2018 (UR) Astronomy 142 Spring 2018 11 / 36

Spiral structure in molecular clouds Face-on view of the first Galactic quadrant: Spiral arms, here delineated in molecular cloud complexes (star formation regions) Molecular ring at r = 3 5 kpc Not much gas within the ring (except closer to center than shown) Spiral arms trail Galactic rotation. Image from MW Galactic Ring Survey (BU). 27 March 2018 (UR) Astronomy 142 Spring 2018 12 / 36

Why would a bar do that? A bar turns out to explain both the noncircular motion of a ring and the spiral structure in the disk. Suppose the mass distribution and gravitational potential of the Galaxy were axisymmetric. Then the orbits of clouds and stars about the Galactic center would be circular. 27 March 2018 (UR) Astronomy 142 Spring 2018 13 / 36

Why would a bar do that? Now suppose the gravitational potential suffered an axisymmetric perturbation like the exaggerated bar-shaped one shown. The stars and clouds would suffer a perturbation in orbital speed at the ends of the bar. As there is additional mass closer to the orbit, the speed increases at the ends according to GM(r) r 2 = v2 r 27 March 2018 (UR) Astronomy 142 Spring 2018 14 / 36

Why would a bar do that? The disk rotates differentially. The bar rotates like a solid body (this is possible), as does the innermost parts of the Galactic stellar distribution. The relationship between orbital periods relative to the bar s orbital period varies with radius within the disk. 27 March 2018 (UR) Astronomy 142 Spring 2018 15 / 36

Why would a bar do that? For orbits with periods that are integer ratios to the bar s rotation period (e.g., 2:1, 4:3, etc.) the perturbations are resonant. For resonant orbits, large deviations from circularity can accumulate. This appears both in coordinate space and in l-v diagrams as noncircular motion. 27 March 2018 (UR) Astronomy 142 Spring 2018 16 / 36

Why does this appear in the distribution of clouds? The arms are due to spiral density waves (Lin & Shu 1964). By analogy, consider a road crew painting line stripes on a highway. They move along at 10 mph. A traffic jam forms behind them and moves along with them. Cars before and after the jam move at 65 mph and are much further apart than in the jam. Cars enter the jam from behind, slow as they move through it, and resume speed as they leave. From a helicopter, it appears that a dense concentration of cars moves along with the road work, though it is composed of different cars at different times. Normal traffic speed normal stellar orbit speed Road crew speed spiral-wave pattern speed Traffic jam density wave (spiral arm) 27 March 2018 (UR) Astronomy 142 Spring 2018 17 / 36

Spiral density waves What plays the role of the road crew in a spiral galaxy disk? Gravity from the rotating non-axisymmetric part of the stellar distribution: Force on orbiting objects larger than average when the ends of the bar make their closest approach; smaller than average between. Other things being equal, this alternately speeds up and slows down the orbital speeds. 27 March 2018 (UR) Astronomy 142 Spring 2018 18 / 36

Can we see the bar? Yes. It is not easy, as the bar indicated by gas motions extends only about 3-4 kpc from the center and is thus heavily extinguished. But at long near-ir wavelengths (3-5 µm), star counts reveal the near end of the bar. Right: power-law exponent of star counts as a function of flux density, plotted vs. magnitude and galactic longitude. From Benjamin et al. (2005). 27 March 2018 (UR) Astronomy 142 Spring 2018 19 / 36

Milky Way structure after measurements with Spitzer Milky Way viewed from Galactic North Pole: Barred spiral with two dominant arms: Scutum-Centaurus and Perseus Molecular ring is superposition of these and the minor Norma/Sagittarius features The 3-kpc expanding arms represent flows closer to the bar. Image from Hurt 2008. 27 March 2018 (UR) Astronomy 142 Spring 2018 20 / 36

The center of the Milky Way: Sagittarius A West The dynamical center of the Galaxy is heavily extinguished. It cannot be seen at visible through longer X-ray wavelengths. It is bright in IR and radio and hard (short-wavelength) X-rays, which are transmitted through the dust. It was the first extraterrestrial object seen in radio by Karl Jansky in 1933. Within the central 3 pc we find a dense cluster of stars, a bright compact radio source, and a swirl of gas clouds. Small bright radio source is called Sagittarius A*, or Sgr A*. Sgr A* lies precisely at the center of the Galaxy (center of rotation): (α, δ) J2000 = 17:45:40.0409, -29:00:28.118. 27 March 2018 (UR) Astronomy 142 Spring 2018 21 / 36

Spectrum of Sgr A* Measured brightness of Sgr A* at radio, IR, and X-ray wavelengths (Melia & Falcke 2001). Dust in ISM hides Sgr A* between near-ir and soft X-ray bands. 27 March 2018 (UR) Astronomy 142 Spring 2018 22 / 36

Central pc of Sgr A West in X-rays Sgr A* is the brightest starlike object in the center of this image taken with the Chandra X-ray observatory (CXC) (Baganoff et al. 2003). 27 March 2018 (UR) Astronomy 142 Spring 2018 23 / 36

Central pc of Sgr A West in Near Infrared Sgr A* does not appear in this picture due to the fact that it is drowned out by light from all of the neighboring stars (Genzel et al. 2003). Color code: blue: 1.6 µm green: 2.2 µm red: 3.8 µm The central square shows the area displayed on the following slide. 27 March 2018 (UR) Astronomy 142 Spring 2018 24 / 36

Central 30 light days of Sgr A West With adaptive optics, it is possible to resolve Sgr A* itself at IR wavelengths. Most of the IR emission comes in short flares. See Genzel et al. 2003, Ghez et al. 2004 and this one-minute zoom-in created at the Max Planck Institute. 27 March 2018 (UR) Astronomy 142 Spring 2018 25 / 36

Orbital motion & the center of the Milky Way Over the course of the past 40 years, astronomers have measured velocities related to orbital motion about the center for many objects that lie within 1 pc of the Galactic center: radial velocities of gas clouds radial velocities of stars proper motions of stars The orbits do not lie in the plane of the rest of the Galaxy, but that does not matter; these are test particles we can use to determine the gravitational potential in the Galactic center. 27 March 2018 (UR) Astronomy 142 Spring 2018 26 / 36

Gas clouds around Sgr A* At radio wavelengths most of the bright objects near Sgr A* are gas clouds in orbit. Top right: false-color radio image of the region around Sgr A*, which appears as the white dot in the center. The swirls are streamers and clouds of ionized gas in orbit around the Galactic center. Bottom right: a color code of the radial velocity of the ionized gas. Red is receding at 200 km/s and blue is approaching at 200 km/s. From Roberts & Goss (1993). 27 March 2018 (UR) Astronomy 142 Spring 2018 27 / 36

Orbital speeds near Sgr A* The closer to Sgr A*, the larger the orbital speeds. The figure shows radial velocities for ionized gas clouds in Sgr A West measured in the IR. Some clouds are found in the IR data that do not appear in radio images (Genzel et al. 1994). 27 March 2018 (UR) Astronomy 142 Spring 2018 28 / 36

Orbital speeds near Sgr A* The brightest stars in the central MW seen at IR over the course of ten years: High speeds (> 5000 km/s) and close approach to Sgr A* by some stars. One has an orbital period of 15.2 yr and passes within 152 AU of the central black hole. From UCLA GC group. 27 March 2018 (UR) Astronomy 142 Spring 2018 29 / 36

Stellar motions near Sgr A* Orbital solutions for stars next to Sgr A*, from the MPE Galactic Center group. 27 March 2018 (UR) Astronomy 142 Spring 2018 30 / 36

Orbital motions & the center of the Milky Way Results of radio and IR observations: The stellar and gas-cloud Doppler shifts get larger the closer the stars or cloud is to Sgr A*. The stellar proper motions are generally larger the closer the star is to Sgr A*. If the central stellar cluster were the only mass present (i.e., no central massive black hole) then the orbital velocities would decrease toward zero as one looked closer to the center since M(r) would decrease with small r. If there is a massive black hole, the mass enclosed by the stellar orbits M(r) = rv2 G should approach the mass of the black hole as r 0. 27 March 2018 (UR) Astronomy 142 Spring 2018 31 / 36

The black hole at the center of the Galaxy Results from stellar and gas-cloud Doppler shifts and proper motions (Schoedel et al. 2003). Assumes r = 8 kpc; note that the current best estimate is 8.34 ± 0.16 kpc (Reid et al. 2014). 27 March 2018 (UR) Astronomy 142 Spring 2018 32 / 36

Spin of the black hole in Sgr A* Occasionally in the near-ir flare emission from Sgr A* is observed a periodic series of peaks (Genzel et al. 2003). This is reminiscent of the fast oscillations or pulses seen in stellar-mass black holes like GRO J1655-40. Is this the beginning of a death spiral at the innermost stable circular orbit, and thus a sign of rotation? 27 March 2018 (UR) Astronomy 142 Spring 2018 33 / 36

Spin of the black hole in Sgr A* If so, the black hole in Sgr A* is spinning at 25% of its maximum rate (e.g., the rotational speed at the horizon is 0.25c). Zero spin is ruled out, within uncertainties. Blue: innermost stable orbits per hour for a black hole with mass 4.3 10 6 M as a function of spin rate, with uncertainties. Red: measured orbits per hour, with uncertainties (Genzel et al. 2003). 27 March 2018 (UR) Astronomy 142 Spring 2018 34 / 36

The black hole at the center of the Galaxy Thus there is a black hole at the center of the Milky Way, and its mass is (4.02 ± 0.16) 10 6 M (Boehle et al. 2016). It spins at 25% of its maximum rate. Presumably the radio and X-ray components of emission from Sgr A* are the outermost and innermost parts of the accretion disk around the black hole. The near-ir flares probably arise from the innermost, hottest part of the accretion disk, with the quasiperiodic oscillations coming from the innermost stable orbit. As we will see, supermassive black holes are very common in galactic nuclei, but the prominent examples those in active galactic nuclei are considerably more massive (10 9 M ). Efforts are currently underway to create a global long-baseline interferometric radio network to image structures around the event horizon... 27 March 2018 (UR) Astronomy 142 Spring 2018 35 / 36

The Event Horizon Telescope The global mm-wavelength network comprising the Event Horizon Telescope (Castelvecchi 2017). 27 March 2018 (UR) Astronomy 142 Spring 2018 36 / 36