HW#4 is due next Monday Part of it is to read a paper and answer questions about what you read If you have questions, ask me!

Transcription

1 Sep 21, 2015 Elliptical galaxies overview Shapes: evidence for triaxiality, isophotal twists, structure What causes flattening in ellipticals? Classes of ellipticals: Disky vs boxy Flat core vs cuspy/power-law core HW#4 is due next Monday Part of it is to read a paper and answer questions about what you read If you have questions, ask me! Reading: Chapter 3 There is a handout today Next week: special lectures by Reinhard Genzel speaking on the Galactic Center and its SMBH and on high redshift galaxies (Hopefully) Arecibo remote observing Fri&Sat Oct 16 & 17

2 HW #2 Wien s Law: For a blackbody, the wavelength where the Planck function peaks, (B max ) X Temp is a constant Rayleigh-Jeans approx. at low freq => low Temp For in cm: 0.29 cm (B max ) = T(K)

3 Evolution in the CMD Ferreras adapted from Faber dry merger: no gas involved wet merger: gas and vigorous star formation/feeding of active nucleus (accretion onto SMBH)

4 Elliptical Galaxies Morphology-density relation => found in regions of high galaxy density M87 jet Often show hints of interactions/merger Offset M87 field: Point sources are globular clusters CenA

5 APOD credit J. Bers Dwarf elliptical galaxies Credit A. Block/NOAO High surface brightness APOD credit J. Schedler Sculptor dsph Credit: ROE/AAO Low surface brightness

6 Normal Ells vs dwarfs Dwarf ellipticals (and dsphs) following different scaling relations in concentration and surface brightness than normal/giant ellipticals Concentration: R50/R90 where 50,90 refer to % of L tot

7 Ellipticals are not so simple ATLAS3D/P-A. Duc

8 Ellipticals: Not all that simple Elliptical galaxies constitute the brightest and faintest galaxies known This statement lumps the des and dsphs; to be discussed later. Apparent simple structure roundish appearance Light is smoothly distributed Lack star formation patches Lack strong internal obscuration by dust. Many fit by R 1/4 law => Sersic n=4 But actual complexity Shapes (from oblate to triaxial) Large range of L and light concentration Fast and slow rotation; even counter-rotation Flat core vs power-law cusp

9 The Sersic index luminosity relation b n ~ 1.99 n High L galaxies fit with higher n Low L galaxies have n closer to 1 = inner slope Ferrarese et al. ACS Virgo Cluster Survey (2006, ApJS 164, 334) Warning: some works use 1/n instead of n

10 Shapes of E gals The contours of constant density are ellipsoids of m 2 = const. Ellipticity 1 - / where, are the apparent major & minor axes : triaxial = < : prolate (cigar-shaped) = > : oblate (rugby-ball) Assume Es are oblate with q = / Along the z-axis: => EO Viewed an an angles => q o = b/a How is q o related to,? Evidence from the observed ellipticity distribution as well as from kinematics suggests that, on mean, ellipticals are modestly triaxial ~ 1:0.95:0.7

11 Twisted isophotes in M32

12 Fine structure in Es: Evidence for Mergers? Probably result from accretion/merger of a small galaxy on a very elongated (radial) orbit Quinn (1984)

13 Exceptions: cd galaxies cd galaxies - extended power-law envelopes seen predominantly in dominant cluster galaxies cd = cluster diffuse Found in regions of local high galaxy number density, often at center of potential (low relative velocity) - clusters, compact groups SB excess at large R caused by remnants of captured galaxies? OR Envelope belongs to the cluster of galaxies (not just central galaxy) -- ellipticity of envelope follows curves of constant # density of gals Multiple nuclei common galactic cannibalism? dynamical friction? later

14 Constraints from velocity fields Spectroscopic observations => Doppler shifts, dispersion/rotation. Galaxies have complex spectra, contributed by the whole stellar pop. S( ) = Stellar Template = spectrum of single star N(v) = relative (normalized) # of stars of projected velocity v (i.e. v los ) N(v) is usually called the LOSVD (Line of-sight Velocity Distribution) G( ) = observed (broadened) galaxy spectrum =>G( ) is the same as S( ) convolved (smoothed) by N(v) Observe G( ) and S( ) and use them to try to obtain N(v).

15 Reconstructed images and first two stellar velocity moment maps (velocity and velocity dispersion) for six galaxies selected near the ellipticity anisotropy relation (magenta line, see text). Eric Emsellem et al. MNRAS 2011;414: The Authors Monthly Notices of the Royal Astronomical Society 2011 RAS

16 Ellipticity and flattening Ellipticity 1 - / => Classication En where n = 10(1-b/a) If the galaxy is an ideal, oblate rotator with an isotropic stellar velocity dispersion, then we can show that: Hence, if the observed ellipticity of 0.4 is due purely to rotation, then V rot / should be 0.8 We define the rotation parameter (V/ ) as (V/ ) (V rot / ) observed (V rot / ) isotopic Although it is somewhat arbitrary, a galaxy is considered to be primarily rotationally supported if (V/ ) > 0.7.

17 Flattening of ellipticals Open circles: lower L galaxies; filled circles: brighter galaxies Dashed line shows fastest rotation expected for a given flattening. Massive ellipticals are not rotationally supported Low mass ellipticals and bulges (crosses) are rotationally supported

18 Kinematically decoupled cores: mergers/accretion? Some ellipticals show kinematically distinct inner components, sometimes even counter-rotating disks Evidence for minor mergers (more on this later) Bertola & Bettoni 1988 ApJ

19 Deviations from ellipses Often, isophotes are not perfect ellipses disky : excess of light on the major axis boxy : excess of light on the minor axis Bender et al., 1988, A&AS, 74, 385 Disky isophotes can be explained by a superposition of an elliptical bulge and a faint edge-on disk. Isophotal analysis is perhaps the only way to detect weak disks in Ells. It is likely that disky Ells are an intermediate class between boxy Ell s and S0 s

20 Disky vs Boxy => Embedded disk From R. Bender

21 Power-law Cusp vs Flat Cores The central regions of elliptical galaxies include two types of profiles: flat and power-law centers. Characterize the profile by its central slope and break radius r b : Flat core flattening of profile towards center Power-law cusp: increases to high value at center Common usage today defines a flat core galaxy as one with a flat inner profile, such than < 0.3 and a power-law galaxy has > 0.5 From R. Bender

22 Boxy vs disky; core vs cusp Faint ellipticals (and bulges) are rotationally flattened; bright ellipticals are often anisotropic Strong correlation between rotational properties and the shape of the isophotes and core properties: - Boxy isophotes, flat (core) centers: anisotropic, peculiar velocity fields (high L) - Disky isophotes, powerlaw (steep/cusp) centers: usually rotationally flattened (low L)

23 Boxy vs disky (lots of differences) boxy disky Schneider Fig 3.16

24 Flat core galaxies are boxy and slow rotators Power-law galaxies are disky and fast rotators Revised tuning fork Flatcore Power-law core Does the difference in Es have to do with the formation of SMBH ( central deficit of stars ; gravitational slingshot => HW#4) and whether the merger was dry or wet? Kormendy & Bender 1996, ApJL 464, L119