Galaxy Rotation: How we know AS413 10/28/2014 D. Clemens

Outline Ways to detect that galaxies rotate Measuring rotations of external galaxies Our problematic location within the Milky Way Clues from the nearby stars Success from Radio Astronomy: HI Sampling the cold ISM: CO (for H2) Outer Galaxy probes: gas+stars New(er) Milky Way probes: APOGEE, Gaia Mass models, dark matter, galaxy assemblage

Evidence that Galaxies Rotate Optical spectroscopy of galaxies – Large-aperture observations reveal absorption lines that are too broad to be from single stars Would imply impossibly high surface gravities Must be due to Doppler shifting of many stars with a range of radial velocities (RVs) wrt us Velocity dispersion of that galaxy (gravitational potential) – Emission lines from large- apertures don’t necessarily trace velocity dispersion Emission regions don’t span galaxy uniformly Elliptical galaxies don’t generally have emission lines Multi-Object or IFU (Integral Field Unit) RV observations – Velocity dispersion from scatter in RVs (MOS) – Velocity dispersion from IFU images

Multi-object fiber-feed – 2dF One fiber per galaxy Good for measuring galaxy RVs and cluster RV dispersions

Lots of spectra taken simultaneously See atmosphere as well as stellar/galaxy absorptions, emission lines Most in this image are earth’s atmosphere

Integral Field Unit (3 types shown) – feeding light to multiple spectrographs Resolve individual galaxies to elucidate RVs

All Measure Velocity Dispersions Alternatively – put spectrograph slit along spiral galaxy axis (long axis) – If spiral galaxy shows a long-axis, it has some inclination (no E7 spirals!) – If inclination angle can be deduced (from apparent axis ratio a/b, say), can correct apparent RVs to disk circular velocities – As a function of offset from the galaxy center, too – Holland, Ford, Rubin (1970s)

Convert wavelength shift to RV, correct for inclination – Mostly due to HII region emission, so spiral arms well-represented – “Fold” curve of velocity vs offset about center Buta,et al. 1987

Lots-o-galaxies – similar ‘Rotation Curves’ Rapid rise from center Then flattening

Wrong Answer! (or so they/we thought) Add up light from luminous matter (stars) Compute mass enclosed to some radius Predict circular rotations at each radius Too little! Need more matter – dark matter Begeman, Broels and Sanders (1991)

What about our Milky Way Galaxy? We are in a lousy location – inside the disk, far from the center – Can’t see (at optical wavelengths) very far along directions in the disk (~ 1 kpc). – The Sun is highly likely to be participating in the local circular orbits of stars about the Galactic Center Moving reference frame (Ugh) – Maybe measure ‘Differential Rotation’ locally? “Flat Rotation Curve” + increasing radii = differential rotation Speed the same, distance isn’t

Jan Oort and his constants

Galactic Longitude L Radial Velocity Of Nearby stars Radial Velocity of distant stars Tangential Velocity of distant stars 000>0 900< <0 2700> >0

In equation form…

org/wikipedia/commons /0/0b/Oortmeasure.jpg

Enter Radio Astronomy Radio wavelengths don’t suffer the extinction seen at optical and near-infrared wavelengths Can ‘see’ through the entire Galactic disk Great! No, wait… I don’t see stars… ‘Clouds’ of gas (‘atomic’ if H I, ‘molecular’ if H 2 – or its tracer CO) Complex emission spectral lines along each line of sight that goes through multiple clouds

Stack up spectra versus Longitude

Find the “Tangent Points” vs L kinematics.jpg Then, remap Tangent Velocities with L projection of  0 to reveal circular velocity dependence on R OK, but also trying to find Milky Way spiral arms – they are associated with star formation, which HI isn’t Survey in CO and repeat tangent analysis UMASS-Stony Brook CO survey of the 1980s

CO traces H 2, which traces Star Formation Potential Dame, Hartmann, & Thaddeus (2001) ApJ, 547, 792

Run of RV with L

Covert RV vs L to  (R)

Rotation curve + full CO survey = remap H 2 distribution as ‘face-on’ view

Criticisms Dips unphysical – too fast for Keplerian R 0,  0 now different than assumed (Reid+) Circular rotation assumption likely not fully correct – Spiral arms have kinematic perturbations Tangent analysis doesn’t work in outer galaxy – Had to adopt other, weaker, methods Others have updated with modern data 13 CO less optically-thick than 12 CO – Better at isolating clouds and arms – Galactic Ring Survey (Jackson+06)

HII Region Discovery Survey (Bania+)

HII Regions trace spiral arms best news.com/images/enlarge/image_1649e- Milky-Way-Arms.jpg

Galactic Rotation: Back to the Stars APOGEE – multi- fiber high-resolution near-infrared spectroscopy of stars in the Milky Way – Spectral types, luminosity classes, RVs GAIA – direct parallaxes, RVs for up to 1 billion stars in the Milky Way

Analysis = Bayesian (a story for another day…) 2012

Mass Models, Dark Matter, Galaxy Assemblage ges/gal_rotation_curve.png hisweek1/thisweek/cloudstr eam.jpg