Introduction to Galaxies Robert Minchin

What is a galaxy?

How would you define a galaxy?

Is M83 a galaxy?

Is M31 a galaxy?

Is M87 a galaxy?

Is NGC 1087 a galaxy?

Is Arp 220 a galaxy?

Is M78 a galaxy?

Is the LMC a galaxy?

Is NGC 104 a galaxy?

Is Leo T a galaxy?

Is UGC 9792 a galaxy?

Is NGC 5139 a galaxy?

Is And XII a galaxy?

NameMagnitudeLuminosityClass Messier ×10 10 Galaxy Messier ×10 10 Galaxy Arp ×10 10 Galaxy Messier ×10 10 Galaxy NGC ×10 10 Galaxy LMC0.41.2×10 9 Galaxy NGC 104/47 Tuc4.03.6×10 5 Glob. C. Andromeda XII Galaxy NGC 5139/ω Cen3.78.3×10 4 Glob. C. ? Leo T165×10 4 Galaxy UGC 9792/Pal ×10 3 Glob. C. Messier Refl. Neb.

What is a galaxy? Small dwarf galaxies and large globular clusters have similar morphologies and similar luminosities. How do we decide what is a globular cluster and what is a galaxy?

Galaxy Rotation Curves The surface-brightness of a disc galaxy is given by σ(r) = σ 0 e -r/h If M/L is constant, ρ(r) = ρ 0 e -r/h Mass within the radius r is then M(r) = 2πρ 0 ∫ 0 r r e -r/h dr Integrating by parts gives M(r) = 2πρ 0 h 2 [1 - e -r/h (1 + r/h)]

Galaxy Rotation Curves Centripetal acceleration is a = v(r) 2 /r Gravitation acceleration is g = GM(r)/r 2 Equating these, v(r) 2 = GM(r)/r Thus, v(r) = (2πρ 0 Gh 2 [1-e -r/h (1+r/h)]/r) 1/2 This is a function that rises steeply to peak near r/h = 2 and then falls off.

The Observed Rotation Curve In the 1970s, interferometers such as the VLA and WSRT observed the rotation curves of spiral galaxies They were able to trace the rotation curves out beyond the stellar disc. The resulting curves were flat or slightly rising, not falling as predicted

The Observed Rotation Curve Clearly there was some other component to the galactic mass GM(r)/r 2 = v 2 /r, thus v = (GM(r)/r) 1/2, implying that M(r) is proportional to r If this new mass is spherically distributed, M(r) = 4π ∫ 0 r r 2 ρ(r) dr Thus ρ(r) is proportional to 1/r 2

The Observed Rotation Curve This density profile is characteristic of a isothermal sphere. This implies that the unknown mass is only interacting gravitationally. Lead to the acceptance of the existence of dark matter. As a general result, galaxies contain more mass than is detectable.

What is a galaxy? Small dwarf galaxies and large globular clusters have similar morphologies and similar luminosities. How do we decide what is a globular cluster and what is a galaxy?

What is a galaxy? Small dwarf galaxies and large globular clusters have similar morphologies and similar luminosities. How do we decide what is a globular cluster and what is a galaxy? Dynamics!

Measuring Dynamics Dynamics can be measured by looking at emission and absorption lines from stars and star-forming regions, or from the gas in the ISM. Dynamics for spiral and irregular galaxies are often measured using the 21-cm line of neutral hydrogen. For elliptical and spheroidal galaxies, which are gas poor, velocity dispersions from stellar spectroscopy can be used

Single-dish HI profiles Interferometry observations give the most accurate measurements, but are expensive in terms of telescope time. Using optical information on inclinations, single-dish profiles can be used to determine the rotational velocity.

What else does HI give us? The recessional velocity gives a measure of how distant a galaxy is. d Mpc = V/H 0 From the distance and the flux, can find the HI mass of the galaxy: M HI = × 10 5 d 2 F HI

What else does HI give us? The HI mass to luminosity ratio (M HI /L) tells us how gas-rich a galaxy is. –Gas-rich galaxies are often blue, late-type galaxies with active star formation. –Some are more intriguing objects with low SF rates. A number of these have been turned up by HI surveys.

Let’s follow up this galaxy! ΔV 20 S peak Velocity

Let’s follow up this galaxy! F HI

Interferometry: Continuum L-band continuum can give information on star formation or nuclear activity. Here, data from the NRAO VLA Sky Survey (NVSS) shows a high level of star formation – termed a ‘Star Burst’

Interferometry: HI Moment 0 From HI observations, a map can be made showing where the neutral hydrogen is in the galaxy. These data from the VLA show an HI ‘tail’ – indicating that this galaxy has interacted with another recently.

Interferometry: Moment 1 HI observations also give information about the velocity field of the galaxy. These data show that the velocity field in the tail is disturbed, but is fairly normal in the rest of the galaxy.

HI Surveys Can turn up interesting objects with high HI masses or high gas fractions. Can side-step optical selection effects. Give an instant measure of the recessional velocity, velocity width, and HI flux for every object detected. Do not find gas-poor galaxies and require optical data for inclinations, etc.

Case Study - NGC 7332 Optical Studies: NGC (Dreyer 1888): 2 galaxies –NGC 7332, NGC 7339 Karachentseva et al. 1999): 2 more –KKR 72, KKR 73 AGES optical (Karachentsev): 3 more –J , J , J Look at your ‘Galaxy Card’

Case Study - NGC 7332 HI Studies: AGES HI: 2 new galaxies –AGES J , AGESJ Also sees NGC 7339 Look at your ‘Galaxy Card’

Case Study - NGC 7332 J J NGC 7339

Case Study - NGC 7332 Have 6 galaxies from optical, 2 from HI, 1 from both BUT – HI Studies show: 2 of the optical galaxies not in the group –AGES J = J –AGES J = J Look at your ‘Galaxy Card’

What happened to NGC 7332?

HI Surveys HIPASS (white) – ALFALFA (red) – present AGES (green) – present AUDS (blue) – present

Summary What is a galaxy? –A galaxy contains stars within a DM halo. –May also contain gas and dust –DM halos extend well beyond optical disc HI gives us dynamics and a wealth of other information HI surveys can detect gas-rich galaxies missed by optical surveys, but miss gas- poor galaxies seen in the optical.