introduction to galaxies

Introduction to Galaxies

Robert Minchin

What is a galaxy?

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 Leo T a galaxy?

Is UGC 9792 a galaxy?

Is And XII a galaxy?

Name Magnitude Luminosity Class

Messier 87 8.6 6.3×1010 Galaxy


Arp 220 13.2 2.6×1010 Galaxy


NGC 1087 11.1 1.2×1010 Galaxy

LMC 0.4 1.2×109 Galaxy

NGC 104/47 Tuc 4.0 3.6×105 Glob. C.

Andromeda XII 17 105 Galaxy

NGC 5139/ω Cen 3.7 8.3×104 Glob. C. ?

Leo T 16 5×104 Galaxy

UGC 9792/Pal 5 11.8 8.5×103 Glob. C.

Messier 78 8.3 95 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 0e-r/h

• If M/L is constant, ρ(r) = ρ0e-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 h2 [1 - e-r/h(1 + r/h)]

Galaxy Rotation Curves

• Centripetal acceleration is a = v(r)2/r

• Gravitation acceleration is g = GM(r)/r2

• Equating these, v(r)2 = GM(r)/r

• Thus, v(r) = (2πρ0Gh2[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


• Clearly there was some other component to the galactic mass

• GM(r)/r2 = v2/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 r2 ρ(r) dr

• Thus ρ(r) is proportional to 1/r2


• 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?



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.

dMpc = V/H0

• From the distance and the flux, can find the HI mass of the galaxy:

MHI = 2.356 × 105 d2 FHI

What else does HI give us?

• The HI mass to luminosity ratio (MHI/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!

ΔV20

Speak

Velocity

Let’s follow up this galaxy!

FHI

Interferometry: Continuum

L-band continuumcan give informationon star formation ornuclear activity.

Here, data from theNRAO 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 alsogive information aboutthe velocity field of thegalaxy.

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– J223450+240757, J223558+234825, J223631+240814

Look at your ‘Galaxy Card’


HI Studies:

• AGES HI: 2 new galaxies– AGES J223627+234258, AGESJ223829+235135

• Also sees NGC 7339



J223829+235135

J223627+234258

NGC 7339


• 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 J223449+240756 = J223450+240757– AGES J223631+240823 = J223631+240814


What happened to NGC 7332?

HI Surveys

• HIPASS (white) – 1997 - 2005• ALFALFA (red) – 2005 - present• AGES (green) – 2005 - present• AUDS (blue) – 2008 - 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.

introduction to galaxies

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