correction

Correcting for red-shifting of light out of wavelength region

This is most important for high-redshift galaxies. If it is not taken

into account conclusions can easily be in error.

Spitzer IR

Galex NUV+FUV

Optical

M31 - different wavelengths

Spirals in ultraviolett (dominated by massive stars) and visual (average population), Ultraviolet Imaging Telescope, Astro mission.

Note: Redshifted spirals observed in the optical will show rest-frame UV morphology!

Galaxies UV & visual wavelengths

What is a Spiral Galaxy ?

bulge

thin disk thick disk

100+ sq deg

Sensitive image of M31 and environment

M31

What is a Spiral Galaxy ?

stellar halo bulge

thin disk thick disk

optical M81-group HI

Looking at the gas

Neutral gas

Rotation Curves of Galaxies

dark matter

What is a Spiral Galaxy ?

stellar halo bulge

thin disk thick disk

+ interstellar medium dark matter halo

Deconstructing Galaxies

Kormendy

Galaxy Components

Surface Brightness Profiles

of galaxy components as a function of (R,z)

See Binney & Merrifield, section 4.4

Surface Brightness Profiles

bulges of spiral galaxies are treated similarly to ellipticals, as the surface brightness distribution typically follows an r^1/4 law (de Vaucouleurs):

mag arcsec^-2

disks of spiral galaxies are frequently modelled with an exponential decay:

mag arcsec^-2

h_r is the characteristic scale length of a disk along its mid-plane

Sb Spiral, NGC7331

From Sparke & Gallagher

Photometric properties of NGC 7331

NGC 7331

If a disk is circular & very thin, it will appear as an ellipse with axis ratio cos i when we view it at an angle i from face-on.

In this case the diameter along the minor axis of the disk isophotes is only 0.35 that measured along the major axis, and so we can infer that the galaxy is inclined at about 75^o from face-on.

This means that the surface brightness is larger by a factor 1/cos i than if we saw the disk face on. Using this we can correct to what we would observe to find the correct

average surface brightness at distance R from the centre.

Photometric properties of NGC 7331

Bulges

•Luminosity profiles fit r^1/4 or r^1/n laws

•Structure appears similar to E’s, except bulges are more

“flattened” (and bulges can be quite different from E’s dynamically)

Disk scale length Central surface

brightness

Disks

•Many are well-represented by an exponential profile I(R) = I_oe^-R/Rd (Freeman 1970)

NGC 7331 Rd

R. Peletier

Photometric properties of NGC 7331

Rd

At the centre of NGC7331 the I band surface brightness is I_I(0)=15 mag arcsec^-2. Each square arcsec at the centre of the galaxy emits about 10000 times as much light as the same area at R= 300”; the centre is 100 times brighter than the sky, while the outer regions fade to about 1% of the sky brightness.

For historical and technical reasons usually measure the outer edge of the galaxy as the radius of the isophote I_B = 25 mag arcsec^-2. For NGC7331, R₂₅ = 315”.

Integrating the surface brightness over the whole image and extrapolating for the parts of the galaxy too faint to measure give the TOTAL APPARENT MAGNITUDE.

Profile fitting: Spirals & S0s

2-component

bulge

disk

Surface Brightness of Disks

Freeman 1970 (ApJ, 161, 802) , for a wide range in luminosity, little scatter

•almost all spirals have disk surface brightness around I_o (B-band) = 21.5 ± 0.5

•partly a selection effect since low-surface

brightness (LSB) galaxies are harder to identify

•Many LSB disks identified since e.g., extreme case - Malin 1 (I_o = 25.5 and R_d=55 kpc!)

µ(B)

SO Sa Sb Sc Sd Im TYPE

The Milky Way

The Milky Way (optical)

The Milky Way (IR)

The effect of dust….

Near-IR: sensitive to giant stars and dust

Gamma-rays: neutron stars and X-ray binaries X-rays: hot supernovae remnants

Optical :dark nebulae

Far-Infrared: concentration of old stars in the bulge Radio (21cm): HI in disc, avoids the centre

Sbc-galaxy (MW) in different wavebands

Milky Way’s Components

More Types of Galaxies from Hubble Sequence

Kormendy

Irregular (Irr) Galaxies:

LMC: Irr-galaxy

V_hel = 278 km/s (51 kpc) 645 x 550 arcmin

M=0.9

SMC: Irr-galaxy

V_hel = 158 km/s (64 kpc) 320 x 185 arcmin

M=2.7

Dwarf galaxies

Leo I : dSph galaxy

V_hel = 285 km/s (260 kpc) 9.8 x 7.4 arcmin

M=11.2

NGC205: dE-galaxy

V_hel = -241 km/s (830 kpc) 21.9 x 11 arcmin

M=8.9

More Dwarf galaxies

I Zw 18 : BCD galaxy V_hel = 751 km/s

0.3 x 0.3 arcmin

Leo A: dIrr-galaxy

V_hel = 24 km/s (800 kpc) 5.1 x 3.1 arcmin

M=12.92

Large HI halos

Dwarf irregular NGC 2915 yellow: optical blue: HI

NGC 3115: S0-galaxy V_hel = 663 km/s

7.2 x 2.5 arcmin M=9.87

Lenticular (S0) Galaxies

NGC 4371: SB0-galaxy V_hel = 943 km/s

4 x 2.2 arcmin M=11.79

cD galaxies

M87 in Virgo cluster V_hel = 1307 km/s 8.3 x 6.6 arcmin M=9.59

Abell 3827

V_hel = 29500 km/s

- found in regions of high density - extremely high-L (4x10¹⁰L_) - multiple nuclei common

Profile Fitting: cD galaxies

Profile departure caused by remnants of captured galaxies OR the

envelope belongs to the cluster of galaxies (not just central galaxy). The ellipticity of the envelope follows curves of constant number density of cluster galaxies.

Early-Type Galaxies

Hubble’s classification scheme for early-type galaxies, based only on apparent ellipticity, is virtually irrelevant. Most physical characteristics are independent of ellipticity. It has proved more useful to focus on other properties: size, absolute magnitude and surface brightness.

cD: huge (sometimes ~1Mpc across), rare, bright objects

Normal Es: centrally condensed objects with relatively high central surface

brightness, giant & compact versions.

dE: lower surface brightness at same M_B compared to Es

dSph: extremely low luminosity and SB mostly detected in vicinity of Milky Way.

BCDs: blue compact dwarf galaxies.

Late-Type Galaxies

Hubble’s classification scheme for late-type galaxies has proved to be very

successful in organising our study of these objects: bulge-to-disk ratio; tightness of spiral arms; ability to resolve arms into stars and HII regions all correlate well with Hubble type. But so do a host of other physical parameters.

e.g., if we compare an Sa galaxy with an Sc galaxy of comparable luminosity, the Sa will be more massive (large M/L_B), have a higher peak in its rotation curve (V_max) have a smaller

mass fraction of gas and dust and contain a higher proportion of older, red stars.

Galaxy Types

Global Properties

M. Verheijen Galaxies get bluer

and fainter

Ursa Major Group

Spectra of Different galaxies

NGC 7742, a Seyfert galaxy

Active Galaxies

SAb

V_hel = 1663 km/s 1.7 x 1.7 arcmin M=12.35

Radio Galaxies, Jets

SA0

V_hel = 5098 km/s 1.6 x 1.4 arcmin M=13.38

Cen A

S0-pec

V_hel = 547 km/s 25.7 x 20 arcmin M=7.84

Interacting and merging galaxies

Interacting galaxy pair. Note that spiral disks are not optically thick!

Ring Galaxies

Collisions: Antennae

Starbursting Galaxies

M 82, a starburst galaxy, white/brown: stellar light and dust, red: hot expanding gas in Hα (Subaru telescope)

I0

V_hel = 203 km/s 11.2 x 4.3 arcmin M=9.3

How mergers progress

Various evolutionary steps of spiral-spiral mergers

Groups of Galaxies

NGC 2300 group (black&white = optical, blue/pink = X-rays)

Galaxy Clusters

Galaxies in “field” vs. “cluster”

In document Points of the Day (pagina 31-82)