Quantifying Surface Brightness Quantifying SB profiles - - PDF document

Padova Lecture Series 2007 Lecture 3 53

Quantifying Surface Brightness

Parametric

CSB : 0

The extrapolated central

surface brightness from the fit to the galaxy disk. Non-Parametric

Effective SB : e

Defined as the surface

brightness level (e) at the half-light radius.

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Quantifying SB profiles

Non-Parametric

Effective parameters:

e, re

Defined as the surface

brightness level (e) at the half-light radius (re)

Concentration: C28

The log of the ratio of

radii which contain 80% and 20% of the total flux

2rI(r)dr = 2 2rI(r)dr

re

• µe µ(r

e)

C28 = 5log r

80

r

20

• Padova Lecture Series 2007

Lecture 3 55

A few words about concentration

r80 and r20 are the radii at 80% and 20% of total light C28 is wavelength dependent (use reddest bands), but non-parametric (unlike e.g. B/T) C28 is independent of absolute calibration (unlike Re, µe) Pure disks have C28=2.8 but Freeman Type II disks can have C28<2.8 Concentration is slightly degraded (lower values) by seeing; the effect is strongest (a few %) at higher C28 We measured C28 from SDSS i-band images for 32 spirals, 12 ellipticals, and 33 SO’s in our sample.

C28 5log(r

80 /r 20)

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Properties of C28

For 300 Virgo galaxies; McDonald, Courteau & Tully (2008)

C28,H log re,H C28,i

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Sersic Light Profiles

Ibulge(R) = I0 exp R r

• 1/ n
• Padova Lecture Series 2007

Lecture 3 60

Freeman (1970) introduced two types of disk light profiles:

MacArthur, Courteau & Holtzman (2003)

Photometric data

Padova Lecture Series 2007 Lecture 3 65

Caveats in BD decompositions

The entire formulation of the B/D decomposition presented so far relies on the assumption of continuity of the disk below the bulge and the bulge below the disk. The structure of bulge and disk relies on detailed kinematics of bulge and disk stars (tomorrow’s lecture). Consider M31 for example...

Padova Lecture Series 2007 Lecture 3 70

• structural coupling

between bulge and disk (re/h = 0.22 ± 0.09)

• independent of

wavelength

• consistent with

scenarios of bulge formation via secular evolution Bulge/Disk Scale Length Ratios

MacArthur et al. (2003)

(taking B/D decompositions at face value)

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Virgo Cluster Galaxies Scaling Relations

Bender et al.(1992)

McDonald et al. (2008)

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Spirals + Irregulars Ellipticals + dE,dS

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Most disk galaxies have additional, thick, disk

(Yoachim/Dalcanton)

Modest mass fraction,

but increases below vc~120 km s-1

Some counter-rotate:

external origin

Heavier counter-rotating

disks seen in S0s (Kannappan)

[/Fe] vs [Fe/H] in MW thick disk it cannot have

formed by accretion of small stellar lumps (Venn)

Origin in gas-rich merger (Sommer-Larsen)

Thick Disks

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Two pure disk galaxies

NGC 4244 NGC 5907 Hint of a thick disk vdK&S

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NGC 4656: small bulge and prominent thick disk

vdK&S

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M33 surface brightness profile (Ferguson etal 2003)

Sharp decrease in SB beyond 5 disk scale lengths. Star counts Light profile

(reaches V~31 mag arcsec -2 !)

DISK!TRUNCATIONS

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DISK!TRUNCATIONS

What is the origin of this disk truncation? Common and seen more easily in edge-on rather than in face-on galaxies

Kregel etal (2002): Rmax/hR = 3.6 ± 0.6 for 34 edge-on disk galaxies Perez (2004) finds similar results at 0.6 <z <1

Kregel etal 2002

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Disk truncations

NGC 300* surface brightness profile (Bland-Hawthorne etal 2005) Exponential profile extends to 10 disk scale lengths without a truncation! Star counts

NGC 300 M33

* Similar to M33 Light profile Star counts

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Interpretations of disk truncations

• The radius associated with the maximum angular momentum
• f the disk baryons in the proto-galaxy -- unlikely -- many

disks have HI extending far beyond the truncation radius

• The radius to which the disk has grown today -- unlikely --

The outer disks may be younger but still typically many Gyr old (eg Bell & de Jong 2000; Ferguson et al 2003; MacArthur et al. 2004). In some galaxies (eg M83, Milky Way), star formation continues in the outer disk but there is an underlying old component.

• The radius where the gas density goes below the critical value

for star formation (Kennicutt 1989) - star formation regulated by disk stability -- likely (van den Bosch 2001)

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Bruzual & Charlot models

• lifted with the addition of infrared photometry

Padova Lecture Series 2007 Lecture 3 87 Padova Lecture Series 2007 Lecture 3 88

Freeman (1970) introduced two types of disk light profiles:

MacArthur, Courteau & Holtzman (2003)

Photometric data

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MacArthur, Courteau, Bell, & Holtzman (2004)

• Compare colour gradients with stellar population models
• Determine main drivers of SFH as a function of galaxy structural

parameters (e.g. surface density, total mass)

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MacArthur etal (2004)

Age-metallicity gradients from colors in disk galaxies Disks grow inside-out; Age gradients stronger than metallicity gradients; upturn at large radii for early-type systems.

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Galaxy Spectra

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Relative Flux Wavelength (Å)

• Worthey & Ottaviani 1997
• set of 21 spectral absorption

line indices to monitor the strength of spectral features in stars and composite systems (galaxies)

• 11 strongest & most reliable

indices calibrated as a function

• f colour, surface g, and Z for

semi-empirical population models (e.g Worthey 1994, Vazdekis 1999,

Maraston et al. 2003, Bruzual & Charlot 2003)