Preliminaries. Brief History of Extragalactic...

Preliminaries. Brief History of Extragalactic Astronomy

18th-19th centuries - “Nebulae” Cataloging• Charles Messier cataloged 103 nebulous objects. Messier Catalog (M87, M51, M100).

Includes famous nearby objects.• William & Caroline Herschel cataloged 2500 objects. Later extended by Sir John

Herschel and John Dreyer to 7840 objects. This is the New General Catalog (NGC). Later Index Catalog (IC), 5086 more objects.

1850 - 1925 : The Great Debates• 1885 : Nova (actually Supernova) in M31. 10% brightness of entire nebula seen as

strong evidence against "island universes"• 1912 : Henrietta Leavitt establishes Period-Lumimosity relation for Cepheids in

Magellanic Clouds. Tool for measuring distances• 1914 : Vesto Slipher takes spectra of spiral nebulae. Finds large velocities (eg M31 is

-300 km/s), much larger than any MW stars• 1917 : Herber Curtis measures more novae in spirals, places them 100x farther than

galactic novae, and suggests 1885 "nova" in M31 was anomalous.

Monday, August 20, 2012

Charles Messier (1730-1817)

Virgo Cluster of Galaxies sky.google.com

Charles Messier cataloged 103 “fuzzy” objects while looking for comets. This is still referred to as the Messier Catalog. Objects are given as “M#”, such as those below. It contains

planetary nebulae, spiral and elliptical nebulae.


Charles Messier (1730-1817)

Virgo Cluster of Galaxies sky.google.com

M84

M86

M87

Charles Messier cataloged 103 “fuzzy” objects while looking for comets. This is still referred to as the Messier Catalog. Objects are given as “M#”, such as those below. It contains

planetary nebulae, spiral and elliptical nebulae.

M58

M88

M89

M90

M91



18th-19th centuries - “Nebulae” Cataloging• Charles Messier cataloged 103 nebulous objects. Messier Catalog (M87, M51, M100).

Includes famous nearby objects.• William & Caroline Herschel cataloged 2500 objects. Later extended by Sir John

Herschel and John Dreyer to 7840 objects. This is the New General Catalog (NGC). Later Index Catalog (IC), 5086 more objects.

1850 - 1925 : The Great Debates• 1885 : Nova (actually Supernova) in M31. 10% brightness of entire nebula seen as

strong evidence against "island universes"• 1912 : Henrietta Leavitt establishes Period-Lumimosity relation for Cepheids in

Magellanic Clouds. Tool for measuring distances• 1914 : Vesto Slipher takes spectra of spiral nebulae. Finds large velocities (eg M31 is

-300 km/s), much larger than any MW stars• 1917 : Herber Curtis measures more novae in spirals, places them 100x farther than

galactic novae, and suggests 1885 "nova" in M31 was anomalous.


1850 - 1925 : The Great Debates (continued) : • 1920 : Famous Curtis-Shapley debate at National Academy of Science, "Are Spiral

Nebulae Island Universes" (Curtis argued in favor, Shapley argued against.) Consensus was that Shapley "won" the debate, though Curtis was right.

• Shapley : Major point: If Andromeda nebulae were as large as the Milky Way (100 kpc), then its angular size (3o x 1o) would imply such a great distance (2 Mpc) that the nova he observed in Andromeda would be much, much more luminous than anything observed in the Milky Way.

• Curtis : Novae showed that spiral nebulae are at least 150 kpc away to have same luminosities as those in the Milky Way. Doppler velocities (>500-1000 km/s) of spiral nebulae implied they would not remain bound to the Galaxy. If transverse velocities were just as high and they are in the Galaxy, we should be able to measure their proper motion (which they could not and we still cannot).

• 1923 : Edwin Hubble uses the new 100" telescope on Mt. Wilson and finds Cepheids in M31, which gave a distance of ~300 kpc (now 770 kpc). Proves M31 is an external galaxy. Settles debate.



1925 - 1950 : Expanding Horizons

• 1929 : Edwin Hubble derives redshift-distance relation, v = H0 d, (now called “Hubble's Law”). Hubble used Cepheids and “brightest stars in galaxies” to derive distances. Derived H0 = 530 km s-1 Mpc-1, which implied Hubble time of ~2 Gyr.

• 1936 : Hubble publishes galaxy morphological classification (tuning fork).

• 1930s : Fritz Zwicky measures galaxy velocities in the Coma cluster, and infers a mass-to-light ratio, M/L ~ 1000 M⊙/L⊙. “Average” for stars is M/L ~ 3 M⊙/L⊙. Zwicky postulates the need for dark matter needed if clusters are virialized (gravitationally bound). No one believes him.

• 1944 : Walter Baade observes Spiral and Elliptical galaxies (WWII war time black-outs help). He uncovers blue supergiants in disks (Population I) and red giants in bulges and ellipticals (Population II).



1950 - Present : Modern Developments

• 1952 : Baade uses Mt. Wilson 200" telescope to recalibrate Cepheid period-luminosity relation, which depends on stellar population (Pop I or II). All distances doubled. M31 is similar in size to MW, and Universe doubled in size.

• 1962 : Eggen, Lynden-Bell & Sandage (ELS) propose collapse model for formation of MW galaxy, which accounts for spatial, kinematic, and metallicity distributions.

• 1965 : Arno Penzias and Robert Wilson discover Cosmic Microwave Background (CMB), which supports a Hot Big Bang model.

• 1970s : Vera Rubin et al. measure velocity rotation curves in spiral galaxies, which indicates presence of dark matter to explain large rotational velocities at large radii. This inspires Cold Dark Matter (CDM) galaxy formation models during 1980s-90s.

• 1983-1985 : Marc Davis, George Efstathiou, Carlos Frenk & Simon White use “numerical methods” (=computer simulation of 32,768 particles) to show that the spatial distribution of galaxies supports Cold Dark Matter.



1950 - Present : Modern Developments

• 1992 : NASA launches COBE satellite to measure black body spectrum of CMB. Finds slight (10-5) anisotropies in CMB, which are seeds for structure formation.

• 1990s-2000s : NASA launches Great Observatories (Hubble, Chandra, Spitzer). HST does a targeted pointing in one patch of sky, the “Hubble Deep Field”. Identifies galaxies down to 29 mag with redshifts out to 5. Strong evolution in distant galaxies. Early star formation rate (~8-10 Gyr in the past) is 10x higher than at present.

• 1990s : Wendy Freeman et al. use HST to measure cepheid stars in galaxies as far as the Virgo cluster. Derive Hubble constant of H0 = 70 km s-1 Mpc-1. (Systematics still debated by people such as Allan Sandage who favor H0 = 40-50 km s-1 Mpc-1.)

• 1998 : Surveys conducted by two groups using Type Ia Supernovae as standard candles out to z ~ 1. Both find evidence for non-zero cosmological constant (expansion of universe accelerating).

• 2003 : NASA launches WMAP, which measures CMB power spectrum (strength and scale of anisotropies in CMB). Establishes “concordance model”, with high-accuracy cosmological parameters (errors of few %). Combined with SNIa, and HST cepheids (and other work) that Universe is very nearly flat, with 70% Dark Energy, 26% Dark Matter, 4% Baryonic Matter, present age is ~13.7 Gyr, with initial fluctuation spectrum index of -1 (consistent with inflation), and an epoch of reionization near z~15-20.



Preliminaries. Units in Astronomy.

• 99% of Astronomy uses cgs system :

- length=[cm], mass=[g], time=[s], energy=[erg].

- c= speed of light =2.998 x 1010 cm s-1.

- h= Planck’s constant = 6.625 x 10-27 erg s.

- k= Boltzmann’s constant = 1.38 x 10-16 erg K-1.

- M⊙ = Solar Mass = 1.99 x 1033 g.

- L⊙ = Solar Luminosity = 3.839 x 1033 erg s-1.

- 1 Mpc = one million parsecs = 3.086 x 1024 cm.


Luminosity is intrinsic amount of energy per unit time emitted by an object. Units of erg s-1.

Specific Luminosity, or Luminosity density, is the differential amount of luminosity emitted per unit frequency, Lν, or per unit wavelength, Lλ. Such that:

L =�

Lν dν =�

Lλ dλ

Flux is amount of energy per unit time (luminosity) per unit area. Units are erg s-1 cm-2.

Specific Flux, or Flux density, is the differential amount of flux emitted per unit frequency, Fν, [erg s-1 cm-2 Hz-1] or per unit wavelength, Fλ [erg s-1 cm-2 Å-1]. Such that:

radius r

Luminosity L

Area of Sphere of radius r = 4πr2.

Flux = L / Area = L / 4πr2F =

�Fν dν =

�Fλ dλ

Preliminaries. Radiation, Luminosity, Flux, Intensity.


The Specific Intensity, Iν, is the amount of flux density, fν, that passes through a unit solid angle dω. Units of erg s-1 cm-2 ster-1 Hz-1. The specific intensity is also called the Surface Brightness.

The radiation energy which passes through a surface area element dA per unit time dt within a solid angle dω around a direction defined by the unit vector n in the frequency range ν and ν + dν is :

Poynting Vector and Surface area aligned: cos θ = 1

θ



The Specific Flux, Fν, is then the amount of specific intensity integrated over solid angle. Units are erg s-1 cm-2 Hz-1.

for θ << 1. (i.e., angular size of objects is very, very small.)

Fν =�

dω Iν cos θ =�

dω Iν

The Mean Specific Intensity, Jν, is the average of Iν over all angles:For an isotropic radiation field no net flux Jν = Iν.

The Specific Energy Density, uν, is related to Jν by

uν is the energy of the radiation field per unit volume per unit frequency. Units are erg cm-3 Hz-1.



Thermal Radiation

For a pure blackbody the source function Sν is solely a function of the Temperature

Consider radiation propagating through matter in thermal equilibrium at Temperature, T.

For large optical depth τν (>>1, called ``optically thick”), then Iν = Bν(T).

For example, radiation in a box with opaque walls at a constant temperature T satisfies this: a so-called “Black” body.

Preliminaries. Radiation, Part II


The function Bν(T) was first obtained by Max Planck (called a “Planck” Function):

Strictly speaking, Thermal Radiation is when Sν = Bν(T) and Blackbody radiation is when Iν = Bν(T). For large optical depths thermal radiation converges to blackbody radiation.

Astronomers will usually use the words “Thermal” radiation.

or

Preliminaries. Radiation, Part III


Thermal Radiation



Thermal Radiation

T=10,000 K T=8000 K T=5800 KT=3000 K


Thermal Radiation

T=10,000 K T=8000 K T=5800 KT=3000 K

Observed Spectra of Vega-type StarSolar-type Star


Thermal Radiation

T=10,000 K T=8000 K T=5800 KT=3000 K


Here Stars behave nearly like blackbodies


Thermal Radiation

T=10,000 K T=8000 K T=5800 KT=3000 K


Here Stars behave nearly like blackbodies

Lots of absorption from atoms in the stars’ atmospheres


Thermal Radiation

The energy density of thermal radiation is given by integrating over the Planck Function

The Flux from the surface of a blackbody is then given by

where



Preliminaries. The Magnitude Scale

Given two sources, with flux S1 and S2, the difference in their magnitudes is

Chosen to be close to historical magnitudes (thanks to Hipparcus, ca. 100 BCE).

A magnitude change of Δm = 1 corresponds to a change in flux of 2.51x.A flux change of a factor of 10 corresponds to Δm = 2.5.


Absolute Magnitude, M, is the magnitude of a an object if it were placed at a distance of 10 parsecs (definition). It is

intrinsic to an object. (Like an object’s Luminosity.)

Apparent Magnitude, m, is the magnitude of an object as it appears to be. It depends on how far away the object is from the

observer. (Like an object’s Flux.)

They are related by the Distance Modulus,

μ= m - M = 5 log10(d) + 5 = 5 log10(d/10 pc)

Preliminaries. The Magnitude Scale


Preliminaries. Filters and Colors


Conventional definitions for apparent magnitudes, B = mB, V = mV, K=mK, etc.

Apparent magnitudes are calibrated using photometric standard stars.

These are compared to Vega-type stars (A0 type).

The apparent magnitudes of a Vega-type star are defined to be

U = B = V = R = I = J= H = K = . . . = 0 mag



For the actual definition, must consider the telescope+detector+optics

Transmission function, TX(ν), which gives the fraction of incoming photons

with frequency ν that are detected. Note the formula in your book is incorrect.

Good reference is Fukugita et al. (1995), PASP, 107, 945.

For a source with flux density, Sν, the the magnitude is

mX = −2.5 log� R

dν TX(ν) Sν/hνRdν TX(ν)/hν

�+constant

where the constant is determined from reference stars, i.e.,

mX −mX(Vega) = −2.5 log� R

dνTX(ν)Sν/hνRdνTX(ν)/hν

�

+2.5 log� R

dνTX(ν)Sν(Vega)/hνRdνTX(ν)/hν

�



mX −mX(Vega) = −2.5 log� R

dνTX(ν)Sν/hνRdνTX(ν)/hν

�

+2.5 log� R

dνTX(ν)Sν(Vega)/hνRdνTX(ν)/hν

�


or, simplifying: mX −mX(Vega) = −2.5 log� R

dν TX Sν/hνRdν TX Sν(Vega)/hν

�

or, generally : mX −mX(ref) = −2.5 log� R

dν TX Sν/hνRdν TX Sν(ref)/hν

�

For example, the “Absolute Bolometric” reference system (Oke & Gunn 1983, AJ, 266, 713),

uses a reference spectrum that is “flat” in Sν (Sν ~ ν0) in which case:

mAB = -48.6 mag for Sν in erg s-1 cm-2 Hz-1,

mAB = 31.4 for Sν in nJy (1 nJy =10-9 Jy),

mAB = 23.9 for Sν in μJy, etc.

mX =

mX = mAB − 2.5 log� R

dν TX Sν/hνRdν TX/hν

�


The Bolometric magnitude is the magnitude over all wavelengths (no filter,

hypothetical detector sensitive to all wavelengths or frequencies).

Absolute bolometric magnitude isApparent bolometric magnitude is

Solar Magnitudes and Relative-to-Solar

m⊙,bol = -26.83 mag. For 1 AU, μ=-31.47 mag, which gives, M⊙,bol = 4.74 mag.

For other objects,

You will see people say that the

“Blue” luminosity of an object is

such-and-such. They mean (for

X=“Blue”)



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