Extragalactic Astrophysics 1 A.A. 2011-2012

Prof. L. A. Antonelliangelo.antonelli@oa-roma.inaf.it

http://www.oa-roma.inaf.it/a.antonelli/lectures/

chapters 1,2galaxies

Milky Way, Local Group, disk and elliptical galaxies, irregular and starburst galaxies, etc

chapters 3,4,5active galactic nucleiBH paradygm, line and continuum spectrum, BLR and NLR, unified models, host galaxies and environment, cosmological framework, surveys, luminosity function, etc

Sparke & GallagherGalaxies in the UniverseCambridge University Press

PetersonAn Introduction

to Active Galactic NucleiCambridge University

Press

chapter 7high redshift UniverseLy-alpha forest, high-z galaxies, passive and active evolution, downsizing, etc

Ch.1

Milky Way

main dataradius RG~15 kpc (stars), ~18-20 kpc (HI); Sun distance from galactic center 8.5 kpcluminosity mass 75-80% DM, 15-20% disk, <5% bulge+halocentral BH rotation periods: Solar neibourhood ~240 Myr (galactic year), bulge ~10 Myr

300-400 pc

1000-1500 pc

8.5 kpc

15 kpc2 kpc

distances and velocities within the Milky Way

http://www.atlasoftheuniverse.com/

•catalogue: of the order of 1 billion stars

•accuracies: median parallaxes of 4 µas at V=10 mag, 11 µas at V=15 mag, 160 µas at V=20 mag

•distance accuracies: 21 million better than 1 per cent, 46 million better than 2 per cent, 116 million better than 5 per cent, 220 million better than 10 per cent

GAIA (~2012)

for nearby stars it is used the trigonometric parallax, based on Earth orbit

proxima centauri: p=0.8”, d=1.3 pc

Hipparcos, 1989-93: 12000 stars up to ~500 pc,precision ~10-3 arcsec

mobile cluster method

Hiades Cluster is very nearby, and it is possible to measurea decreasing of its apparent diameter due to its outwards motion

receding velocity is measured by Doppler shift

masers in the Galactic Center

Doppler shift

proper motion

example: Sagittarius B2 (North), star cluster in the Galactic Center. radiation by massive stars excites H2O maser sources within circumstar gas, very strong in spectral line at 22.2 GHz.

VLBI observations allow to measure relative positions with precision 10-5 arcsec

the observed motion is mainly radially directed with respect to the cluster center

assume

i.e.

it is found average of all the maser sources

if it is known (or if it can be assumed) how Vt and Vr are related for a particular object, then distance can be determined by the combined measures

the uncertainty is due to the relative low number of bright maser sources

light echo from Supernova 1987A in LMC

~ 85 days after SN observation narrow emission lines of ionized C and N have been detected from a ring, probably circular but inclined

from the delay we can measure ring radius, and then distance

inclination is deduced from apparent axial ratio:

measured delays are t-=86 d and t+=413 dand corresponding path differences are:

it is found:

LMC distance

0.83”

spectroscopic parallaxes

if a star’s spectral type is known, we can derive its luminosity from HR diagram, once calibrated with parallax measurements of nearer stars, so we can measure distance, if we can estimate interstellar absorptionfor MS stars it works well: uncertainties ~ 10% luminosity, ~5% distancefor giants HR diagram is ~vertical: uncertainty ~50% luminosity, ~25% distance

photometric variant, estimate spectral type from colorexample: looking orthogonally to galactic plane, red stars fainter than mV~14 are almost all K ed M dwarfs (for giants instead MV~0 and mv~MV+5logd-5~10, with d~1 kpc)from color, we get MS luminosity. there is little dust normal to galactic disc, then distance measurements are reliable enoughwe can measure the spatial distribution of stars:

thick diskthindisk

scaleheight

scalelength

spectraltype

spectral typelum

inosi

ty

R z

thin disk and thick diskolder stars have larger velocity dispersions and scale heights, because they suffer for a longer time the gravitational potential irregularities (giant clouds, star clusters) which tend to make their motion disordered

metal-poor stars ( )

thick disk stars are usually metal-poor ( )

the average velocity of stars with respect to Sun is negative because Sun has positive velocity (+7 km/s) with respect to LSR

F main sequence stars in the Solar neiborhood (< ~40 pc)

thick disk could be the result of a “gas-rich merger” with a satellite galaxy, where most thick disk stars were born in situ

open clusters

for open and globular clusters more precise distance determinations are possible, because all the stars of the same cluster have about the same age, chemical composition, and distance. optimal agreement of isochrones with HR diagram can be found

open clusters are absorbed by dust within galactic disk, we can see them only up to ~5 kpc. we know ~1200 of them

isochronebinary sist

pleiades

globular clusters

globular clusters are old up to ~12-15 Gyr. taking account of uncertainties in stellar evolution theory, age might go down to 11-12 Gyr. problems with the age of the Universe: to=2/3 Ho

-1~9 Gyr (Einstein-deSitter, Ho=75);concordance model ok:Ho=70, Ω=0.3ΩΛ=0.7: to~13 Gyr

for globular clusters there is no absorption problem. ~130 are known

metal-rich globular clustersZ=1/3-1/10 of solar valueflattened distribution, may be part of thick disk

metal-poor globular clustersZ~1/300 of solar valuenearly spherical distribution

RR Lyrae variables

another method to estimate distance of globular clusters is to use RR Lyrae stars, which have periods ~ 0.5 days and mean luminosity about uniform

from measurement of apparent magnitude, distance can be determined

similarly, for external galaxies, cepheid variables are used, which have a P-L relation

they cannot be used in the Milky Way because they are in the disk and are absorbed by dust

infrared

in the galactic plane, visible light is absorbed by dust. it is convenient to observe in the IR, which is less absorbed.it is found, both for thick and thin disk, scale length 2 kpc < hR < 4 kpc

it is observed a flattened bulge, larger on one side, probably due to a bar, with semilength ~2-3 kpc from center

probably Milky Way type is between Sbc e Sc, not clearly barred as in SB types, it sometimes classified SBA, intermediate type between S and SB

central cluster

Sagittarius B2, ~150 pc galactic centercentral density , halves at ~ 2-3 pc from centerresembles a globular cluster, ma still forms starsthere is gas inflowtotal mass

central black hole

central cluster

BH

DM

model of distribution of mass in the central region, to account for observed rotational velocities:

stellar orbits around Milky Way central BH

Gillessen et al 2008

MBH=(4.31±0.06±0.36) x106

28 well determined orbits S2 completed orbit

S2

QuickTime™ and aH.264 decompressor

are needed to see this picture.

Unprecedented 16-Year Long Study Tracks Stars Orbiting Milky Way Black Hole

http://www.eso.org/public/outreach/press-rel/pr-2008/phot-46-08.html

stat astrom

differential rotation

stars and gas rotate in the galactic plane with nearly circular orbits, but with angular velocity increasing toward galactic center

differential rotation affects transverse and radial velocities with respect to Sun, and was indeed discovered from proper motions of nearby stars

towards galactic center, we see stars going ahead and in the opposite direction stars remain behind, with respect to Sun. stars in the same galactocentric orbit as Sun have same velocity in absolute value, but relative velocity has a transverse component as in figure

this configuration of proper motions was already noted around 1900 and explained by Oort in 1927 with the differential rotation

radial velocitiesS

+

+

+

-

-

-

this is valid not only for stars, but also for the gas, which is best observed in the radio band, e.g. HI 21 cm, CO 2.6 mm

Oort constants

for d<<R we can approximate:

proper motions:

rotation curve

if we can measure Vr

at various distances:

for the stars there is a problem: absorption by dustfor HI at 21 cm we miss information on distance, use the tangent-point method: orbit of cloud No. 4 is tangent to line of sight, we get . and Vr is maximum

but method doesn’t apply for R>Ro, in such case we must use associations of young stars, measure distance with spectroscopic parallaxes and measure Vr through emission lines from circumstellar gasit is found that V(R) doesn’t decrease either in the external parts of Milky Way

contributions of various clouds along the line of sight

Ro

R

Oort 1952

dark matter

for a spherically simmetric configuration, centripetal acceleration of a star in circular orbit at galactocentric radius R is determined by mass internal to radius R, M(<R):thus, measurement of V(R) provides a mass determination:

there must be other matter, other than that visible in stars, theDark Matter, which is believed to be distributed within a dark halo

because V(R) doesn’t decrease, M must increase at least as R

if M is confined to a given radius Ro , M(<R)=const for R>Ro

so V~R-1/2 (keplerian case)

for a flattened configuration like a disk F≠GM/R2

and formula gives M(<R) with error ~10-15%

[ ]

Ro

dark matter

DM can account for 80% of the total mass: what is it done of?

WIMPs:

MACHOs:

weakly interacting massive particles (neutrinos, neutralinos, gravitinos ...)difficult to detect directly [non-barionic dark matter]

massive compact halo objects (black holes, planets, brown dwarfs, white dwarfs ...) detectable by their effects of gravitational microlensing [barionic dark matter]

and/or

most matter is DM

in the Universe:

most DM is non-barionic

(from Big Bang nucleosynthesis)

also some is barionic

M31

M33

Local Group

1Mpc

Local Group galaxies

the 3 dominant galaxies~90% of the LG Luminosity

the only ellipticaldistances measured through Cepheids P=L relationknown within ~10% for brightest galaxies

boldface: Milky Way satellites

italics: M31 satellites

most are dwarfs: dSph, dIrr, dE

nearest one (low surface brightness, discovered 1994)

carina dSph

sagittarius dSph

LMC

SMC

sextans dSph draco dSphumi dSph

sculptor dSph

fornax dSph

most lie close to a plane

Milky Way satellites

NGC 147 IC 10

NGC 3109

M32M31

NGC 205

M33

(Irr) (Irr)(dE)

(E2)

(dE)

(Sb)

(Sc)

NGC 185

(dE)

M31

M33

Local Group

velocities

radial velocities

easily measurablesubtracting solar motion, it is found that Milky Way and M31 approach each other at V~120 km/smost other galaxies have velocities within ~60 km/s from MilkyWay+M31 center of mass, not enough to escape from LG:

transverse velocities

we can measure them only for nearest satellites:at d~100 kpc, with Vt~100 km/s, need to select distant quasars and galaxies in order to define a non-moving reference frame

Local Group represents a typical galactic environment: less dense than a galaxy cluster like Virgo or Coma, but contains enough mass to bind the galaxies together

Local Group constitutes a great opportunity to study stellar systems close-up: we can resolve stars, analyse their HR diagrams, and determine their ages and chemical compositions

[ ]

Magellanic Clouds

they are the most prominent Milky Way companions, clearly visible with naked eye in the southern sky, they form stars and star clusters in abundance

LMC measures 15o x 13o on the sky and is ~14 kpc long

SMC measures 7o x 4o on the sky and is ~8 kpc long

it is a disc, tilted ~45o from plane of the sky, with a strong bar. rotation velocity reaches ~80 km/s.very gas-rich: M(HI)/LB~0.3 (compare MW, M(HI)/LB~0.1)

it is an elongated structure seen roughly end-on, with depth ~15 kpcno rotation motion. M(HI)/LB~1

Magellanic Bridge: bridge of gas connecting the two cloudsMagellanic Stream: long tail of gas behind SMC, contains ~Leading Arm: stream of gas between LMC and MW

LMC and SMC orbit around their common center of mass, and also orbit the Milky Way.orbit of the Clouds is slowly decaying as energy is transferred to random motions of MW stars.position and motion of the Clouds suggest that their orbit is strongly eccentric, with a period ~2Gyr, and that ~200-400 Myr have elapsed from their closest approach to MW.

Magellanic Clouds

distance between LMC and SMC ~ 20 kpc, but could have been shorter (~10 kpc) at epoch of closest approach, and gravitational attraction by LMC has likely extracted some gas from SMC, so forming the Magellanic Stream

Magellanic Clouds

they are rich in star clusters. can use HR diagrams to determine age, chemical composition, and distance

LMC: it is found dLMC ~ 50 kpc, in agreement with measurement obtained through SN1987A.from HI rotation curveit is foundSMC: from globular clusters and fromvariable stars, it is found dSMC ~ 60 kpcglobular clusters (LMC), bimodal age dist:many old (>~10 Gyr) andmetal-poor ( ), do not form a halo, instead lie in a thick disk, with larger velocities than the gas:

few clusters between 4 and 10 Gyr, many young clusters and associations, some very populous(~100 times MW open clusters), may be young version of LMC GCsages of SMC clusters are continuously distributed between few and ~12 Gyr, with no gap

HI map

LMC

30 Doradus(Tarantula)

SN1987A

LMC

radio image in HI

SMC

47 Tucanae(Milky Way)

NGC 362(Milky Way)

search for isolated galaxies with luminosity similar to MW + satellites 2-4 mag fainter

search for isolated galaxies with luminosity similar to MW + satellites 2-4 mag fainter

Cepheid variables

Cepheids are massive, Helium-burning, pulsating stars, with luminosities up to and periods between 1 and 50 days

also RR Lyrae can be used:

Henrietta Leavitt found in 1912 that brighter Cepheids in LMC had longer periods: as distance is the same for all, brighter Cepheids have also higher Luminosity, and a period-luminosity relation is found:

from measurement of period it is determined the luminosity (standard candle), and then from apparent magnitude the distance is found

factor due in part to the different distance of LMC and SMC,and also to the different chemical composition and interstellar absorption

with Hubble Space Telescope we can use RR Lyrae up to ~2-3 Mpc and Cepheids up to ~30 Mpc

Cepheid variables

cosmic distance ladder

HST

trig

onom

etr

icpara

llax

Cepheids constitute an important step in the cosmic distance ladder. each measurement method must be calibrated through the previous one

dwarf spheroidals

Milky Way subsystem includes also 9 dwarf spheroidals with low surface brightness, ~1/100 than Magellanic Clouds

they are gas-free systems, with no stars younger than 1-2 Gyr

many of them contain RR Lyrae variables, with ages at least ~8 Gyr

some have luminosities similar to Milky Way GCs, but with much larger sizes (~102pc vs ~pc or less)

however they are true galaxies: fornax and sagittarius possess GC systems. spheroidals did not form stars at same epoch like in GCs, but distributed on many Gyr, from gas with different metallicities, e.g. Carina ->

from radial velocities and sizes Virial Theorem gives estimates of mass, and M/L, which in some cases is much higher than for MW

e.g. Carina M/L~75 => large abundance of DM

3 Gyr

7 Gyr15 Gyr

Carina

chemical abundances are relatively low <~1/30 than Solar.metal rich gas could have been lost and transferred to Intra Group Medium

spirals

M 31, Andromeda

larger than Milky Way:•50% more luminous•larger disk, scale length hR~6-7 kpc, twice MW•higher rotation velocity, V(R)~260 km/s, ~20% more than MW•more numerous globular clusters, 300 (vs 130)

M32(E2)

M31(Sb)

NGC 205(dE)

satellites: M32 (E2) + 3 dE + at least 6 dSph

two central concentrations, ~ 0.5 arcsec apart (2pc): BH with + star cluster

luminous star forming ring around the bulge at R~10 kpc

no clear large scale spiral pattern

radio observations of HI show S-shaped disk in the outer parts, similar to MW

IR: Spitzer

IR: IRAS

optical

star-forming ring

HI disk very extended, ~3 Holmberg radii, i.e. ~30 kpc, appreciable fraction of the distance M33-M31 (200 kpc)

very luminous nuclear cluster with old, intermediate, and young stars (differently than for GCs)

spirals

M 33, Triangulum

tiny bulge: Sc or Scd

smaller than MW: hR~1.7 kpcV(R)~120 km/s

in shows loops, filaments and shells due to SNe and stellar winds, which heat the gas and stop star formation (feedback)

no evidence of a central BH

strong central X-ray source + many weaker sources

optical spectrum: strong emission line by NII

radio map of M33 false colors indicate radial velocity and show rotation

NGC 604giant gaseous

nebula with strong star formation

X-8

X-7

M33 in X-raysit is one of the best studied galaxies in X-rays. there is diffuse emission + many tens of point-like sources, among which most conspicuous are X-8 Ultra Luminous X-ray source (ULX) with LX~1039 erg/s, and X-7 with a BH of ~15 solar masses. normal galaxies have total LX(0.5-10 keV) ~1038-1041 erg/s

formation of Local Group galaxies

protogalaxies form close to each other (Universe was smaller than now) and gain angular momentum through tidal torques

condensations which will later form galaxies (protogalaxies) begin to grow in regions of higher density

recombination epoch: T~3000 K, z~1100, t~300,000 yr

from this epoch:- H atoms are neutral, not ionized- photons do not interact with matter any more- Universe is transparent to radiation- matter is not supported by photon pressure, and can collapse to form condensations

first stars must have been born at z~6, when cosmic background radiation cooled to ~20 K, so that protostars could be able to radiate heat away and collapse

born from clouds with masses , they had primordial chemical composition, then at their death they polluted the residual gas with heavy elements, raising it to abundance ~10-3-10-2 solar

Bulge stars are younger than globular clusters (age < ~8-10 Gyr). they could have been formed in the densest region of the protogalactic gas, or in a dense region of the disk, or they could be remnants of globular clusters fallen in the center because of dynamical friction. Once formed the Bulge, the galactic gravitational field helps confining the gas enriched by SNe and enables the birth of metal-rich stars

dark matter is located mainly in external regions. in fact, DM is supposed to be very weakly interacting matter, so it doesn’t lose energy and remains in elongated orbits

during protogalaxy collapse, many gas clouds gave rise to globular clusters, forming stars inside them. other less dense clouds continued collapse, and collided, increasing gas density and forming a disk, rotating due to conservation of angular momentum previously gained, locating themselves in nearly circular orbits, those with minimum energy for a given angular momentum

on the contrary, stars and globular clusters born during collapse do not lose a significant amount of energy in collisions and move on elongated orbits with random orientations, and with negligible total angular momentum

formation of Local Group galaxies

chemical evolutionsimplified scheme:1 zone model: well mixed gas, with homogeneous chemical compositioninstantaneous recycle: enriched gas poured rapidly into ISM, before forming stars closed box: gas doesn’t enter or escape from galaxy

gas mass at time t

mass in low mass stars and remnants of high mass stars, at time t

mass in heavy elements at time t

metallicity

a given amount of stars dMs produces a mass of heavy elements dMh, which partly goes back to the gas, with a mass pdMs, while a mass ZdMs is subtracted to form new stars

yield (fraction of heavy elements in the gas returned from stars)

morever, it is supposed p indipendent of Z (primary elements: their production doesn’t depend on the presence of other elements)

(closed box)

chemical evolution

metallicity increases as gas is consumed

mass in stars born up to time t, with Z<Z(t):

mass in stars between Z and Z+dZ:

in agreement with the observationsof the Galactic Bulge, withZ(0)=0 e

dwarf ellipticals and dwarf spheroidals

dSph are slightly more luminous than GCs but much more diffuse

dE are more luminous versionsof dSph, or

e.g. M31 satellites:NGC 147, NGC 185, NGC 205

they are vulnerable to tidal stripping

no evidence of rotation, probably triaxial shape

relatively old stars > ~5 Gyr, but alsoyoung (100-500 Myr) in the central parts

M32 elliptical with very high central surface brightnessno stars younger than some Gyrcentral BH

dE dSph

M32 might be small version of a giant elliptical, or a central remnant, deprived of the envelope and of GCs

warm system: in M32, compare MW ~7 (cold), dSph<<1 (hot)

called dE inthe original paper

(Binggeli 1994), now called dSph

dwarf irregulars

irregular galaxies have asymmetric shapestar formation occurs in disorganized regionscalled dwarf irregulars below ~

moderate rotation: in giant irregulars and in dwarf irregulars

relatively metal-poor: <10% than Solar

relatively brighter than dwarf spheroidals only because they have young stars

many similarities between dIrr and dSph, the former have gas, the latter, which have closer orbits, have possibly lost it in interactions with MW or M31

past and future of Local Group

Local Group galaxies do not expand with Hubble flow. their gravitational attraction was strong enough to keep them together

Milky Way and M31 approach each other and probably they will get close to a collision within few Gyr. from their relative distance and velocity, r~770 kpc and dr/dt~ -120 km/s,we can estimate the total mass of LG within the central region where they are located

as a two-body system,they obey the equation:

orbit is almost radial.compute free-fall time:

we can find an approximate solution:

then, from the previous two eqs:

and solving for the mass:

a factor 5 greater than the combined mass of the two galaxies

probably MW and M31 will finally collide and merge in a unique galaxy

- with few exceptions, only groups including at least one elliptical display X-ray emission while groups with only spirals as LG do not show it. why? intragroup gas in groups with only spirals could have too low density and/or temperature to emit appreciably in X-rays

- any X-ray emission from LG would be seen from inside and would appear as an additional component to X-ray background

- observations and models of the XRB put upper limits

- it has been searched for an effect on the anisotropies of the cosmic microwave background trhoughSunyaev-Zeldovich effect (inverse Compton on CMB photons by relativistic electrons) but this also appears negligible

- an intragroup medium with moderate/low density and temperature can however be observed in absorption in the spectra of AGNs and quasars in X-rays (O VII 21.6Å) and FUV (O VI 1031,1037Å), the so called WHIM (warm-hot intergalactic medium). absorption features have been detected, but it is not clear if they are associated with the intragroup medium

intragroup gas and X-ray emission

clusters of galaxies and many groups of galaxies emit in the X-ray band by thermal bremsstrahlung from intracluster or intragroup gas

•is there an intragroup gas within Local Group?•is there associated X-ray emission?

[ ]

( T ~ 106 K)

EXTRA SLIDES on MACHO’s and

Gravitational Lensing

gravitational lensing

according to General Relativity, light passing at distance b from a mass M is deflected by an anglewe can calculate where the image of a star should appear if in front of it is placed a mass M which acts as a gravitational lensin absense of the lens, we would see the star in S’, at an angle with the lens L (supposing )because light deviates of an angle , we

see it in I, at an angle with the lens L moreove

r and

then:

Einstein radius

is called Einstein radius

one gets: and

if the lens and the source are perfectly aligned, with ,we expect to see a ring of light (Einstein ring) with radius

with we have two images: lies outside Einstein radius,

is inverted and lies within the Einstein radius, on the opposite side of the lens

examples

1) light rays grazing the Sun surface

dL=1 AUdLS>>dL

S

I

the light of a star is deflected by ~ 2 arcsec

radius , area within Einstein radius is max when

examples

2) star at distance dS: area of Einstein ring is maximum if Lens is half way

max when x=1/2

in fact: remember , say

*dS

dL

3) suppose Lens is an object with

and that you observe a star at distance dS

= 2dL

examples

*LensStar

then the Einstein radius is:

[ ]this is called microlensing, due to smallness of the angle

magnification

the two images are too close and we cannot separate them, however the sum of the two images appears brighter

a small area of the source is seen like two areas in the plane of the image

it can be demonstrated that gravitational lensing leaves surface brightness unaltered, so the flux of each image is proportional to its area

image I occupies same angle as the source S’and is modified in distance and thickness so that

e

generally the farther image is brighter than the sourceand the closer one is fainter

if the Lens moves so that its Einstein radius passes in front of the source, the image of the star becomes brighter and then fainter

moving lens, stationary source

SL

assume that

then

if the Lens has proper motion , then:

magnification is a function of time:

MACHOs

microlensing events are achromatic, because gravitational deflection is independent on wavelength. so they are observed in two bands to check this and to distinguish them from other variable sources (stars, quasars, planetary occultations)

http://sirius.astrouw.edu.pl/~ogle/ http://wwwmacho.anu.edu.au/

MACHOs

if we assume that the Dark Halo of the Milky Way is done by MACHOs, the probability of alignment between source and MACHO within Einstein radius depends on the density of MACHOs and not on their individual masses. such probability is estimated 10-6, thus millions of stars are being observed to have the chance of finding some microlensing events

star-rich regions in the Galactic Center and in the Magellanic Clouds are continuously monitored

as a by-product, it is obtained a database with the light-curves of thousands variable stars, tens of quasar, some planetary occultations, besides several microlensing events

MACHOs

if the lens is constituted by a binary star or by a planetary system, then multiple images are produced, and the light-curve is more complex