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Extragalactic Astrophysics 1 A.A. 2011-2012
Prof. L. A. [email protected]
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
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
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
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
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
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
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)
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