Download Extragalactic Astrophysics 1 AA 2011-2012 Prof. LA Antonelli

Extragalactic Astrophysics 1
A.A. 2011-2012
Prof. L. A. Antonelli
[email protected]
http://www.oa-roma.inaf.it/a.antonelli/lectures/
chapters 1,2
galaxies
chapters 3,4,5
active galactic nuclei
Milky Way, Local Group, disk
and elliptical galaxies, irregular
and starburst galaxies, etc
BH paradygm, line and continuum
spectrum, BLR and NLR, unified models,
host galaxies and environment,
cosmological framework, surveys,
luminosity function, etc
Sparke & Gallagher
Galaxies in the Universe
Cambridge University Press
Peterson
An Introduction
to Active Galactic Nuclei
Cambridge University Press
chapter 7
high redshift Universe
Ly-alpha forest, high-z galaxies,
passive and active evolution,
Ch.1
Milky Way
1000-1500 pc
300-400 pc
15 kpc
2 kpc
8.5 kpc
main data
radius RG~15 kpc (stars), ~18-20 kpc (HI); Sun distance from galactic center 8.5
kpc
luminosity
mass
75-80% DM, 15-20% disk, <5% bulge+halo
central BH
rotation periods: Solar neibourhood ~240 Myr (galactic year), bulge ~10 Myr
distances and velocities within the Milky Way
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
GAIA (~2012)
•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
http://www.atlasoftheuniverse.com/
mobile cluster method
Hiades Cluster is very nearby, and it is possible to
measure
a 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
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
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
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 d
and corresponding path differences are:
it is found:
LMC distance
0.83”
spectroscopic parallaxes
luminosity
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 absorption
for MS stars it works well: uncertainties ~ 10% luminosity, ~5% distance
for giants HR diagram is ~vertical: uncertainty ~50% luminosity, ~25% distance
spectral type
photometric variant, estimate spectral type from color
example: 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 enough
scale
scale
spectral
we can measure the spatial distribution of stars:
length
height
R
thin
disk
z
thick disk
type
thin disk and thick disk
older 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
F main sequence stars in
the Solar neiborhood (< ~40
pc)
the average velocity of
stars with respect to Sun
is negative because Sun
has positive velocity (+7
km/s) with respect to LSR
thick disk stars are usually metalpoor
(
)
thick disk could be the result of a “gas-rich merger” with a
satellite galaxy, where most thick disk stars were born in
situ
metal-poor stars
(
)
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
pleiades
isochrone
binary sist
open clusters are
absorbed by dust within
galactic disk, we can see
them only up to ~5 kpc. we
know ~1200 of them
globular clusters
for globular clusters there is no
absorption problem. ~130 are
known
globular clusters are old up to ~1215 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, m=0.3 =0.7: to~13 Gyr
metal-rich globular clusters
Z=1/3-1/10 of solar value
flattened distribution, may be part of
thick disk
metal-poor globular clusters
Z~1/300 of solar value
nearly 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 center
central density
, halves at ~ 2-3 pc from
center
resembles a globular cluster, ma still forms stars
there is gas inflow
total mass
central black hole
BH
model of distribution of
mass in the central region,
to account for observed
rotational velocities:
DM
central cluster
stellar orbits around Milky Way central BH
S2
Gillessen et al 2008
MBH=(4.31±0.06±0.36) x106
stat astrom
28 well determined orbits
S2 completed orbit
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
QuickTime™ and a
H.264 decompressor
are needed to see this picture.
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 velocities
+
S
-
+
+
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
dust
for 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
Oort 1952
Ro
R
contributions
of various
clouds along
the line of
sight
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 gas
it is found that V(R) doesn’t decrease either in the external parts of Milky Way
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:
[
for a flattened configuration like a disk
F≠GM/R2
and formula gives M(<R) with error ~10-15%
]
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)
Ro
there must be other matter, other than that visible in stars, the
Dark Matter, which is believed to be distributed within a dark
halo
dark matter
DM can account
for 80% of the
total mass: what
is it done of?
weakly interacting massive particles (neutrinos, neutralinos,
WIMPs: gravitinos ...)
and/or
difficult to detect directly [non-barionic dark matter]
massive compact halo objects (black
MACHOs: holes, planets, brown dwarfs, white
dwarfs ...) detectable by their effects of
gravitational microlensing [barionic dark
in the
matter]
(from Big Bang nucleosynthesis)
Universe:
most matter is DM
most DM is non-barionic also some is barionic
Local Group
1Mpc
M31
M33
Local Group galaxies
the 3 dominant galaxies
~90% of the LG Luminosity
the only elliptical
distances measured
through Cepheids P=L
relation
known within ~10% for
brightest
galaxies
nearest one
(low surface
brightness, discovered
1994)
boldface: Milky Way satellites
italics: M31 satellites
most are dwarfs:
dSph, dIrr, dE
Milky Way satellites
most lie close to a plane
carina dSph
LMC
SMC
fornax dSph
sculptor dSph
sagittarius dSph
sextans dSph
umi dSph draco dSph
NGC 147
Local Group
IC 10
M31(Sb)
NGC 185
NGC 3109 (Irr)
M32(E2)
(dE) (dE)
(Irr)
NGC 205(dE)
M31
M33
M33
(Sc)
velocities
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
radial velocities
easily measurable
subtracting solar motion, it is found that Milky Way and M31 approach each other at
V~120 km/s
most other galaxies have velocities within ~60 km/s from MilkyWay+M31 center of mass,
not enough to escape from LG:
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
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)
SMC measures 7o x 4o on the sky and is ~8 kpc
long
Magellanic Bridge: bridge of gas connecting the two
clouds
Magellanic Stream: long tail of gas behind SMC,
it is an elongated structure seen
roughly end-on, with depth ~15
kpc
no rotation motion. M(HI)/LB~1
Magellanic Clouds
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.
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 curve
it is found
SMC: from globular clusters and from
variable stars, it is found dSMC ~ 60 kpc
globular clusters (LMC), bimodal age dist:
many old (>~10 Gyr) and
metal-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
ages ofversion
SMC clusters
young
of LMC are
GCscontinuously
distributed between few and ~12 Gyr, with no
gap
HI map
LMC
30 Doradus
(Tarantula)
SN1987A
LMC
radio image in HI
SMC
NGC 362
(Milky Way)
47 Tucanae
(Milky Way)
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
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 periodluminosity relation is found:
from measurement of period it is determined the luminosity (standard
candle), and then from apparent magnitude the distance is found
also RR Lyrae can be
used:
Cepheid variables
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
cosmic distance ladder
Cepheids constitute an important
step in the cosmic distance ladder.
each measurement method must be
calibrated through the previous one
trigonometric
parallax
HST
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)
Carina
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
chemical abundances are relatively low <~1/30 than Solar.
metal rich gas could have been lost and transferred to Intra Group Medium
3 Gyr
7 Gyr
15 Gyr
spirals
M 31, Andromeda
M31(Sb)
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)
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
optical
IR: Spitzer
star-forming ring
IR: IRAS
spirals
in
shows loops, filaments and
shells due to SNe and stellar winds,
which heat the gas and stop star
formation (feedback)
M 33, Triangulum
tiny bulge: Sc or
Scd
smaller than MW:
hR~1.7 kpc
V(R)~120 km/s
HI disk very extended, ~3 Holmberg radii, i.e. ~30 kpc,
appreciable fraction of the distance M33-M31 (200 kpc)
very luminous nuclear cluster
stars (differently than for GCs)
with old, intermediate, and young
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 604
giant gaseous
nebula with strong
star formation
M33 in X-rays
it 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) ~10381041 erg/s
X-8
X-7
formation of Local Group galaxies
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
condensations which will later form
galaxies (protogalaxies) begin to
grow in regions of higher density
protogalaxies form close to each other
(Universe was smaller than now) and gain
angular momentum through tidal torques
formation of Local Group galaxies
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
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
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
chemical evolution
simplified scheme:
1 zone model: well mixed gas, with homogeneous chemical composition
instantaneous 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
yield (fraction of heavy elements in the gas returned from stars)
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
(closed box)
morever, it is supposed p indipendent of Z (primary elements: their
production doesn’t depend on the presence of other elements)
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 observations
of the Galactic Bulge, with
Z(0)=0 e
dwarf ellipticals and dwarf spheroidals
dSph are slightly more luminous
than GCs but much more diffuse
dE are more luminous versions
of 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
dE
dSph
relatively old stars > ~5 Gyr, but also
called dE in
young (100-500 Myr) in the central
the original paper
parts
(Binggeli 1994), now called dSph
M32 might be small version of a giant elliptical,
M32 elliptical with very high centralor a central remnant, deprived of the envelope and of
surface brightness
GCs
warm
in M32, compare MW ~7 (cold), dSph<<1
no stars younger than some
Gyrsystem:
(hot)
dwarf irregulars
irregular galaxies have asymmetric shape
star formation occurs in disorganized regions
called dwarf irregulars below ~
moderate rotation:
in dwarf irregulars
in giant irregulars and
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
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 associated X-ray
•is there an intragroup gas within Local Group?
emission?
- 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
( T ~ 106 K)
- it has been searched for an effect on the anisotropies of the cosmic microwave background
trhough
Sunyaev-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
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
angle
we 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 lens
in 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
moreover
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
I
S
dLS>>dL
dL=1 AU
the light of a star is deflected by ~ 2
arcsec
examples
2) star at distance dS: area of Einstein ring is maximum if Lens is half
way
dL
*
radius
when
dS
, area within Einstein radius
in fact: remember
is max
, say
max when x=1/2
examples
3) suppose Lens is an object with
Star
and that you observe a star at distance dS = 2dL Lens
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 source
and 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
assume that
then
if the Lens has proper motion
then:
S
L
,
magnification is a function of
time:
MACHOs
http://sirius.astrouw.edu.pl/~ogle/
http://wwwmacho.anu.edu.au/
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)
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