A4 Galaxies > GAL2 - Galaxy Formation & Evolution > Flashcards

GAL2 - Galaxy Formation & Evolution Flashcards

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1
Q

what happens when galaxies merge?

their stars do not collide but

A

some of the galaxies’ kinetic energy is transferred to the random motion of the stars

(analogy- busy train station - crowds will slow everyone down but not very many collisions)

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2
Q

galaxies experience a ‘drag’ force known as

A

dynamical friction

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3
Q

drag force of each galaxy depends on

A

mass of galaxy

velocity of galaxy

mass density of neighbour

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4
Q

mass of galaxy enters via Newton’s law of gravitation so the drag force will be a function of

A

GM

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5
Q

drag force can be shown via dimensional analysis to be

A

Fdrag prop to (GM)^2p/v^2

can also use this to estimate the timescale for eg a globular cluster orbiting a parent galaxy to spiral into the galaxy’s centre

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6
Q

why a drag force?

consider a star cluster (approx as point mass) moving through a background of less dense stars and dark matter

A

gravitational pull of the sat cluster attracts background matter towards it

cluster will have already moved on whilst matter attracted to where it used to be

get overdensity where cluster used to be

this acts like a drag force, gravitationally pulling star cluster and slowing it down

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7
Q

high speed collision of two disk galxies

when two disk galaxies interact with great enough velocity, they are

A

not slowed down enough to become a bound pair

the galaxies separate but their disks are ‘disheveled’ - stars acquire random motions, causing disks to ‘puff up’

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8
Q

flyby galaxies

the interactions from stars acquiring random motions, can lead to

A

the formation of spiral arms or bars

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9
Q

multiple ‘close encounters’ may

A

destroy disks all together

explains lack of disk galaxies in the core of rich clusters (interacting more often given the density)

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10
Q

slower collisions or fly by causes

A

a much greater disturbance - particularly if co-planar and the direction of the fly-by aligned with direction of motion

travelling slower = stars spend a lot more time in close proximity

interaction draws out a long tidal tail which can persist for several Gyr

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11
Q

analogy to help remember what fast/slow collision/fly-by causes

A

ripping a sheet of paper

if you rip is quickly get a clean cut

if you rip it slower gets messier

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12
Q

observations of antenae galaxies show

A

lots of star formation - kickstarted by the collision of galaxies

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13
Q

observations of mice (with tail) galaxies show

A

in the process of interacting - predicted to merge in the future

can use simulations to predict this

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14
Q

sometimes, instead of the ‘disturber’ drawing out a tidal tail from the first galaxy, the first galaxy can

A

tidally strip gas and dust from the disturber

this leaves behind a dust lane

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15
Q

what is a dust lane

A

a fresh supply of gas and dust which can kick-start new star formation, even in an elliptical galaxy

well known examples are Centaurus A and the Sombrero galaxy

often see star formation around dust lane

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16
Q

a close passage of two gas-rich spirals can produce a

A

starburst galaxy (interaction really kicks off the star formation)

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17
Q

a close passage of two gas-rich spirals produces a starburst galaxy

how does this happen

A
  1. disk gas is pulled away from near-circular orbits
  2. gas clouds collide at high speed, causing shocks
  3. this compresses gas to very high density (also high T) which are perfect conditions to trigger lots of star formation
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18
Q

a head on collision can produce a

A

polar ring galaxy

disruption of diffuse galaxy and creation of ring, we know these should exist through simulations and have also observed eg cartwheel galaxy

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19
Q

why should a polar ring form?

overview of derivation

A

virial theorem applies before collision

happens quickly so potential energy does not have time to change at first

stars gain KE from the disturbing galaxy - thrown out of equilibrium

after some time, re-virialises

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20
Q

why do polar rings form?

just after interaction, galaxy has gained ΔK of KE and when virialised agin, has lost ΔK of KE

one way this happens is

A

convert excess KE into PE

ie a shell of galactic material expands outwards, losing KE and gaining PE

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21
Q

polar ring forming

stars in the galaxy gain KE from the disturber and the galaxy KE increases to

A

Kafter = Kinit +ΔK

galaxy is thrown out of equilibrium

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22
Q

polar ring forming

just after the interaction, Eafter=

A

Kafter + Uinit (potential not had time to change)

=Kinit +ΔK +Uinit

=Einit +ΔK

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23
Q

polar ring forming

when virialised, the galaxy’s total energy satisfies

A

E=K+U

= approx K-2K = -K

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24
Q

polar ring forming

once virial equilibrium has been restored, Kfinal=

A

-Efinal = -(Efinal+ΔK)

=Kinit -ΔK

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25
what do observations of ring galaxies show
expanding ring of star forming material perfect ring galaxy observed when aligned perfectly with LOS
26
observation of ring galaxy no sign of galaxy it collided with this could be due to
either collided a long time ago and ahs since moved on or the other galaxy was broken up during the collision
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interactions with lower approach velocities may lead to a merger - possibly after an elaborate 'courtship' the final appearance of the galaxies depends on
the mass and speed of the perturber and their orientation during the interaction
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since galaxies contain stars, to understand galaxy formation we need to
understand star formation (much more complicated problem than following the evolution of dark matter - which only requires gravity)
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to understand star formation, we need to understand
-How and where do stars form in galaxies? -What determines stellar masses? -What determines stellar luminosities? -What determines stellae chemical compositions? -How do all of these depend on galaxy type, age and redshift?
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a good way to compare star formation models with observation is by
spectral synthesis
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we can compute a synthetic spectrum for our model galaxy, accounting for
age of galaxy chemical composition (metallicity) initial mass of stars and gas rate at which new stars form redshift of observation (can then compare with observed spectrum to best fit galaxy properties)
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by metallicity, what do we mean?
abundances of heavy metals (in astro anything other than H or He)
33
what do the spectral energy distributions (SED) from eg Bruzual and Charlot 2003 assume
solar metallicity (ie the same chemical abundances as the ISM in the vicinity of the solar system) a single burst of star formation at time t=0 with ages ranging from 0.001 Gyr to 13 Gyr
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points to note from spectral energy distribution plot at short wavelength
young=huge luminosity at short wavelength (new stars - blue/UV also get blue/uv from white dwarfs
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points to note from spectral energy distribution plot at long wavelength
over time a stars evolution depends on mass - larger stars evolve quicker (less time on MS) get peak at redder end from older red giants spectrum shape evolves very little between 4 and 13 Gyr (hard to distinguish age of galaxy)
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what is the Lyman Break?
sharp drop in flux for wavelength <100nm
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why does the Lyman break occur?
1. there aren't many stars hot enough to produce UV photons in great numbers 2. any UV photons that are produced can ionise HI clouds so a large fraction of them are absorbed before they reach us
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what is the 4000 Angstrom break?
sharp drop in flux for wavelengths <400nm
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why does the 4000 Angstrom break occur?
1. there aren't many stars hot enough to produce UV photons in great numbers 2. Metal lines absorb much of the light at wavelengths shorter than 4000 angstrom
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where else have we seen the 4000 angstrom break
in spectra for S0 and Sb galaxies
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although spectrum shape varies little with age, the strength of spectral absorption lines depends on
metallicity (we see stronger absorption features i metal rich galaxies)
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lines associated with metals are stronger in
metal-rich galaxies (but note degeneracy here, as older galaxies are usually also more metal-rich, as metals build up after many generations of star formation
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we define the stellar birthrate function B(M,t) as
B(M,t)dMdt = number of stars per unit volume with masses between M and M+dM formed between t and t+dt (equivalently per unit surface area for disk galaxies instead of per unit volume)
44
stellar birthrate function usually we assume
B(M,t)dMdt = ψ(t) ξ(M) dMdt where ψ(t) is the star formation rate (SFR) and ξ(M) is the initial mass function (IMF)
45
star formation rate various models can be adopted for ψ(t) including
1. instantaneous burst (delta function) 2. constant SFR 3. steep rise and exponential decay or can model a combination eg (a)+(c) for feedback models
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the initial mass function is generally modelled as
a power law ξ(M) prop to M^-(1+x) different forms for this including different values of x for different mass ranges
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as new stars form and old stars leave the MS, what changes?
the chemical composition of the interstellar medium
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how to most easily model the change in the chemical composition of the interstellar medium
a drastic over-simplification known as the one zone, instantaneous recycling closed box model
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assumption for the one zone, instantaneous recycling closed box model
-the galaxy's gas is well mixed; same composition everywhere -massive stars return their nuclear products to the ISM rapidly -no gas escapes from the galaxy or is added to it -all elements heavier than He maintain the same proportion relative to each other
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assumption for the one zone, instantaneous recycling closed box model what is meant by 'massive stars return their nuclear products to the ISM rapdily'
this is the 'instantaneous' part of the model stars go through the MS instantaneously
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assumption for the one zone, instantaneous recycling closed box model what is meant by 'no gas escapes from the galaxy or is added to it'
this is the 'closed box' part of the model no mergers/collisions etc
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assumption for the one zone, instantaneous recycling closed box model what is meant by 'all elements heavier than He maintain the same proportion relative to each other'
lump together everything other than H and He
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chemical evolution models what is Mg(t)
mass of gas in the ISM at time t
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chemical evolution models what is Ms(t)
mass in lower mass stars and remnants of high mass stars (white dwarfs, neutron stars, black holes...)
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chemical evolution models what is Mh(t)
mass of metals: elements heavier than He in the ISM
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chemical evolution models what is Z(t)
=Mh(t) / Mg(t) the metallicity of the ISM
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chemical evolution models we can model the evolution of the contents of the box by
considering small time steps of Δt
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in one time step, what is formed?
A mass ΔMs' is formed from the interstellar gas
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massive stars go through their lives rapidly while low-mass stars are long-lived so in one time step, Ms(t)
increases by ΔMs(t) (a mix of low mass stars and remnants from high mass stars)
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in one time-step, what is returned (instantaneously) to the ISM?
a mass pΔMs of metals is returned (instantaneously) to the ISM, where p is the yield.
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the yield will depend on
stellar nucleosynthesis and the proportions of low and high mass stars
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in a time step, the mass of metals in the ISM increases by
ΔMh=pΔMs - ZΔMs pΔMs (the mass returned to the ISM) ZΔMs (the mass of metals that are locked away in low mass stars and stellar remnants)
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mathematically, ΔMh=
Δ(MgZ) =ZΔMg + MgΔZ (from the product rule)
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since = ΔMh = ZΔMg + MgΔZ, we can write ΔZ as
pΔMs - Z(ΔMs+ΔMg) / Mg
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if no gas enters or leaves the system then
ΔMs + ΔMg =0 ΔMs = -ΔMg
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since ΔMs + ΔMg =0, we can rewrite the equation for ΔZ as
ΔZ/ΔMg = - p/Mg
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if p is independent of Z, then we can solve for Z(t) Z(t)=
Z(t)=-plnMg(t) +const or Z(t)=Z(t=0) + pln[Mg(t=0) / Mg(t) ]
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what does Z(t)=Z(t=0) + pln[Mg(t=0) / Mg(t) ] tell us
metallicity increases with time, as stars are formed and the gas is in the ISM is steadily used up (from eqn, Mg goes down over time so [ ]<1 so ln gives +ve number and then Z(t) increases)
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the mass of stars formed before time t, and hence with Z
Mg(0) - Mg(t)
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rearranging Ms(
Ms(
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differentiating, the mass of stars with metallicity between Z and Z+ΔZ is dMs/dZ ΔZ=
Mg(0)/p exp[-Z(t)-Z(0)/p]ΔZ
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our model predicts that the distribution should fall off exponentially we can test the model using
galactic bulge observations (through Baade's window) gives reasonably good fit with Z=0 at t=0 and p=0.7Zsun but some issues
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we can also apply the model to the solar neighbourhood what are Mg, Z and p?
Mg around 50 solar masses per pc squared the sun is more metal-rich than the solar neighbourhood gas =0.7 Zsun assuming Z(0)=0, Znow=pln(50/13) so p=0.5Zsun
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solar neighbourhood gives p=0.5Zsun this is lower than for the bulge (doesn't fit box model) adaptation to model
'leaky box' model could some of the metal-enriched gas, recycled by supernovae, have leaked away from the solar neighbourhood? (and then not available to later stars)
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another problem with model from expression for Ms(
Ms(<0.25Zsun) / Ms(<0.7Zsun) = 0.52 ie more than half of the stars in the solar neighbourhood should have metallicities less than a quarter of the Sun's this has NOT been observed in surveys
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G-dwarf problem
in a survey of 132 G-dwarfs, only 33 had less than 25% of solar iron abundance (expect 60 odd) and only 1 had a less than 25% solar oxygen abundance
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the G-dwarf problem highlights the limitations of
the one zone, instantaneous recycling closed box model
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we can resolve the G-dwarf problem if we suppose that the
initial metallicity of the solar neighbourhood was not zero ie the gas was pre-enriched (by earlier star formation?) when it arrived at the solar neighbourhood
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G-dwarf problem The abundances of different heavy elements vary relative to each other. Also, abundances show a large scatter even for stars of a given age this suggests
subsequent inflow of fresh, metal-poor gas may have diluted the ISM but may not have mixed evenly with the gas already present
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simple extensions to the closed box model are
the leaky box model (gas is lost from the system over time) the accreting box model (gas enters the system over time)
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increasingly, gas dynamics and sophisticated star formation models are being incorporated into numerical galaxy simulations so we no longer need to
consider only simplified approximations such as the closed-box models much still to be done and a lot of complicated physics remains to be treated properly
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our recipe for galaxy formation also depends on the
background cosmology we develop this further in section 4
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A4 Galaxies
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