Astronomy 1G 2011-12 Galaxy Distances and the Hubble Flow Prof Andy Lawrence Astronomy 1G 2011-12 Distance ladder recap • Step-1 : Venus-radar + Keplers law → size of Earth's orbit (AU) • Step-2 : Parallax of nearest stars → calibrate stellar luminosities vs type • Step-3 : Main sequence fitting / recognising stellar types → distances throughout Milky Way • External galaxies : can we recognise individual stars ? Astronomy 1G 2011-12 Can we see stars in external galaxies ? • Detecting a solar-like star – absolute mag M = -5 – at D= 1Mpc apparent mag m = 25 : very hard to detect • Resolving a solar-like star – Consider a 1 arcsec box on image – At D = 1pc is 1AU across – At D = 1Mpc is 4.8 pc across contains light from dozens of overlapping stars • ==> even the nearest galaxies are unresolvable • Supergiant stars : M= -7 – D=1Mpc m=13 ; also they are further apart – the most luminous stars can be detected above the galaxy background • ... but only for very nearby galaxies.....
Transcript
Astronomy 1G 2011-12
Galaxy Distances and the Hubble Flow
Prof Andy Lawrence
Astronomy 1G 2011-12
Distance ladder recap
• Step-1 : Venus-radar + Keplers law → size of Earth's orbit (AU)
• Step-2 : Parallax of nearest stars → calibrate stellar luminosities vs type
• Step-3 : Main sequence fitting / recognising stellar types→ distances throughout Milky Way
• External galaxies : can we recognise individual stars ?
Astronomy 1G 2011-12
Can we see stars in external galaxies ?• Detecting a solar-like star
– absolute mag M = -5– at D= 1Mpc apparent mag m = 25 : very hard to detect
• Resolving a solar-like star– Consider a 1 arcsec box on image – At D = 1pc is 1AU across– At D = 1Mpc is 4.8 pc across
contains light from dozens of overlapping stars
• ==> even the nearest galaxies are unresolvable• Supergiant stars : M= -7
– D=1Mpc m=13 ; also they are further apart– the most luminous stars can be detected above the galaxy background
• ... but only for very nearby galaxies.....
Pulsating variables
The easiest stars to spot are pulsating variables, which change in brightness periodically over 10-100 days : Cepheids and similar stars
From ATNF websitehttp://outreach.atnf.csiro.au/education
Why do they do this ?
In normal stars gravity balances pressure. What would happen if you could expand a star ? It would oscillate ...
In most stars, if this oscillation starts, it will damp down, and star returns to equilibrium. In some ranges of physical conditions, the oscillation does not die down, and the star oscillates in both luminosity and temperature
Period-luminosity lawAAVSO
Henrietta Leavitt
Henrietta Leavitt showed by observing Cepheids in the LMC that more luminous stars have longer periods. So if you measure the period, you know the luminosity and can get the distance.
The oscillation period must be related to the free-fall time near the surface of the star. This will be longer for bigger, more luminous stars.
ATNF
Cepheids in the Local Group
Edwin HubbleObservatories of the Carnegie Institution of Washington
In the 1920s Hubble made the first Cepheid measurements in M31 and M33 and established that these were external galaxies.
Modern values of distance :M31 = 780kpcM33 = 900kpc
Hubble's 1926 photograph of M33 with Cepheids marked and some example light curves
Astrophysical Journal, 1926
Cepheids in nearby groups
Cepheid variables in M81 NASA/STScI
The Hubble Key Project gave reliable distances to eighteen nearby galaxies, enough to make the next rung in the Distance Ladder
Hubble Space Telescope can resolve out Cepheids several times further away than we can from the ground.
M81 D=3.4Mpc
Freedman et al 1994
Supernovae as standard candles• Supernovae are rare : one
per century per galaxy• But there are thousands of
nearby galaxies ... so we catch supernovae in nearby galaxies quite often
• There are two main types of supernova• Core collapse (Type II)• White dwarf thermonuclear runaway (Type I)
• SN1a always have the same abs.mag at peak : MB=-19.3• So even at 100Mpc is m=16.3... can be seen far away
Tully-Fisher method
Measure velocity width of HI emission from whole galaxy : gives rotation velocity
This gives mass of galaxy. If typical stellar mix is constant, and ratio of dark matter to stars, then the rotation velocity will also be proportional to the luminosity of the galaxy.
In practice, we calibrate velocity-width versus luminosity for nearby galaxies of known distance.
Then this calibration can be used to get luminosities of other galaxies, and so their distances.
From Scholarpedia pagemaintained by Brent Tully
Galaxy velocities
• Velocities measured by Doppler shift of starlight or HI emission
• Very nearby galaxies show both blueshifts and redshifts .. e.g. M31 is approaching at 295 km/s
• But more distant galaxies always redshifts.• Hubble first showed that more distant galaxies have
larger redshifts : the Hubble law
V = cz = H0 D H0 = "Hubble's constant"
• Caused by the expansion of the Universe - see later.
The Hubble Law
From John Huchra's web page
Combining Cepheid, Tully-Fisher, and Supernova distances, the Hubble Law can be clearly seen.
With D in Mpc, and V in km/s, the best-fit value of Hubble's constant is
H0 = 72 km s-1 Mpc-1
e.g. if you measure the redshift of a galaxy to be z=0.032, then the recession velocity is v=cz=9593 km/s, and the distance is D=V/H0 = 133.2 Mpc
Hubble Wars
The scale of the Universe has only recently become clear. Hubble had the Cepheid calibration wrong; and for several decades there was a kind of war between the H0=100 camp and the H0=50 camp.
They were both wrong.
From John Huchra's web page
Astronomy 1G 2011-12
Distance ladder recap• Step-1 : Venus-radar + Keplers law → size of Earth's orbit (AU)
• Step-2 : Parallax of nearest stars → calibrate stellar luminosities vs type
• Step-3 : Main sequence fitting / recognising stellar types→ distances throughout Milky Way
• Step-4 : Light curves of Cepheids and similar stars→ distances to ~10Mpc
• Step-5 : Tully-Fisher and Supernova methods→ distances to ~300 Mpc→ calibrate Hubble flow
