The Milky Way - Detailed Structure

Swinburne Online Education Exploring Galaxies and the Cosmos

© Swinburne University of Technology

Activity:

The Galactic Centre

‘Our Galaxy' modelled by ESO VLT Image of NGC2997

NROA VLA radio image

• Appreciate the challenges to observing the centre of our Galaxy.

• Know the observing tools which do penetrate to the Galactic centre.

• Learn of the objects currently observed at the Galactic centre.

Summary

This Activity should enable you to:

AAT 028 Absorbing material blocking visual observation of the Galactic centre

Recent historyThe story of the study of the central regions of our Milky Way Galaxy parallels:

• the progress in observation at wavelengths other than visual - infrared radio X-ray gamma ray• the progress in telescopes using these wavelengths - higher resolutions and orbiting telescopes.• the discovery of phenomena associated with the central regions of other galaxies black holes jets

Let’s briefly see some examples of these, in order to see what features we might expect to find at the centre of our galaxy.

High resolution of galactic centres

Even in visible light, the Hubble Space Telescope resolved a bright split source at the centre of the M31 galaxy in Andromeda.

180,000 light years

NOAO

Ground view of M31 core2,000 light years

40 light years

HST view of M31 nucleus

HST Black Hole ‘images’

A black hole itself, by definition, cannot be imaged.Radiation, emitted by gas and dust orbiting a massive object at high speed, is detectable.

Estimation of the rotation speed and the orbit radius leads (by Kepler’s 3rd Law) to the central mass and an upper limit to its diameter.

The HST has imaged several objects meeting black hole* criteria; this dramatic image is from an object at the centre of galaxy NGC4261.

*Click here to find out about black holes

Model: Material orbiting a central object - with higher velocities toward centre

slit

HST Imaging Spectrograph

UKS 024 The Virgo cluster

M84

HST M84 nucleus

slit

Finding: Orbital speeds of 400 km/sec within 26 light years of the central object.

HST Spectrograph black hole evidence

The HST imaged a spectrum of the core of the M84 galaxy (May 1997).

Doppler shift of central line

M84 central object mass

Compute the mass using the form of Kepler’s 3rd law*:

In the above units, d=1,641,000 AU and P=122,523 years.

Assuming m can be neglected compared with M, d3/P2 gives M=294 million solar masses!

The escape velocity for such a massive object, of upper size limit set by HST resolution, is greater than c, the velocity of light. By definition, a black hole is inferred.

Where mass m (solar masses) orbits mass M at a distance d (astronomical units) in a period of P (years).

d3/P2 = M + md3/P2 = M + m

Use the M84 finding of orbital speeds of 400 km/sec within 26 light years of the central object.

*Click here to revise Kepler’s 3rd Law

Jets from Galactic NucleiSome galaxies show jets of material emitted in opposite directions from their nuclei.

This galaxy's nucleus is the small point in the centre of the image. These jets impact material surrounding the galaxy, giving rise to the giant "lobes" of radio emission seen in this image. The energy required to produce these jets is believed to be due to the influence of a black hole millions of times more massive than the Sun.

Cygnus A radio source VLA

Back to our own Galaxy

With our appetite whetted by what may lurk at our Galaxy’s centre, what are the observational difficulties involved?

The previous Activity showed the difficulty in identifying spiral arms in our own Galaxy, though they are clearly evident in external galaxies.

The difficulty is worse when our target is the very nucleus of our Galaxy - some 8Kpc away across the densest regions of absorbing gas and dust in the Galactic plane.

Visible light is reduced by 28 magnitudes.

As with infrared night vision and for observations through fog or dust on Earth, other wavelengths are needed for astronomy of the Galactic centre.

Radiation reaching us from the centre

Astronomy now utilizes a wide range of the electromagnetic spectrum.

< 0.6nm X-rays detected from central region by orbiting Einstein X-Ray Observatory

< 0.6nm X-rays detected from central region by orbiting Einstein X-Ray Observatory

21 cm radio detects H I regions >100pc from centre

21 cm radio detects H I regions >100pc from centre

511 keV gamma rays detected from Galactic nucleus

511 keV gamma rays detected from Galactic nucleus

2.2m infrared enables detection of central old Population I K and M giant stars (temperature ~4000oK)

2.2m infrared enables detection of central old Population I K and M giant stars (temperature ~4000oK)

Instruments

A selection of ground based and satellite telescopes

Australia TelescopeNRAO 12m millimetre telescope at Kitt Peak

COBE SatelliteEinstein X-Ray satellite

Parkes 64m

Very Large Array

Abbreviations

From this point on, certain terminology (eg ‘centre’) will save repetition of full terms such as ‘the Galactic centre’.

Bulge - central ~3kpc diameter region of the Galaxy.Central region - central ~300pc diameter region.Centre - ~20pc diameter centre of Galaxy.Nucleus - ~3 parsec diameter core of Galaxy.

The region of the electromagnetic spectrum used for an observation will appear simply as 21cm, 2.2m, <0.6nm etc

Radio (0.4 GHz*)

Atomic hydrogen

Radio (2.7 GHz*)

Molecular hydrogen

Infrared

Near Infrared

Visual

X-Ray

Gamma Ray

Location key

Multi wave-length imagesThe website http://adc.gsfc.nasa.gov/mw/

nicely presents multi- wavelength panoramic views along the plane of the Milky Way, of which just 60o either side of the centre (l=0o) are reproduced here.

The website includes references to authors, observations and background material.

*Click here to be reminded about GHz

Highlights

Radio (0.4 GHz*)

Atomic Hydrogen

Radio (2.7 GHz)

Molecular H2

Infrared

Near Infrared

Visual

X-Ray

Gamma Ray

Location key

Note the hopelessness of visual observations of the Galactic centre.

Note the high central intensities in:

• near infrared

• 0.4GHz radio

• gamma ray

Note the quiet centre for atomic hydrogen.

The Galactic Bulge

IRAS 12m shows strong sources from asymptotic giant branch (AGB) stars in the H-R diagram*.

Apart from the very centre, the rotation curve* shows that bulge stars rotate with similar periods (like a solid body) with higher velocities for larger orbits about the centre.

The bulge, about 3kpc diameter, comprises heavy element enriched stars, especially type M giants, Pop I K giants, and a few metal rich RR Lyrae stars*.

As we journey to the centre of the Galaxy, we take a quick glance at its central bulge.

* Click here to revise H-R diagrams * Click here to find out about RR Lyrae stars

*we met rotation curves in the Activity on Galactic Rotation

The derived bulge mass is some 10 billion solar masses.

106

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Radius (pc)0.1 1 10 100 1000

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The inner Galactic Bulge

The high central stellar density affects the velocity curve.

Mechanical energy exchange from close stellar encounters should lead to a close-to-flat rotation curve, with enclosed mass proportional to orbit radius.This is confirmed, from various indicators, down to r = 2pc.Closer in, velocities increase significantly, suggesting

~4x106solar masses in the inner 0.5pc.

rM r

2.2m from old Pop I K and M giants indicates a high central stellar density.

Radio Map of Central Region

Continuous radio emission from the Galactic central region shows a string of radio sources in the galactic plane.

Centre of GalaxyCentre of Galaxy+15’

0o00’

-15’

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Sagittarius B Sagittarius A

The strongest source is Sagittarius A (Sgr A), followed, like the split source in M31, by nearby Sgr B.

This region is about 270x90 parsecs.longitude 0o00’0o30’1o00’ 359o30’Galactic

Galactic equatorGalactic equatorB1B2

C

Sources of energyAs we introduce each type of source detected in the Galactic centre region, we will consider what it might consist of - from the point of view of energy production or mass involved (in solar units).

For example, some of the sources in the last frame show characteristics of HII regions.

The O and B stars necessary to keep these regions ionized and emitting radiation, is estimated to be equivalent to about five million Suns (close to that from velocity measures). To a first approximation this is about 7 times the density of stars in the solar neighbourhood.

… Night skies would be rather bright!

Clues to magnetic fields

Unusual filamentary features appear near Sgr A.

20cm radiation produced by synchrotron radiation* reveals filaments which stretch for 20pc, at right angles to the galactic plane, and then make an almost right-angle turn.

Sgr A

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From the strength and polarization of the radiation, magnetic fields would be two to four orders of magnitude weaker than the Earth’s magnetic field.

*Click here to find out about synchrotron radiation

High resolution radio information

We now turn to high resolution radio mapping of the Galactic centre, for which the Very Large Array (VLA) - introduced in the next frame - has been at the forefront.

The Very Large Array (VLA)

A high resolution radio interferometer.

Near Socorro, New Mexico, the VLA consists of 27 antennas arranged in a huge Y pattern up to 36km across. Each antenna is 25 meters in diameter. They are combined electronically to give the resolution of an antenna 36km across, with the sensitivity of a dish 130 meters in diameter. At its highest frequency, 43GHz, its resolution is 0.04” arc.

NRAO Photo by Dave Finley

Internet: http://www.nrao.edu/vla/html/VLAintro.shtml

RA

Dec

Sgr A EastSgr A East

Sgr A WestSgr A West

Sgr A*Sgr A*

The VLA resolves Sagittarius AThis image, from 6cm and 20cm radiation, resolves detail down to ~2” arc. It shows the following components:

Sgr East: A non-thermal shell-like structure, usually interpreted as a supernova remnant.

Sgr West: A spiral shaped thermal source, like an HII region.

Within Sgr West is a non-thermal point source <0.1” diameter, given the name Sgr A* --pronounced Sadge-A-Star

Sagittarius A West

Here we show the various named regions of this complex.

RA 17h42m31s 29s 27s 25s

1 parsec

Northern arm

Western arc

Dust and gas disk

Galactic equator

Background

Sgr A*far infrared 40-300m

2cm microwave

Diagram indicative only

Eastern arm

Bar

-29o00’

-28o59’

-28o58’

N

E

Dec

Doppler shifts, from NeII infrared emission at 12.8m, reveal high velocities in the ‘bar’ region.

The various ‘arms’ of the mini-spiral pattern are as labelled.

The Sgr A* radio luminosity is ~2x1027 W from within a diameter of less than 20AU.

The next frame gives a mass estimate from the higher velocities within the ‘bar’ region.

The general nature of their radial velocity (recession, approach) is indicated (up to ~130km/sec).

Sgr A*Sgr A*

1 parsec

The Sgr A West mini-spiral and Sgr A*

Rotation velocities increase toward the site of Sgr A*.

BarBarEastern armEastern arm

Northern armNorthern arm

Western arcWestern arc

Mass estimate within Sgr A*

A gas cloud r=0.3pc from the centre has a measured velocity of v=260km/sec. If this is orbiting a central mass, calculate that mass.

Use either M=v2r/G in standard units and work through to a result in solar masses, orKepler’s law, r3/P2=M, which gives M in solar masses if we first calculate distance r in AU and period P in years.

In this case r=0.3 parsecs or ~61679 AU and P=7089 years, leading to M = 4.7 million solar masses! Could Sgr A* be a massive black hole? The Schwarzschild radius (within which light cannot escape) is Rs=2GM/c2 = 0.09AU

This is well below the current resolution limit.

X-ray emission

Time variable X-rays have been detected from the region of Sgr A West including Sgr A*.

One X-ray mechanism involves accretion disks around dense stars - white dwarfs, neutron stars or black holes - another hint to the nature of Sgr A*.

X-ray images of the Galactic nuclear region

The speed of light limits the diameter d of an object from which time fluctuations t of radiation are observed: d<ct

The upper limit for the Sgr A West source is 0.1pc.

Gamma rays

Gamma rays at 511 keV have been observed from a source less than 0.3pc diameter almost coincident with the Galactic centre.

511keV, the rest mass energy of an electron, is a signature of electron-positron annihilation.

Since it is believed black holes can produce positrons in the space around them, this seems to support a black hole as a candidate for Sgr A*. However the enormous 511keV luminosity of about 5x104 times the solar luminosity implies a smaller black hole (~500 solar masses) than that envisioned for the Galactic centre.

VLA =90cm - central region, wide field

Compare with the earlier Radio Map.Note the shell-like structure of super-novae remnants (SNR’s).

Note the fine ‘thread’s at high angles to the Galactic plane and extending for tens of parsecs.

NRAOImage: Kassim, LaRosa, Lazio & Hyman 1999

Supernovae activity

Even higher energy 1.8MeV gamma rays have been detected.The 1.8MeV line is produced by the decay of 26Al to 26Mg.

26Al has a half-life of 716,000 years and is only produced in small amounts in supernovae & novae explosions

and possibly Wolf-Rayet * stars.

The detected presence of ~5 solar masses of 26Al suggests that a large number of supernovae have occurred in the Galactic centre over the last million years.

It certainly appears to be an active environment!

*Click here to find out about Wolf-Rayet stars

The importance of Galactic centre studiesOther galaxies also appear to have black holes at their centres. Some are relatively quiet while others have extremely active nuclei.

Back to our own Galaxy, the rotational dynamics, mass distribution and energy processes of the overall Galaxy may lead to the production of high mass density (including black hole(s) at the centre, or

if supermassive black holes were produced at the galaxy’s embrionic stage they may in some way power other features of the Galaxy - even spiral arms.

Wavelength Telescope Region Sourceradio nucleus Sgr A and a string of sources; HII

and SNR characteristicsIR/Radio Nucleus metal-rich giants, low mass dwarfs

40-300m Dust at Sgr A heated by OB stars12-20M centre Dust heated by PopI and O stars12.8m nucleus NeII emission; Doppler shift

200km/sec within 1.5pc of centre 12m IRAS Bulge AGB stars 2.2m centre PopI K giants <0.6 nm X-ray Einstein <100pc weak sources in weaker halo~10-3 nm ray nucleus <0.3pc size, at or near nucleus

Wavelength Telescope Region Sourceradio nucleus Sgr A and a string of sources; HII

and SNR characteristicsIR/Radio Nucleus metal-rich giants, low mass dwarfs

40-300m Dust at Sgr A heated by OB stars12-20M centre Dust heated by PopI and O stars12.8m nucleus NeII emission; Doppler shift

200km/sec within 1.5pc of centre 12m IRAS Bulge AGB stars 2.2m centre PopI K giants <0.6 nm X-ray Einstein <100pc weak sources in weaker halo~10-3 nm ray nucleus <0.3pc size, at or near nucleus

Summary i)

Some of the wavelengths and sources we’ve visited.

Summary ii)

All the observations point to massive objects within a very small radius of the Galactic centre.

They may take the form of:

a) a massive black hole of ~4x106 solar masses,

b) a very dense star cluster of ~106 solar masses within 2pc of the centre.

Additional support for the black hole scenario comes from similar evidence in other galaxies.

AAT images © David Malin (used with permission):http://www.aao.gov.au/local/www/dfmIndividual Malin images (© David Malin (used with permission)), shown with a 6 character code - such as AAT028, - are found at the website ending with that code; eg:http://www.aao.gov.au/local/www/dfm/aat028.html

Multiwave galactic plane imageshttp://adc.gsfc.nasa.gov/mw/

Galactic Centre X-ray imagehttp://antwrp.gsfc.nasa.gov/apod/image/9807/galcen_sigma2.gif

Australia Telescope Compact Array and Parke Telescopeshttp://www.atnf.csiro.au/overview/telescope.html

Image Credits

Hubble Space Telescope images indexed by subject:http://oposite.stsci.edu/pubinfo/subject.html

ESO (European Southern Observatory) VLT images:http://www.eso.org/outreach/info-events/ut1fl/astroimages.html

NRAO VLA 90cm radio image of Galactic centre regionhttp://www.nrao.edu/intro/galsrc.html

NRAO VLA site imageshttp://info.aoc.nrao.edu/

VLA: Cygnus A http://www.nrao.edu/vla/html/VLA-images.shtml

COBE and Einstein satellite pictures:http://www.gsfc.nasa.gov/astro/cobe/slide_captions.htmlhttp://asca.gsfc.nasa.gov/docs/einstein/heao2.html

Image Credits

VLA

The origin of the Milky Way is the subject of the next Activities.

Hit the Esc key (escape) to return to the Index Page

Thus: f=c/ or =c/f c=3x105 km/sec

What wavelengths would 2.7Ghz and 0.4 GHz be?

Frequencies and wavelength

Explanation

Since the days of tinkering with valves, radio astronomers often refer to frequencies (f) rather than wavelengths ().The frequency of passing wavecrests = speed of wave / wavelength

2.7GHz: = 3x105/(2.7x109) = 1.11x10-4 km = 11.1 cm

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The previous frame showed radio observations expressed in GHz. (GigaHertz or 109 cycles per second)

What frequency is the 21cm hydrogen line?

f = 3x105/(.21x10-3) = 1.428x109 = ~1.4 GHz

0.4GHz: = 3x105/(0.4x109) = 7.5 x10-4 km = 75 cm

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Introduction to Synchrotron Radiation

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The following section is a brief introduction to thermal and non-thermal processes, and in particular, synchrotron radiation.

Thermal Radiation

Conventionally, thermal radiation refers to black body radiation at a given temperature.

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In astrophysics the term thermal includes absorption, emission and scattering processes arising from any interactions between electrons and atoms or molecules in a hot medium - including:• excitation/de-excitation1 within the atom• ionization/recombination2 to/from free electrons• free-free processes3 between electrons, photons and ions.

1 2 3

Electrons (mass m, charge q) spiral, in magnetic field B, at angular frequency =qB/(mc)c = speed of lightv = electron velocity= 22 cv11 //

Non-thermal radiation

Involving physical processes not dependent on temperature. (Including the MASER process, not covered here.)

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Non-thermal processes include synchrotron radiation from electrons, moving at near light speeds, and spiralling along magnetic flux lines. The radiation is polarized and the process relativistic.

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About Wolf-Rayet StarsAlthough we feel that we know a lot about stellar evolution, (even if only through indirect evidence), there are still some fascinating stellar objects which are hard to explain.

Wolf-Rayet stars are very hot (T~30,000 K), massive (perhaps 10 to 40 M) stars which are often found in binary systems (which we use to estimate their mass), are losing mass at very high rates, and exhibit strong, wide emission lines of nitrogen, oxygen and carbon and weak or nonexistent hydrogen lines.

It is believed that the high rate of mass loss in these (probably) post-main-sequence stars has stripped them of most of their hydrogen envelopes, exposing nuclear processed material in inner layers near their cores.

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If indeed these stars turn out to be typically in binary systems, they may turn out to be the more massive and faster evolving partners. Theoretical models suggest that a Wolf-Rayet star in a binary system is just past its red supergiant stage, where much of its envelope has swollen up and spilled over onto its companion,

and just before it undergoes a supernova explosion!However without more evidence, we can’t be sure exactly what these intriguing stars are.

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Giants

Lu

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Temperature (K)

The HORIZONTAL BRANCH

Inst

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…that also happen to fall within the “Instability Strip”, the region of the H-R diagram which contains variable stars.

RR Lyrae Variables are stars on a region of the H-R diagram called the “helium burning Horizontal Branch”

RR Lyrae Starsb

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The great thing about RR Lyrae Variables is that they are bright and all at about the same absolute magnitude

This is because RR Lyrae Variables all have about the same mass and are all at the same phase in their evolution.

As long as we can find such stars their brightness immediately tells us their distance, and therefore the distance to the cluster they are in.

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H-R Diagrams

In 1905 the Danish astronomer Ejnar Hertzsprung noticed that a graph of the absolute magnitudes of stars versus their colour showed a few very regular groupings.

A bit later on, Henry Russell in America noticed the same thing, although he used spectral type rather than colour.

blueblue yellowyellow redredA

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+15faint

Spectral typeSpectral typeO5O5 M8M8

That’s why the diagrams you are about to study are called Hertzsprung-Russell Diagrams (H-R for short).

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nd Temperature versus Type

Later on, when the link between spectral type and temperature was realised, H-R diagrams began to appear with temperature along the horizontal axis instead.

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Spectral typeSpectral typeO5O5 M8M8

Boring! Why have yousuddenly gone all historical?

Boring! Why have yousuddenly gone all historical?

4000040000 temperaturetemperature 25002500

Because we have to explain why temperature goes down along the horizontal axis of an H-R diagram: a long time ago,

astronomers listed stars by colour, from blue (hot) to red (cool).

Ahhhh.Ahhhh.blueblue Colour Colour redred

O B A F G K M

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White dwarfs

Red dwarfs

Main sequence

Super-giants

Giants

H-R diagrams and spectral classes

We’ll use this version of an H-R diagram to show how spectral classes appear in that format.

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Looking for patterns

high temperature low

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hHuge, cool stars appear in the top right, and small, hot stars tend to gather in the bottom left.

But the rest of the stars lie somewhere along the main sequence.

T increasingT increasing

L in

crea

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L in

crea

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Mass increasing

Mass increasing

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M mradius

d period P

G and 42 are constants

M is the mass of the Sun

Johannes Kepler’s third law for planets: There is a fixed relationship between the cube of the radius (d) of a planet’s orbit and the square of its period (P) of orbit.

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In other situations where objects are in orbit the law still applies, but if the mass m is not tiny compared to M then the formula becomes

d3/P2 = M + md3/P2 = M + m

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Black HolesWhen a star more massive than 8 M reaches the end of its life, the star’s gravity is so strong that it collapses into an object of zero radius and infinite density - a black hole.

The gravitational field of a black hole is so strong that even light cannot escape. For this reason, black holes are not directly observable.

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