X-rays represent a tiny fraction of the energy output of galaxies.
e.g. Arp 220:
This is unlike AGN (e.g. NGC 5548) where X-rays a dominant component.
So why do we want to study them?
Let’s see where the emission comes from…
Galaxies are historically classified based on their aspect at optical wavelengths. The most famous of these classification schemes is the Hubble diagram (figure on the right).
In normal galaxies, e.g. those which are not dominated by an active nucleus (see next lectures), the high-energy component is due to the integrated emission of several different processes which were discussed in previous lectures:
As discussed in a previous lecture, accretion of material from primary to compact secondary (black hole or neutron star): infalling material heats up to ~106 K giving off X-rays.
Low-mass X-ray binaries: primary mass similar or lower to that of the Sun, slow evolution.
High-mass X-ray binaries: primary mass >3MΘ, fast evolution timescale.
Major contributor to integrated X-ray emission of our Galaxy (~50%).
As discussed earlier, explosion in Supernovae produces shock wave that accelerates electrons and heats up material.
X-rays due to thermal, synchrotron and bremsstrahlung radiation.
However, small contribution to total X-ray emission of Galaxy (<1-2%).
Like the Sun, most stars have atmosphere of hot plasma (T~106K): thermal X-ray emission.
However, main sequence stars represent a minor contribution to the total Galaxy X-ray emission (<1-2%).
An exception is represented by pre-main sequence stars, as discussed in an earlier lecture, but those dominate only in strong starburst galaxies.
Gas heated by SN explosions trapped in galaxy potential well.
Relativisitc electrons produce X-rays by bremsstrahlung.
Major contribution to total X-ray emission of galaxies (~50-100%).
The presence of multiple X-ray components in galaxies was predicted comparing the expectations from different emission models.
While Late-types are well explained by the integrated emission of discrete sources (binaries and SNR) Early-type galaxies have an additional component that is represented by the emission of the hot interstellar medium.
The Antennae galaxies represent one of the best examples of gas enrichment during a major merger, as a consequence of vigorous star formation.
The image at the lower right is processed and color-coded to show regions rich in iron (red), magnesium (green) and silicon (blue). These are the types of elements that form the ultimate building blocks for habitable planets.
Intense star formation can produce superwinds that blow the interstellar medium out of the galaxy and enrich the intergalactic medium:
Ellipticals:
Gas origin:
Gas heating mechanism:
Late type system are dominated by large number of X-ray binaries (in addition to some hot gas) which allow to study the formation & evolution of these systems and, as they are linked to star formation, of the whole galaxy.
Point sources (mostly XRBs) follow spiral structure (link to star-formation).
Ultra luminous X-ray sources: 10-1000 X-ray power compared to typical X-ray binaries. Accretion on intermediate mass black holes, 100-104 MΘ. How are such massive compact objects created?
Elliptical galaxy NGC 4649:
Starburst galaxy NGC 3310:
Linear relation between X-ray luminosity and star-formation indicators (e.g. far-infrared luminosity) for spirals.
X-ray emission can be used as a census of the star-formation rate (SFR) in late type galaxies.
X-ray/SFR correlation is driven by short-lived HMXB population.
The X-ray binary luminosity function (i.e. number of sources with luminosity LX) for galaxies with SFRs in the range ~0.1-50 MΘyr-1 can vary considerably from galaxy to galaxy.
However if we rescale by the SFR of the galaxy, the different XLF agree quite well: the number of binaries is proportional to the SFR.
N(LX>2×1038)=2.9xSFR[MΘyr-1]
In conclusion the X-ray luminosity can be used to trace star formation.
2. Absorption and scattering processes – Part I
3. Absorption and scattering processes – Part II
4. Emission processes – Part I
5. Emission processes – Part II
6. Instruments for X-ray and γ-ray Astrophysics – Part I
7. Instruments for X-ray and γ-ray Astrophysics – Part II
8. X-rays from the solar system
9. X-rays from low-mass and PMS stars
12. Evolution of Shell-type Supernova remnants
13. X-ray binaries
14. X-ray emission in normal galaxies
15. Active Galactic Nuclei – part I
16. Active Galactic Nuclei – Part II
17. Active Galactic Nuclei – Part III
18. Clusters of Galaxies – Part I