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Maurizio Paolillo » 14.X-ray emission in normal galaxies


Contents

  1. Classification of galaxies;
  2. High energy emission in Early-type galaxies;
  3. High energy emission in Late-type galaxies;
  4. X-ray emission and star formation;
  5. The galaxy contribution to the extragalactic X-ray background.

X-rays from galaxies

X-rays represent a tiny fraction of the energy output of galaxies.

e.g. Arp 220:

  • most of the energy at the infrared
  • X-rays: 5dex lower flux!

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…


The optical classification of galaxies

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).

  • Ellipticals
    • smooth profiles
    • old stars
  • Spirals
    • spiral arms
    • young stars
  • Lenticulars
    • smooth profile + spiral structure
  • Irregulars
    • amorphous
    • young stars
The Hubble diagram: Early-type galaxies are on the left, while Late-types are on the right.

The Hubble diagram: Early-type galaxies are on the left, while Late-types are on the right.


High energy emission from galaxies

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:

  • X-ray binaries;
  • Supernovae remnants;
  • Hot gas;
  • Stellar coronae.

X-ray binaries

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%).

A schematic viw of a Low Mass X-ray binary system.

A schematic viw of a Low Mass X-ray binary system.


Supernovae remnants

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%).

Sequence of the Supernova explosion.

Sequence of the Supernova explosion.

The Supernova remnant CAS-A exploded  300 years ago, 10 ly across and T=5×106K

The Supernova remnant CAS-A exploded 300 years ago, 10 ly across and T=5x106K


Stellar coronae

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.

The solar corona in X-rays.

The solar corona in X-rays.


Gaseous X-ray halos

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%).

X-ray halo (left) compared with the optical image (right) of a the elliptical galaxy NGC 4555.

X-ray halo (left) compared with the optical image (right) of a the elliptical galaxy NGC 4555.

The composite X-ray (blue) / optical (red) image of the lenticular galaxy NGC 5746.

The composite X-ray (blue) / optical (red) image of the lenticular galaxy NGC 5746.


Emission components

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.

Optical vs X-ray luminosity of Early-type galaxies. Also shown is the predicted emission due to discrete sources (LDSCR) and to Supernova winds (LSN).

Optical vs X-ray luminosity of Early-type galaxies. Also shown is the predicted emission due to discrete sources (LDSCR) and to Supernova winds (LSN).


Why study the X-ray emission….

  • Hot gas component
    • Metal enrichment
    • formation history of ellipticals.
  • Binary star population
    • formation & evolution
    • link to star-formation.

Enrichment of the 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.

The interacting system of thde Antennae galaxies at optical.

The interacting system of thde Antennae galaxies at optical.

Credit: NASA/CXC/SAO/G. Fabbiano et al.

Credit: NASA/CXC/SAO/G. Fabbiano et al.


Enrichment of the Interstellar Medium

Intense star formation can produce superwinds that blow the interstellar medium out of the galaxy and enrich the intergalactic medium:

  • Bubble of warm (104K; red) and hot (107K; blue) gas blown out of the galaxy.
  • SN explosions or AGN activity can both be responsible for the ejection.
  • This mechanism is believed to regulate the star-formation/AGN activity by depleting the galaxy center of fuel (cold gas).
Composite optical/X-ray image of NGC3979. 
Blue: X-rays 107K, red is warm 104K
Credit: NASA/CXC/STScI/U.North Carolina/G.Cecil.

Composite optical/X-ray image of NGC3979. Blue: X-rays 107K, red is warm 104K Credit: NASA/CXC/STScI/U.North Carolina/G.Cecil.


Formation of halos in Elliptical galaxies

Ellipticals:

  • X-rays dominated by diffuse emission: thermal Bremsstrahlung (see previous lecture)
  • Hot gas distribution inhomogeneous (unlike optical)
  • Some stirring mechanism in operation: possible periodic AGN activity

Gas origin:

  • mass ejected by old stars
  • gas falling back on the galaxy

Gas heating mechanism:

  • shock heating
  • Supernovae
  • AGN feedback?
A gallery of optical (white) and X-ray (blue) images of Elliptical galaxies.
Credit: X-ray: NASA/CXC/U. Ohio/T. Statler & S. Diehl.

A gallery of optical (white) and X-ray (blue) images of Elliptical galaxies. Credit: X-ray: NASA/CXC/U. Ohio/T. Statler & S. Diehl.


Evolution of binary star population

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.

Optical image of M31.

Optical image of M31.

X-ray image of M31: combination of point sources (X-ray binaries) and diffuse hot gas (~106-107K). Credit: NASA/UMass/Z.Li & Q.D.Wang.

X-ray image of M31: combination of point sources (X-ray binaries) and diffuse hot gas (~106-107K). Credit: NASA/UMass/Z.Li & Q.D.Wang.


Evolution of binary star population (cont’ed)

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?

Credit: X-ray: NASA/CXC/U. of Michigan/J.Liu et al.; Optical: NOAO/AURA/NSF/T. Boroson.

Credit: X-ray: NASA/CXC/U. of Michigan/J.Liu et al.; Optical: NOAO/AURA/NSF/T. Boroson.


X-ray spectra of galaxies

Elliptical galaxy NGC 4649:

  • Two temperature hot gas components:
    • T=8×106 & 2×107K
    • ~85% of LX
  • Power-law, Γ~1.8
    • stellar sources (e.g. binaries)
    • ~15% of the luminosity
X-ray image of NGC 4649: both the diffuse gas and the pointlike bynary population are clearly visible.

X-ray image of NGC 4649: both the diffuse gas and the pointlike bynary population are clearly visible.

X-ray spectrum of NGC 4649, due to the superposition of the different components on the left.

X-ray spectrum of NGC 4649, due to the superposition of the different components on the left.


X-ray spectra of galaxies (cont’ed)

Starburst galaxy NGC 3310:

  • Two temperature hot gas components:
    • T=3×106 & 7×106K
    • 25% of the 0.3-10 keV LX
  • Power-law, Γ=1.8
    • stellar sources (e.g. binaries)
    • 75% of the 0.3-10keV luminosity
XMM X-ray spectrum of NGC3310. The dotted, dot-dashed and dashed lines show the different components discussed on the left.

XMM X-ray spectrum of NGC3310. The dotted, dot-dashed and dashed lines show the different components discussed on the left.

Optical image of NGC3310.

Optical image of NGC3310.


X-ray emission and star-formation

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.

Correlation between X-ray and IR luminosity in spiral galaxies.

Correlation between X-ray and IR luminosity in spiral galaxies.


X-ray emission and star-formation (cont’ed)

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.

X-ray luminosity functions for a sample of starforming galaxies.

X-ray luminosity functions for a sample of starforming galaxies.

X-ray luminosity functions normalized to the star formation rate of each galaxy.

X-ray luminosity functions normalized to the star formation rate of each galaxy.


The galaxy contribution to the X-ray background

AGN and galaxy number counts. At the faintest fluxes galaxies are expected to dominate the X-ray background. However current instruments do not allow to resolve these sources individually.

AGN and galaxy number counts. At the faintest fluxes galaxies are expected to dominate the X-ray background. However current instruments do not allow to resolve these sources individually.


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