X-ray Astronomy: An Overview

X-ray Astronomy: An Overview
Jimmy Irwin University of Alabama Day 4 XXIII Ciclo de Cursos Especiais - August 14-17, 2018

Two Problems With the Hot Gas in Ellipticals
LX – Loptical dispersion problem Two galaxies that look identical optically can have vastly different amounts of hot X-ray gas 2) Low metal abundance problem Some galaxies, particularly those with small amounts of X-ray gas, have abnormally low measured metal abundances in the hot gaseous phase

X-ray Bright vs. X-ray Faint Ellipticals
NGC4636 (MV = -21.3) NGC4494 (MV = -21.5) Two ellipticals look nearly identical optically – but differ by a factor of ~100 in the amount of X-ray gas they contain! Why?

LX vs. Loptical Relation
X-ray bright galaxies O’Sullivan, Forbes, & Ponman (2001) factor of ~100 dispersion in the relation (e.g, Trinchieri & Fabbiano 1985; Beuing et al. 1999; O’Sullivan et al. 2001; Boroson 2011) X-ray faint galaxies LX  Lopt

LX vs. Loptical Relation
X-ray bright galaxies (high LX/Loptical) - gas dominated O’Sullivan, Forbes, & Ponman (2001) Spread remains even after the X-ray emission from any supermassive black hole is removed. X-ray faint galaxies (low LX/Loptical) - XRB dominated LX  Lopt

Possible Causes of LX vs. Lopt Dispersion
Environmental? ram pressure stripping: LX  - cluster environment ICM pressure confinement: LX  “stifling” Internal? - variation in Type Ia supernovae-driven winds - variation in depth of dark matter gravitational potential Complicated interplay of all these factors likely leads to the larger dispersion in the observed LX/Loptical relation.

Ram Pressure Stripping
ROSAT PSPC X-ray image of the Virgo Cluster, showing some of the constituent galaxies’ X-ray halos. For this asymmetric cluster, the density is not uniform at a constant radius from the center. Low density ICM high density ICM Bohringer et al.

Chandra image of M86 in Virgo Cluster (Randall et al. 2008) ESO in A1367 (Sun et al. 2006) Galaxies move fast (300 – 1000 km s-1) through the galaxy cluster in which it might reside. The intracluster medium (ICM) through which the galaxies move is dense, and can strip out gas in galaxies that pass through it.

Chandra image of M86 in Virgo Cluster (Randall et al. 2008) ESO in A1367 (Sun et al. 2006) The ram pressure force is given by Pram = ρICM vgal2, where ρICM is the mass density of the intracluster medium, and vgal is the velocity of the galaxy through the cluster. Galaxies moving through the center of the cluster (high ρICM ) and/or at a large velocity, will have their hot gas stripped away.

ICM Pressure Confinement
PICM = ne * k * TICM (Ideal Gas Law) Alternatively, if the galaxy is moving slowly, the intracluster medium will actually help keep gas in – gas that would have otherwise escaped the galaxy in an outward wind.

Internal Factors Type Ia supernova winds + dark matter potentials Not only do Type Ia supernova contribute metals to the hot ISM, they also inject a lot of energy into the galaxy that is transformed to kinetic energy winds that sweep up material. Stronger winds will remove more gas from the galaxy. Also, if there is a variation in the amount, or distribution, of dark matter in galaxies, galaxies with more dark matter will have greater gravity, and will be more able to hold on to its hot gas content.

Internal Factors Type Ia supernova winds + dark matter potentials Not only do Type Ia supernova contribute metals to the hot ISM, they also inject a lot of energy into the galaxy that is transformed to kinetic energy winds that sweep up material. Stronger winds will remove more gas from the galaxy. Galaxies with smaller Type Ia supernova rates (weaker galactic winds) and/or deeper dark matter potential wells (can more easily hold in gas) will retain more hot gas.  will be X-ray brighter (larger LX/Loptical)

A Possible (Partial) Solution?
The ability of a galaxy to retain its hot ISM seems somewhat dependent on the stellar mass density of the galaxy. Stellar mass density = mass (< reff) /π reff2, where reff= half-light radius in the optical Low central stellar mass density High central stellar mass density

A Possible (Partial) Solution?
The ability of a galaxy to retain its hot ISM seems somewhat dependent on the stellar mass density of the galaxy. LX/Loptical for galaxies in the ATLAS3D and MASSIVE surveys with central stellar mass density 3-5 x 109 M/kpc2 Spread in LX/Loptical significantly smaller.

Metallicity of Gas in Low LX/LOptical Galaxies
Since the source of the hot gas within elliptical galaxies is: 1) stellar mass loss from red giants in the galaxy 2) metal-enriched supernovae ejecta we might expect the metal content of the hot gas to be at least the solar value to match that of the stars that contributed the gas in the first place. And this is true for the X-ray brightest galaxies…….

Metallicity of Hot Gas in X-ray Bright Ellipticals
XMM-Newton MOS+PN for NGC4472, NGC4649, and NGC1399 Solar value For X-ray luminous galaxies, the metal content of the hot gas is similar to the metal content of the stars (determined optically) – as expected.

For the few X-ray faint ellipticals for which the metallicity is reported in the literature, highly sub-solar values were found: NGC4697: 7% solar (Sarazin, Irwin, & Bregman 2001) NGC1291: 13% solar (Irwin, Sarazin, & Bregman 2002) NGC4494, NGC3585, NGC5322 : <10% solar (O’Sullivan & Ponman 2004) NGC1553 : ~15% (Humphrey & Buote 2006) More recent analysis of the spectra with more sophisticated models raise the measured abundances somewhat, but still they are only around 25% solar (Su & Irwin 2013).

For the few X-ray faint ellipticals for which the metallicity is reported in the literature, highly sub-solar values were found: NGC4697: 7% solar (Sarazin, Irwin, & Bregman 2001) NGC1291: 13% solar (Irwin, Sarazin, & Bregman 2002) NGC4494, NGC3585, NGC5322 : <10% solar (O’Sullivan & Ponman 2004) NGC1553 : ~15% (Humphrey & Buote 2006) If these galaxies had particularly strong (supernovae-driven) winds that removed most of the gas from the galaxy, then the remaining gas should be over-abundant in metals, not under-abundant.

Su & Irwin (2013) Strong trend of iron (Fe) abundance with LX/Loptical. X-ray faint galaxies are metal-poor X-ray bright galaxies have near solar abundance X-ray faint X-ray bright

Source of Low Metallicity Gas
How are both low LX/Lopt and low metallicity achieved? One solution: ongoing accretion of pristine (metal-free) gas surrounding galaxies dilutes gas to subsolar metallicities

Extended HI structures around Ellipticals
Oosterloo et al. (2007) HI radio contours around 12 isolated elliptical galaxies. HI (neutral, un-ionized hydrogen) masses of up to 1010 M Neutral gas tends to extend to very large radii (tens of kiloparsecs), explaining why it was missed in previous surveys. Gas probably left over from the epoch of galaxy formation.

Extended HI structures around Ellipticals
Oosterloo et al. (2007) Galaxies within clusters tend not to have these HI halos  the hot intracluster medium of clusters evaporates and/or strips off any cold gas surrounding galaxies. Cold gas halos are less commmon around cluster elliptical galaxies (Oosterloo et al. 2010).

Source of Low Metallicity Gas
How are both low LX/Lopt and low metallicity achieved? One solution: ongoing accretion of pristine (metal-free) gas surrounding galaxies dilutes gas to subsolar metallicities observational evidence: extended HI and H2 structures observed around some ellipticals (Oosterloo et al. 2007) X-ray faint elliptical (~108 M hot gas)  only need to accrete few x 108 M of accreted pristine gas to dilute sufficiently X-ray bright elliptical (~1010 M hot gas) accreting few x 108 M of accreted pristine gas will be ineffective to dilute

Cold Gas Content vs. Metal Abundance
Su & Irwin (2013) Weak dependence of metallicity on amount of neutral HI. Somewhat stronger dependence of metallicity on amount of molecular H2.

Groups and Clusters of Galaxies

The Milky Way in the Local Group
The Milky Way and M31 (Andromeda Galaxy) are the two dominant large galaxies, along with a couple dozen dwarf galaxies in the Local Group of galaxies. Small groups tend to be dominated by spiral galaxies.

Groups and Clusters Coma cluster NGC5044 group Abell 2199 Larger groups and clusters of galaxies tend to be dominated by elliptical galaxies, often times with a very large elliptical galaxy (sometimes two) at the center of the galaxy distribution.

X-ray Haloes of Galaxy Groups/Clusters
Abell 383 ‘Regular’ clusters CIZA 2242 Bullet Cluster Abell 1795 ‘Irregular’ clusters MS

Larger and larger z represents earlier and earlier times.
z=0 represents today Larger and larger z represents earlier and earlier times. A fixed volume cube Andrey Kravtsov/U. Chicago Anatoly Klypin/NMSU,NCSA At early times (large redshift value z) matter was very uniformly distributed. Slightly over-dense regions condensed into galaxies and clusters of galaxies.

z=0 represents today Larger and larger z represents earlier and earlier times. A fixed volume cube Andrey Kravtsov/U. Chicago Anatoly Klypin/NMSU, NCSA The nodes of the network are where galaxy clusters are formed. Energy emitted is proportional to T1/2 * ne * ni,

z=0 represents today Larger and larger z represents earlier and earlier times. A fixed volume cube Andrey Kravtsov/U. Chicago Anatoly Klypin/NMSU, NCSA Filmentary network of hot, very low-density gas that connects larger structures like clusters called the Warm-Hot Intergalactic Medium (WHIM) Energy emitted is proportional to T1/2 * ne * ni,

Heating the ICM A parcel of gas entering the ICM (either from galaxy mass loss or infalling primordial gas) will have a large velocity relative to other parcels of gas. When parcels of gas collide, they are shock heated up to high, X-ray—emitting temperatures. The more massive the cluster, the higher infall velocity primordial gas will have, and the faster galaxies will be moving  even more shock heating and even higher temperatures. Temperature of hot gas strongly linked to mass of cluster.

Metal Abundance Determination
Once again, we fit a thermal bremsstrahlung model with metal emission lines to determine both the temperature and the metal abundance of the hot gas. Fe Kα is typically the strongest emission line for very hot (> 3 keV) gas. Dupke & White (2000)

Iron Kα Emission line Fe Kα is a transition from the n=2 level to the n=1 level of an iron atom that has been ionized 25 times (only one electron remains) Characteristic energy of 6.7 keV that is very strong in hot gas that is metal-enriched. Only see one line at the spectral resolution of X-ray instruments.

Groups vs. Clusters Groups Clusters
dozen - few 100 galaxies few few thousand galaxies 0.1 – 1 Megaparsecs in size ~1 – several Megaparsecs in size galaxy mass ~ hot gas mass galaxy mass ~2 – 5x hot gas mass total mass: 1013 – 1014 MSun total mass: 1014 – 1015 MSun hot gas temperature: 1 – 3 keV hot gas temperature: 3 – 15 keV typical galaxy: km/s typical galaxy: 300 – > 1000 km/s velocity velocity

The Problem With Cooling Flows
The hot gas at the centers of clusters cools by emitting X-rays via thermal bremsstrahlung: Emissivity ~ T1/2 * ne * n i, so the higher the density of the gas, the more it radiates, and the faster it cools. But as it cools, it loses thermal support (since P = ne * k * T), causing the gas to sink to the center. As gas cools and sinks to the center, the density, ne, increases, which increases the cooling rate, which lowers the temperature, which reduces the thermal support, which increases the density, etc., etc.  run-away cooling!

The Problem With Cooling Flows
Thermal bremsstrahlung cooling time ~ T1/2 /ne , so for the densest part of a cluster (at its center) the cooling time can be less than the age of the Universe. So what happens to all the cooled gas? Big mystery! It does not form new stars (this would be easily detectable) It does not condense to molecular gas (harder to detect but still should have been seen). Where does it go?

What Happens to the Hot Gas?
X-ray astronomers insisted that cooling flows existed: 1) Clusters showed a decrease in their measured temperatures toward the center, where cooling flows were expected. Pointecouteau, Arnaud & Pratt 2005

What Happens to the Hot Gas?
X-ray astronomers insisted that cooling flows existed: 2) These same clusters showed a spike in X-ray surface brightness in the centers A478 – strong predicted cooling flow based on density, temperature Coma – no strong predicted cooling flow based on density, temperature

A Partial Solution Peterson et al. (2003) Predicted model for an XMM-Newton RGS observation for hot gas with a temperature of 6 keV, and an abundance of 1/3 solar. Note in particular the Fe, Ne and O lines between 12 – 17 Angstroms.

A Partial Solution 3 – 6 keV gas (red) 1.5 – 3 keV (yellow) 0.75 – 1.5 keV (green) 0.375 – 0.75 keV (blue) Peterson et al. (2003) Lower temperature gas (< 1.5 keV) predicts strong emission lines between 12 – 17 Angstroms, whereas these lines are predicted to be much weaker at higher temperatures (> 1.5 keV) relative to the bremsstrahlung continuum.

A Partial Solution Peterson et al. (2003) Actual XMM-Newton RGS spectra did not show evidence of gas with T < 2 keV as expected from the cooling flow model. In fact, regardless of the cluster temperature, T0, there was no evidence for gas cooling below T0/3. So clusters have “cool cores”, not “cooling flows”.

What Keeps the Gas From Cooling?
So the cooling flow problem has gone away, but a new problem has arisen: Why is the gas not cooling below T0/3? Current theories revolve around heating by an energetic active galactic nuclei (AGN) in the central dominant elliptical galaxy.

Radio jets (red or green below) from the outburst of supermassive black holes seem to fill in cavities in the X-ray emission (blue or pink below) Hydra-A (McNamara et al. 2000) MS (McNamara et al. 2006)

AGN outbursts have the energetics to heat the gas, but there are still a few problems: Temperature maps of clusters are usually quite azimuthally symmetric, but models show that the heating from a bipolar outburst should be quite patchy -- how is the heat so evenly distributed? 2) Why is there such apparent fine-tuning between heating and cooling (the gas cools, but not by too much)? Why doesn’t the gas get over-heated or under-heated? Both issues potentially solved by episodic AGN outbursts triggered by gas inflow into the AGN.

Episodic AGN Outbursts
Material accretes onto supermassive black hole, triggering an energetic outburst Hot X-ray gas cools somewhat and flows toward supermassive black hole Self-Regulating System Jets from outburst heats surrounding intracluster gas Heated gas stops flowing in, starving the black hole and ending the AGN outburst If the jet precesses with time, the jet will heat different parts of the cluster each outburst  more uniform heating of ICM

So Do All Clusters Have Cool Cores?
Clusters must be relaxed and relatively undisturbed in order to establish a cool core. What happens if there is a violent collision with another cluster? The cool core is generally heated in a collision, and sometimes the X-ray emission is severely distorted. The cluster is then referred to as a “non-cool core cluster” Abell 520 (

How Do We Measure the Masses of Clusters of Galaxies?
Galaxy Motions X-ray emission from hot gas Gravitational lenses (weak and strong)

Velocity Dispersion Owers et al. (2011) Not all galaxies have the same velocity, but the galaxy velocities of a relaxed cluster follow a Gaussian distribution. σ is the width of the Gaussian, and is called the velocity dispersion. The square of the velocity dispersion is proportional to the mass of the cluster. σ

Velocity Dispersion Not all galaxies have the same velocity, but the galaxy velocities of a relaxed cluster follow a Gaussian distribution. Unrelaxed clusters with non-Gaussian velocity distributions are not suitable for this method Irwin et al. (2015)

Finding the Mass of Clusters - Galaxy Velocities
Virial theorem: 2 x kinetic energy = gravitational potential energy 2 x ½ mgalaxyv2galaxy,average = GMclustermgalaxy/R where vgalaxy,average can be approximated by the velocity dispersion, σtrue, 3D M = σ2true,3DR/G = 3σ2line of sight R/G (where σ2line of sight is the measured 1-dimensional radial velocity dispersion). Only valid to where σ can be determined accurately (inner regions)

Finding the Mass of Clusters - Hot Gas Method
IF hot gas is in hydrstatic equilibrium in the gravitational potential well of the cluster, the kinetic energy of the electrons + protons reflects the gravitational potential of the well ½ mv2 ~ GMclusterm/r Coma Cluster

The temperature of the gas reflects the velocity of the electrons ½ mv2 ~ 3/2 kT Mcluster(<r)  rT More carefully, assuming T and gas density ρ vary with radius: Coma Cluster

IF hot gas is NOT in hydrostatic equilibrium, this mass determination method does NOT work. Hydrostatic equilibrium: condition where the force of gravity is equally balanced by thermal gas pressure. Hydrostatic equilibrium often destroyed by cluster mergers. Keep in mind: hot gas mass a factor of ~5 greater than sum of galaxy masses. Bullet Cluster

Gravitational Lensing
Curved spacetime alters the paths of light rays, shifting the apparent positions of objects in an effect called gravitational lensing. Observed shifts precisely agree with general relativity.

Strong Gravitational Lensing
Gravity of foreground galaxy (center) bends light from an object directly behind it A ring of light from the background object appears in the sky (Einstein Ring).

same galaxy The amount of lensing gives us an estimate of the mass doing the lensing (the cluster). This utilizes Einstein’s General Relativity Law and is independent of gravity laws. Measure , R  Mass.

Weak Gravitational Lensing
Less concentrated collections of mass cannot form a strong gravitational lens. BUT the gravity well can cause background galaxies to be slightly bent and distorted. By looking at many of these slight distortions, we get a map of the mass distribution that is doing the lensing. Individual background galaxies appear slightly rotated (distorted) in a particular direction

Agreement of Mass Methods
By and large, the four mass determination method yield consistent results. Galaxy velocity method valid near center, where there are many galaxies. Strong lensing method valid near center, where mass concentration is high, at the radius of the arc/ring. X-ray gas method valid near center and farther out, as long as gas is relaxed and in hydrostatic equilibrium. Weak lensing method is valid everywhere, assuming one has enough background galaxies to statistically determine mass of intervening matter.

Agreement…..But Too Much Mass!
The galaxy velocity method was first feasible in the 1930s, long before the other methods became available. Fritz Zwicky first determined the mass of the Coma Cluster. Found a mass for Coma cluster that was 50x greater than implied by the sum of the galaxy masses (he didn’t know about the hot intracluster medium component). Result was largely ignored because it was not understood. Fritz Zwicky

Dark Matter and Clusters of Galaxies
The discovery of the hot intracluster medium (ICM) by X-ray satellites in the 1970s-1980s narrowed the gap between the mass inferred from galaxy velocities and the mass within the galaxies. But the addition of the ICM was not enough. The inferred mass was still ~5x too high  Dark Matter! Dark matter: 85% of mass Hot Gas: 13% of mass Stars/Galaxies: 2% of mass (Hot gas/stars/galaxies called the baryonic content of clusters)

The Bullet Cluster Spells Trouble for MOND
A direct collision between two clusters of galaxies. Pink light: X-ray gas in two (post)-colliding galaxy clusters

The Bullet Cluster Spells Trouble for MOND
Galaxies of Cluster #1 Hot gas of Galaxies of Cluster #2 Hot gas of Cluster #2 The collision has separated the hot gas from its host cluster, while the galaxies have passed unhindered.

Response From MOND Supporters
MOND supporters aren’t quite ready to give up, but…… “…..even after taking MOND into account, we infer more mass from dynamical measures than we currently see in all known baryonic components. Is this bad for the theory? Yes.” Stacy McGaugh, MOND Supporter

The Bullet Cluster and Dark Matter
Clowe, Markevitch et al. Pink: hot gas Blue: mass distribution from weak lensing measurements Weak lensing says most of the mass is STILL with the galaxies even though we know there is 5x more mass in the gas than in the galaxies. This implies that dark matter must exist and has passed through the collision unhindered just like the galaxies.

X-ray Astronomy: An Overview

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