An Introduction to Galaxies and the Interstellar Medium
IIA Summer School Instructor : Mousumi Das, [email protected] July 2021 Topics that we plan to cover ...
● What are Galaxies?
● Spirals and ellipticals ==> the 2 main types of galaxies.
● The main components of spiral and elliptical galaxies.
● Our Galaxy : the Milky Way and the interstellar medium
● The morphological classes ==> the Hubble Sequence.
● Galaxy clustering : galaxy pairs, groups, clusters and the large scale structure of our universe.
● Our galaxy neighbourhood.
● Galaxy interactions.
● Galaxy evolution and the color-magnitude plot.
● Galaxy rotation and dark matter.
The Milky Way in the night sky
● The Milky Way (MW) appears in the night sky as a stream of diffuse emission. This term was coined by the Greeks and it means “river of milk”.
● Galileo in 1610 observed it with his telescope and found it to be composed of stars. Hence the MW was confirmed to be a stellar system.
● The MW is actually the plane of our own galaxy. Our galaxy is called the MW and is always denoted by Galaxy i.e. with a capital G.
● We can see the plane of our Galaxy because the solar system is tilted out of the Galactic plane.
Galactic Plane and Island Universes
● Immanuel Kant (in approx 1787) was the first to give a theory of the sun and solar system moving under their mutual gravitational field.
● He also explained the formation of the galactic plane and hence the origin of the MW in the sky.
● He also explained that the fuzzy patches in the sky may other galaxies or “island universes”.
● Comet hunter Charles Messier catalogued may more fainter nebulae (both stellar and planetary nebulae). His catalogue is the Messier catalogue.
● William and Caroline Herschel (1800's) also compiled a list. Dreyer (1895) compiled the New General Catalogue (NGC).
Photographic plates : revolutionized astronomy
● With the advent of photography, astronomers were able to record spectra of stars and images of the night sky.
● Kapetyn used stellar spectra to build a model of our Galaxy, called the Kapteyn Universe. However, he did not correct for interstellar extinction. Hence his model put our solar system too close to the galaxy center.
Barnard Cloud and and a nearby star forming region
Advent of radioastronomy
● In 1932 Karl Jansky discovered that our universe emits in a broad range of radio waves.
● van der Hulst (1944) predicted that we should be able to detect 21cm radio emission from neutral hydrogen (HI) from galaxies and the MW.
● Detection (approx 1950) was done by several scientists e.g. Oort and Muller (Netherlands), Ewan and Purcell (Harvard), Christiansen (Australia).
● Stellar and radio observations helped us understand how stars and gas rotate in the disks of galaxies (e.g. Lindblad, Oort).
In Pune, India we have one of the largest arrays of radio telescopes in the world called the Giant Meterwave radio telescope (GMRT) which is used for doing low frequency radioastronomy.
Multiwavelength Approach to Astronomy
● Research in astronomy is now done at a variety of wavelengths. The different wavelengths enable us to study different physical processes in space.
● Both space based and ground based observatories are used to study galactic and extragalactic sources.
ImagingImaging ofof galaxiesgalaxies :: studyingstudying galaxiesgalaxies atat differentdifferent wavelengthswavelengths
We observe galaxies at different wavelengths. It helps us understand the different processes taking place in galaxies.
X-ray : ROSAT Ultraviolet : GALEX Visible : DSS
Radio : Near-infrared : IRAS Mid-infrared : Far-infrared : SPITZER ISO Effelsberg Stars : the basic building blocks of galaxies
A Main sequence type star. Massive and has a high temperature
The star cluster M103. Note the reddish stars.
A molecular cloud with newly formed stars. The stars are formed as a cluster and have varying masses. The stellar winds can drive the gas away. WhatWhat areare galaxiesgalaxies andand thethe 22 mainmain typestypes
● Galaxies are collections of stars (106 to 1011 in number). The smallest galaxies may have even 105 number of stars.
● Galaxies are mainly of 2 types : elliptical or spiral depending on their morphology. Spiral galaxies have a disk of stars that are rotating in a plane whereas elliptical galaxies are ellipsoid in shape. There are also irregular and peculiar shaped galaxies.
An elliptical galaxy : ellipsoidal in shape
A spiral Galaxy : has a disky shape Image Credit HST : Above is the Hubble Deep
Field showing different types of galaxies. What are galaxies made of?
A typical spiral or disk galaxy is made up of stars, interstellar gas (both ionized and neutral gas (mainly hydrogen denoted by HI and ionized H gas by HII), interstellar dust and dark matter.
A typical elliptical galaxy is mainly composed of stars, hot ionized gas and dark matter.
The gas is mainly neutral hydrogen (called HI), molecular hydrogen and ionized gas. The stars can vary from the most massive O and A types in the star forming disks to the really old stars in globular clusters.
An elliptical galaxy: NGC 4458 The spiral Galaxy M101
Image Credit : en.wikipedia.com Image Credit : Hubblesite.org Important distance scales :
The fundamental distance unit in astronomy is the parsec (pc) scale. It is defined as the distance at which a star has a parallax of 1”. Parallax is the angular shift in the sky of a star over a period of 6 months
1 pc = 206 265 AU = 3.086 x 1018 cm = 3.26 light years (Where 1 AU = earth – sun distance approx = 149,597,871 km 1 light year = distance light travels in one year )
Important distance scales for galaxies : Kiloparsec (kpc) : example, our galaxy has a radius of at least 15 kpc Megaparsec (Mpc) : example, nearby large galaxies are a few Mpc away.
The differences between spirals and ellipticals
KinematicKinematic DifferenceDifference betweenbetween EllipticalsEllipticals andand SpiralsSpirals
● Spirals are rotating disks of stars and gas and dark matter, where the support against gravity is due to rotation. This is similar to the satellite problem that you may have studied in high school. GM(r)/r2 = v2/r
● In an elliptical galaxy the gravitational support is due to the random motion of the stars. So we have to use the virial equation 2T + V =0 Σm v 2 = Σ Gm m /r i i i j ij Or σ2 = GM/R
TheThe MassMass inin aa DiskDisk galaxygalaxy
● A disk galaxy has a rotation velocity of 220 km/s at a radius of 8.5 kpc from the galaxy center. Calculate the mass within that radius. What is the mass in solar mass units where M = 2x1033gm and 1kpc=3x1021 cm sun
● Answer : M = v2xR/G = (220x105)2 x (8.5x3x1021 cm)/6.67x10-8 = 1850x1039 gm = 6.16 x 108 M sun
TheThe MassMass ofof anan EllipticalElliptical galaxygalaxy
● An elliptical galaxy has a mean velocity dispersion of 100 km/s and a radius of 20 kpc. Calculate the mass of the elliptical galaxy in solar mass units where M = 2x1033gm sun
SpiralSpiral GalaxiesGalaxies :: disksdisks embeddedembedded inin haloshalos
● The stars in spiral galaxies are in disks and are supported by rotational motion about their galaxy centers.
● It is composed of a disk of stars and gas that are embedded in a very massive dark matter halo. There maybe a bulge in the center of the disk and an elongated feature called a bar.
● There are compact old, star clusters called globular clusters in the halo.
● The name `spiral' comes from the spiral features seen in the disks. Our Galaxy Structure Image credit : astronomyonline.org EllipticalElliptical GalaxiesGalaxies :: SupportedSupported byby randomrandom motionmotion oror non-circularnon-circular orbitsorbits ofof starsstars
● They are ellipsoidal in shape and M87 Galaxy supported by the the motion of the stars that are in various non-circular orbits (loop orbits, boxy orbits, etc) .
● They often appear fairly featureless in optical images. Their stars are old and generally no ongoing star formation is seen.
● The most massive black holes are found in the centers of elliptical galaxies. They usually show strong nuclear activity Image Credit : APOD, star.ucl.uk
Elliptical Galaxies : their halos
● The stars are embedded in a very massive dark matter halo. There may small nuclear disks of dust and gas, possibly due to mergers.
● They are often surrounded by massive halos of hot gas emitting in X-rays. The gas is a remnant from early star formation.
NGC6482 : 2MASS image K band NGC6482 : Xray halo (image credit image (2MASS survey) Chandra/NASA) Elliptical Galaxies : nuclear activity
● They have the most massive nuclear black holes (107 to 1010 solar mass) in the Universe. A good example is M87, the nearest elliptical galaxy. The recent Event Horizon Telescope (EVT) detection of a SMBH “shadow” in a nearby galaxy was from M87.
● They show the high nuclear activity due to accretion onto the supermassive black holes (SMBHs). Quasar host galaxies are ellipticals. They often have radio jets. They show emission in the highest frequencies – gamma rays and X-rays.
Centaurus Galaxy (with jets overlaid) Image credit : apod.nasa.gov
Differences between spiral (or disk) and elliptical galaxies
● They are rotationally supported ● Supported by velocity dispersion of against gravity. Disky shapes. stars. Ellipsoidal shapes.
● Contain an interstellar gas in the ● Usually do not contain interstellar form of molecular hydrogen gas neutral hydrogen or molecular (H ) and neutral hydrogen (HI) gas. 2 hydrogen gas.
● Made up of a mixture of old and ● They are made up of old stars and do young stars. They have ongoing not show signs of star formation. star formation and hence appear Hence they are optically not bright. bright. ● They usually have hot ionized gas ● Spirals usually do not have halos of halos, remnant from early epoch star hot ionized gas. formation.
● Spirals are mainly found in the ● They are found in the denser parts of outer parts of galaxy clusters where clusters, mainly in the inner regions the galaxy number densities are and close to the cluster center. low. Our Galaxy (the Milky Way)
Our Galaxy : a medium size disk galaxy
● Galactic Halo
● Disk (stars, gas, dust)
● Globular Clusters
● Halo Stars
image credit : icc.durham.ac.uk
image credit :
bartol.udel.edu The Milky Way Disk : Mapping the Spiral Arms
Since we are sitting close to the disk it is difficult to map our Galaxy. We can do it by tracing the stellar velocities.
There are several spiral arm parts. The arms are the sites of massive star formation (O and B type stars). The arms are alao associated with the dense molecular and neutral hydrogen gas Our Galaxy may also have a bar. The evidence comes from the velocities of the stars.
image credit : physast.uga.edu Gas and Dust in our Galaxy : Neutral Hydrogen (HI)
● Neutral hydrogen is distributed over the entire disk and extends even beyond the optical disk.
● Its total mass is only 1-5% of the stellar mass but it is dynamically a very important component of the disk.
● It traces the mass all the way to the outer radius (rotation curves).
● The outer parts of the HI disk appears to be warped.
Image credit : teacherlink.ed.usu.edu Molecular Hydrogen Gas (H ) in our Galaxy 2 Molecular hydrogen is the coldest gas in galaxies. It is the site of star formation and hence is very important for star formation and galaxy evolution.
Molecular hydrogen forms on dust particles in the interstellar medium. Hence H is always found associated with dust in the Galaxy plane. It is 2 clumpy in nature and can be easily dissociated by UV radiation from stars.
mage credit : [email protected] Dust in the interstellar medium
The interstellar medium is also composed of small dust particels that are not more than a few micron in size.
Dust particles are ususally elongated and paramagnetic. They are made of silicon, carbon and other heavy elements. Dust absorbs optical light and re-radiates in the infra-red.
Image credit : [email protected] ● All-sky view of the IRAS 100 micron imaging data, representing a MONTAGE-generated combination of the individual images created by Schlegel, Finkbeiner & Davis (1998).
Detecting the BH in our Galaxy center
● It took 15 years of regular monitoring of the Galactic Centre with ESO telescopes at the La Silla Paranal Observatory to finally obtain conclusive evidence of the BH in the Galactic Center.
● First detected by Reinhard Genzel and his group at the Max-Planck Institute for Extraterrestrial Physics. Later detected by Andrea Ghez at UCLA (both received the Nobel prize)..
● The center was detected by tracking the orbits of stars near the BH over long period of time. A technique called speckle imaging was used.
6 ● The BH in our Galaxy center has a mass of approximately 4.2 x 10 solar mass.
Galaxy Morphologies : the Hubble Sequence
HubbleHubble SequenceSequence :: AA ClassificationClassification ofof GalaxiesGalaxies
Variation of Properties along the Hubble Tuning Fork
● Elliptical galaxies vary in ellipticity, becoming flatter towards the fork. Ellipticity is given by (1-b/a) and it varies from E1 to E7 for ellipticals. Galaxies are unstable for ellipticity>0.7 and so we do not have galaxies with larger axis ratios.
● The spirals vary from left to right according to the prominence of the bulge, tightness of the spiral arms and how well the arms can be distinguished.
Spirals are two types – barred and unbarred. Bars are elongated features in the centers of galaxies.
Lenticular Galaxies and irregular spirals
● Lenticular or S0 galaxies have a smooth light distribution like ellipticals but also disks like spiral galaxies. They often have a lot of dust and prominent bulges.
● Irregulars have disks, but lack symmetry and do not have well defined spiral arms.
Sombreo Galaxy : an S0 galaxy NGC1427A : an irregular galaxy
Barred galaxies and interacting spirals
A prominent bar in the center of the barred galaxy NGC1300. The dust lanes along the bar can be clearly as well as the star formation.
The tidally interacting galaxy pair M51 and its companion dwarf galaxy. There is a bridge of gas connecting both galaxies.
Galaxy clustering and the large scale structure of our universe
Galaxy Groups
● Spiral, irregular and sometimes elliptical galaxies are often found in associations of 3 -10 galaxies, called groups.
● Group galaxies have relative speeds of 500 km/s and may affect each other through interactions. Gas is pulled out from the galaxies. Gas is also stripped from the galaxies by ram pressure of the ISM. Some galaxies are merging.
● Groups are often surrounded by hot x-ray emitting gas.
Stephens Quintet : galaxy group Galaxy Clusters
● About half the galaxies that we see are found in clusters. They are a few Mpc in size and composed of 100 to 1000 galaxies.
● The giant members are bright ellipticals and found near the center. Clusters are more dominated by large ellipticals and dwarfs than spirals.
● They are embedded in very large halos of hot ionized gas; these are detected in x- ray emission.
● Most clusters are in virial equilbrium and some are found to be colliding.
Optical (above) and X-ray (below) image of a cluster (using the Chandra X-ray telescope). The hot ionized gaseous halo emitts x-rays. Large Scale Structure of Universe : Groups, Clusters, Super-clusters and Voids
Our Local Group and Virgo supercluster
We have a peculiar motion towards the Virgo cluster as it is the largest cluster close to us. These velocities on top of the Hubble flow give rise to the peculiar velocities of galaxies.
Image credit : University of California, San Diego
Image credit : Wikipedia
Galaxy Evolution
Galaxy Evolution : external and internal influences
● Galaxies evolve due to interactions and collisions with other galaxies. The dark halo potential reduces the impact of tidal interactions, but close interactions can still strip galaxies of Interacting galaxies their gas and sometimes even their stars.
● The internal evolution of galaxies is due to the effect of spiral arms and bars on the disk gas and stars.
● Galaxy evolution can be both slow (called secular evolution) or fast due to interactions.
Galaxy Interactions and star formation
● galaxy interactions lead to star formation. This leads to changes in galaxy morphology. Bars form, bulges grow, spiral arms are triggered. This leads to changes in galaxy morphology.
The interacting galaxies M101 : Pinwheel galaxy M31 : Andromeda NGC2207 and IC 2163
NGC1300 : Barred Galaxy The Antennae Galaxies : a colliding system, NGC4038 and NGC4039 Importance of Galaxy Mergers and Interactions : star formation and galaxy evolution
● Very large bursts of star formation are triggered by galaxy collisions or interactions.
● Galaxy interactions or mergers disturbs the galaxy potentials and causes cloud collisions leading to star formation.
● The interactions/mergers disturbs the orbits of stars and gas in the galaxy disks, leading to gas falling inwards and collecting in the galaxy nuclei. This can lead to nuclear starburst.
Image credit : nasa.gov Cycle star formation in galaxies
Image Credit : SPICA Collaboration Star formation and the galaxy evolution sequence
● Star forming galaxies are generally blue in color whereas red galaxies have finished their SF (quenched galaxies). This is clear in the color magnitude plot for galaxies.
● The gas rich, star forming galaxies are in the blue cloud. The ellipticals and S0 galaxies that have finished their star formation are in the red cloud.
Image Credit : Galaxy Zoo galaxy evolution sequence : red and blue cloud (also a green valley)
● Just like the main sequence for stars, galaxies follow a blue to red galaxy sequence, through the driving force of star formation.
Image Credit : University of Oxford
Galaxy Rotation
● The disks of spiral galaxies are rotating. This rotation stabilizes Galaxy Rotation the disk against gravity. Hence we have : v2/R = GM/R2
● The approaching side of a galaxy has blue shifted emssion while the receding side has red shifted emission.
● The inner parts of disks (in the bulges) have solid body rotation. The outer parts have differential rotation (the angular velocity varies with R).
● We can follow disk rotation by studying the HI distribution of galaxies. For spirals, the HI disks is much more extended and hence is used to trace the disk rotation.
Galaxy Spider ● Galaxies have a systemic diagrams and velocity due to Hubble expansion, V . They also sys rotation curves have rotational motion which when deprojected is V . r Rotation curve ● V (R,i) = V + V(R)SiniCosφ r sys
● When this is plotted, the contours of equal V give the r typical galaxy “spider diagram” .
● When rotation is deprojected and plotted with V against r radius, we get the rotation curve.
Figure from Begeman (1989)
The Calculated and Observed Rotation Curves
Flat Rotation Curves : indicates the presence of dark matter in galaxies
● The flat rotation curves observed in nearly all disk galaxies indicates that there are dark matter halos surrounding them.
● The excess dynamical mass is called dark matter. It can only be detected through its gravitational effects. We do not know what dark matter is made of.
● The flat rotation velocity indicates also that the disk has differential rotation. This shear can affect matter in the disk.
SUMMARY
6 ● Galaxies contain a huge number of stars, approximately 10 to 1011 in number.
● There are basically 2 types of galaxies, disk or spiral galaxies and elliptical galaxies.
● Galaxies evolve through internal processes and via galaxy mergers or interactions. They evolve from being gas rich, blue star forming galaxies into red ellipticals.
● Galaxies are embedded in large halos of dark matter.