Lecture 19 Yogesh Wadadekar Jan-Feb 2018yogesh/galaxies_2018/lect19.pdf · animation by animate[2017/08/11] ncralogo. Connection between star formation history and morphology. Galaxy

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Galaxies: Structure, formation and evolutionLecture 19

Yogesh Wadadekar

Jan-Feb 2018

IUCAA-NCRA Grad School 1 / 26

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Measuring star formation - IR

Emission in the far infrared (FIR). This is radiation emitted by warmdust which is heated by hot young stars. For the relation of FIRluminosity to the SFR, observation yields the approximate relation

SFRFIR

M�/yr∼ LFIR

5.8 × 109L�

IUCAA-NCRA Grad School 2 / 26

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Measuring the SFR - Radio

A very tight correlation exists between the radio luminosity of galaxiesand their luminosity in the FIR, over many orders of magnitude of thecorresponding luminosities. Since LFIR is a good indicator of thestar-formation rate, this should apply for radiation in the radio as well.

SFR1.4GHz

M�/yr∼ L1.4GHz

8.4 × 1027erg s−1Hz−1

IUCAA-NCRA Grad School 3 / 26

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Measuring the SFR - Hα

This line emission comes mainly from the HII regions that form aroundyoung hot stars.

SFRHα

M�/yr∼ LHα

1.3 × 1041erg s−1

IUCAA-NCRA Grad School 4 / 26

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Measuring the SFR - UV

This is only emitted by hot young stars, thus indicating the SFR in themost recent past - an instanteneous SFR

SFRUV

M�/yr∼ LUV

7.2 × 1027erg s−1Hz−1

IUCAA-NCRA Grad School 5 / 26

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SFR estimators compared

IUCAA-NCRA Grad School 6 / 26

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Other star formation indicators

The fine-structure line of singly ionized carbon at λ = 157.7µm is ofparticular importance as it is one of the brightest emission lines ingalaxies, which can account for a fraction of a percent of their totalluminosity. The emission is produced in regions which are subject toUV radiation from hot stars, and thus associated with star-formationactivity. Due to its wavelength, this line is difficult to observe and has,until recently, been detected only in star-forming regions in our Galaxyand in other local galaxies. All this has changed with ALMA.

IUCAA-NCRA Grad School 7 / 26

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Measuring SFR from X-ray emission

In star-forming galaxies, the X-ray luminosity is produced by high-massX-ray binaries, massive stars, and supernovae, but non-negligiblecontributions from low-mass X-ray binaries are also present.The latter are not directly related to recent star formation, andrepresent a source of uncertainty in the calibration of SFR(X-ray).

IUCAA-NCRA Grad School 8 / 26

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ULX in star forming galaxies

Why resolution is important?IUCAA-NCRA Grad School 9 / 26

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ULX in center of M31

IUCAA-NCRA Grad School 10 / 26

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Connection between star formation history andmorphology

Galaxy morphology, star formation history, stellar mass and projectednumber density are all correlated.

IUCAA-NCRA Grad School 11 / 26

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Red sequence, blue cloud and the green valley

IUCAA-NCRA Grad School 12 / 26

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Another view

IUCAA-NCRA Grad School 13 / 26

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Red and blue galaxies

Galaxies are well known to divide into two large families: red, oldellipticals and blue, star-forming spirals. The SDSS has shown that, inthe local universe, the division into these two large families happens ata stellar mass of ∼ 3 × 1010Msun.

IUCAA-NCRA Grad School 14 / 26

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Galaxy main sequence

Note that both axes are log scale.IUCAA-NCRA Grad School 15 / 26

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Galaxies differentiated by stellar mass

Low mass High Massyoung stellar populations old stellar populationslow surface mass densities high surface mass densitieslow concentration high concentrationlots of gas little or no gasblue cloud Red sequencefast rotators Slow rotatorsspiral elliptical

Gas supply and quenching seems to be most important parametersregulating this red/blue division

IUCAA-NCRA Grad School 16 / 26

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Question

Where does the Milky Way sit on this diagram? Where will it sit in thefuture? What kind of galaxies are present in the green valley?

IUCAA-NCRA Grad School 17 / 26

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What inhibits star formation in the most massiveellipticals?

Various possibilities exist: Has star formation stalled in these galaxiesbecause the gas supply has been fully consumed? Or has the gasbeen pushed to the outskirts of these galaxies e.g. by tidal effects orram pressure stripping, or heated to temperatures that inhibit thegravitational collapse needed to form new stars?For the most massive ellipticals, which mostly sit at the centers of richclusters, it is the hot halo (seen in X-ray observations) that preventsfurther infall of gas.

IUCAA-NCRA Grad School 18 / 26

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The hot halo of the Sombrero galaxy

Pre-Chandra LMXBs unresolved.IUCAA-NCRA Grad School 19 / 26

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Milky Way hot gas halo discovered in 2012

IUCAA-NCRA Grad School 20 / 26

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Evolution of the hot gas halo

The evolution of the hot halos is the result of a tug of war between thepull of gravity and the push of feedback from stellar evolution (SNe Ia)and AGNs, as well as interactions with the circum-galactic medium andgalaxy encounters.If there is equilibrium, then one can measure the mass of a galaxy orcluster via hydrostatic equilibrium.The discovery of these hot halos and the characterization of theirproperties have been unique contributions of high resolution X-raytelescopes (Chandra/XMM) to astronomy.

IUCAA-NCRA Grad School 21 / 26

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The most important parameter

that defines how a galaxy will evolve seems to be stellar mass andmorphology.

IUCAA-NCRA Grad School 22 / 26

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Morphology density relation

IUCAA-NCRA Grad School 23 / 26

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Dressler (1980)

classified 6000 galaxies in 55 clusters plus 15 field regions. Herecorded positions and local projected galaxy density. He found:

Strong dependencies of f(Sp), and f(E) on projected local galaxydensity. In poor clusters, the trend is stronger with local densitythan with simple cluster radiusThe dependency of f(S0) is weaker than f(E) or f(Sp)The effect occurs in regions of sufficiently low density that gasstripping or encounters cannot operate.

He claimed: The primary effect is with local galaxy density NOT clusterradius. The effect occurs at galaxy formation, and is not an ongoingevolutionary process.

IUCAA-NCRA Grad School 24 / 26

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But there is major debate on its origin

High densities inhibit the formation of spirals.Spirals may be stripped of gas to make S0sBulges may “grow” by accretion of dwarfsS0s may not be a homogeneous class: some originate as spirals,others not. Stellar mass important.Spirals experiencing “harrassment” (Moore et al 1996) canresemble S0s. rapid gravitational shocks disturb spiral structureand “heat” the disk stars.Spiral mergers may create S0s and/or Es.

IUCAA-NCRA Grad School 25 / 26

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Ram pressure stripping - Gunn and Gott (1972)

One of the (several) possible environmental effects on galaxies inclusters is the stripping of ISM due to ram pressure as the galaxymoves through the ICM. We can measure the HI deficiency which isdefined as (M - <M>)/<M>, where <M> is the mean HI mass forgalaxies of the same Hubble type. HI deficiency is found to increase(a) towards the center of clusters , in richer clusters of higher X-rayluminosity,However, whether this is sufficiently efficient is unclear :(a) Studies show CO is not removed (denser and deeper in galaxypotential) (b) Only the outer HI is stripped (eg HI map of virgo showssmaller sizes in the core)

IUCAA-NCRA Grad School 26 / 26