Lectures on stellar astrophysics

THE FORMATION OF THE GALAXY:Constraints from the CMD

Antonino Milone

Tumlinson 2010 Credit: Tom Brown

Dark Matter distribution around a large spiral galaxy

Bright dwarf galaxies, where star formation continued beyond reionization

Tumlinson 2010 Credit: Tom Brown

Tumlinson 2010 Credit: Tom Brown

Low-luminosity dwarf galaxy where star-formation was shut of by reionization

Most dark-matter clumps never form stars

Tumlinson 2010 Credit: Tom Brown

If this is true, there should be faint dwarf galaxies beyond the bright dwarf galaxies

that we have known from decades

The missing satellite problemThe missing satellite problemDwarf galaxies could follow one of three evolutionary paths:

True fossils (?) That formed most of their stars prior to reionization;

Polluted fossils with star formation continuing beyond reionization;

Survivors That largely formed their stars after reionization.

Ricotti & Gnedin (2005)

Discovery of a new class of galaxies in the SDSS.The Ultra-Faint Dwarfs.

Belokurov et al. (2007) Credit: Tom Brown

Portrait of ultra-faint dwarf

Leo IV

Portrait of ultra-faint dwarf

Portrait of ultra-faint dwarf

Giant elliptical galaxy~100 billion stars

Large city~10 million people

Giant elliptical galaxy~100 billion stars

Classical dwarf galaxy~10 million stars

Large city~10 million people

Theatre~thousand people

Giant elliptical galaxy~100 billion stars

Classical dwarf galaxy~10 million stars

Ultra-faint dwarf galaxy~10 thousand stars

Large city~10 million people

Theatre~thousand people

Giant elliptical galaxy~100 billion stars

Classical dwarf galaxy~10 million stars

Ultra-faint dwarf galaxy~10 thousand stars

Large city~10 million people

Theatre~thousand people

Single person

Ultra-faint dwarfsUltra-faint dwarfs

The ultra-faint dwarf (UFD) galaxies have luminosities of MV > −7 (M~10,000 solar masses).The internal kinematics reveal that UFD have mass to light ratio M/L>100, hence they are dark-matter dominated.

Ultra-faint Dwarfs VS. Globular Clusters

Globular Clusters are consistent with having no dark matter.Their mass to light ratio is M/L~2

Galaxies have significant dark matterClassical dwarfs have mass to light ratio M/L~10Ultra-faint dwarfs have mass to light ratio M/L>100

Ultra-faint Dwarfs VS. Globular Clusters

Ultra-faint dwarfs look like extension of dwarf galaxies. And not star clusters!

The Big question:The Big question:

Do true-fossil galaxies exist ?Are the ultra-faint dwarfs true-fossil galaxies?

The UFD Eridanus II. Belokurov

Brown et al. 2014

HST infers the age of six ultra-faint dwarfsHST infers the age of six ultra-faint dwarfs

80 % of stars formed by z=6 and 100% of stars formed by z=3.The similarly ancient populations suggest that star formation in the smallest dark-matter sub-halos was suppressed by a global outside influence (e.g. the reionization).

Investigating UFDs: The receipe

Binaries

ACS/WFC data

Stellar isochrones

CMDs GCs and UFDs

If we consider the two components of an unresolved binary system and indicate with m1, m2, F1, and F2 their magnitudes and fluxes.

The binary system will appear as a single point-like source with magnitude:

Indeed:

m1=-2.5 log(F1)m2=-2.5 log(F2)

mbin=-2.5 log(F1+F2) =-2.5 log[F1 (1+F2/F1)] =-2.5 log(F1) -2.5 log(1+F2/F1) = m1-2.5 log(1+F2/F1)

Unresolved binariesUnresolved binaries

In the case of a simple stellar population the fluxes are related to the stellar masses, M1, M2.

As a consequence, the luminosity of the binary system will depend on the mass ratio q=M2/M1

For simplicity we assume: M1≥M2 0≤q≤1

Unresolved binariesUnresolved binaries

Milone et al. 2012Milone et al. 2012

Equal-mass binaries are binary systems formed by two stars withthe same mass M1=M2.

In a simple stellar population: m1=m2, F1=F2

As a consequence: mbin= m1-2.5 log(1+F2/F1) = m1-2.5 log(1+1) = m1-2.5 log(2) = m1-0.752

The binary system will appear0.752 mag brighter than eachsingle star.

Unresolved binariesUnresolved binaries

Milone et al. 2012Milone et al. 2012

As an example, the V and I magnitudes of two equal-mass binaries are:

Vbin=V1-0.752Ibin=I1-0.752

Their color is

Vbin-Ibin=(V1-0.752)-(I1-0.752)=V1-I1

The binary system composed of equal-mass stars has the same color as each single star.

Unresolved binariesUnresolved binaries

Milone et al. 2012Milone et al. 2012

Mirror diameter: 2.4 metersWavelength range: ~0.1–1.7 μm (WFC3)

– Launched in 1990It orbits in low Earth orbit.Altitude ~540 km, period ~55 min. – Five Shuttle servicing missions

– Cost > 10 billion $

Imaging with HubbleImaging with Hubble

The Wide Field Planetary Camera 2 (1993-2009)

The most-used instruments on HSTThe most-used instruments on HST

It includes four cameras composed of 800x800 pixels each. – WF2, WF3, WF4Plate scale 0.10x0.10 arcsec/pixel

– Planetary Camera Plate scale 0.05x0.05 arcsec/pixel

Wavelength range: ~1200–10000 Å

The ‘Pillars of Creation’. Star forming region in the Eagle Nebula.

The Advanced Camera for Surveys (ACS) 2009 – present

The most-used instruments on HSTThe most-used instruments on HST

It includes three channels:

1) High Resolution Channel (HRC) Field of view of 29x26 square arcsec Wavelength range 1700 – 11000 Å Plate-scale: 0.027 arcsec/pixel.

2) Solar Blind Channel (SBC)Field of view: of 34.6x30.5 arcsecWavelength range: 1150 – 1700 Å Plate-scale: 0.032 arcsec/pixel.

Ceres (2005, HRC/ACS)

Mars (2003, HRC/ACS)

The most-used instruments on HSTThe most-used instruments on HST

3) Wide Field Channel (WFC) of ACS

– field of view: 202x202 square arcsec – Plate-scale: 0.05 arcsec/pixel.– Wavelength range: 3500-11000 Å

The most-used instruments on HSTThe most-used instruments on HST3) Wide Field Channel (WFC) of ACS

– field of view: 202x202 square arcsec – Plate-scale: 0.05 arcsec/pixel.– Wavelength range: 3500-11000 Å

Imaging with HubbleImaging with Hubble

Imaging with HubbleImaging with Hubble

Imaging with HubbleImaging with Hubble

The fraction of electrons that are successfully moved from one pixel to another during read-out is described by the charge transfer efficiency (CTE).

Normal charge transfer efficiencies are 0.99999 – 0.999999, (one photoelectron is lost for every 100000 to 1000000 shifts!)

If the CTE is only 0.999, you couldn't read most of the CCD.

CCDs that have a very low CTE will leave streaks which are caused by charge/electrons being left behind after a transfer.

Charge Transfer Efficience lossCharge Transfer Efficience loss

Imaging with HubbleImaging with Hubbleoriginal

Corrected forCTE

Imaging with HubbleImaging with Hubble

Point-Spread Function PhotometryPoint-Spread Function PhotometryAll the point-like sources imaged by the telescope system can be represented by a point-spread function (PSF).

The PSF gives ‘the shape’ of a star on the detector.

Its amplitude will scale linearly with the brightness of the star forming the image.

ACS/WFC images ofNGC2158.Bedin et al. (2010)

Point-Spread Function PhotometryPoint-Spread Function Photometry

In principle the receipe to derive the PSF model is simple:

– we must identify stars in our image– determine the sky under the stars– use isolate stars to derive the PSF model

Point-Spread Function PhotometryPoint-Spread Function Photometry

To measure stellar fluxes and positions we must fit the PSF model to all the star observed in the image. (allowing for the fact that the stellar image sits on top of the sky).

Countour plots of the PSF obtained from WFPC2 images (WF2). Anderson & King (2000).

Homework:Homework:

The file ‘HOROLOGIUMI.XYVIRDS’ includes high-precision HST photometry of a deep field centred on the UFD Horologium I.

This is the deepest CMD of Horologium I ever, and is not properly analyzed yet.

It includes stellar coordinates and magnitudes in the filters F606W and F814W of ACS/WFC (see header).

The ACS survey of Galactic Globular ClustersThe ACS survey of Galactic Globular Clusters

A. P. Milone Padova 2020

We obtained homogeneous photometry of stars in 68 Globular Clusters, from the RGB tip to ~0.2 solar-mass MS stars through the F606W and F814W filters of ACS/WFC

Sarajedini et al. (2007),

Anderson et al. (2008), Dotter et al. (2011), Milone et al. (2012)

Results indicate a rapid chemical enrichment in the inner Galaxy and suggest prolonged GC formation in the outer halo.

The latter is consistent with the outer halo GCs forming in dwarf galaxies and later being accreted by the Milky Way.

Age-metallicity relation and Galaxy formationAge-metallicity relation and Galaxy formation

Dotter et al. (2011) ApJ, 738, 74

Homework:Homework:

– Derive age, distance and reddening of Horologium I*.

– Compare your results with those by Brown et al. (2014) based on six UFDs and those from Dotter et al.(2010) based on GCs.

– Try to address the following ‘big questions’ of stellar astrophysics:

Do true-fossil galaxies exist ?Is the UFD ‘Horologium I’ a true-fossil galaxy?

* You can assume [Fe/H]=-2.49, and the following relations for reddening law (from A. Dotter, private communication):AF606W=2.8782 E(B-V) AF814W=1.8420 E(B-V)