View
2
Download
0
Category
Preview:
Citation preview
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)
Recommended