ModernModern observationalobservational astronomy:astronomy: fromfrom starsstars toto galaxiesgalaxies Universidad Nacional de Colombia, Bogota, Agosto 2005

V – Star clusters & Stellar populations Dr. Michael Hilker (Sternwarte Bonn) 47 Tuc NGC 6093 GC1 in M31 Star clusters near and far E in Coma cluster

Dr. Michael Hilker (Sternwarte Bonn) What is the definition of a star cluster? Historically, the term “globular cluster” was dedicated to the old, massive and metal-poor clusters in the Milky Way. In contrast, the less massive, less dense young clusters in the Milky Way disk were named “open clusters”.

Today, this picture has changed a little bit: 1) Milky Way globular clusters are not only metal-poor bulge/disk clusters 2) there exist young clusters that are as massive and dense as old globular clusters in mergers, starburst galaxies, and even disks of normal spirals The total assembly of globular clusters (GCs) around a galaxy is called a “globular cluster system” (GCS), the

galaxy being its “host galaxy” .

Dr. Michael Hilker (Sternwarte Bonn) Distribution of open clusters in the Milky Way

Properties: mass 10 -10 M

Dr. Michael Hilker (Sternwarte Bonn)

diameter ~5 pc

2 4 

 The open cluster M37

Dr. Michael Hilker (Sternwarte Bonn) Color-magnitude diagram of the Pleiades cluster

solid line: isochrone for stars with an age of 100 Myr dotted line: without correction for dust reddening dashed line: isochrone for age 16 Myr

Dr. Michael Hilker (Sternwarte Bonn) What is the link between star clusters and star formation? In order to relate cluster formation with the underlying star formation history one has to investigate what is the cluster formation efficiency in different environments.

• Does clusters always form when stars are forming? • Is there a one-to-one relation between cluster and star formation? • Is the cluster formation efficiency the same for metal-poor and metal-rich environments? This questions have been partly answered by studying the formation of massive clusters in late-type galaxies and merger/starburst galaxies as well as the metallicity distri- bution of globular clusters and field stars in the halos of nearby ellipticals.

Dr. Michael Hilker (Sternwarte Bonn) Massive cluster formation in spirals

Larsen & Richtler (1999)

Dr. Michael Hilker (Sternwarte Bonn) The cluster formation efficiency strongly depends on the local star forma- tion rate

TL=100(Lcl/Lgal)

Larsen & Richtler (1999)

Dr. Michael Hilker (Sternwarte Bonn) Distribution of old globular clusters in the Milky Way

Properties: mass 10 -10 M

Dr. Michael Hilker (Sternwarte Bonn)

diameter 20-50 pc

5 6 

 The globular cluster M71

Dr. Michael Hilker (Sternwarte Bonn) Surface density profile of Galactic globular clusters

cores cusps

core-collapse clusters

Dr. Michael Hilker (Sternwarte Bonn) Color magnitude diagrams of Galactic globular clusters

Dr. Michael Hilker (Sternwarte Bonn) The faintest stars in globular clusters

White dwarfs in M4

Dr. Michael Hilker (Sternwarte Bonn) Proper motion of stars One of the deepest globular cluster color-magnitude diagrams (M4)

(V-I)

Dr. Michael Hilker (Sternwarte Bonn) Deep combined color-magnitude diagram of various Galactic globular clusters

Dr. Michael Hilker (Sternwarte Bonn) What can we learn from globular cluster HR diagrams? • Since all cluster stars formed together from the same parent cloud, they have a common age, chemical composition and distance. • The HR diagram can be used to measure these properties of clusters: age, composition and distance. • In particular, the most important measurement is age, which is impossible (or extremely difficult) to measure for isolated field stars.

• Cluster ages are measured using the color and luminosity of the turn-off

point at the top of the main sequence. Dr. Michael Hilker (Sternwarte Bonn) DependenceDependence ofof CMCM diagramsdiagrams uponupon ageage andand metallicitymetallicity

The turn-off point of a younger cluster is brighter and bluer than that of an older cluster (at equal metallicity). Lowering the metallicity causes individual stars to become both brighter and hotter (= bluer).

Dr. Michael Hilker (Sternwarte Bonn) Color-magnitude diagrams for two globular clusters with different metallicities

47 Tuc: [Fe/H] = -0.83 M 30: [Fe/H] = -2.31

Isochrone fitting shows that 47 Tuc probably is about 2 Gyr younger than M 30 clues to the evolution of the Milky Way 10, 12, 14 Gyr 12, 14, 16 Gyr

Dr. Michael Hilker (Sternwarte Bonn) Metallicity distribution of Milky Way and M31 globular clusters

Discovery of sub- populations in the GCS of the Milky Way and M31: halo: metal-poor bulge: metal-rich

Dr. Michael Hilker (Sternwarte Bonn) The Galactic Globular Cluster System

Dr. Michael Hilker (Sternwarte Bonn) Each major galaxy possesses many globular clusters out to large radii. This makes GCs to valuable test particles for galaxy evolution and the study of galaxy environments.

NGC 1399 in the Fornax Cluster

Dr. Michael Hilker (Sternwarte Bonn) The color distribution of globular clusters in nearby ellipticals

Larsen et al. (2001)

there always are blue and red clusters found, often divided by a gap (bimodal distribution)

Dr. Michael Hilker (Sternwarte Bonn) SummarySummary ofof findingsfindings forfor sub-populationssub-populations inin globularglobular clustercluster systemssystems blue red metal-poor metal-rich old some have intermediate ages ~ same metallicity in all broad metallicity distribution galaxies larger linear diameter smaller linear diameter radially extended radially concentrated higher specific frequency small specific frequency single “halo”-population various populations, associated to the bulge formed in shallow potential formed in deep potential wells, wells with gas loss, some in like in galaxy mergers galaxy’s halos, others in satellites that were accreted

Dr. Michael Hilker (Sternwarte Bonn) ClusterCluster formationformation inin mergersmergers

The Antennae galaxy

Dr. Michael Hilker (Sternwarte Bonn) Three-color images of the cluster formation in Antennae

Whitmore et al. (1999)

Dr. Michael Hilker (Sternwarte Bonn) Color evolution of globular cluster systems in merges

Dr. Michael Hilker (Sternwarte Bonn) StellarStellar populationspopulations The most simple stellar population is that of a globular cluster because of the same age and metallicity of its stars.

However, in galaxies stellar populations of different ages and metallicities are mixed. They can hardly be disentangled in color magnitude diagrams.

Difficulty besides the population mix: the large distances of galaxies stars are barely resolved only brightest stars are seen in CMD

Dr. Michael Hilker (Sternwarte Bonn) What ingredients do we need to make the step from the stellar evolution of individual stars to the evolution of a whole stellar population of a galaxy?

1) Star formation • initial conditions: stellar mass/luminosity function • time dependence: star formation rate SFR SFR SFR t t t constant exponentially falling SF burst

2) Chemical enrichment • synthesis of chemical elements • models of chemical enrichment

Dr. Michael Hilker (Sternwarte Bonn) AA simplifiedsimplified picturepicture Let’s follow stellar evolution in time: t (Gyr)

metals de- first metals stellar metals fines population to- second day’s stellar CMD population

further stellar population(s) This sketch illustrates the star formation history and the chemical enrichment history of a galaxy (which can be quite complicated).

Dr. Michael Hilker (Sternwarte Bonn) LuminosityLuminosity functionsfunctions andand massmass functionsfunctions

V The luminosity function Φ (M ) describes how many stars of each luminosity are present in each pc3. V V Φ (M ) dM is the density of stars with magnitudes in the V V V V intervalThe initial [M ,Mluminosity+dM ]. function Ψ(M ) is related to the number of stars when they were on the main sequence Assuming a uniform star formation rate for the disk of the Milky Way gal over its lifetime of τ ~ 10 Gyr, one can calculate its initial luminosity V V MS V MS V gal functionΨ(M from) = Φ its (Mpresent-day) luminosity functionfor τ Φ(M (M) >) τlike this: MS V gal MS V MS V = Φ (M ) x τ /τ (M ) when τ (M ) gal < τ Dr. Michael Hilker (Sternwarte Bonn) Present-day and initial luminosity function of nearby stars

Dr. Michael Hilker (Sternwarte Bonn) V With the initial luminosity function Ψ(M ) in hand, one can derive an IMF, the initial mass function ξ (M)dM This is the number of stars that have born with masses between M and M+dM 0 -2.35 ξ ξ Near the Sun one finds: ( M ) = M Salpeter IMF with ξ 0 being the local stellar density. The Salpeter function gives a good fit in various parts of the Galaxy and the Magellanic Clouds here plotted for the Pleiades cluster Dr. Michael Hilker (Sternwarte Bonn) The IMF with different assumption on the star-formation rate

Dr. Michael Hilker (Sternwarte Bonn) FittingFitting lawslaws forfor thethe IMFIMF Simple power law function:

Salpeter IMF

Three power law functions:

Scalo IMF

Kroupa IMF

steeper fall-off at high mass end

Dr. Michael Hilker (Sternwarte Bonn) EvolutionEvolution ofof stellarstellar populationspopulations For most galaxies we can only analyse the integrated light of their stellar populations. Mostly, the stellar population of a galaxy as a whole is a mix of stars with different ages and metallicities. So what can we learn from the spectra and photometric colors of superposed stellar populations? First, one can estimate the mean metallicity and can get some constraints on the age of the galaxy. But moreover, one can regard a galaxy as a superposition of coeval stellar populations of different ages. The only thing one has to know is the evolution of a coeval, so-called single stellar population. There exist extensive numerical models for that.

Dr. Michael Hilker (Sternwarte Bonn) NumericalNumerical modelsmodels ofof populationpopulation evolutionevolution With the models of stellar evolution, that predict apparent magnitudes, broad-band colors and spectral indices for individual stars, one can construct a theoretical CM diagram, when assuming a reasonable initial luminosity function. By adding together the contributions of all the model stars in the theoretical CMD one can derive the integrated magnitudes, colors, and spectral indices.

Many groups have developed codes for evolution models. Uncertainties in the derived colors arise mainly from the assumptions made regarding the evolution of stars once they leave the main sequence.

Let’s have a look to some results of the models ….

Dr. Michael Hilker (Sternwarte Bonn) Aging of the integrated spectrum

Dr. Michael Hilker (Sternwarte Bonn) Metallicity and age effects on the integrated spectrum of a single stellar population

Dr. Michael Hilker (Sternwarte Bonn) Luminosity evolution for different star formation histories

Ψ1: constant SFR starburst Ψ2: expontionally decreasing SFR

starburst of 5 Myr duration in a 10 Gyr old galaxy (constant SFR)

Krüger & Fritze-v. Alvensleben (1994)

Dr. Michael Hilker (Sternwarte Bonn) metallicity [dex] Broad-band color evo- lution

Age-metallicity degeneracy

3.3 Gyr 2 Gyr 8 Gyr 13.5 Gyr Worthey 0.3 dex 0 dex -0.5 dex -0.3 dex (1994)

Dr. Michael Hilker (Sternwarte Bonn)

Dr. Michael Hilker (Sternwarte Bonn)

Dr. Michael Hilker (Sternwarte Bonn) ModernModern observationalobservational astronomy:astronomy: fromfrom starsstars toto galaxiesgalaxies Universidad Nacional de Colombia, Bogota, Agosto 2005 shown to the same linear scale and the same level of surface brightness

VI – Milky Way & Local Group Dr. Michael Hilker (Sternwarte Bonn) TheThe LocalLocal GroupGroup The Local Group contains roughly 35 galaxies within a sphere of about one megaparsec in radius. It forms a gravitationally bound system. The Local Group is a typical environment in the Universe. More than 50% of all galaxies are found in small groups or associations, which are distributed along walls or filaments between the dense concentration of galaxy clusters. Due to the close vicinity of all galaxies in the Local Group, there evolutionary histories can be studied in great detail. • in color magnitude histograms of resolved stars one can study the evolution of their stellar populations • the interstellar medium can be mapped and its physical conditions analyzed • kinematic studies give clues how the galaxies have been assembled Dr. Michael Hilker (Sternwarte Bonn) Sketch of the Local Group with Milky Way in the center

3 spirals ~14 (dwarf) irregulars 4 dwarf ellipticals >14 dwarf spheroidals

Dr. Michael Hilker (Sternwarte Bonn) TheThe LocalLocal GroupGroup -- somesome factsfacts 90% of the visible light is emitted by the three spirals Andromeda, M33 and our Milky Way. The center of the Local Group is located between Andromeda and the Milky Way. The Milky Way and Andromeda are approaching each other with a velocity of about 120 km/s. Most of the smaller galaxies orbit around Andromeda or the Milky Way. Most of the MW satellites lie close to a single plane. The only elliptical galaxy is the small compact elliptical (cE) M32, a satellite of Andromeda.

Dr. Michael Hilker (Sternwarte Bonn) Galaxy luminosity function in the Local Group

Pritchet et al. 1999

Dr. Michael Hilker (Sternwarte Bonn) Distribution of Local Group galaxies

Sparke & Gallagher

Dr. Michael Hilker (Sternwarte Bonn) The Milky Way in optical and infrared

Dr. Michael Hilker (Sternwarte Bonn) Disk components in the Milky Way Interpretation: z • σ increases with age • stars born “cold” from molecular clouds • then gradually “heated” by scattering on DMCs and spiral arms • or: heating of disk over time by satellite passage density old stars and/or minor mergers

gas young stars

Distance from Galactic plane

Dr. Michael Hilker (Sternwarte Bonn) TheThe GalacticGalactic CenterCenter

Orbits of stars from 10 years observations

Radio source Sagittarius A* = Galactic Center

Evidence for a black hole with > 2x106 solar masses Dr. Michael Hilker (Sternwarte Bonn) TheThe GalacticGalactic CenterCenter

Measured orbits Predicted orbits (until 2010)

Dr. Michael Hilker (Sternwarte Bonn) The neighbouring spiral galaxy: Andromeda (M31) Galaxy 50% more type: Sb luminous than the Milky Way

contains twice as many glo- bulars as the MW

has a warped disk

has more than 10 satellite galaxies

Dr. Michael Hilker (Sternwarte Bonn) Tidal debris around the Andromeda galaxy

Dr. Michael Hilker (Sternwarte Bonn)

Dr. Michael Hilker (Sternwarte Bonn) The light profile follows a de Vaucouleurs law

It possesses no globular clusters and only old stars

There probably exists a black hole in the center

M32 might be the remnant of a formerly larger galaxy which lost its outer parts in the tidal field of M31 (remnant bulge?)

Dr. Michael Hilker (Sternwarte Bonn) M33 Galaxy 20% as lumi- type: Scd nous as the Milky Way

has a tiny bulge and a dense nuclear star cluster

is richer in HI gas than M31 and Milky Way

Dr. Michael Hilker (Sternwarte Bonn) The Magellanic Clouds

The Magellanic Clouds:

dLMC = 50 kpc

dSMC = 60 kpc

vLSR,LMC = 266 km/s shown is the number density of stars in the infrared vLSR,SMC = 149 km/s Magellanic Stream Magellanic Clouds

Dr. Michael Hilker (Sternwarte Bonn) TheThe MagellanicMagellanic Clouds:Clouds:

• bridge of gas and stars in between them • trailing stream of gas distributed in the Galactic halo (Magellanic Stream) • orbit about each other • are on an eccentric plunging orbit around the Galaxy, with a period of about 2 Gyr • the closest approach to the MW happened about 200-400 Myr ago

Dr. Michael Hilker (Sternwarte Bonn) Our neighbours: the Magellanic Clouds

Small Magellanic Cloud Large Magellanic Cloud

Dr. Michael Hilker (Sternwarte Bonn) CMDs of the bulge of the Milky Way and the disk of the LMC

MW LMC

bluer than in MW broad: range of bulge: lower stellar ages metallicity

Dr. Michael Hilker (Sternwarte Bonn) Hipparcos CMD in Milky Way CMD of the SMC

Perryman 1997

SMC Red Clump (Stanek et al. 2001)

Prominent red clump: metal-rich horizontal branch stars Dr. Michael Hilker (Sternwarte Bonn) 2MASS: colour magnitude diagram of LMC stars

van der Marel (2004) Dr. Michael Hilker (Sternwarte Bonn) Number density of LMC stars in the 2MASS survey

deprojected: LMC disk is not round but elongated due to tidal forces of MW

Dr. Michael Hilker (Sternwarte Bonn) 2MASS: distances to Cepheids in the LMC

van der Marel (2004) Dr. Michael Hilker (Sternwarte Bonn) Distribution of young stars in the Large Magellanic Cloud

Grebel (2004) Dr. Michael Hilker (Sternwarte Bonn) In the Magellanic Clouds star clusters still can be resolved into individual stars. Investigations show that the population of star clusters is very different from that of the Milky Way, reflecting the different star formation histories of the galaxies. Whereas only few old globular clusters exist in the Large Magellanic Cloud, it is dominated by a large population of young populous clusters. Most of them might be de- structed within a Hubble time (like the Milky Way open clusters), but some have masses comparable to Galactic globular clusters. Although being close companions and being connected by a bridge of neutral hydrogen, both galaxies seem to have had a very different evolutionary history, as deduced from their cluster populations … let’s have a look...

Dr. Michael Hilker (Sternwarte Bonn)

Dr. Michael Hilker (Sternwarte Bonn) Star clusters and the star formation history in the Magellanic Clouds

Large Magellanic Cloud

Small Magellanic Cloud

Dr. Michael Hilker (Sternwarte Bonn) The Magellanic System in HI – An Overview

column density radial velocity Dr. Michael Hilker (Sternwarte Bonn) Stars (color) & HI gas (contours) Gas and stars in the LMC

Stellar surface density of AGB and RGB stars (van der Marel 2001) overlaid with HI data from the Parkes survey.

The HI gas is less extended towards the north-western region => stripping or compression?

HI column density Stars

Location of the Sagittarius dwarf spheroidal

Dr. Michael Hilker (Sternwarte Bonn) The Sagittarius stream – 2MASS counts of M-giant stars

Ibata et al. 2002

Dr. Michael Hilker (Sternwarte Bonn) Sagittarius dwarf spheroidal – artist´s view

Dr. Michael Hilker (Sternwarte Bonn) The tidal tails of Palomar 5

Dr. Michael Hilker (Sternwarte Bonn) Simulation of a dissolving dwarf galaxy

QuickTimeﾪ and a YUV420 codec decompressor are needed to see this picture.

Dr. Michael Hilker (Sternwarte Bonn) Simulation of dynamical friction for a dissolving dwarf galaxy

QuickTimeﾪ and a YUV420 codec decompressor are needed to see this picture.

dv M r v de-acceleration is related to mass and velocity of ~ The moving object and the density of the environment: dt | v |3

Dr. Michael Hilker (Sternwarte Bonn) A nearby dwarf spheroidal galaxy

Leo 1

Dr. Michael Hilker (Sternwarte Bonn)

Dr. Michael Hilker (Sternwarte Bonn)

Dr. Michael Hilker (Sternwarte Bonn) CMD of the dwarf spheroidal Carina

metal-poor isochrones

15 Gyr

3 Gyr

7 Gyr

15 Gyr

Dr. Michael Hilker (Sternwarte Bonn) Detection of new dwarf spheroidal galaxies near Andromeda

Zucker et al. (2004) Dr. Michael Hilker (Sternwarte Bonn) The new dwarf spheroidal And IX near Andromeda

Zucker et al. (2004) Dr. Michael Hilker (Sternwarte Bonn) IC10

Dr. Michael Hilker (Sternwarte Bonn) The dwarf irregular galaxy IC 10

HI contours superposed on Hα emission from R-band image ionized gas

Dr. Michael Hilker (Sternwarte Bonn) Sextans A: dwarf irregular of the Local Group

Dr. Michael Hilker (Sternwarte Bonn) CMD for the irregular dwarf galaxy Sextans A

Dr. Michael Hilker (Sternwarte Bonn) Hydrogen mass of dSphs vs. distance to their host galaxy

Conclusion: dwarf galaxies lose their gas due to tidal stripping in the vicinity of Andromeda and the Milky Way

Dr. Michael Hilker (Sternwarte Bonn) Simulating the fate of the Milky Way and Andromeda

QuickTimeﾪ and a YUV420 codec decompressor are needed to see this picture.

Dubinski et al. Dr. Michael Hilker (Sternwarte Bonn)

Dr. Michael Hilker (Sternwarte Bonn)

Dr. Michael Hilker (Sternwarte Bonn)