Using the HR Diagram to Measure the Star Formation Histories of Galaxies. Tammy Smecker-Hane University of California, Irvine

Transcription

1 Using the HR Diagram to Measure the Star Formation Histories of Galaxies Tammy Smecker-Hane University of California, Irvine Irvine, California 1

2 Outline 1. Stellar Evolution Hertzsprung-Russell Diagram (HRD) or Color- Magnitude Diagram (CMD) 2. Determining the Age of Star Clusters & Field Stars What they tell us about Galaxy Evolution (SMC, LMC, M33) In a star cluster, stars have different masses but each has the same distance, age, and chemical composition Field stars probe the whole range in ages The Physics Governing Stars & Stellar Evolution 1. Gravity Surface Gravity g = GM/R 2 2. Nuclear Fusion Net reaction: 4 H He + γs s + e + s + νs Only happens in the core of the star (r < 0.25 R) 3. Energy Transport Radiative Transport in the core (photons move the energy outward) Convective Transport at large radii (physical motion of gas moves the energy outward) 2

3 The Physics Governing Stars & Stellar Evolution 4. Pressure Ideal Gas Law: P = ρkt/<m> 5. Hydrostatic Equilibrium At every radius in the star, the force exerted by the outward gas pressure must balance the weight of the material on top Theoretical Stellar Evolutionary Models successfully predict the properties of the Sun and other stars from these basic inputs! The Interior of a Star 3

4 The Interior of a Star The Hertzsprung-Russell Diagram (HRD) The Main Stages of Stellar Evolution Note: By convention, the T eff axis in an HRD is reversed. Hotter, bluer stars are to the left and cooler, redder stars are to the right. 4

5 The Hertzsprung-Russell Diagram (HRD) Massive Stars change T eff at fixed L when they evolve and become Supergiant stars Low Mass Stars change in both T eff and L when they evolve and become Red Giant stars Star Clusters As the cluster ages, stars evolve off the main-sequence, go through their various evolutionary stages, then they die and disappear from the HR Diagram 5

6 Quiz on Cluster Ages Here is a table of the temperatures of stars at the main sequence turnoff in four clusters. Which cluster is the oldest? Cluster A B C D Name Orion NGC Tuc M 67 eff ( K) 22,000 9,000 5,000 7,000 T eff Hipparcos Catalog: Stars Nearest the Sun The Sun is a very typical star. Most of the stars are seen places where they spend the longest time. Stellar Lifetimes: 90% on MS, ~5% on RGB, ~5% HB 6

7 Main Sequence Lifetime Lifetime = Energy Available / Rate Used t MS M / L The mass M dictates the L of a star. In fact, L M 3.5 for most main sequence stars. Therefore, t MS M / L M / M 3.5 M -2.5 MS = yrs x (M/M ) -2.5 t MS Main Sequence Lifetime Stars have masses from 0.08 M < M < 130 M What is the main sequence lifetimes of a 10 M star? t MS = 32 million yr What is the lifetime of a 0.5 M star? t MS = 56 Gyr, which is > the age of the Universe, 13.7 Gyr! All low mass stars with m < 0.85 M that ever formed in a galaxy is still there. Measuring the # stars at different parts of the HR Diagram can tell you a galaxy s star formation history Hess Diagram: : diagram showing the density of stars in bins of color & magnitude in an HR Diagram 7

8 Apply HRD to Star Clusters Star Cluster: a group of stars with different masses that formed at roughly the same time from the same gas cloud. All the stars have the same chemical composition, age and distance Globular Cluster M80 Open Cluster NGC 2420 Milky Way Star Clusters 150 Globular Clusters: Gyr, metal-poor, stellar halo Thousands of Open Clusters: 0 to ~10 Gyr old, approxi- mately solar metallicity,, disk 8

9 Metallicity A star s properties (L, T, R, t MS ) also depend on the chemical composition of the star. Nomenclature: X = the fraction by mass of H = M H /M Y = the fraction by mass of He Z = fraction by mass of metals (>H,He) [Fe/H] = log [ (X Fe /X H ) / (X Fe /X H ) ] ~ log (Z/Z ) Sun: Z = (1.9%) and [Fe/H]=0 [Fe/H] = 1 1/10 solar [Fe/H] = 2 1/100 solar M30 A Metal-Poor, Galactic Globular Cluster 9

10 Globular Cluster M30 Ground-based photometry taken with the CTIO 4m (Smecker- Hane, et al.) Horizontal Branch Stars (core He burning) MSTO (sensitive to age) Main Sequence (unevolved stars) Red Giant Branch Nomenclature: Apparent Magnitude: m = -2.5 log(flux) + m 0 e.g., UBVRI, m V V Color: (B - V) m B -m V Absolute Magnitude: M m M + A λ 5 log(d/10 pc) M M = -2.5 log(l/l ) Extinction: A λ Reddening: E(B V) A B A V B V = (B V) 0 + E(B V) Distance Modulus: (m M) 0 Distances in pc; 1 pc = 3.2 lyr Globular Cluster M30 10

11 Globular Cluster M30 Vandenberg et al. (2002) stellar evolutionary model isochrone Comparing the observed fiducial points to the theoretical isochrone allows you to deteremine the cluster s reddening, distance, & age (chemical composition) d = 81 pc Globular Clusters Two different clusters with about the same age, but very different chemical abundances ( 25)( M30: [Fe/H] = Tuc: [Fe/H] =

12 Star Clusters Star Clusters are useful (although( biased) probes of the stellar populations of galaxies First an explanation of the classification scheme for galaxies Spiral Galaxies: : thin, rotating disk & bulge Elliptical Galaxies: : entirely stars, little or no HI gas, shape supported by velocity dispersions of the stars Dwarf Galaxies: : galaxies fainter than M B = -16, such as the LMC & SMC or the dsphs Dwarf Irregular (dir( dir): : gas rich dwarfs Dwarf Spheroidal (dsph):: all stars, no HI gas Four Galaxies are visible here M31, the Andromeda Galaxy M32 NGC 205 Stars in our own Milky Way Gary Stephens 12

13 The Fornax dsph Galaxy Two Star Clusters in the Fornax dsph 13

14 Open Cluster C in the Fornax dsph Globular Cluster 4 in the Fornax dsph 14

15 Star Clusters Some very interesting pieces of information about galaxy evolution come from studying the star clusters in galaxies Nearly all the Milky Way globular clusters have ages of Gyr although they span a range of chemical abundances. Initially, the Milky Way formed very chaotically globulars were probably born out of violent collisions, similar to the bright, compact clusters we see forming in star-burst galaxies today. Star Clusters Cosmological simulations of galaxy formation predict that large galaxies form first, and small galaxies form later. But what do we see in nearby galaxies? All galaxies have Gyr old stars! There are no late blooming galaxies in the Local Group (galaxies with d < 1 Mpc), although their star formation histories are very different. The Milky Way, LMC, Fornax and Sagittarius dsphs have globular clusters that are essentially co-eval eval,, Gyr in age. However the SMC has only 1 globular cluster, and its age is Gyr younger than in other galaxies. 15

16 Large and Small Magellanic Clouds Large and Small Magellanic Clouds LMC/SMC are interacting with one another as they orbit the Milky Way; bridge & tidal tails HI Maps - neutral hydrogen gas (M. Putman) Magellanic Stream 16

17 The Stellar Populations of the SMC HST survey of star clusters and field stars in the Small Magellanic Cloud done by a large collaboration: Gallagher, Grebel, Nota, Tosi, Sabbi, Glatt,, etc. including myself Goals: Accurately measure the ages (±0.5( Gyr) and distances of star clusters in the SMC Use the star clusters to trace the evolution of the metallicity over time in this dwarf galaxy Use the field stars to derive the star formation and metal enrichment history Identify how cluster formation compares to the star formation SMC Globular Cluster NGC 121 Glatt, et al. (2008) Assumed [Fe/H] spec = 1.46 Dartmouth Isochrones (Dotter et al. 2007) [α/fe] = +0.2 (m-m) 0 = d = 61.9 kpc age = 10.5 ± 0.5 Gyr 17

18 SMC Cluster NGC 121 Glatt, et al. (2008) 5 isochrones displayed at 0.5 Gyr intervals age = 10.5 ± 0.5 Gyr SMC Star Clusters NGC 121 has an age of 10.5 Gyr and [Fe/H] = 1.46 ( intermediate( intermediate metallicity) NGC 121 is the only globular cluster in the SMC, and is its oldest star cluster Disruption of Clusters? Total disruption is not likely for globulars because of their very high spatial density So is the SMC a young galaxy? No. 18

19 The Stellar Populations of the SMC 6 SMC Field Star Areas in Gallagher et al. The Stellar Populations of the SMC Field Stars in 6 SMC Locations (Sabbi,, et al. 2009) Z = Ages = 50, 100, & 500 Myr Z = Ages = 3, 5, & 12 Gyr 19

20 The Stellar Populations of the SMC Full analysis of the CMDs is not complete yet, but simply from comparing the CMDs to isochrones, we find that the SMC does have Gyr old stars. Just not that many. Thus the SMC had a slow start to its formation which maybe a reason so few globular clusters formed in it. Mapping the Spatial Distribution of SMC in 3D Using Star Clusters delta [deg] NGC411 NGC416 BS90 NGC419 Kron 28 Kron 44 NGC152 NGC alpha [deg] Lindsay 38 NGC121 Kron 3 Lindsay distances [kpc] age [Gyr] 20

21 Age-Metallicity Relationship in SMC 0.5 Kayser et al Parisi et al Da Costa et al [Fe/H] [dex] (CG97) 1 Bica et al age [Gyr] Large Magellanic Cloud 21

22 LMC Star Clusters LMC Star Clusters show a large age gap.. Only 1 cluster has an age of 3-13 Gyr, e.g., Da Costa (2002) 43 Large Magellanic Cloud Smecker-Hane, Cole, Gallagher & Stetson (2002) imaged star fields in the LMC with the WFPC2 on the Hubble Space Telescope (HST) Derived SFHs for the Bar and Disk 1 fields from the # stars as a function of magnitude on the main-sequence 5% of WFPC2 area shown at right; mean separation of stars with V 25 mag is ~ 6 pix =

23 Large Magellanic Cloud LMC Disk LMC Bar 23

24 A Differential Hess Diagram LMC Field Stars Black = Larger # stars in the Bar Field White = Larger # stars in the Disk 1 Field V I LMC Field Stars Bar: Open Histogram Disk 1: Hatched Histogram Bar Formation 24

25 LMC Field Stars SFR of the LMC Disk 1 field was nearly constant with time, not varying by more than a factor of 2, in the last ~ 1 to 14 Gyr SFH of the LMC Bar field is very different from that of the Disk 1 field Initial formation of the bar ~ 4 to 6 Gyr ago, exact age depends on the assumed metallicity SFR in last 1 to 2 Gyr also has been high in Bar We note a distinct lack of metal-poor stars in both fields, but not a lack of old stars Stellar Populations of Galaxies Collisions of proto-galactic fragments early in the evolution of galaxies are thought cause dissipation of energy & funneling of gas to the centers, which may create galactic bulges (red, old stars ) In addition, early merging of the proto-galacitc fragments and later the continual canniblizaton of dwarf galaxy satelites are thought to make the stellar halos of galaxies out of merger debris 25

26 Hierarchical Formation of Galaxies ΛCDM N-body N simulation (dark matter only) of the evolution of a Milky Way type galaxy from Bullock & Johnston (2005) Questions: Stellar Populations of Galaxies Do all galaxies have stellar halos? What about a bulge-less less spiral like M33? Do the ages and metallicities of the stars in the halos match the predictions of sophisticated hierarchical galaxy formation simulations, and can they be tested over a range of galaxy luminosity? 26

27 With Michael Hood, Matt Teig, Annette Ferguson & Mike Irwin, and myself Spectra taken w/ Keck II 10-m telescope and the DEep Imaging Multi-Object Spectrograph (DEIMOS) M33 Spectroscopic Survey The areas studied in different parts of this project DEIMOS spectrscopic fields are the long & narrow fields. 27

28 M33 Spectroscopic Survey Dispersion = 0.47 Å/pix, Resolution = 1.8 Å Exposure time = 3 hrs Average S/N per pixel = 6 (3.5 to 15) Average Velocity Error = 9 km/s Field of View over which slits are placed is 16.3 x 5.0 Multiplexing is key to getting to our eventual goal: observing 400 M33 RGB stars M33 Spectroscopic Survey Initial runs selected stars for spectroscopy based on Ferguson et al. s photometric survey (Ferguson et al. 2006) Judge whether or not M33 or Milky Way stars after the fact using our DDO51 photometry Kinematic results presented here for 173 stars which are likely M33 members based on DDO51 photometry and relative densities of stars in the Hess diagrams of the cleaned MWay and M33 CMDs 28

29 Spectra of the Calcium Lines 3 Calcium Absorption Lines: Wavelength star s s velocity along the line of sight Depth abundance of Calcium in the star (Ca/H) Heliocentric Velocity vs Position Angle 29

30 M33 Velocity Results Thin Disk Intrinsic σ = 15 km/s The Stellar Halo of M33 Omitting Rotating Disk Stars ( v < 35 km/s), what is the intrinsic dispersion in heliocentric velocity? N = 34 stars Simple Calculation: <Vhelio> = 170 km/s RMS implies a Halo intrinsic σ = 72 km/s 30

31 M33 Star Clusters M33 does have a population of star clusters that have much higher velocity dispersion than the HI disk (Chandar et al. 2002) Clusters w/ age > 1 Gyr have σ = 68 km/s However only 18 clusters have kinematics that are inconsistent with disk rotation Conclusions 1. The Hertzsprung-Russell (HR) Diagram is a very valuable tool that allows us to measure distances, reddening and ages of star clusters and stellar populations of galaxies 2. Application to Star Clusters and Field Stars in Galaxies Milky Way, LMC, Fornax dsph and Sagittarius dsph all have globular clusters that are Gyr in age. SMC has only 1 globular cluster and it is only 10.5 ± 0.5 Gyr old, however it does have field stars that span the fulll range in age from 0 12 Gyr. The SMC started forming slowing, then after Gyr the star formation rate increased, and this gas-rich dwarf galaxy is still actively forming stars today. 31

32 Conclusions 2. Application to Star Clusters and Field Stars in Galaxies LMC formed stars at nearly a constant rate throughout the age of the Universe, probably had a burst about Gyr ago when the bar formed. Galaxies form star clusters at different rates than they form field stars. The formation rate of clusters is not the same as the star formation rate. Star clusters can be very useful probes of the age- metallicity relationship in a galaxy, overcoming the degeneracies in age/metallicity inherent in the CMD, but you need to study field stars to determine a galaxy s s star formation histiory. 3. More about that in tomorrow s lecture Thank you for your attention. Do you have any Questions? 32