Galactic Structure

Heidi Newberg

Rensselaer Polytechnic Institute

Overview

• Overview of results from SDSS (this will go quickly, so I hope most peole have some familiarity with it)

• SEGUE – Sloan Extension for Galactic Underpinnings and Evolution

• Galactic Structure in SNAP/Destiny

The Milky Way Galaxy

40,000 square degrees on the skyComposed of stars, gas and dust, dark matterDark matter: not detected at all (yet)Gas and dust: Rv, density, compositionStars: 3D velocities, distances, mass, age, chemical compositionThe only galaxy for which we can hope to get comprehensive

stellar information in the next two decades (RAVE, QUEST, SEGUE, Pan-STARRS, GAIA, etc.).

The dark matter is only revealed by the motions of stars. The better we understand the detailed motions of stars in the Galaxy, the better resolution we will have on the spatial distribution of dark matter.

How are we going to understand all of that “detail” found in external galaxies if we don’t even know how our own galaxy is put together?

The Standard Galactic ModelRadial scale length (kpc)

0.22.94-53.5-52-3

BulgeSpheroidThick DiskThin DiskDark Halo

Vertical scale height or c/a

0.40.6-11.3 kpc325 kpc1?

Density near Sun (Msol/pc3)

0.000260.0260.1240.009

Metallicity [Fe/H]

0.3-1.5-0.6-0.1

Vrot at Rsol (km/s)

0-501802200?

Allen’s Astrophysical Quantities, 2000

Yanny et al. 2000

Pal 5 globular cluster

GalacticCenter

Yanny et al. 2000

Newberg et al. 2003

Celestial equator

Lookat theseBHB/Astarsat g=20.3near(l,b)=(190,30)degrees.

NGC2419 rightnearby the80 kpc streampiece! GlobularCluster wasperhaps onceassociatedwith Sagittarius

GalacticCenter

Yanny et al. 2000

Newberg et al. 2002

Newberg et al. 2002

Squashedhalo

Sphericalhalo

Exponentialdisk

Prolatehalo

Newberg et al. 2002

Newberg et al. 2002

Newberg et al. 2002

New

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et a

l. 20

02

Yanny et al. 2003

Yanny et al. 2003

Disk

“Ring”

2MA

SS

M s

tars

fro

m R

ocha

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t al.

Disk

“Ring”

2MA

SS

M s

tars

fro

m R

ocha

-Pin

to e

t al.

Frinchaboy et al. 2004

GCs from Crane et al. 2003

GC from Harris 1996

Open clusters

Martin et al., 2003

Martin et al., 2003

Canis Major

Canis Major

Canis Major

Southern Arc

Northern Arc

A

AB

B

Disk

“Ring”

2MA

SS

M s

tars

fro

m R

ocha

-Pin

to e

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Frinchaboy et al. 2004

GCs from Crane et al. 2003

GC from Harris 1996

Open clusters

Following Helmi et al. 1999

(Vx, Vy, Vz) = (-65, 135, -249), (σx, σy, σz) = (62, 33, 17)

Density of dark matter in Sagittarius Stream

[2, 190] x 104 M/kpc3

[0.001, 0.07] Gev/cm3

0.3-25% of the local density of the isothermal Galactic halo, assumed to be 0.3 GeV/cm3

Left : z=10, small haloes dominate. Red indicates possible site of star formation at this time (very dense regions). Right: Present time, many of the small haloes have merged into the model Milky Way halo; oldest stars found throughout the Milky Way and in satellites.

CDM simulation Moore et al. 2001

Michael Odenkirchen, MPIA

Ibata et al. 2001q=1.0

Ibata et al. 2001q=0.9

Ibata et al. 2001q=0.75

Photometric surveys can:

Discover spatial density through statistical photometric parallax

Separate stellar populations by turnoff color, metallicity

Discover tidal streams from globular clusters and dwarf galaxies in the Galactic halo

Contribute to proper motion/parallax measurements

Find variable objects

Spectroscopy can be used to

Find radial velocities (need < 2 Angstrom resolution to get interesting error bars)

Determine individual stellar properties[SDSS spectroscopy produces radial velocities to ±15 km/sec

(g~20), Temperatures: ± 200 K, and surface gravities to ± 0.4 dex, and [Fe/H] within 0.3 dex]

Stellar populations maintain kinematic coherence long after density coherence is lost, so finer and older structures can be identified this way.

Clearly, we are not yet using all of the information in the data

We have found everything so far by looking by eye at two-dimensional parameter plots (color-magnitude, magnitude vs. angle) of sub-selected stellar catalogs (A stars, F stars).

Clearly, we want to build up a global model of the Galaxy that fits all of the stars, using colors, magnitude, velocities if available.

We want to identify components from the kinematics, age, metallicity, and spatial distribution.

I currently have a graduate student working on this problem, and we have a prototype algorithm that has successfully rediscovered the Sagittarius dwarf tidal stream (Purnell et al., in preparation).

Gerson Goldhaber, Professor of Physics at UC Berkeley

1991 winner of the Panofsky prize of the American Physical Society, in recognition of his discovery of charmed mesons

Leonard Searle and Bob Zinn (1978)

Eggen, Lynden-Bell, and Sandage (1962)

The galaxy was created in a monolithic gravitational collapse

The galaxy was created by hierarchical merging

Eggen’sSpaghettisky

Rigatoni's ridges and holes are perfect with any sauce, from cream or cheese to the chunkiest meat sauces. National Pasta Association.

SDSS Contributions to Galactic Structure

(1) Measurement of the scale height of the thick disk(2) Discovery of the Sagittarius tidal stream in A-type stars(3) Discovery of additional tidal debris in the Galactic halo, including a

stream of debris in the Galactic plane (Monoceros, GASS, Canis Major)

(4) Discovery and analysis of the tidal tails of Pal 5(5) Discovery that the Sagittarius tidal stream extends to a distance of

90 kpc from the Galactic center(6) Found globular cluster in Sagittarius tidal stream(7) Draco dwarf galaxy has no tidal tails(8) Tidal tail discovered in Andromeda(9) Tracing the Sagittarius tidal stream in RR Lyrae stars

From a survey that was designed to avoid as many Galactic stars as possible

SEGUE

Sloan Extension for Galactic Underpinnings and Evolution (SEGUE) 

Segue (v.) – to proceed to what follows without pauseHeidi Newberg1, Kurt Anderson2,3, Timothy Beers4, Jon Brinkmann3, Bing Chen5, Eva Grebel6, Jim Gunn7, Hugh Harris8, Greg Hennessy9, Zeljko Ivezic7, Jill Knapp7, Alexei Kniazev6, Steve Levine8, Robert Lupton7, David Martinez-Delgado6, Peregrine McGehee2,10, Dave Monet8, Jeff Munn8, Michael Odenkirchen6, Jeff Pier8, Connie Rockosi11, Regina Schulte-Ladbeck12, J. Allyn Smith10, Paula Szokody11, Alan Uomoto13, Rosie Wyse13, Brian Yanny14

1 Rensselaer Polytechnic Inst.2 New Mexico State University3 Apache Point Observatory4 Michigan State University5 ESA/Vilspa, Madrid, Spain6 Max-Planck Heidelberg7 Princeton University

8 US Naval Observatory, Flagstaff9 US Naval Observatory, DC10 Los Alamos National Laboratory11 University of Washington12 University of Pittsburgh13 The Johns Hopkins University14 Fermi National Accelerator Laboratory

•Legacy – complete spectroscopy in the contiguous area of the N. Galactic Cap•SEGUE – new survey (4000 square degrees of low latitude imaging + 250,000 stellar spectra) for Galactic structure •Supernovae – light curves for ~200 Type Ia supernovae (on the south celestial equator, two or three three three-month photometry campaigns)

Elements of the SDSS extension

Three years, $15 million dollars

Legacy fills in spectroscopy in this region

Designed to sample the galaxy every 10-20 degrees, with ~10 distance bins per blue dot

l

b

SDSS + SEGUE Sky Coverage

Test Stripeat l=110 deg:

Turnoffs and Giant Branches visible, even at low latitudes

b =

E(B-V)=

Spectroscopic Samples

(1) 20,000 stars within 2 kpc of the Sun. This and the next category will be valuable to normalize Galactic components at the solar position.

(2) 40,000 stars within 4 kpc of the Sun. (3) 20,000 BHB stars, from 6 kpc to 70 kpc from the Sun (A nearer

sample of BS stars will also be obtained.) (4) 15,000 K giant stars, to distances of 80 kpc from the Sun (5) 4800 local white dwarf stars(6) 50,000 G dwarfs from 3 to 12 kpc from the Sun, which will sample

birth rate of stars, in each component, since these stars are selected to be redder than the turnoff of all Galactic components.

(7) 55,000 stars which sample all areas of color space, primarily low metallicity, in search of unusual things we did not expect - in search of those rare low metallicity stars that can tell us about the heavy element production in the very first generation of stars.

Current Status of SEGUE

Sloan Foundation has promised $5.2 million

Negotiations for institutional support underway

Proposal for ~$5 million NSF funding will be submitted in June

A few hundred square degrees already obtained

M(V) D(20) D(27) D(30)O5V -5.7 1.4 Mpc 35 Mpc 140 MpcB5V -1.2 170 kpc 4 Mpc 17 MpcA5V 1.95 250 kpc 1.0 Mpc 4.1 MpcF5V 3.5 20 kpc 500 kpc 2.0 MpcG5V 5.1 10 kpc 240 kpc 950 kpcK5V 7.35 3.4 kpc 85 kpc 340 kpcM5V 12.3 0.35 kpc 8.7 kpc 35 kpcG5III 0.9 66 kpc 1.7 Mpc 6.6 MpcK5III -0.2 110 kpc 2.8 Mpc 11 MpcM5III -0.3 115 kpc 2.9 Mpc 11 Mpc

Distance at which we can see individual stars

Radius of Galactic disk: 15 kpc Distance to Virgo galaxy cluster: 19 MpcKnown Extent of stellar halo: 100 kpc The Great Wall: 100 MpcDist. to Andromeda 700 kpc Distance to edge of visible Universe: 4000 Mpc

Galactic structure projects

• RAVE – eventually (starting 2006 if funded) 5 x 107 stellar spectra to V=16 and R~ 10,000. Currently running pilot to get 105 stars to V=12 with R~4000.

• RVS on GAIA (Launch 2010??) – All sky. Imaging to V=20, distance and space motion to V~18, spectra of everything to V~16 with R=11,500.

• Pan-STARRS – deeper repeated photometry of northern sky for variability and better astrometry

• UKIDSS/VISTA – deeper 2MASS to K~18.5

Galactic Structure and SN Spectroscopy

The spectra are too low resolution to get interesting radial velocities.

They are probably too low resolution to learn anything interesting about stellar properties, beyond what we would get from photometry.

The DESTINY survey would allow us to use synthetic magnitudes in passbands that we understand, by convolving the spectra, but only the long wavelength bands that are less important for discriminating stellar properties.

Parallax

0.1” pixels

If you centroid to 1/10th of a pixel, then we have 10 mas astrometry (parallaxes to 100 pc).

Parallaxes for very nearby brown dwarfs.

Proper motions for intrinsically faint, solar neighborhood stars (depends on time between repeat images).

Directions that could be explored

(1) Looking through many magnitudes of extinction in the Galactic plane.

(2) Looking at stellar populations in external galaxies.

(3) Mapping main sequence stars in our galaxy, understanding the IMF.

(4) Study extremely faint red “stars” in the solar neighborhood.