The near-IR window to study mysteries of the Milky Way disk

Noriyuki Matsunaga(The University of Tokyo, Japan)

Acknowledgement

• To J. Baba (TiTEC) and M. Fujii (NAOJ)– General discussions on the Galactic evolution

• To N. Gouda (NAOJ)– Showing his talk slides in advance

• Section 2.11 in the last Japanese WISH book, prepared by J. Baba et al., is dedicated to a proposal of the Galactic disk survey (for spiral-arm dynamics and stellar cluster evolution).

Contents• Why we want WISH to observe the Galaxy.

• Useful tracers and possible projects with WISH(1) Pulsating stars (Cepheids, Miras, RR Lyrs, …)(2) Stellar clusters

• What should be kept in mind when we use WISH for studying the Galactic objects.

The Galaxy: the nearest spiral• Detailed information within reach.– A lot of the information needs to be collected yet.

• Details are necessary for the comprehensive understanding of the galactic evolution.– Bridging individual stars, clouds, and the whole galaxy

Dame et al. (2001)

Motivations• What does the Galaxy look like? How did it evolve?• Almost all kinds of objects gather in the bulge/disk.– But, these are difficult regions to observe in the optical.

• An example of big questions to study– Are spiral arms predicted in density-wave theory or

transient as suggested in recent simulations?

Baba et al. (2009)suggested that spiral arms are transient in a short time scale of 100 Myr based on their cutting-edge N-body/SPH simulations.

Gaia to reveal the Galaxy• To be launched by ESA on 19th Dec 2013• The final catalogue with astrometric (parallaxes &

proper motions) as well as photometric and velocity data will be released in 2021.

• G-band: 0.33-1.0 micron, 6 mag < G < 20 mag– Parallax accuracy: 7μas (G<12) down to 100μas (G~18)

• 109 stars at almost the whole Galaxy to be observed.

The Galactic disk behind the veil• The disk, the main body of the Galaxy, is

obscured by the interstellar extinction.• Infrared (astrometric) observations are

necessary.

Stellar density of the bulgeseen down to G=20

Absolute magnitude MV

just reached at G=20

Reyle et al. (2008)

Tracers of the Galactic disk/bulge• Normal stars– Statistically large samples; but errors tend to be large.

• Red clump giants– Useful, but necessary to be treated statistically

• Pulsating variable stars– Distances and ages can be easily/accurately estimated.

• Stellar clusters– Distances and ages can be easily/accurately estimated.

• Star forming regions– Good tracers (especially with VLBI data).– Complementary to our optical/near-IR tracers, but tracing

only star-forming sites.

Two kinds of possible projects• Survey of pulsating stars and their proper

motions– Some dense and obscured regions remain to be

discovered.• Deep photometry of stellar clusters and their

proper motions– Many interesting targets will be available until the

WISH days.• Both projects would provide us with valuable

information on individual objects with distances and 3D velocities as well as ages. (x, v, t).

Why WISH is good for our purpose?

• High spatial resolution– Diff. Limits = 0.3arcsec, 0.4arcsec in filters 2 and 3.– Significantly better than 0.8arcsec in the VVV survey.– In many cases, the crowding effect limits our surveys.

• Efficient survey ability– Wide-field (0.23 deg2). Deep with a short exposure.

• Multi-color photometry– Need to make the CMD and correct for reddenings.

• Astrometry– Proper motions (even without parallaxes) would be

very useful to constrain the kinematic model.

Remarks on our requirements• High signal-to-noise ratio needed– Almost nothing can do with 3-σ detection!– Photometric errors on individual distance tracers

should be less than 0.1 mag.– To estimate distances, we need S/N=30 at least.

The CMD must go down well below the feature we try to determine.

(1) PULSATING STARSUsefulness and possible WISH projects

Pulsating stars as tracers• Distance indicators• Population indicators• Kinematic tracers• Chemical tracers

Mira

Classical Cepheid

Type II Cepheid

Gautchy & Saio (1995)

RR Lyr

Variable stars as distance indicators

log(Period/day)

Ks [m

ag]

ClassicalCepheid

Type IICepheid

Mira

Period-Luminosity relations (from Matsunaga et al. 2009, 2011)

For Miras, P-L relations are narrow only in the IR (not in the optical)

Variable stars as population indicatorsType Initial Mass AgeClassical Cepheids 4—10 Msun 20 –300 MyrMiras 1—6 Msun 100 Myr –10 GyrType II Cepheids ~1 Msun ~10 GyrRR Lyrs ~1 Msun ~10 Gyr

Period-Age relation of classical Cepheids

(Bono et al. 2005)

log (Age/yr)

log (Period/day)1 20

8.5

7.5

7.0

8.0

300Myr

100Myr

10Myr

30Myr

Sun

GC

The distribution of ~500 Cepheids from DDO database:http://www.astro.utronto.ca/DDO/research/cepheids/overlaid on the illustration by GLIMPSE project (2008)

The earlier status of Cepheid surveys• Cepheids distant in the Galactic disk are obscured.• Many remain to be discovered (by infrared surveys).

Recent and the expected progress

• Near-infrared surveys in progress– VISTA/VVV (K-band): Bulge 300 deg2, Disk 220 deg2

– OGLE (I-band), Pan-STARRS (optical)– and LSST in the future

• Our works– IRSF/SIRIUS (South Africa), Galactic center, etc.• the first near-IR (JHKs) survey of pulsating stars which

successfully discovered Cepheids

– KWFC (Kiso), northern Galactic disk

IRSF variable survey of the Galactic Center• With 8 year survey using the IRSF/SIRIUS in South Africa,

we detected 3 classical Cepheids, 16 type II Cepheids, and >500 Miras towards 30min ×20min around the GC (Matsunaga et al. 2009, 2011, 2013).

• We discussed the GC distance and star formation there.>500 Miras

3 ClassicalCepheids

16 Type IICepheids

WISH and pulsating stars• Fainter pulsating stars in the obscured and dense

regions probably remain elusive for a decade.• The bulge survey presented by Gouda-san would also

provide us with a good dataset.• Some additional fields towards the disk (eg. towards

spiral arms)

A plan of the bulge survey from Gouda-san’s slides.

RR Lyrs in the Galactic Center

• K=17～ 18mag including the foreground extinction.

• The best measurement of the GC distance based on a photometric distance indicator.

• Considering their confusion limit, VISTA is not expected to be good enough to detect them.

> 500 Miras

3 ClassicalCepheids

16 Type IICepheids

RR Lyr

How far can we reach?• RR Lyr stars (MK=-1mag) across the disk are

within the reach.• Limig mag. 20 Vega mag (@S/N=30; with 300sec)– VVV’s limit: Ks=15.5 mag(@S/N=30)– K(AB)=K(Vega)+1.9

WISH (20 mag)

VISTA(17 mag)

Curves show the limits to detect RR Lyrs (MK=-1mag) on the (AK, D) diagram.

RR Lyrs inthe far-side disk

GC

Story about IR surveys of pulsating stars

• Our surveys have been demonstrating that infrared surveys are important to study pulsating stars across the Galaxy.

• Large-scale surveys like VISTA/VVV will find a lot of (bright) pulsating stars in 5 years.

• WISH will be able to– Add proper motions which are crucial to study

the kinematic model– Expand our census to fainter (and more

numerous) pulsating stars like RR Lyrs.

(2) STELLAR CLUSTERSUsefulness and possible WISH projects

Stellar clusters• See the section by Baba-san in the WISH book• Distances and ages can be estimated based on

the CMD. Certainly useful tracers of the disk.– New deep photometric data will give us a chance to

use a new distance indicator, a faint kink of the MS.

The faint kink of the MS appears in the infrared CMD and is useful to determine distances and reddenings accurately.

New clusters being found by IR surveys• Massive and young (and obscured) stellar clusters

have been discovered with various IR data (2MASS, Spitzer-GLIMPSE, VISTA-VVV, etc).

Davies et al. (2012)Added a new massive (≥104 Msun) young (a few Myr) clusters to others discovered in recent infrared surveys.

GC

Sun

WISH and stellar clusters• Many interesting targets will be available.• Measuring distances (mainly based on the CMD)

and proper motions.• Accurate measurements of proper motions may

be useful to find runaway stars in young clusters (Fujii-san’s works; eg Fujii & Portegies Zwart, 2011, Science, 334, 1380).

Fujii+11(Illustration by 4D2U)

How far can we reach?• The faint MS kinks (MJ=+6 mag) of many clusters can be

detected for the first time.– Still challenging, but a great jump with WISH.– Also useful for more distance clusters with bright stars.

• Limig mag. 20 Vega mag (@S/N=100, with 900 sec)23 Vega mag (@S/N=50, with 3.3 hr)– VVV’s limit: Ks=15.5 mag(@S/N=30)– J(AB)=J(Vega)+0.9

WISH (20mag)

VISTA (17 mag)

Curves show the limits to study the MS kink (MJ=+6mag) on the (AK, D) diagram.

GC

WISH (23mag)

On the system and what we need• Filters and the photometric system– Self photometric calibrations needed.– Otherwise, systematic errors can be sticky.

• Efficiency and overhead– Observing objects in the Galaxy tend to require

relatively short exposure times.• Visibility: The Galactic Center is near the Ecliptic

equator, but 20 day monitoring would good enough.

Summary• WISH surveys of the Galaxy will be important:– Infrared—Severe interstellar extinction– Wide FOV—Need for a wide coverage of sky– High resolution—Crowding towards the Milky Way– Depth—More interests in fainter features

• Useful tracers of the Galactic disk:– Pulsating stars: RR Lyrs will be within reach.– Stellar clusters: More distant and obscured clusters.

• Time-series observations required:– Each integration time is relatively short.

End

KISOGP survey of the northern disk• Using Kiso Schmidt telescope (1.05m) and KWFC

camera (4 deg2 FOV) to survey variable stars.• We are around the middle of 3 year survey plan and

have found hundreds of new variables.

10kpc

4kpc

α~21.2h

δ~+24d

eg

α~23.9h

δ~+48deg

α~4.1h

δ~+52d

eg

α~5.8h

δ~+29d

eg

α~0.4h

δ~+63d

eg α~6.8h

δ~+03d

eg

WFIRST-2.4 Science Cases seemingly under consideration

• from A collection of one page science programs (Version 1.0; Mar 2013)– Proper motions and parallaxes of disk and bulge

stars (by S. Gaudi, OSU)– The most distant star- forming regions in the Milky

Way (by J. Stauffer, IPAC)– The infrared color-magnitude relation (by J. Kalirai,

STScI)

The Galaxy: some interesting features• Bulge: boxy and barred structure, X shape, …• Disk: warp, flare, large peculiar motions of SFRs, …• Halo: streams, ultra-faint dwarfs, …

Reid et al. (2009)Finding large motions of SFRs based on VLBI data

Levine et al. (2006)investigated the warp of the disk with revised datasets

変光星研究：可視から赤外へ赤外線の周期光度関係可視光の周期光度関係

赤色変光星

セファイド

赤色変光星

セファイド

RRライリ

横軸・周期、縦軸・明るさ

Ita et al. (2004)

Soszynski et al. (2011)

Period-luminosity relations of RR Lyrs• 可視と赤外のそれぞれの指標

σ＝ ±0.03 mag

K band

WVI=I-(V-I)

V band

σ＝ ±0.3 mag

σ＝ ±0.2 mag

古典的セファイドは ±0.1 mag

Cluster “Reticulum”In the LMC(Dall’Ora et al. 2004)

24,000 RR Lyrs in the LMC(Soszynski et al. 2004)

Classical Cepheids• 3 classical Cepheids discovered near the GC.• The first discovery in this important region.

(Matsunaga et al., 2013, Nature, 477, 188)