局所銀河群で探る銀河進化：矮小不規則銀河NGC6822

国立天文台
小宮山 裕

第3回銀河進化研究会

2016/6/21

局所銀河群で探る銀河進化• 近傍銀河＝育ってしまった銀河• だがその進化の歴史を内包している• 近くにいるため、多波長に渡って情報が豊富– 星  Star Formation History, Chemical Evolution– ガス  Current Star Formation– 分光  Abundance, Dynamics• 矮小銀河＝大きな銀河の’building block’• 特に興味深い矮小不規則銀河– MW,M31等の大きな銀河と孤立した環境– 現在もガスを保持し、星生成を行っている

 銀河というシステムの進化を研究するうえで格好の天体• Hyper Suprime-Cam (HSC) 見かけの大きな近傍銀河研究には最適なカメラ
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Dwarf Irregular: NGC6822••••

Star-forming dwarf irregular galaxy at the distance of 500kpc.
Mv = -15.2mag, r_h=354pc (2’.65), M*=10^9Ms, M_HI=1.3x10^9Ms
Current SFR ~0.01-0.06Ms/yr
SFH studies show enhanced SFR at 100-200Myr ago (e.g., Gallart et al.
1996)• Observation by HSC
 deep broad-band (g, r, i) data, Halpha (NB0656) data are available.

3What is known for NGC6822• Old stars (>>1 Gyr, represented by RGB, RC, C-star) show round andsmooth distribution (e.g. Battinelli et al. 2006).
• Young stars (<1 Gyr, represented by MS O~A stars) show elongateddistribution (from SE to NW) beyond the main body of NGC6822. The
distribution traces the HI distribution very well (e.g., Komiyama et al. 2003).

HSC ImageYoung Stars (MS)Old Stars (RC)Area incomplete4

What is known for NGC6822• HI: elongated distribution, extended beyond R25 (de Blok & Walter 2000)– Complex structure with density peak at north-west (NW cloud), extended arm to southeast (SE arm), and giant hole next to the center (hole).

• UV: many sources centered on NGC6822 (Efremova et al. 2011)– Less luminous extended structure toward NW-SE direction traces HI and blue star
distributions.

HSC ImageHIGALEX FUV image

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Old Population(>>10Gyr)
Young Population(<1Gyr); HIUV sources

Main Body

g, r, Ha Composite Image
Integration Time: 1hr15min6

This Study• Shed light on current star-formation traced by Halpha (and UV)– Bright (in Halpha and UV) regions at center are investigated by Efremova et al. 2011.
– Outer regions and faint regions can be probed by HSC.
– How is the star forming activities at the outer regions?
– How significant are these regions in total star formation rate?

HSC Image

GALEX FUV image

Efremova et al. 2011

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NGC6822: Halpha Imageg, r, Ha Composite Image
Integration Time: 1hr15min
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NGC6822: Continuum Subtracted Halpha Image

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Outer Diffuse HII Regions• Diffuse HII regions are detected from Halpha-r image. 49 are detected andphotometric properties (Halpha flux, size) are measured.
NWSE

FUV, g, Ha Composite Image
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NGC6822: Continuum Subtracted Halpha Image

Overlaid: Efremova+ 2011
Blue: FUV selected SF region
11Red: Halpha bright region

NGC6822: Continuum Subtracted Halpha Image

Overlaid: Efremova+ 2011
Blue: FUV selected SF region
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Red: HalphaGALEX
brightFUVregion
image

HI map (deBlok & Walter 2000)

NGC6822: Continuum Subtracted Halpha Image

1. Halpha emission regions are found in the outskirts of NGC6822.
2. They are coincident with GALEX FUV sources and show good
Overlaid: Efremova+ 2011
correlation with HI.
Blue: FUV selected SF region

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Red: HalphaGALEX
brightFUV
regionimage

Blue Compact HII Regions• Compact HII regions are selected based on 2-color diagram. 1698 objects
are detected.

Ha-r (mag)g-r <0.3-1<Ha-r<0.07

g-r (mag)14

Spatial Distribution• Most compact HII regions are found around center, following thedistribution of UV sources. Diffuse HII regions are widely distributed.

All survey area

Zoom to center

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Size vs Halpha Flux
• Compact HII regions follow the size-flux relation of central HII regions.
Outer diffuse HII regions follow different relation; Halpha fluxes are x0.10.01 fainter or sizes are x10-100 larger.

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Coincidence with UV Sources• Many HII regions coincide with UV Sources.
• Compact: 263 (FUV+NUV), 354 (NUV) out of 1698 regions (except forcentral crowded regions)• Diffuse: 29 (FUV+NUV), 30 (NUV) out of 49 regions• Halpha-UV relation is same for both Compact and Diffuse HII regions.
FUVNUV17

Total Star Formation Rate
• Total star formation rate is calculated for each group.
• SFR(central) ~ 10xSFR(compact) ~ 100xSFR(diffuse)• Significant fraction of SF in NGC6882 occurs at a few large HII regions.

18SummaryLocal Group dwarf irregular NGC6822 is investigated using Hyper SuprimeCam (HSC) on Subaru Telescope. Halpha image is used to study current star
forming activities in particular at the outskirts of this galaxy.
• Deep Halpha image reveals 49 diffuse (faint and extended) Halphaemission regions in the outskirts of NGC6822.
• Most of these regions are coincident with GALEX FUV sources. They show
good correlation with HI.
• Their Halpha fluxes are x0.1-0.01 fainter (or sizes are x10-100 larger) than‘normal’ HII regions found at the center of NGC6822.
• Blue compact HII regions are located at the fainter extension of ‘normal’HII regions.
• Diffuse HII regions and blue compact HII regions follow same relation onf(Halpha)-f(UV) plane.
• Contribution to total SFR from diffuse and compact HII regions is not
significant: SFR(central) ~ 10xSFR(compact) ~ 100xSFR(diffuse)19