The Milky-Way spiral arm tracers to the actual pattern D. Russeil LAM, Marseille, France

What are the best spiral arm tracers? Examples: M31, M83, M51

Visible stars UV Young stars (OB) Near-IR old stars FIR dust COHI M31

M51 2MASS J,H,K VisibleUV (GALEX) NIR spitzer (four band composite) FIR Herschel (70,100,160 m m) HaHa

H  – R image UV image (GALEX)‏M83 Spiral arms = overdensity of gas (ionised + molecular), stars (especially OB stars) and dust

We use the same tracers as external galaxies The spiral structure: traced by the most massive clouds and brightest HII regions e.g. Baade (1963), Rumstey and Kaufman (1983), Boulanger et al. (1981) Structure: distorted, spurs, bridges, arm splitting... e.g. Kennicutt (1981) Problem Problem: we are inside the Galaxy !!!! - Information along the line of sight is superimposed - Distance ambiguity problem (inner Galaxy)‏ The structure of our Galaxy

AAO/UKST H  Survey l = 290° ; b = 0° Georgelin et al., 2000 Marseille H  Survey (Scanning PF interferometer) Vlsr=20 kms -1 Vlsr= -25 kms -1 d ~ 2.7 kpc d ~ 8 kpc Parker et al., 1999 Kinematic information

Distance Determination Stellar distance – Distance uncertainties on individual star:~25% (e.g. Humphreys, 1976; Conti and Vacca, 1980; Brand and Wouterloot, 1988; Reed, 1996) ● Spectral type determination ● Mv - Spectral type relationship ● HII regions: identification of exciting stars – Interstellar Extinction => Local structure (< 6 kpc ) Identification of the exciting star(s): UBV photometry + spectro. Kinematic distance Systemic velocity + choice of a rotation curve Circular Velocity departures Near/far ambiguity Brand & Blitz (1993) Rotation curve Multiwavelenght information: H , radio recomb., Molecular, HI, Absorbtion lines Solar parameters: R 0, q 0

Kinematic information: velocity departures Spiral arm density wave streaming motion (e.g. Burton, 1973) Typical values: 10 – 40 km/s Brand & Blitz 1993 Russeil et al => velocity departure corrected if stellar distance known !! McClure-Griffiths et al HI Terminal velocity

I: Distribution of stellar tracers OB stars, young clusters, HII regions HII regions (Crampton & Georgelin 1975) OB associations (Mel’nik & Efremov 1995)

Dias & Lépine, 2005 Open Clusters in the Solar neighborhood Importance of the cluster age: only the youngest are still located in the arms Perseus arm Sagittarius arm Local (Orion) arm

II: Adding velocity information (e.g. Mezger 1970, Downes 1980) Radio lines => probing (almost) the whole galactic plane Recomb. lines (H109 , H110  ) Abs. lines ( H 2 CO, OH …) Molecular lines (CO) Caswell & Haynes 1987 HII regions (radio data) Mol. clouds Cohen et al. 1985

Solomon & Rivolo, < M < Msun Molecular clouds

III: Star-forming complexes distribution Method/strategy (Bok, 1971; Georgelin and Georgelin, 1976) Goal: reduce spatial and kinematic spread of objects belonging to the same complex. Kinematic + Multiwavelength information: Grouping young objects into complexes Determination of the systemic velocity (taking into account internal motions …) Determination of the kinematic distance solving the distance ambiguity problem Determination of the stellar distance

The spiral structure 1': Norma-Cygnus arm (or external arm) 2': Norma-Perseus arm 1: Sagittarius-Carina arm 2: Scutum- crux arm Symbole size -> excitation parameter U (Russeil 2007) U > 70 pc/cm 2 Fit weighting: excitation parameter

Distance Determination « New methods » Maser parallaxeStellar distance IR-NIR Stellar parallaxe ?? e.g. Hanson et al. 1996, 2005, Figer et al Spectral classification ~2.12 m m, ~8000 A Classification not easy e.g. Xu et al. 2008, Zhang et al. 2008, 2009 VERA project (e.g. Honna et al. 2005, 2007) Very good distance accuracy Only few (very close) OB stars observed with Hypparcos GAIA: capability to observe/classify OB stars ??

Hou et al Updated view of the spiral arms (including the most recent data): - Molecular clouds + HII regions - dkin: ambiguity distance solving (e.g. Kolpack et al. 2003, Watson et al. 2003, Sewilo et al. 2004, Busfield et al. 2006, Anderson & Bania 2009) - distance: stellar distance + maser parallaxe (e.g. Xu et al. 2008, Zhang et al. 2008) - Fit weighting: Exc. param. and Mol. Mass The spiral structure

FIR emission (Herschel- HiGal) a good ‏spiral arm tracer! Herschel Hi-GAL Survey: P.I.: S. Molinari (INAF, Roma, Italy) Survey of the Galactic plane: -70°< l<70° ; -1°<b<+1° Wavelenghts: 70, 100, 250, 350 and 500 m m

The filamentary structure of the cold ISM First results from Herschel-Hi-GAL survey (Molinari et al. 2010)

The filamentary structure of the cold ISM First results from Herschel-Hi-GAL survey GRS 13CO isocontours (Jackson et al. 2006)

The filamentary structure of the cold ISM First results from Herschel-Hi-GAL survey Molinari et al. 2010

Herschel Hi-GAL Survey: FIR as the spiral arm fitting weight Star formation rate through the Galaxy Emission profile/sources count plots to find arm's tangent direction...

The spiral arms from star counts and Emission profiles Arms tangent directions Old star population and dust Benjamin 2008 and Drimmel 2000 => 2 major spiral arms !! Scutum-Crux and Perseus Churchwell et al., 2009 (JHK 2Mass, 4.5 m m Spitzer)

Scutum Crux (south) Scutum Crux (North) Norma (North) Norma (South) (JHK 2Mass, 4.5 m m Spitzer) Sagittarius Carina (North) Russeil 2003

Bronfman 1992, CO Hou et al. 2009, 4 Poly. arms. Hou et al. 2009, 4 log. arms. Sagittarius Carina Sagittarius Carina Drimmel, 2000 (COBE/DIRBE data) Scutum Crux Scutum Crux Norma Orion spur

Rygl et al Spitzer IRAC m m Mean color excess Norma Scutum Crux Norma Sagittarius Carina Scutum Crux

The HI to trace our Galaxy's outer structure HI data - Poor contrast: arm to inter-arm density ratio ~3 - High spatial filling factor - Largely dominant in the outer Galaxy Molecular data - High contrast: e.g. arm to inter-arm density ratio ~28 in 12 CO for Perseus arm (Brunt, 2003) - But fast decline with Galactocentric radius - Few far outer molecular clouds (R G 13.5 – 17.5 kpc) with embedded stellar cluster (e.g. Yun et al. 2009, 2007, Snell et al. 2002, Santos et al. 2000)

McClure-Griffiths et al (SGPS HI data) Vazquez et al (photometric distance of clusters + CO mol. clouds) Russeil et al HI data

Nakanishi & Sofue (2003) - Outer: kinematic distance - Inner: kinematic distance + vertical distribution model ajustement HI map

OuterPerseus Sag. Carina Nakanishi & Sofue (2003) - Logaritmic HI arms - Not detected in HI: Scutum-Crux and Norma arms

Levine et al., 2006 Connection Outer- inner structures (Hou et al. 2009) HI map - Map of perturbation HI surface density - « Unsharp masking » technique - HI thickness smaller in the amrs

Distance determination : GAIA, maser Parallaxes, Infrared spectroscopy/photometry New/recent and incoming Large surveys of the galactic plane : Molecular: GRS, IGPS HI:all plane surveys Ionised gas: radio (MAGPIS, CORNISH), Halpha (IPHAS, WHAM)... Dust/PAH: MSX, Spitzer-GLIMPSE, Spitzer-MIPSGAL Cold dust: ATLASGAL, SCUBA2 (sub-mm continuum), Herschel-Hi-GAL (FIR)‏ Perspectives To resolve distance ambiguity SF Complexes delimitation Optical counterpart, HII regions morphology/velocity/excitation parameter New tracer!? FIR longitude profiles Direct distance; precise the rotation curve; velocity departures

Molecular data Nakanishi & Sofue (2006) from 12 CO(1-0) data - Outer: kinematic distance - Inner: kinematic distance + vertical distribution model

Interstellar medium global view in our Galaxy Ionised gas: HII regions + WIM HII regions: around OB stars, cm -3, < 30 pc WIM: widespread (h ~ 1 kpc), 20% of the disk volume, 0.08 cm -3 Molecular gas: GMC, clumps, cores... Contrast arm-interarm ~ 28:1 GMC: M sun, pc, cm -3 Clumps and cores: <1 pc, cm -3 Atomic gas: Contrast arm-interarm ~ 3:1 Widespread (h ~ pc), Warp, 1 – 500 cm -3

Galaxy's outer structure: the warp Observed througth all tracers: - Young stars (e.g. Smart et al. 1997, Reed 1996 Vazquez et al.2008) - HII regions, Ionised gas (e.g. Guibert et al. 1978, Fich & Blitz, 1981, Paladini et al. 2004, Cersosimo, 2009) - HI & CO (e.g. Burton 1988, Nakanishi & Sofue 2003, Levine et al. 2006) Nakanishi & Sofue (2003)

Russeil 2003 Paladini et al HII regions - Starts ~ 13 kpc - Northern warp more pronounced

The filamentary structure of the cold ISM First results from Herschel-Hi-GAL survey IPHAS

SH2-86 Halpha image Isocontours= 13CO

H  – R image UV image (GALEX)‏M83 NIR image 4.5  m NIR image 8  m