Canis Major overdensity and Monoceros Ring explained in terms of pure Milky Way structure Martín López-Corredoira Instituto de Astrofísica de Canarias (Tenerife, Spain) Leiden; July 14th, 2009 Colaborators: P. L. Hammersley, Y. Momany, S. Zaggia, A. Cabrera-Lavers
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Canis Major overdensity andMonoceros Ring explained in
terms of pure Milky Waystructure
Martín López-CorredoiraInstituto de Astrofísica de Canarias (Tenerife, Spain)
Leiden; July 14th, 2009
Colaborators: P. L. Hammersley, Y. Momany, S. Zaggia, A. Cabrera-Lavers
Canis Major and its stream?
Martin & Ibata (2003)
Monoceros Stream
A new streamdiscovered:
(e.g., Newberg et al. 2002, Rocha-Pinto et al. 2003,
Conn et al. 2005)
Canis Major: a galaxy covering
~50º of sky and at only ~8 kpc from
the Sun, fromwhich we had notrealized before?
A new galaxy discovered! (e.g., Martin et al. 2004, Martínez-Delgado et al. 2005,
Bellazzini et al. 2006)
Or just the warpof our Galaxy?
Stellar warp(+flare) in the Milky Way
López-Corredoira et al. (2002):zw=Cw Rεw sin (Φ-Φw) – (15 pc)
(valid for R<13 kpc)zw: height of disc over plane b=0
Cw: amplitude of the warp=1.2e-3 pcεw: exponent of the warp=5.25±0.50Φw: galactocentric azimuth of warp
Model from López-Corredoira et al. (2002)Φw=-5o lmax=270o
Φw=+5o lmax=248o
lmax=244oBellazzini et al. (2006): 5o<|b|<15o
López-Corredoira (2006)
Canis Major: Blue plume
Carraro et al. (2005): open squares open clustersfilled triangles BP population
Moitinho et al. (2006): open cluster NGC 2362, l=238.2o, b=-5.5o
Blue Plume: B5-A5 stars with <100 Myr in the Norma-Cygnus or Perseus spiral arms
Canis Major: Metallicity
-0.4<[Fe/H]<-0.7 (Bellazzini et al. 2006)
(as expected in the outer disc, at R=13 kpc)
Canis Major: velocity of stars- Radial velocities: distribution explained in terms
of the Galactic rotation (Momany et al. 2006).- Tangential velocities perpendicular to the disc:
* The selected stars might be contamination notassociated with CMa (Momany et al. 2006) [blue plume].* The expected warp signature is compatible with the negative vertical velocity (Momany et al. 2006) [7σ came from errors in distancedetermination of Hipparcos OB stars in Drimmel et al. (2000)].
- Unknown warp velocities:* They depend on the model of warp formation.* Asymmetries in the warp. Dinescu et al. (2005)
Canis Major (2nd. round): discovering flaws in 2007 papers withinterpretations different to a Galactic warp
López-Corredoira et al. (2007)
Butler et al. (B07), Conn et al. (C07), de Jong et al. 2007 (d07) claims against the warp:
1) The warp should give similar star counts and CM diagrams at l=2400, b=-30±Δb and we seedifferences (B07,C07)
2) The warp produces a maximum of the counts at(m-M)~10.5 [d=1.3 kpc] instead of the obs. 7 kpcand it is a wider structure than Canis Major (B07)
3) There is a blue plume population in our CM diagrams typical of dwarf galaxies…Young pop. in spiral arm in our Galaxy? hummm, maybe…! But this would be a conspiracy of two effects (warps and spiral arms) (B07, d07)
4) [Fe/H] down to -1.0±0.2 at some fields (d07)
Reply:
1) The mid plane of the warp at b=-30
is not a plane of symmetry
2) B07 calculations are wrong.Possible source of error: maybe theyhave used dN/dm α ρr instead of
dN/dm α ρr3
3) There is no conspiracy, but just oneeffect: the warp which includes the oldand the young populations of the disc(spiral arms)
4) The thick disc has metallicity in theouter Galaxy around -1.0 while thethin disc around -0.5 with r.m.s. of0.4-0.5 mag. NO PROBLEM!!
1) Warp: no vertical symmetry
López-Corredoira et al. (2007)
2) Differential star counts:For a given direction (l,b), without extinction a population
with absolute magnitude M gives total star counts up to m:
N(<m)=ω∫0r(m) dx x2 ρ(x)
r(m)=10(m-M+5)/5
Thus, the differential star counts (counts per unit magnitude)
A(m) ≡ dN(m)/dm = ω ρ[r(m)] r(m)3 ln(10)/5
3) No conspiracy of two effects
Carraro et al. (2005)
Simply: Spiral armsdo also warp
4) No population older than MW thick disc/halo
- No stars with lowermetallicity than expected(López-Corredoira et al.
2007)- No excess of RR Lyrae
(Mateu et al. 2009) - No excess of Open
Clusters (Piatti & Clariá2008)
MONOCEROS RING
Conn et al. (2008)
Monoceros ring
SDSS data
Galactic models with cut-off at R=14 kpc (Besançon) or without flaredo not reproduce the CM diagram