Galactic Warp explains the overdensity of the
Canis Major Region
M. López-Corredoira1
Instituto de Astrofı́sica de Canarias. C/.Vı́a Láctea, s/n, E-38200 La Laguna (S/C
de Tenerife), Spain. martinlc@iac.es
Summary. In this paper, I present the arguments, which were explained in detail
in López-Corredoira (2006) and López-Corredoira et al. (2006), to think that the
Canis Major overdensity is just a part of the Galactic warp+flare, instead of a dwarf
galaxy or a new substructure in the Galaxy.
1 Introduction
Some authors (e.g. Martin et al. 2004, Martı́nez-Delgado et al. 2005, Bellazzini
et al. 2006b) have suggested the existence of a nearby (at distance of only 7-8
kpc) dwarf galaxy associated to the Canis Major (CMa) overdensity. Momany
et al. (2004, 2006[hereafter M06]) and López-Corredoira (2006, hereafter L06)
showed that these effects can also be reproduced by the Galactic warp+flare.
Butler et al. (2006) insisted that a smooth Galactic warp cannot explain the
CMa overdensity and at least some substructure within the Galaxy should
be necessary rather than a smooth warp, but this is again counted (LópezCorredoira et al. 2006).
2 Galactic warp vs. dwarf galaxy hypothesis
2.1 Canis Major overdensity
The height of the López-Corredoira et al. (2002, hereafter L02) warp over the
plane defined by the central disc as a function of the galactocentric distance
and azimuth is
zw (R, φ)[pc] = CW R(kpc)²W sin[φ − φw ],
◦
(1)
−3
with ²W = 5.25 ± 0.5, φW = −5 ± 5 and CW = 1.2 × 10 (for the given
values of ²W = 5.25 and φW = −5◦ ; the normalization will change if ²W
2
M. López-Corredoira
and/or φw vary). Bellazzini et al. (2006b)1 calculate in their fig. 6(down) the
south/north overdensity of the red clump stars due to this L02 warp and the
flared disc parameters given in L02. The maximum of the overdensity is at
l ≈ 270◦ , which is not in agreement with the observed value of l ≈ 240 − 244◦ .
L02 have given an approximate model of the “average” warp for the whole
sky assuming north–south symmetry and a power law for the amplitude of
this warp. This assumption is just a first-order approximation since, as is well
known, the Galactic warp is not symmetric (Burton 1988; Voskes & Burton
2006, Levine et al. 2006); our warp is somewhere between an L-warp and a
S-warp rather than being a pure S-warp. Moreover, the position of the Sun
also affects our perspective of the northern and southern warp. There are
some parameters of the disc (galactocentric distance of the Sun, height of the
Sun above the plane, flare parameters, scale length, scale height at the Sun
galactocentric distance, etc.) whose variations affect the result. Figure 1d of
L06 shows the density maps for angles φw = +5◦ instead of the original value
φw = −5 ± 5◦ given by L02: the angle of the maximum is 248◦ , not very far
from the observed value of 240-244◦ by Bellazzini et al. (2006b).
We must also consider that the error of 5 degrees for φW in the analysis of
L02 is merely statistical, but there might be further systematic errors due, for
instance, to errors in the assumed luminosity function of stars, or deviations
from equation (1). As M06 say, some uncertainties of the L02 model could
also come from gaps in the region around l = 270◦ in the data used for the
fit. Given the irregularities of the southern warp, it is quite possible that an
extra shift in the azimuth in the southern with respect the average L02-warp
might occur. Indeed, the analysis by Voskes & Burton (2006) gives precisely
the best fit for φw = +5◦ (warp maximum at φ = 95◦ ) for their gas warp. The
asymmetries in the gas warp (similar to the stellar warp according to L02)
are shown by Voskes & Burton (2006) or Levine et al. (2006), which should
serve to indicate that there is no unique large-scale symmetric warp model of
the type eq. (1); therefore, we cannot extrapolate the exact result at around
l ≈ 240◦ to the whole Galaxy. Figure 11 of Levine et al. (2006) shows how the
line of nodes defined by φw varies depending on the galactocentric distance
and from north to south. Indeed, Levine et al. (2006) explain the asymmetries
with a warp dependence
zw (R, φ) = W0 (R) + W1 (R) sin[φ − φ1 (R)] + W2 (R) sin[2φ − φ2 (R)].
(2)
The fact that φ1 6= φ2 [the difference is up to 12 degrees according to Levine
et al. (2006)] and depending on R, causes the equivalent φw in expression
(1) not to be constant. The first term, the mode of m = 1, is dominant for
1
I was the referee of this paper for MNRAS and accepted it for publication after
a pair of revisions because I considered that the authors had done an interesting
analysis which should be known, even though I did not agree with some points.
I had recommended the authors to analyze some weak points but they did not
follow my advice, so I performed in L06 the analysis of these weak points.
Canis Major warp
3
R < 13 kpc, so eq. (1) can be a relatively good approximation, but if we aim
to provide an accurate explanation for all the deviations from it, perhaps eq.
(1) is insufficient.
2.2 Bump
Another comment concerning Bellazzini et al. (2006b) paper is their claim
that the bump in their fig. 1, in the region 238◦ < l < 244◦ , −11◦ < b < −6◦ ,
is due to the dwarf galaxy. In this case, they compared it with the predictions
of another warp + flare model (Robin et al. 2003) and failed to reproduce
it. However, the bump could be explainable in terms of the warp with an
appropriate extrapolation for values of R > 13 kpc. Taking the southern warp
equal to the northern warp is inappropriate because there is an abundance
of data for the gas emission of the Galaxy that show the asymmetry. At
R > 13 kpc, the m = 2 mode becomes important and the extrapolation of
the southern and northern warp are not equal. Burton (1988) shows that the
southern warp is approximately of constant height between 1.6R¯ and 2R¯
(beyond 2R¯ is unimportant for our analysis) instead of the monotonically
increasing northern warp. Something similar is observed in Voskes & Burton
(2006, fig. 16), and we know that the gas warp and stellar warp are similar
(L02, M06), so the adopted approximate extrapolation for stars is justified.
Compare Fig. 1 with fig. 1 of Bellazzini et al. (2006b). In Fig. 1, we see
a bump with a maximum peak at mK = 13.4 (equivalent to R = 15.4 kpc),
close to the peak at mK = 13–13.5 obtained in Bellazzini et al. (2006b). Note,
however, that only the red clump giants are plotted in Fig. 1, while fig. 1
(right panel) of Bellazzini et al. (2006b) includes all contaminants, especially
dwarfs over mK = 13.5.
Further, the calculations for Fig. 14 of Butler et al. (2006) are wrong
(López-Corredoira et al. 2006). According to L02 or L06 model, the peak of
the read clump stars is around mK = 13.4 (see Fig. 1), and taking into account
that MK,red clump ≈ −1.65 we get a distance modulus of (m−M ) ≈ 15 instead
of (m − M ) ≈ 10.5 obtained by Butler et al. (2006).
2.3 Colour–magnitude diagram and Canis Major depth
Martı́nez-Delgado et al. (2005) claim the line-of-sight depth of Canis Major
to have a r.m.s. of 0.8±0.3 kpc, a result difficult to reconcile with the warp
hypothesis. However, Martı́nez-Delgado et al. (2005) paper contains errors: i)
the Gaussian fit of their fig. 3 gives σM S,total = 0.57 mag; once the contribution
of intrinsic broadening (0.19 mag) and latitude dispersion (0.29 mag) are
subtracted, it would give σM S,CM a = 0.45 mag (equivalent to 1.6 kpc at
d = 7.9 kpc instead of 0.8 kpc as they calculate); ii) a Gaussian distribution of
bins of constant magnitude does not give a Gaussian distribution in the density
distribution along the line of sight (the second distribution is proportional to
the first multiplied by a factor 1/d3 ), so we cannot translate σ in magnitudes
4
M. López-Corredoira
1200
φw=-5
1000
o
o
φw=+5
φw=+14
800
o
600
400
200
0
10
10,5
11
11,5
12
mK
12,5
13
13,5
14
Fig. 1. Prediction of the warp model using L02 parameters for the counts of the
Fig. 1 in the paper by Bellazzini et al. (2006b) (238◦ ≤ l ≤ 244◦ , −11◦ ≤ b ≤ −6◦ )
with extrapolation of constant height in 13 < R < 16 kpc and null beyond.
into σ in distances, that is, σ = 1.6 kpc would be the r.m.s. of the function
r3 ρ(r), not the r.m.s. of ρ(r); iii) the assumed constant distribution for the
underlying Milky Way stars (dotted line in fig. 3 of Martı́nez-Delgado et al.
2005) is not even a good first order approximation.
In any case, if we forget the analysis by Martı́nez-Delgado et al. (2005)
but take as correct their data in their fig. 3 (a FWHM≈ 1.6 magnitudes in
the distribution in bins of constant magnitude), we can compare them with
our predictions of the warp and see that it is not so different with respect to
the predictions of the warp (see Fig. 3 of L06, or also Fig. 1 in this paper):
2.2 mag (2.4 mag if we took into account the intrinsic broadening and the
latitude dispersion) instead of 1.6 mag. The calculations on it in Fig. 14 of
Butler et al. (2006) are wrong (López-Corredoira et al. 2006). The prediction
of the L02 warp gives a somewhat broader distribution, possibly because the
radial dependence is not very accurate. The extinction would also make the
width of the peak narrower, specially if we put an important fraction of the
total extinction along the line of sight associated to the distance of CMa.
The population attributed by Martı́nez-Delgado et al. (2005) to be a 1–
2 Gyr old population in the blue plume of intermediate-age belonging to
the CMa dwarf galaxy is indeed a young stellar population (≤ 100 Myr)
of the Galactic spiral arms, not placed in the putative CMa galaxy (Carraro
et al. 2005). Butler et al. (2006) think that this hypothesis is a different to
the warp+flare explanation, but it is not. Both the old population and the
young population are warped so this produces that the spiral arms are shifted
towards the south in the external disc, and consequently we see along the line
Canis Major warp
5
of sight in the southern Galactic latitudes an excess of apparent blue plume
stars.
Another paper by Bellazzini et al. (2006a) claims that the metallicity of the
core of CMa is −0.4 ≤ [M/H] ≤ −0.7, a relatively old population; however,
this is also within the expectation for the outer disc; for an R = 13.1 kpc,
[F e/H] ≈ −0.57 is expected for the Galactic disc according to the metallicity
distribution by Cameron (1985).
Butler et al. (2006) suggest that the strong over-density towards the anticentre (l = 175.26◦ , b = −3.40◦ ) detected by Bragaglia et al. (2006) cannot
be due to the warp, and it is compatible with having intercepted a population
similar to CMa at a distance around 4.8 kpc from the Sun, tentatively interpreted as the orbit of the disrupting CMa dwarf galaxy. It is compatible but
there are other solutions in Galactic terms (López-Corredoira et al. 2006).
In conclusion, I do not see in the analysis of colour–magnitude diagrams
of the Canis Major region any conclusive proof that we are observing a population different from that of our own warped Galaxy.
2.4 Velocity of the CMa stars
The bimodal distribution in the radial velocity of M-stars (Martin et al. 2004),
presented as a proof that Canis Major is not the warp, reflects two kinds of origins for the sources: one was artificially produced by template issues resulting
from a fluctuating line spread function asymmetry during the different observing nights, as recognized by the authors in a later paper (Martin et al.
2005); and the other peak can be reproduced by the Galactic rotation (M06).
In any case, even if M06 were wrong, the kinematics of the warp is somewhat
complex and unknown, so we cannot discard it. Another recent claim of measured motion of CMa perpendicular to the disc should not be considered as
inconsistent with the expected motion of the warp because indeed we do not
know very much about how the warp was formed or its subsequent evolution,
whether it is steady or still oscillating with respect to the plane—there is
no unique scenario, and there are at least four possible hypotheses of warp
formation (Castro-Rodrı́guez et al. 2002, Sect. 1), each one offering different predictions on its motion—or whether the northern and southern parts
should have similar kinematics (given the north–south asymmetry, it is possible that we cannot compare them). Moreover, as M06 state, the stars selected
to measure the proper motions might be contamination not associated with
CMa, and “the expected warp signature can be and is compatible with negative
vertical velocity”.
3 Discussion
The question now arises as to whether it is absolutely necessary to invoke
the existence of a new dwarf galaxy or new substructure in our Galaxy to
6
M. López-Corredoira
explain the CMa overdensity. In principle, unless new convincing proofs are
presented, there are no reasons to think there is something else than a Galactic
warp+flare. Moreover, the effect of a dwarf galaxy or massive substructure
with extension around 10 kpc embedded in the Milky Way at only 13 kpc of
the Galactic center should present some distortion/asymmetry of the Galactic
disc and spiral arms which has not been observed. My guess (only a guess) is
that there is nothing extraordinary in Canis Major.
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