Issue |
A&A
Volume 509, January 2010
|
|
---|---|---|
Article Number | L1 | |
Number of page(s) | 4 | |
Section | Letters | |
DOI | https://doi.org/10.1051/0004-6361/200912715 | |
Published online | 12 January 2010 |
LETTER TO THE EDITOR
Probing the origin of the microwave anomalous foreground
N. Ysard - M. A. Miville-Deschênes - L. Verstraete
Institut d'Astrophysique Spatiale, UMR8617, Université Paris-Sud, 91405 Orsay, France
Received 18 June 2009 / Accepted 28 July 2009
Abstract
Context. The Galactic anomalous microwave emission detected
between 10 and 90 GHz is a major foreground to CMB fluctuations.
Well correlated with dust emission at 100 m,
the anomalous foreground is interstellar but its origin is still
debated. Possible carriers for this emission are spinning, small dust
grains carrying a permanent electric dipole.
Aims. To probe the origin of the anomalous foreground, we compare microwave data to dust IR emission on an angular scale of 1,
and search for specific signatures predicted by models of spinning dust.
Methods. For the anomalous foreground, we use the 23 GHz
all-sky map deduced from WMAP data by Miville-Deschênes and
collaborators. The IR dust emission is traced by IRAS data. Models show
that spinning dust emission is little sensitive to the intensity of the
radiation field (G0) for
GHz, while the mid-IR emission produced by the same small dust grains is proportional to G0. To test this behaviour in our comparison, we derive G0 from the dust temperature maps of Schlegel and collaborators.
Results. From all-sky maps, we show that the anomalous
foreground is more strongly correlated with the emission of small
grains (at 12 m) than with that of large grains (at 100
m). In addition, we show that the former correlation is significantly improved when the 12
m flux is divided by G0, as predicted by current models of spinning dust. The results apply to angular scales greater than 1
.
Finally, from a model fit of the anomalous foreground, we deduce
physical properties for Polycyclic Aromatic Hydrocarbons that are in
good agreement with those deduced from mid-IR spectroscopy.
Key words: dust, extinction - ISM: general
1 Introduction
As part of an effort towards accurate measurements of CMB fluctuations, experiments have motivated a detailed study of the Galactic foregrounds in the GHz range. Kogut et al. (1996), Leitch et al. (1997), and de Oliveira-Costa et al. (1997) found an unexpected emission excess between 10 and 90 GHz, which is correlated with dust far-IR but not with synchrotron emission. To avoid an inaccurate interpretation, this excess has been referred to as an anomalous foreground. Low frequency observations have shown that it has a rising spectrum for





The paper is organized as follows. Section 2 describes the observational predictions of spinning dust models and how they can be used to probe the origin of the anomalous foreground. Section 3 presents the data sets used to reach this goal. Section 4 shows how the anomalous foreground correlates with dust emission. Section 5 lists some remarkable fields and presents the type of information that we expect to derive from the anomalous foreground study. Finally, Sect. 6 presents our conclusions.
![]() |
Figure 1:
Results obtained with the model of YV09, for a MRN size
distribution (
|
Open with DEXTER |
2 Behaviour of the spinning dust emission
Nanometric-sized grains or Polycyclic Aromatic Hydrocarbons (PAHs) emit mostly in the mid-IR, whereas large grains dominate the FIR emission. The PAH emission is known to scale with the intensity of the radiation field G0![[*]](/icons/foot_motif.png)



![]() |
Figure 2:
All-sky correlations. Left panels show the anomalous
foreground in the 23 GHz WMAP band versus the dust IR emission in
the 12 and 100 |
Open with DEXTER |
3 Data sets
To carry out these correlations, we need maps of the anomalous and
dust emission as well as for the G0-values. IRAS is a natural
data set to study dust IR emission. Our IR template is the new
generation of IRAS images, called IRIS (Miville-Deschênes & Lagache 2005), in the 12 and
100 m bands, which is corrected for most of the remaining
instrumental problems of the IRAS/ISSA data set. Point sources
were removed in the IRIS plates (at 5 arcmin resolution) using the
method described in Miville-Deschênes & Lagache (2005). The plates were then projected
onto the Healpix grid, where an ecliptic-oriented filtering was
applied to remove residual zodiacal light emission
(Miville-Deschênes et al. in preparation). Finally, the IRIS
all-sky maps were convolved with a 1 degree FWHM Gaussian,
smoothing out any imperfections related to the point source
subtraction.
Miville-Deschênes et al. (2008) performed a separation of components in the WMAP bands, using a physical approach to describe the Galactic foregrounds. We use this anomalous template at 23 GHz, inferred from their ``Model 4''. The main assumption made to obtain this map is that polarized emission at 23 GHz is dominated by synchrotron (no assumption about any correlation with dust).
Finally, the G0-map is deduced from the BG temperature map of
Schlegel et al. (1998) that is inferred from the 140/240 DIRBE
ratio. We assume that the interstellar radiation field has the
same spectral distribution as the standard field of Mathis et al. (1983),
everywhere in the Galaxy, and that the BG spectral index is
(Draine & Lee 1984). The energy balance of a single grain of
size 0.1
m then yields
.
All of these maps have been smoothed to the same
angular resolution of 1
.
4 Correlations
Figure 2 shows the all-sky correlation of the
23 GHz anomalous flux with the dust IR emission. The anomalous
foreground clearly correlates more strongly with the 12 m
band than the 100
m (the Pearson correlation factor P is
0.90 and 0.82, respectively). A similar result was obtained by
Casassus et al. (2006) towards the LDN 1622 cloud, but here it is
the first time that it has been shown to also be true for the
entire sky, following the removal of zodiacal light residuals at
12
m. The correlation is also improved significantly when the
dust IR emission is divided by G0 (P= 0.90-0.95, in the case
of the 12
m band
). This improvement concerns
60% of the
sky at 12
m. These regions are 1.4-1.6 times brighter at
23 GHz than the regions for which the division does not improve
the correlation. However, in most of the regions where the
division by G0 does not improve the correlation, it also does
not make it poorer. It does only for 5% of the sky, which could
be explained by the uncertainties in the G0-values. These
correlations show the independence of the anomalous foreground of
G0 at 23 GHz and its link with the smallest grains. However,
since the all-sky correlation is almost as strong with BG emission
as with small grains, we are unable to draw firm conclusions at
this stage. Across the entire sky, the emissions of PAHs and BGs
are known to be correlated well. This is no longer true for
particular fields, as we now discuss.
5 Selected fields
![]() |
Figure 3:
Field centred on
|
Open with DEXTER |
To test the spinning dust hypothesis further, we searched for
fields of a few squared degrees according to the following
criteria: location outside of the Galactic plane, and bright at
both 23 GHz and 12 m with G0 variations as large as
possible. Searching the sky maps by areas of 5
squared, we
identified 27 such fields. Figure 3 is an
example of one of them. The anomalous and dust brightness maps
correlate far more tightly when the latter is divided by
G0
.
The correlation plots indeed clearly illustrate two cases
(Fig. 4a) corresponding to different values of
G0. The difference disappears when the 12
m brightness is
divided by G0 (Fig. 4b), as expected if the
anomalous foreground is produced by the emission of spinning PAHs.
![]() |
Figure 4:
Field centred on
|
Open with DEXTER |
![]() |
Figure 5:
A
representative anomalous spectrum of our selected fields (taken
from Model 4 of MD08). The solid line is the fit with our model
(YV09) comprising contributions from the CNM (10%, dashed line)
and from the WNM (90%, dotted line). PAH parameters (see text)
are m=0.3 D, and
|
Open with DEXTER |
For 5 of the 27 selected fields, we observe significant spatial
variations between the 12 and 100 m brightness maps (as shown
in Fig. 3). In these fields, we note that the
correlation between the anomalous foreground and the BG
100
m/G0 is worse
(Pearson's correlation
factor P=0.7 for the field in Fig. 3) than
with the smaller grains 12
m/G0 (P=0.86). This shows
that the anomalous foreground is correlated well with BG emission,
only if BG emission is well correlated with IR emission
characteristics of smaller grains. These results are consistent
with spinning dust emission.
We further test the spinning hypothesis and attempt to constrain
the electric dipole moment of PAHs in the selected fields. As
discussed by YV09, the brightness of spinning PAHs at 23 GHz,
S23, is given by
,
where
is the proton column density,
is
the abundance of PAHs, m is a scaling factor inferring the
electric dipole moment of PAHs,
(where N is the number of atoms in the PAH), and
is the rotational luminosity per solid angle and per PAH
molecule. The PAH IR brightness in the 12
m band, I12,
is proportional to
,
where
is the IR luminosity
per solid angle and per PAH. The correlation coefficient between
anomalous and IR brightness divided by G0 is then
,
where
depends
on the number of carbon atoms in the smallest PAH molecules
(
)
and the fractions of neutral cold (CNM) and warm
(WNM) diffuse gas. In Fig. 5, we show a representative
fit to the observed anomalous foreground with our model. From the
27 selected fields, we find a mean ratio
with a standard deviation
of
.
Our model fits yield m = 0.3-0.4 D,
20-60 and about 10% of CNM to account for both
the 23 GHz and 12
m emission. These sizes are currently
invoked to explain the 3.3
m profile in interstellar clouds
(Pech et al. 2002; Verstraete et al. 2001) and the m-value is in good
agreement with laboratory measurements for organic molecules
(DL98). Thus, the rotational and vibrational emission of PAHs, as
in current models, can consistently explain the anomalous and
12
m emission for plausible properties of PAHs.
6 Conclusions
From an all-sky, degree-scale comparison of the 23 GHz anomalous
map with dust IR emission, we have found that the anomalous
foreground is well correlated with the 100 m IRAS band. Using
an enhanced set of IRAS maps, we have shown for the first time
that the anomalous foreground is correlated with the 12
m
band across the entire sky and that the correlation is tighter
than with the 100
m flux. This correlation becomes even
tighter when the 12
m flux is corrected for the intensity of
the radiation field G0, indicating that the anomalous emission
is independent of G0 at 23 GHz on a 1 degree scale. These
findings strongly argue in favour of a spinning dust origin to the
anomalous foreground. Current models predict that the spinning
dust emission is dominated by the smallest dust grains (PAHs)
carrying the 12
m flux and that the corresponding 23 GHz
emission is almost independent of G0. From a model fit of both
microwave and IR data in selected fields with strong G0contrast, we deduce the physical properties of PAHs (sizes,
electric dipole moment) that are in good agreement with results
obtained from mid-IR spectroscopy.
We thank our referee, Simon Casassus, for his insightful comments that helped in improving the content of this letter. Some of the results in this paper have been derived using the HEALPix package (Górski et al. 2005). This paper used the photoionization code CLOUDY (Ferland et al. 1998).
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Footnotes
- ...G
- Scaling factor for the
radiation field integrated between 6 and 13.6 eV. The standard
radiation field corresponds to G0=1
and to an intensity of
erg s-1cm-2 (Parravano et al. 2003).
- ... band
- Using a Monte Carlo method to
simulate thermal noise in the 12
m map, we find that the Pearson coefficent 0.95 differs significantly from 0.90 with a confidence level greater than 99.9% (using a map containing 786 432 pixels).
- ...G
- The improvement in the correlations is significant to a confidence level greater than 99.7% for the 27 regions.
- ... worse
- The correlation is also poor
with the 60
m/G0 and for 3 of them with 25
m/G0(P = 0.18 and P = 0.7 for the field in Fig. 3, respectively).
All Figures
![]() |
Figure 1:
Results obtained with the model of YV09, for a MRN size
distribution (
|
Open with DEXTER | |
In the text |
![]() |
Figure 2:
All-sky correlations. Left panels show the anomalous
foreground in the 23 GHz WMAP band versus the dust IR emission in
the 12 and 100 |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Field centred on
|
Open with DEXTER | |
In the text |
![]() |
Figure 4:
Field centred on
|
Open with DEXTER | |
In the text |
![]() |
Figure 5:
A
representative anomalous spectrum of our selected fields (taken
from Model 4 of MD08). The solid line is the fit with our model
(YV09) comprising contributions from the CNM (10%, dashed line)
and from the WNM (90%, dotted line). PAH parameters (see text)
are m=0.3 D, and
|
Open with DEXTER | |
In the text |
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