The Spectral Energy Distribution of Galaxies
Proceedings IAU Symposium No. 284, 2011
R.J. Tuffs & C.C.Popescu, eds.
c ? 2011 International Astronomical Union
Spectral Energy Distributions of a set of Hii
regions in M33 (HerM33es)
M. Rela˜ no1, S. Verley1, I. P´ erez1, C. Kramer2, E. M. Xilouris3, M.
Boquien4, J. Braine5, D. Calzetti6, C. Henkel7, and HerM33es Team
1Dept. de F´ ısica Te´ orica y del Cosmos, Universidad de Granada, Spain,2Instituto de
Radioastronom´ ıa Milim´ etrica, Granada, Spain,3Institute of Astronomy and Astrophysics,
NOA, Athens, Greece,4Laboratoire d’Astrophysique de Marseille, Marseille, France,
5Laboratoire d’Astrophysique de Bordeaux, France,6Deparment of Astronomy, University of
Massachusetts, USA,7MPI f¨ ur Radioastronomie, Bonn, Germany
Abstract. Within the framework of the HerM33es Key Project for Herschel and in combination
with multi-wavelength data, we study the Spectral Energy Distribution (SED) of a set of Hii
regions in the Local Group Galaxy M33. Using the Hα emission, we perform a classification of a
selected Hii region sample in terms of morphology, separating the objects in filled, mixed, shell
and clear shell objects. We obtain the SED for each Hii region as well as a representative SED
for each class of objects. We also study the emission distribution of each band within the regions.
We find different trends in the SEDs for each morphological type that are related to properties
of the dust and their associated stellar cluster. The emission distribution of each band within
the region is different for each morphological type of object.
Keywords. (ISM:) dust, ISM: evolution, (ISM:) Hii regions, galaxies: M33.
The study of the star formation rate (SFR) in galaxies of different types has been lately
improved due to the new available data. Remarkably, the focus has been turned to the
closest galaxies as well as to star-forming regions within our Galaxy. For these nearby
objects, the resolution of the data offers us an opportunity to test whether the proposed
SFR calibrators trace indeed the location of the stellar births (Churchwell et al. 2006;
Rela˜ no & Kennicutt 2009, among others). Recently, a new study on the star-forming
regions in the Magellanic Clouds has analysed the relation of the amount of flux at the
different wavelengths via the SEDs of these objects (Lawton et al. 2010).
Within the HerM33es Key Project (Kramer et al. 2010) we are obtaining maps of the
entire galaxy M33 at wavelengths between 100µm and 500µm using PACS and SPIRE
instruments on Herschel. Verley et al (2010) identified a set of Hii regions in the north
of M33 showing a shell-like morphology in these infrared bands, and also traced by the
Hα emission. In order to further study this phenomenon we have analysed the SEDs of
a set of Hii regions in M33 covering different morphologies. The resolution of our data
(from ∼ 2??to ∼ 20??) is good enough to perform such as study.
2. SED of Hii regions
We use an Hα image of M33 (Hoopes & Walterbos 2000) to identify a set of Hii
regions for which clear morphology can be recognised. A morphological classification was
obtained with the following criteria: filled regions are objects showing a compact knot,
mixed regions are those presenting several compact knots and filamentary structures,
and shells are regions showing arcs. We add another classification for the most spherical,
arXiv:1111.5427v1 [astro-ph.CO] 23 Nov 2011
2 M. Rela˜ no & the HerM33es Team
log(Flux) [mJy Hz]
IRAC 3.6IRAC 4.5 IRAC 5.8 IRAC 8.0
MIPS 24MIPS 70
PACS 100PACS 160
Figure 1. Left: Location of the Hii region sample on the continuum-subtracted Hα image
of M33 (Hoopes & Walterbos 2000). Circles correspond to the apertures used to perform the
photometry. Right: SED for our set of Hii regions. Typical errors for the fluxes are shown in
the lower left corner of the figure.
closed shells called clear shells. Out of the 120 selected Hii regions, 9 are filled, 47 mixed,
37 shell and 27 clear shells. Our sample is distributed over the whole disk of M33 (see
Fig. 1, left).
We use multi-wavelength data from FUV (GALEX) to 250µm (Herschel) smoothed to
a common 20??resolution and regridded to a 6??pixel size (corresponding to those of the
250µm Herschel image) to obtain the SED for each region. Photometry was performed
using individual apertures for each object and local background was subtracted to elimi-
nate the contribution of the diffuse medium to the Hii region fluxes. In Fig. 1 (right) we
show the SEDs for our objects together with a characteristic SED for each classification
obtained with the mean values in each band for all the Hii regions in the corresponding
classification. From Fig. 1 (right) we observe the following trends: (i) mixed regions are
more luminous in all bands as they normally have several knots of star formation, (ii)
the slope of the SED between the FUV-NUV wavelength range and Hα is steeper for
shells and clear shells than for filled and mixed, (iii) filled and mixed objects have more
24µm relative to 8µm than the shells and clear shells do.
3. Dust Temperature
The 100µm/70µm, 160µm/70µm or 160µm/100µm ratios normally trace the tem-
perature of the warm dust emitting from 24µm to 160µm. In Fig. 2 (left) we plot the
100µm/70µm ratio versus the Hα surface brightness for our sample. At high Hα surface
brightness the 100µm/70µm flux density ratio decreases showing that highly luminous
Hii regions tend to have warmer dust. This agrees with the correlation observed by Bo-
quien et al. (2010, 2011) in M33, and in a similar study by Bendo et al. (2011) who
observed M81, M83 and NGC 2403 with Herschel.
However, the filled regions seem to have a constant 100µm/70µm ratio, independent
of the Hα surface brightness. For shells and clear shells there is a dispersion in the
100µm/70µm ratio showing that these regions present a range of dust temperatures. For
the shells, the relative location between stars and dust can affect more the temperature
of the dust than the intensity of the stellar radiation field and therefore they tend to
present a wider dust temperature range.
SED for Hii region in M333
log[100 µm/70 µm]
400 300 2001000 100200 300400
Distance from centre [pc]
Source number 1
400 300200 1000100200300400
Distance from centre [pc]
Distance from centre [pc]
Figure 2. Left: 100µm/70µm ratio versus the Hα surface brightness for our sample. Colour
code is the same as in Fig.1. Right: Normalised emission line profiles for a shell region in the
horizontal direction. Profiles are separated in three panels (top: Hα, FUV, and NUV, middle:
3.6µm, 4.5µm, 5.8µm, and 8.0µm and bottom: 24µm, 70µm, 100µm, 160µm, and 250µm).
At top-left corner in each panel we show the location of the profile in the region in the Hα,
4.5µm, and 250µm images for the top, middle and bottom panels, respectively. The Hα profile
is depicted in grey in all the panels for reference.
4. Emission line profiles for clear shells
We have performed a multi-wavelength study of the emission distribution in the interior
of the clear shells. From FUV to 250µm we have obtained profiles in the horizontal (East-
West) direction crossing the centre of the Hii regions. Each profile corresponds to the
integration of a line of 4 pixels (∼ 24??) width perpendicular to the direction of the profile.
In Fig. 2 (right) we show the emission line profile for one of the clear shells of our
sample. The Hα profile shows the characteristic double peak of the shell emission, and
there is a displacement between the Hα and FUV/NUV emission, with the FUV/NUV
emission located in the inner part of the region. The ratio Hα/FUV is lower in the centre
than in the boundaries of the shell, which could be due to: (i) the existence of a young
stellar population within the shell, or (ii) the ionising photons from the central cluster
reaching the shell and ionising the gas within the rim. The emission at all IR bands
follows clearly the Hα shape of the shell: at 24µm and 250µm the emission decays in the
centre of the shell and they are enhanced at the boundaries. The same trend, though not
so clear, is seen at 70µm, 100µm, and 160µm. The emission of the Polycyclic Aromatic
Hydrocarbon molecules (PAH) at 8µm is marked by the location of the shell boundaries.
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