Table Of ContentProperties of the HII region populations of M51 and NGC 4449
from Hα images with ACS on HST.
1 Leonel Guti´errez1,2,4, John E. Beckman2,3,4, and Valeria Buenrostro2,4,5
1
[email protected], [email protected], [email protected]
0
2
n
a
J
ABSTRACT
3
1 We have used the images from the ACS on HST in Hα, and in the neighboring continuum,
to produce flux calibrated images of the large spiral galaxy M51, and the dwarf irregular NGC
] 4449. From these images we have derived the absolute luminosities in Hα, the areas, and the
O
positions with respect to the galactic centers as reference points, of over 2600 Hii regions in
C
M51 and over 270 Hii regions in NGC 4449. Using this database we have derived luminosity
.
h (L)–volume(V) relationsfor the regionsinthe twogalaxies,showingthatwithin the errorlimits
p these obey the equation L ∼ V2/3, which differs from the linear relation expected for regions of
-
constant uniform electron density. We discuss briefly possible models which would give rise to
o
r this behavior, notably models with strong density inhomogeneities within the regions. Plotting
t
s the luminosity functions for the two galaxies we find a break in the slope for M51 at log(L) =
a 38.5 dex (units in erg s−1) for M51 in good agreement with the previous ground-basedstudy by
[
Rand,andabovethisluminosityNGC 4449alsoshowsasharpdeclineinitsluminosityfunction,
1 althoughthe number of regionsis too small to plot the function wellat higher luminosities. The
v cumulative diameter distribution for the Hii regions of M51 shows dual behaviour, with a break
4
ata radius closeto 100pc, the radius of regionswith the break luminosity. Here too we indicate
1
the possible physical implications.
6
2
Subject headings: ISM: general — Hiiregions — galaxies: structure)
.
1
0 1. Introduction niques have improved and the precision of the
1
measurementshasincreasedthe scopeofthemea-
1 There is a well grounded tradition of study-
surements has been gradually extended. The
v: ing populations of Hii regions in spiral and ir- methods for discriminating an Hii region from
i regular galaxies, which goes back to the work
X its diffuse surrounding emission have been im-
of Hodge, Kennicutt, and their collaborators
proved, and the luminosity functions have been
r
a (Kennicutt & Hodge1980;Hodge, Lee, & Kennicutt extended to lower limiting values. Although the
1989; Kennicutt et al. 1989). It has become cus-
ultimate aim should be to improve our under-
tomarytomeasuretheirHαluminositiesandtheir
standing of the star formation process and the
radii, and prepare systematic catalogs. As tech-
interaction of, notably massive, stars with the
ISM, working on the improvement of the intrin-
1Universidad Nacional Aut´onoma de M´exico, Instituto sic data base is an objective intrinsically worth
deAstronom´ıa,Ensenada, B.C.M´exico
pursuing. Considerable worthwhile ground based
2Instituto de Astrof´ısica de Canarias, C/ Via L´actea
data have been reported (a representative but in
s/n,38200LaLaguna, Tenerife,Spain
3ConsejoSuperiordeInvestigaciones Cient´ıficas,Spain no way comprehensive list of references is: Rand
4Facultad de F´ısica, Universidad de La Laguna, Avda. (1992); Knapen et al. (1993); Rozas et al. (1996,
Astrof´ısicoFco. S´anchez s/n, 38200, LaLaguna, Tenerife, 1999, 2000); Gonz´alez-Delgado & P´erez (1997);
Spain Feinstein (1997); Youngblood & Hunter (1999);
5UniversidadNacionalAut´onomadeM´exico,Centrode MacKenty et al. (2000); Thilker et al. (2002);
Radioastronom´ıayAstrof´ısica,Morelia,M´exico
1
Buckalew & Kobulnicky (2006); Hakobyan et al. dra Aloisi). The exposures were taken at two
(2008)) and, in the case of M33, Cardwell et al. different pointings along the axis of the galaxy,
(2000) presented a list of over 9,000 detected re- with four exposures per filter at each pointing,
gionsinasinglegalaxy,butitisclearlyofinterest dithered to optimize the removal of cosmic rays
to analyze data of the quality and angular resolu- and bad pixels, and to allow an improved deter-
tion provided by HST for this purpose. However mination of the point spread function. The total,
relatively little work has been presented of this integrated exposure times for the three filters, af-
type, because the required narrow band filters at ter combining the images using MULTIDRIZZLE
arbitrary recession velocities are not included in (Koekemoer et al. 2002) were 2400s, 2000s, and
theopticsoftheHSTinstruments. Anotablepre- 360srespectively,inV, I,andHα. The totalarea
viousstudyofM51basedonWFPC2narrowband imagedonthegalaxyis340×200arcsec2,withpix-
imaging was presented by Scoville et al. (2001), els of 0.05 arcsec.
and a parallel study of the same galaxy based on Thearchivedatawereprocessedon-the-fly,cor-
NICMOS observations in Paα was published by rectingforbiasanddarkcurrent,flat-fielding,and
Alonso-Herrero & Knapen (2001). In this article correctingfornon-lineareffectsusingthestandard
we use ACS data for the first time to examine pipelinecalibrationforACS(CALACS,Hack et al
the statistical properties of Hii regions in whole (2000)). The images were also corrected for de-
galaxies, presenting results for M51, and also for tector defects using MULTIDRIZZLE, which also
the dwarf irregular NGC 4449. In section 2 we corrects each image for the distortion associated
discuss how the archival data were processed in with each filter, and can then be used to com-
order to prepare the Hα maps, and in section 3 bine images, and take out cosmic rays. For M51,
we describe the critically important processes of these processeswereperformedbyMutchler et al.
continuum and background subtraction, followed (2005), but for NGC 4449 they were carried out
by the derivation of the luminosities, areas, and by ourselves, with the help of IRAF, which was
positions of the regions. In section 4 we derive employed to combine all the images for a given
the luminosity-radius relations and the luminos- pointing and a given filter into single images be-
ity functions for both galaxies and the diameter fore joining them in a mosaicto take in the whole
distribution for M51, and in section 5 we offer a galaxy. This was done by making use of the over-
discussion and our conclusions. lap zone in the pointings (∼ 30 × 200 arcsec2).
The alignment errors were less than 0.3 pixels in
2. Observations, and basic data reduction any direction, which we derived from the differ-
ences in the centroids of the positions of the stars
2.1. Basic processing
used for positional calibration.
The images of M51 (NGC 5194) and its com- Images processed with MULTIDRIZZLE con-
panion NGC 5195 were observed with the wide tain the signal in units of electrons/sec, and from
field camera of the ACS on HST in January theimageheadersonecanfindthesensitivitycon-
2005 within the program 10452 (PI: Steve Beck- versionfactor, PHOTFLAM, defined as the mean
with, Hubble Heritage Team). The observa- flux F (units of erg cm−2 s−1 ˚A−1), which yield
tions consisted of 6 pointings through two broad 1 count per second in the observing mode used.
band filters: F555W (close to standard V), and Any gain differences between the ACS detectors
F814W (close to I), and a narrow band Hα filter, are alreadyaccounted for by the photometric cor-
F658N. The exposures per pointing were 340s, rection, so that to obtain a calibrated image in
340s, and 680s for the three filters, respectively. erg s−1 cm−2 ˚A−1 one simply multiplies by the
The observations used here, taken from the HST constantfactorPHOTFLAM.Toconvertthisflux
archive, were corrected, calibrated, and combined imageintoanHSTmagnitudeoneusestheexpres-
by Mutchler et al. (2005). sion
The images of NGC4449 were also taken from
theHSTarchive. TheyweretakenduringNovem-
ber2005throughthesamefiltersasthoseusedfor m=−2.5×log10(F)+PHOTZPT (1)
M51, within the programGO 10585(PI: Alessan-
wherethevalueofPHOTZPTisthezeropointon
2
theSTmagnitudescale,andisusuallylistedinthe fraction of 14%. On the other hand, the surface
image header, but can also be found on the ACS brightness in the F555W filter on the Hii regions
website (http://www.stsci.edu/hst/acs/analysis/ is, on average,∼3 times that in F658N, measured
zeropoints). We list in Table 1 the values of on our images. So, we can infer that the emission
PHOTFLAM and ZPT for the three filters used in [Oiii]λ5007 is on average 5% of the continuum
(Sirianni et al. 2005). (with a maximum fraction of 1/6th). Taking into
account that in the calibration process we found
3. The HII region Hα luminosities that the ratio of the off-band continuum to the
on-band line+continuum intensities is ∼0.06, we
3.1. Continuum and background subtrac-
considerthatweneednotbeworriedaboutsignif-
tions
icantover-subtractionofthecontinuumduetothe
[Oiii] emission, which would make a mean contri-
Once all the images had been aligned we could
bution of order 0.3% (maximum 1%) of the total
subtract the continuum contribution to the total
signal.
fluxes in the Hα images, to isolate the flux from
the emission line itself. Since we did not have Once the Hα flux map had been obtained, we
a narrow band continuum image close to Hα, we went on to determine the luminosities of the in-
used a weighted combination of the V and I im- dividual Hii regions. The difficulty here lies in
ages,derivingtheproportionsofeachbyminimiz- effecting a valid separation of the region from its
ing the residuals of the stellar images when the surrounding background, and above all from any
subtraction of the weighted continuum from the overlapping regions. To separate partially over-
Hαimagehadbeeneffected. Thecombinedimage lapping regions, we opted to count as two regions
chosen for the final subtraction comprised 15% of those whose brightness distribution between the
the I image and 85% of the V image. The factor two central maxima dipped below 2/3 of the cen-
by which this combined VI continuum image had tral brightness of the fainter of the two regions,
to be multiplied before the subtraction in order while those which did not satisfy this criterion
to produce an image in Hα flux alone was found, were treated as single regions. This process was
using a method developed by B¨oker et al. (1999) unambiguousformostcases. Forthe specialcases
and used by Knapen et al. (2004). This consists ofregionsoldenoughfortheircenterstohavebeen
in plotting the intensity in the Hα image, pixel blownoutwardstotheextentthattheirbrightness
by pixel, against the corresponding intensity in distributions are annular (i.e. we are looking at
the continuum image. An example of this pro- expanding shells), we considered such objects as
cess is shown in Fig. 1 where we used the origi- single regions.
nal Hα image and the weighted sum of the V and Asfarasthebackgroundisconcernedwehadto
I continuum images. As most of the pixels do carryoutthe followingprocess: define the bound-
not contain any signal due to Hα, the majority ary of each region, determine a value for the Hα
of points in the plot lie on a straight line, with backgroundinthesurroundingzone,andsubtract
lowscatter,whoseslopegivesthe constantofpro- thisvaluefromeachpixelofthedefinedHiiregion
portionality for the continuum subtraction. The before integrating the remaining flux over all the
continuum-subtractedHα image is thenproduced pixelstogivethetotalfluxfortheregion. TheHα
by subtracting the continuum image, multiplied background is due to the emission of the diffuse
by this constant, from the original Hα image. warmionizedgassurroundingtheregion,andthis
It is possible that the signal from the F555W procedure assumes that this pervades the disk, in
filter (V) includes some contamination from such a way that along the line of sight to an Hii
strong emission lines, of which [Oiii]λ5007 is regionweseethecombinedeffectofemissionfrom
the strongest, because the filter transmission for the regionandfrom the diffuse gas. This assump-
that redshifted line is near maximum. How- tion,andotherrivalassumptions,arediscussedin
ever, the spectroscopic measurements made by detail in Zurita et al. (2004), who reach the con-
Bresolin et al. (2004) on 10 Hii regions in M51 clusion that it is difficult to do better than this,
show that the [Oiii] line emission ranges from even though the assumption may not give a per-
1.6% to 56% of the Hα emission with an average fect description of the local morphology.
3
λ ∆λ PHOTFLAM
Filter (˚Ac) (˚A) erg·cm−2·s−1·˚A−1 ZPTSTMAG
F555W 5359.547 847.78 1.955929E-19 25.672
F658N 6584.015 87.48 1.999181E-18 23.148
F814W 8059.761 1541.56 7.072360E-20 26.776
Table1:ConversionfactorsforsensitivityandzeropointsforACS/WFC.λ isthecentral(pivot)wavelength
c
of the filter and ∆λ is the full width at half maximum (FWHM) of the filter. ZPT is the zero point
STMAG
in the magnitude scale ST.
Fig. 1.—PlotofthepixelintensityintheHαimageofthegalaxyNGC4449against
the intensity of the same pixel in the continuum image. The two images were first
registered in position to a fraction of a pixel (some 0.3 pix) taking bright stars for
refereence. In order not to overcrowdthe plot we use a 1000 x 400 pixel portion of
the galaxy to derive it.
4
ForM51wedeterminedthebackgroundinrings pixels making a continuous object, whose inten-
around the outside of the diaphragm used to de- sity is greater than three times the rms scatter of
termine the Hii region flux. After trials, in which the local background. From this integrated value
the ring width was varied between 2 and 5 pixels we then subtract off the product of the number
we found that the difference in estimated back- of pixels so sampled by the flux value per pixel of
ground,definedasthe medianflux valueper pixel thelocalbackground. Thesefluxeswereconverted
over the ring, varied by no more than 12%, and from electrons s−1 to erg s−1 cm−2 using PHOT-
we confined our measurements to rings of width 2 FLAM as described above, and using the best
pixels, before subtracting this value from the pix- estimated distances to M51 or to NGC 4449 we
els of the region, and integrating to give the total could convert these values to absolute luminosity
flux overeachregion. ForNGC 4449the taskwas inergs−1 foragivenHiiregion. Afterconsidering
morecomplexduetothepresenceofstrongdiffuse and comparing the values of the distance to M51
emission, which had significant gradients, and we by a number of authors, we selected the distance
opted to make a map of the diffuse emission and to M51 given by Feldmeier et al. (1997) of 8.39
correct for it before subtracting the local back- Mpc,basedontheluminosityfunctionofits plan-
grounds. We designed a selective filter to do this, etarynebulae,asthemostreliable. Thismeasure-
sincewehadpreviouslydeterminedthatamedian ment,togetherwiththatofTonry et al.(2001)us-
filter tended to yield backgrounds which were too ing surface brightness fluctuations on NGC 5195,
strong, and whose subtraction led to underesti- with which it agrees within the limits of their er-
mates of the Hii region fluxes. We used boxes of rors, gives the lowest scatter and systematic er-
101 pixels on the side, and to estimate the back- rors of the techniques published to date for this
ground within a box we first calculated the me- observation. At this distance, the scale of the im-
dian value, then discardedthe pixels in the upper age is equivalent to 2.03 pc pixel−1. For NGC
and lower 5% flux ranges, and then tested to see 4449 we have used a distance of 3.82 Mpc from
whether the fluxes in all the pixels below the me- Annibali et al. (2008) who used the tip of the red
dianwerewithina3σ limit(wehadpreviouslyde- giant branch. This value is the most accurate one
terminedthestandarddeviationσinazonewhich publishedsofar,andisalsoinfairagreementwith
did not contain Hii regions). If this test was not theresultsofKarachentsev et al.(2003),whoused
satisfied, the program reduced the percentile pro- the sametechnique,butgavewidererrorbars. At
gressivelyuntil the pixel fluxes did satisfy the cri- this distance the image scale is 0.93 pc pixel−1.
terion, down to a limiting 10 percentile. Finally From the flux measurements we could also de-
theprogramcomputedanimagewhichgaveanex- riveanareaAforeachHiiregion,multiplyingthe
cellentfitto the diffuse background,andwecould area per pixel by the number of pixels counted as
make the appropriate subtraction systematically. part of the region as defined above, and we could
In Fig. 2 we show a scan across a typical zone of thenmeasureanequivalentradius,R ,definedby
eq
the galaxy with one major Hii region and several
R = (A/π). The position of each region was
eq
minor regions (see Fig. 3), to which we had ap-
determipnedbythecentroidoftheregionwhichwas
plied a median filter correction (dotted line), and
found in rectangular coordinate directions, mea-
a correction using the selective filter (solid line),
suredfromthegalacticnucleus,bytakingtheflux
which demonstrates the value of our method. So
weighted sum of the coordinate distances of the
our final procedure for this galaxy was to use our
pixels from the nucleus along each coordinate di-
selective filter to filter out the large scale features
rection divided by the integrated flux along that
correspondingtothediffusebackground,andthen
direction. For regions in the form of shells this
subtract off the local background flux from each
centroid did not, in general, lie within a luminous
region, as described briefly below.
part of the region, though for the majority of re-
gions it lay close to the visually inspected center.
3.2. The luminosities,areas, and positions
The luminosities in Hα for the Hii regions had
of the population of HII regions
to be corrected for the presence of the [Nii] dou-
The flux within an Hii region is derived by in- blet, λλ6548,6584 ˚A, as the narrow band filter
tegrating the intensity over all those contiguous F658N is wide enough to take in both of these
5
Fig. 2.—Intensityinelectronspersecond,atthe400pixellengthscanacrossNGC
4449 shown in Fig. 3. The intensity was measured in the median filtered image
(dotted line) and also in the image treated with the selective filter (solid line) as
described in the text. The scans were calculated by averaging over 10 adjacent
columns.
6
Fig. 3.— Ionized compled in the south-east of NGC 4449. A scan of length 400
pixels for whichwe calculatedthe intensities inFig. 2 is shown. The intensities are
the averagesof ten adjacent columns.
7
emission lines. This cannot be done region by 3a (on-line version of the Journal). In this log-
region, but using the detailed measurements for log plot we can see that there is a reasonablefirst
M51 by Bresolin et al. (2004) of the strengths of orderstraightlinefittothislargecollectionofdata
the two doublet lines in ten regions at distances points, which takes the form:
ranging from 0.19 to 1.04 of the isophotal radius,
we find that to use a constant factor, indepen- log(L )=1.92log(R )+34.55 (2)
Hα eq
dent of radius, of [Nii]λλ6548,6584 equal to 0.33
(±0.06) found as the mean value of Bresolin et where Req is the equivalent radius of the Hii re-
al.´s regions (in agreement with the value used gion defined above. A more accurate fit to the
by Hill et al. (1997)), gives an entirely satisfac- distribution is given by the quadratic expression
tory procedure for M51, given the very moder-
ate metallicity gradient found in the disk of that
galaxy. For the wavelengths of the two [Nii] dou- log(LHα)=35.35+0.78log(Req)+0.40(log(Req))2
blet lines, and of Hα, redshifted for M51´s re- (3)
cession velocity, the differences in transmission We know that if Hii regions were sphericaland
acrosstheF658Nfilterarelessthan1%,andcould with uniform density, the fit should be linear and
be neglected. So, the flux measured in F658N have slope 3, in accordance with “Case B” of
was multiplied by 0.67 to give the Hα flux. For Osterbrock (1989), (or indeed with the classical
the redshift of NGC 4449, however, the transmis- model of Stro¨mgren 1939), as given by the rela-
sions of the filter are 0.730, 0.923, and 0.929, at tion:
the wavelengths of the three lines, in ascending
wavelength order, respectively, and appropriate 1 4πN2α hc
L(Hα)= H B R3 (4)
corrections had to be applied. From Osterbrock 2.2(cid:26) 3 λ (cid:27)
Hα
(1989)weknowthat[Nii]λ6548/[Nii]λ6584=0.33
whereN isthedensityofthehydrogeninthere-
and for NGC 4449 we find from Kennicutt (1992) H
that [Nii]λ6584/Hα =0.12. Making the transmis- gioninatoms cm−3, whichwe assume to be equal
to the electron density n , α is the recombina-
sioncorrectionswe deduce thatthe flux measured e B
tion coefficient for all the excited levels of hydro-
in F658N should be multiplied by 0.862 to give
gen, and λ is 6563˚A.A partof the discrepancy
the Hα flux. This single factor is deemed ade- Hα
between prediction and observation may be due
quate given that the metallicity measured close
to the fact that a fractionof the luminosity of the
to the nucleus by B¨oker et al. (2001) is in agree-
smaller regions is buried in the noise, so that the
ment with the metallicity in the disk measured
slope of the relation should (and does) increase
by Lisenfeld & Ferrara(1998) within the limits of
somewhat with increasing luminosity. However
their mutual errors.
the basic difference cannot be explained in this
We have described in this section the deriva-
way, and we will return to this point in a later
tions of the Hα luminosities, equivalentradii, and
section of the paper. If we do take the expression
positions of the Hii regions measured in M51 and
in Eq. 4 as valid, we can calculate an equivalent
NGC 4449. These are listed in Tables 3a and 4a
meanelectrondensity,whichvariesbetween1and
inthe on-lineversionofthis articleinthe journal,
30 cm−3 with a mean value hn i close to 6 cm−3.
and the data for the 50 most luminous regions in e
In Fig. 5 we show the plot of equivalent radius
each galaxy is listed here in Tables 3 and 4
against Hα luminosity for the population of the
Hii regions of NGC 4449. The best linear fit to
4. Basicphysical propertiesofthe popula-
the points is given by:
tions of HII regions in the two galaxies,
as derived from their Hα emission
log(L )=2.09logR +34.99 (5)
Hα eq
4.1. The luminosity-volume relations
and a more accurate fit is given by the quadratic
In Fig. 4 we have plotted the distribution of
expression:
the radii of the Hii regions in M51 against their
Hαluminosities,usingthedatatabulatedinTable
8
Fig. 4.— Relation between the logarithmof the Hα luminosity in erg s−1 and R
eq
in parseconlogarithmic scales,for the measuredHii regionsofM51. The solidline
showsthebestlinearfittothedistributionlog(L )=1.924×log(R )+34.550and
Hα eq
thedottedlinetheevenbettersecondorderfitlog(L )=35.35+0.78×log(R )+
Hα eq
0.40×(log(R ))2. Error bars are plotted for a random selection of 150 of the Hii
eq
regions (including more error bars would lead to a diagram too confused for ready
interpretation). In generalthe uncertainties indetermining the areasofthe regions
are, as seen, greater than those found in the measurements of their luminosities,
because the major fraction of the luminosity is concentrated in the center of each
region.
9
galaxy above this discriminating exponential was
set to unity (the arms) and the rest of the galaxy
2
log(L(Hα))=35.41+1.26log(Req)+0.38(log(Req)) set to zero (the interarm zones). As the galaxy
(6) is deformed by its interaction with NGC 5195 we
These expressions are very similar to those for had to make manual corrections to the mask for
M51, and in particular for both galaxies the lu- those (relatively small) outer parts of the galaxy
minosities in Hα vary as the square of the radii where the single exponential fit was clearly inad-
of the regions (see Eqs. 2 and 5) rather than as equate. To illustrate our results we show, in Fig.
thecube,whichwouldbepredictedfromstandard 8, the distribution of the Hii regions in the two
models of Hii regions. spiral arms, plotted in polar coordinates on the
deprojected image of the disk. In Fig. 9 we show
4.2. The luminosity functions of the HII the luminosity functions of the arm Hii regions,
regions in both galaxies the interarm regions and, for comparison, of the
full population, which is the same as the plot in
In Fig. 6 we show the luminosity function of
Fig. 6. Intotalwemeasuredluminositiesfor1966
the Hii regions in M51, plotted as the log of the
regions in the arms, or within the crowded zone
numberofregionsinbinsofwidth0.2dexinlumi-
within 1kpc ofthe galaxycenter,and693regions
nosity against the log of the luminosity itself. We
in the interarm zones. The integrated luminosity
havefittedapowerlawfittothedecliningportion
of all the regions in the arms and in the central 1
of the function towards high luminosities, using
kpcis1.25×1041ergs−1(90%ofthetotal)andthe
the equation:
integrated luminosity of the interarm Hii regions
is 1.37×1040 erg s−1 (some 10% of the total). If
N(L)dL=ALαdL, (7)
we exclude the central 1 kpc the integrated lumi-
where N(L)dL is the number of Hii regions with nosity of the regions in the arms is 1.18×1041 erg
−1
s , some 85% of the total.
luminosities in the intervalbetweenL andL+dL
(see Kennicutt et al. 1989). The slope of this We note that the population of the interarm
straight line fit in the log-log diagram is -0.793, zones is lacking in Hii regions in the high lumi-
which implies that α = -1.793, because the lumi- nosity range. There are several possible expla-
nosity function is plotted with logarithmic bins, nations for this. One of them is that the mean
whereas the slope α is defined via the differential age of the regions in the interarm zones is greater
function in Eq. 7 with linear bins. The interval than the mean age of the regions in the arms,
overwhichwehavedeterminedαinFig. 6isfrom butthisscenarioappearsimprobable,becausethe
log(L) = 37.2, to log(L) = 39.2 , i.e. a hundred- only way we could envisage this is that the in-
fold interval in luminosity. Below the lower limit terarm Hii regions are the result of the migra-
the function is incomplete for S:N considerations, tion of regions formed within the arms, so that
and above the upper limit the function becomes the greater distances from the formation sites im-
uncertain due to the paucity of the statistics. ply greater ages. However the numbers do not
WenexteffectaseparationoftheHiiregionsin work out well here, since a characteristic lifetime
for Lyman continuum emission in regions in the
the arms from those in the interarm zones. To do
upper luminosity range is of order 3×106 years,
this we made a mask using the I image, and after
while the migration timescales implied are of or-
applyingamedianspatialfilter,usinga10×10arc-
der3×107years. Anotherpossibilityisthatinthe
sec box, to reduce local effects of dust structure,
arms we are seeing crowded regions so that the
we then binned into 10×10 pixel bins to reduce
larger regions represent the overlapping of more
the computational load, and finally, using a PA
◦ ◦ than one region, a phenomenon which is infre-
of 26 and an inclination of 42 (Bersier et al.
quentintheinterarmzones. Howevertheevidence
1994), we fitted an artificial exponential disk to
from Relan˜o et al. (2005), who obtained two di-
the galaxy in such a way that 50% of the light
mensionalvelocitymappingofcompleteHiiregion
fromthe galaxydisk lay abovethis disk, and 50%
populationsingalaxiesusingFabry-Perotinterfer-
below it. To assign regions to the arms or inter-
ometry,andwerethus able to separatethe contri-
armzones weseta masksuchthatthe partofthe
10
