Embedded Clusters in Giant Extragalactic HII Regions III. Extinction and Star Formation

29 Pages·0.36 MB·

by  Y. D. Mayya

6
9
9
1
Table 1. Sample of Galaxies
n
a
J Galaxy deV. Type A (gal) Incl. VelocityDistance Reference N(Hii )
v
1 NGC (◦) km s−1 (Mpc) dist spect

1
1365 SB(s)b 0.000 56 1502 10.0 1 8,9 5
v
1 2366 SBm 0.136 62 252 3.6 2 10,11 1
0 2903 SAB(rs)bc 0.053 58 467 10.0 3 12 8
0 2997 S(s)c 0.409 37 805 10.0 4 13,14 49
1
3351 SB(r)b 0.038 46 673 8.5 5 12 4
0
4303 SAB(rs)bc 0.000 24 1483 19.0 6 15 10
6
9 5253 IrrIIp 0.152 64 209 2.0 7 16 1
/
h
p
- Quantitiesincolumns2,3,4and5arefromdeVaucouleursetal.(1991)(RC3)
o
r
References for Table 1.
t
s
a (1)Jones& Jones(1980);(2)Sandage& Tammann(1976);(3)Sandage& Tam-
: mann (1981); (4) Peterson (1978); (5) Bottinelli et al. (1984); (6) Kennicutt
v
i (1981); (7) de Vaucouleurs (1979); (8) Roy & Walsh (1988); (9) Alloin et al.
X
(1981); (10) Peimbert et al. (1986); (11) Kennicutt et al. (1980); (12) McCall el
r al. (1985); (13) Walsh & Roy (1989b); (14) Roy & Walsh (1987); (15) Shields et
a
al. (1991); (16) Walsh & Roy (1989a);
Table 2. Derived Properties of GEHRs
Ident 12+ T A V (B−V) (V−R) φ † A (B−V) V−R φ † N † M †
e v ∗ ∗ ∗(cid:16)LB(cid:17)∗ vc 3 3 (cid:16)LB(cid:17)3 L T
log(O) K phs−1 M
H ⊙
N1365-1 9.13 6000 0.91 16.79 0.17 −0.02 −1.24 0.45 0.02 −0.13 −1.48 51.15 5.25
N1365-2 9.20 5626 1.07 15.80 0.16 0.14 −1.15 0.53 −0.01 0.00 −1.43 51.63 5.73
N1365-3 9.11 6083 0.86 16.99 0.24 0.06 −1.18 0.43 0.10 −0.05 −1.40 51.10 5.20
N1365-4 9.20 5627 1.10 16.34 0.24 0.14 −1.35 0.55 0.06 −0.00 −1.64 51.19 5.29
N1365-5 9.13 6014 0.96 16.25 0.34 0.13 −1.70 0.48 0.19 0.01 −1.95 50.83 5.53
N2363 8.06 12180 0.45 14.74 −0.22 0.52 −0.68 0.29 −0.31 0.45 −0.83 51.96 5.56
N2903-1 9.29 5153 1.97 17.35 0.37 0.66 −0.40 1.01 0.05 0.40 −0.92 51.68 5.28
N2903-2 9.26 5317 0.29 16.77 0.01 0.34 −1.23 0.17 −0.04 0.30 −1.32 51.23 4.83
N2903-13 9.37 4671 2.39 16.75 0.15 0.54 −0.33 1.22 −0.24 0.23 −0.97 51.23 4.83
N2997NE-1 9.18 5726 2.13 17.55 0.25 0.60 −0.40 1.27 −0.15 0.28 −1.06 51.65 5.25
N2997NE-2 9.29 5129 1.58 18.10 0.23 0.18 −0.84 1.00 −0.09 −0.07 −1.35 51.00 4.60
N2997NE-7 9.25 5369 1.90 18.76 0.11 0.22 −0.96 1.16 −0.26 −0.07 −1.57 50.66 4.26
N2997NE-9 9.29 5124 1.30 19.13 0.10 0.35 −0.99 0.85 −0.18 0.13 −1.43 50.66 4.76
N2997NE-10 9.32 5000 1.33 17.47 0.07 0.18 −1.23 0.87 −0.21 −0.04 −1.68 50.93 5.03
N2997NE-12 9.30 5100 1.60 18.50 0.18 0.26 −0.89 1.01 −0.14 0.01 −1.42 50.80 4.90
N2997NE-13 9.22 5510 1.81 18.90 0.08 0.12 −1.08 1.11 −0.27 −0.16 −1.65 50.50 4.10
N2997NE-14 9.22 5510 1.81 19.14 0.19 0.30 −1.30 1.11 −0.17 0.02 −1.87 50.14 4.24
N2997NE-15 9.22 5510 1.81 19.56 0.49 0.27 −0.38 1.11 0.14 −0.01 −0.95 50.66 4.26
N2997NE-24 9.17 5771 1.40 18.19 0.16 0.15 −1.31 0.90 −0.13 −0.07 −1.78 50.52 4.62
N2997NE-34 9.21 5537 2.43 17.67 0.31 0.46 −0.82 1.42 −0.14 0.11 −1.56 51.16 5.26
N2997SW-1 9.34 4821 1.63 17.27 0.53 0.50 −0.94 1.02 0.20 0.25 −1.47 51.11 5.21
N2997SW-2 9.29 5136 2.15 17.32 0.38 0.44 −0.37 1.28 −0.03 0.12 −1.04 51.71 5.31
N2997SW-3 9.30 5084 2.04 16.37 0.33 0.40 −0.90 1.23 −0.06 0.09 −1.54 51.59 5.69
N3351-1 9.50 3921 1.75 17.60 −0.05 0.30 −1.26 0.89 −0.33 0.08 −1.72 50.74 4.34
N3351-2 9.42 4374 1.97 14.62 0.55 0.58 −1.26 1.00 0.23 0.33 −1.78 51.65 5.75
N4303-4 9.21 5526 0.43 15.96 0.20 0.31 −1.20 0.22 0.13 0.25 −1.31 52.06 5.66
N4303-9 9.35 4795 0.86 16.50 0.25 0.34 −1.29 0.43 0.11 0.24 −1.51 51.74 5.84
N4303-24 9.33 4927 0.43 16.50 0.20 0.25 −1.53 0.22 0.13 0.20 −1.64 51.52 5.62
N5253-1 8.37 10410 0.95 13.77 0.18 0.35 −0.70 0.55 0.00 0.21 −0.98 51.56 5.16
N2997NE-G1 9.19 5637 2.24 15.76 0.36 0.32 −0.50 1.32 −0.06 −0.01 −1.19 52.22 5.82
N2997NE-G2 9.25 5362 2.00 17.00 0.06 0.42 −0.85 1.21 −0.33 0.12 −1.48 51.50 5.60
N2997NE-G3 9.30 5090 1.38 18.08 0.19 0.28 −0.97 0.89 −0.10 0.06 −1.44 50.89 4.99
N2997NE-G4 9.31 5033 1.81 17.57 0.16 0.51 −0.52 1.11 −0.20 0.24 −1.10 51.55 5.15
N2997NE-G5 9.19 5650 2.09 17.75 0.28 0.19 −0.46 1.25 −0.12 −0.12 −1.11 51.49 5.09
N2997NE-G8 9.21 5537 2.43 16.50 0.18 0.56 −0.94 1.42 −0.27 0.21 −1.67 51.56 5.66
† in log units
Table 3. Star formation Histories of GEHRs
IB IB 2-burst 2-burst
(t<3Myr) (t∼3–5Myr) (t <3Myr)(t ∼3–5Myr)
young young
N2363: N2997NE-13 N1365-1: N1365-5
N2997NE-2: N2997NE-14 N1365-2 N3351-2
N2997NE-7: N2997NE-24 N1365-3
N2997NE-10 N2997SW-3: N1365-4
N3351-1: N2903-1
N2903-2
N2903-13:
N2997NE-1
N2997NE-9
N2997NE-12
N2997NE-15
N2997NE-34
N2997SW-1
N2997SW-2
N4303-4
N4303-9
N4303-24
N5253-1
N2997NE-G1
N2997NE-G2:
N2997NE-G3
N2997NE-G4
N2997NE-G5:
N2997NE-G8:
14% 17% 62% 7%
EMBEDDED CLUSTERS IN GIANT EXTRAGALACTIC Hii
REGIONS:
III. EXTINCTION AND STAR FORMATION
Y.D. Mayya1,2 and T.P. Prabhu2
1Infra Red Astronomy Group, Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Bombay 400
6 005 INDIA
9
2Indian Institute of Astrophysics, Bangalore 560 034 INDIA
9
1 Electronic mail: [email protected] and [email protected]

n To appear in Astronomical Journal
a
J

1

1
v ABSTRACT
1
0 A study of star formation is carried out on 35 giant extragalactic Hii regions
0 (GEHRs) in seven galaxies using optical photometric data in BVR broad bands and
1
0 in the emission line of Hα. Interstellar extinction, metallicity and nebular contributions
6 to the broad bands are estimated using spectroscopic data on these objects. Dimension-
9 less diagrams involving B −V and V −R colors and the flux ratio of Balmer line to B
/
h
bandcontinuumareusedtostudystarformation. Theclustercolorsindicatereducedex-
p
- tinction towards stellar continuum compared to the values derived from Balmer lines for
o the ionized gas. The frequency of detection of classical young (t < 3 Myr) regions with
r
t only one burst of star formation is found to be low as compared to young regions with
s
a an accompanying population rich in red supergiants from a previous burst (t ∼10 Myr).
:
v Reduced extinction towards cluster stars, destruction of ionizing photons and the exis-
i
X tence of older population, often spatially unresolvable from the younger population, all
r conspire to make the observed Balmer line equivalent widths low in a majority of the
a
GEHRs. A scenario of star formation is suggested which explains many of the observed
properties of GEHRs, including the core-halo structure, reduced extinction for the ra-
diation from stars as compared to that from the nebular gas, non-detection of young
single burst regions and the co-existence of two populations of different ages.
1
1. Introduction the above problems in mind, we use the BVRHα
data of only those GEHRs from Mayya (1994) for
Giant extragalactic Hii regions (GEHRs) are sites of
which additional spectroscopy is available in the lit-
current star formation activity in galactic disks. A
erature. The data, after applying corrections for ex-
detailed investigation of star formation in these rel-
tinctionandnebularcontribution,arecomparedwith
atively simpler systems is important towards under-
theevolutionarypopulationsynthesismodeltoinves-
standingstarformationinmorecomplexstar-forming
tigate the properties of star formation in the sample
systems such as blue compact galaxies (BCDs or Hii
regions. Analyses were performed on dimensionless
galaxies), and starburst systems. GEHRs in an indi-
diagrams, thus eliminating the possible errors due to
vidualgalaxyareknowntodifferfromoneanotherin
the uncertainties in the estimation of distance to the
many of the observational properties, but still have
sourceand/orthemassofthestar-formingregion. We
several properties in common. The common proper-
findthattheextinctionexperiencedbytheembedded
tiessuchastheconstancyofluminosityanddiameter
cluster stars is lower compared to the values derived
of the brightest (also largest) Hii regions have been
fromthe Balmerdecrement. Theseresultsareofspe-
used earlier for estimating the distances to galaxies
cial interest, especially after a similar conclusion has
(Sandage & Tammann 1974, Kennicutt 1979). It is been reachedby the independent study of Calzetti et
important to investigate the reasons for similarities al. (1994) using ultraviolet (UV) and optical spectra
as well as differences between different Hii regions in
ofcentralregionsofgalaxies. Ourstudyalsorevealsa
order to make use of GEHRs as reliable distance in-
slightly complex historyof starformationinGEHRs.
dicatorsortracersofstar formationratesin galaxies.
In sec. 2, we describe the sample and the method
Initial Mass Function (IMF), age, history, metal-
followedinextractingthepurestellarcontinuumfrom
licity, extinction, mass or luminosity are among the
the observed BVR data. In sec. 3, the properties of
most important parameters determining the proper-
the embedded cluster are compared with population
ties of GEHRs. In recent years, there have been sev-
synthesis models. Our results are discussed in the
eral attempts to investigate the IMF parameters un-
context of recent results in sec. 4. Physical mod-
der a variety of conditions (see Scalo 1986). Yet, the
els for star formation in GEHRs and the distribution
IMFslopeofα=2.35asobtainedbySalpeter(1955)
of cluster stars, gas and dust are proposed in sec. 5.
remains the most accepted value. Humphreys &
Sec. 6 summarizes the main results of this study.
McElory(1984)havefavouredamuchsteeperIMFfor
high mass stars in the Galaxy, while Parker & Gar- 2. Sources of Data and Preliminary Treat-
many(1993)obtainedanIMFslopeclosetoSalpeter’s ment
value for the stars in 30 Doradus, both studies being
based on star counts. Investigations based on the in- The GEHR data for this study are taken from the
tegrated properties of GEHRs have so far remained BVRHα photometric catalogue of Mayya (1994, Pa-
inconclusive on the exact value of IMF slope and the perI henceforth). The cataloguecontainsCCDaper-
lower and upper cutoff masses. There are some in- ture photometric data on nearly 180 regions in nine
dications that IMF may be flatter or deficient in low nearby galaxies. In these studies, special care was
mass stars in active starburst regions (Scalo 1987). takeninensuringthatthecontaminationbytheback-
Inthis study we aim to investigatethe starforma- groundisminimumandhencethederivedmagnitudes
tionhistoriesofGEHRs,takingintoaccountthe pos- aregenuinelyfromthe embeddedclusterandthesur-
sibleIMFdifferencesfromoneregiontoanother. Itis rounding nebulosity. The usage of apertures rather
howeverimportant to take into account the effects of thanslits,aswellasmeasurementofBVRcontinuum
(i)interstellarextinction,(ii)metallicityand(iii)neb- and Hα emission line fluxes, centered on the same
ular contributions on the observable quantities, be- pixel, makes these measurements ideally suited for
fore these quantities can be compared with the syn- the investigation of star formation properties. Spec-
thetic models. Interstellar extinction is convention- troscopic data are available for about one fourth of
ally derived using the observedratios of Balmer lines these regions (∼45). The sensitiveness of the back-
(see e.g. Shields 1990). Balmer decrements as well as ground values on the catalogued colors is indicated
metallicitiesandnebularcontributionscanbederived by a photometric quality index in the catalogue. Re-
from spectroscopic data on individual regions. With gionshavingphotometricqualityindex4(background
2
uncertainties leading to errorsin excess of0.3 mag in of dust residing internal to Hii regions is negligi-
B−V andV −R colors: see Appendix inPaperI for ble. The large spread in extinction values for regions
a discussion on backgroundsubtraction errors in Hii within a single galaxy implies non-uniform distribu-
region photometry) are dropped to eliminate back- tion of dust within the parent galaxy, either within
ground dominated regions from the sample. The es- or outside the Hii region. There have been stud-
timated rms errors for the entire sample in the cata- ies investigating the relative contribution of the dust
logue are <0.15 mag in colors and <0.05 dex in the mixedwiththeionizedgas,knownas‘internalextinc-
ratio of Hβ to blue band luminosity ( φ ). The re- tion’, to the total extinction from the parent galaxy
LB
gionsselectedforthestudyherearerelativelybrighter (Lequeux et al. 1981). These studies show the inter-
and hence the errors are expected to be smaller. We nal extinction to be non-negligible at least for some
estimate the errors to be a factor of two less as com- regions. Theextinctionvaluesderivedusingtheradio
pared to that for the entire sample. Finally, we are thermalcontinuumisstatisticallyknowntobehigher
leftwith5regionsinNGC1365,3inNGC2903,14in thanthatderivedusingthe Balmerdecrement(Israel
NGC2997,2inNGC3351,3inNGC4303inaddition &Kennicutt1980). Caplan&Deharveng(1986)have
tothegiantHiicomplexNGC2363inNGC2366and studied the effect of different dust configurations, in-
the starburst nucleus of NGC5253. Measurements cluding the dust internal to the Hii regions to ex-
using bigger apertures, which include more than one plaintheobserveddiscrepancybetweentheradioand
emitting knot,arealsoincludedaroundsixregionsin optically derived extinction values. Internal redden-
NGC2997. ing is found to be significant for some regions in the
Thespectroscopicdatainvolving[Oiii]andBalmer Large Magellanic Cloud (LMC). We find the extinc-
lines are used to estimate the abundance of oxy- tion properties of embedded cluster stars to be dif-
gen, electron temperature (T ) and visual extinction ferent from that of the ionized gas and hence suggest
e
(A (Bal)) using a semi-empirical approach (McCall an alternate prescription for extinction correction in
v
et al. 1985). All the observed quantities have to be GEHRs.
corrected for interstellar extinction before comparing The Hα band fluxes have contribution also from
withthemodels. Totalextinctiontowardsextragalac- [Nii]linesatλ6548,6583˚A.TheobservedHα+[Nii]
tic Hii regions consists of three parts — (i) extinc- fluxes are first corrected for interstellar extinction
tionfromtheinterstellarmedium(ISM)ofourgalaxy using A and the galactic extinction curve (Seaton
v
(A (gal)), (ii) extinction from the ISM of the parent 1979). Hα fluxes are computed from the observed
v
galaxy outside the Hii region (A (ext)) and (iii) ex- Hα+[Nii] fluxes using the spectroscopically deter-
v
tinctionduetothedustmixedwiththegaswithinthe mined [NII]/Hα ratios. Lyman continuum luminosi-
Hii region (A (int)). A (gal) values for the program ties are derived from extinction corrected Hα lumi-
v v
galaxies are given in Table 1 for each of the program nosities using
galaxies,astabulatedbydeVaucouleursetal.(1991).
Otherquantitiesinthetablearedistancestothepar- NL =7.32×1011 Hα (1)
ent galaxies, number of regions used in the present phs−1 erg s−1
study and the sources of spectroscopic data and dis-
which assumes an ionization bounded case B nebula
tances. We denote the visual extinction as derived
(Osterbrock 1989). The expected Hβ luminosity (φ)
from the Balmer decrement by A , in the remaining
v is computed from this using
partofthe text,thoughitcontainscontributionfrom
all the three components. We start our study using φ N
=4.78×10−13 L (2)
Av todereddentheclusterandnebularquantities,as- erg s−1 phs−1.
suming that the spectroscopically derived values are
The treatment of broad-band magnitudes is slightly
valid for both the cluster continuum and the nebular
different. First, an estimation of the contribution
radiation integrated over the apertures. In the later
fromthenebula(line+continuum)withineachband
part of this section, we suggest alternative prescrip-
is made based on the derived T and extinction cor-
tions for extinction corrections that are necessary to e
rected Hα luminosity. The strengths of all bright
explain the present data set.
lines relative to Hβ line, aretakenfromMcCallet al.
The usage of A to deredden quantities related to
v (1985). Free-free, free-bound and 2-photon emission
embedded clusters is appropriate only if the amount
mechanisms are consideredin computing the nebular
3
continuum. Details of these computations are pre- Oxygen abundance and visual extinction are based
sentedinMayya(1995). Foramajorityoftheregions, on the sources of spectroscopic data listed in Tab. 1.
the nebular corrections amount to <5% in B and V While deriving the visual extinction, the underlying
bandsand15–25%intheRband,withemissionlines absorption in Balmer lines, which at the most con-
being the main contributors. The estimated nebular tributes about 2˚A in Hβ equivalent width (McCall
contributions to B and V bands exceed that in the et al. 1985), has been accounted for. The V magni-
∗
Rbandinthetwohighexcitationregions(NGC2363 tude,and(B−V) and(V −R) colorsincolumns5,
∗ ∗
and 5253), due to the presence of strong [Oiii] lines. 6 and 7 respectively, are cluster quantities after sub-
However, these estimates heavily depend on the as- tractingthegaseouscontribution. Column8contains
sumedshapeofthepassband,becauseofthe location the log( φ ) = log(φ)−log(L ) values. Note that
of the [Oiii] lines in the steeply falling part of the B A (Bal)LhBas∗beenusedinobtainBing∗theextinctioncor-
v
andV bandresponses. We find thatevena 50˚Ashift rected φ from observations. Lyman continuum lumi-
of the V band response curve to longer wavelengths nosity is given in column 13. Remaining columns in
changes the B−V and V −R colors by as much as the table will be explained in the next section.
0.3 mag inNGC2363. This amountofshift ofthe ef-
fective wavelength is not unusual and hence gaseous 3. Comparison with Models
corrected cluster colors using broad bands are bound
We plot in Fig. 1 the values of log( φ ) against
tohavelargeerrorsforhighexcitationregions. Useof LB 1
(B−V) ,whicharecorrectedonlyforgalacticextinc-
narrower bands situated at emission line free regions 1
tion. Fig. 2 shows a similar plot, but with (V −R)
would have eliminated this source of uncertainty. 1
replacing (B−V) . Hii regions from different galax-
The estimatednebular luminosities aresubtracted 1
iesareshownbydifferentsymbols,asindicatedatthe
from extinction corrected broad band luminosities to
right hand side of the diagram. The small and large
obtain pure cluster quantities. These quantities are
aperture measurements are distinguished in the case
reddenedbacktoobtaintheobservationallyexpected
of NGC2997. Rms errors on the plotted quantities
valueswithoutanycontaminationfromthesurround-
are indicated by the cross at the bottom-left corner
ing nebula. We denote these quantities by the sub-
of the diagram. For the two high excitation regions,
script∗,whiletheobservedquantitiesaredenotedby
NGC2363 and 5253, there is an additional source of
the subscript ‘obs’ (e.g. (B −V) ,(B −V) etc.).
obs ∗ error, arising out of the subtraction of nebular con-
The resultant quantities are corrected for the galac-
tribution, as discussed in the previous section. The
tic andtotalextinctionusingA (gal)andA andare
v v
arrow on the right side of the figure is the reddening
denoted by suffixes 1 and 2 respectively. The Galac-
vector, with its length corresponding to 1 magnitude
tic extinction curve as tabulated by Seaton (1979) is
of visual extinction.
used in these computations. The process is summa-
The observed quantities are compared with the
rized below.
evolutionary population synthesis model, which is
(B−V) −−−gas subt−→(B−V) , (3) briefly described below. The computational details
obs ∗
and results for different star formation scenarios can
(B−V) −−−A (gal)−→(B−V) , (4)
∗ v 1
be found in Mayya (1995: Paper II henceforth). The
(B−V) −−−A (Bal)−→(B−V) , (5)
∗ v 2 evolutionary population synthesis model is based on
the input stellar evolutionary data of Schaller et al.
Table2containstheresultsforindividualHiiregions.
(1992),andstellaratmospheredataofKurucz(1992).
Column1containsidentificationnumbersforGEHRs
Theclusterisdefinedbyaninitialmassfunction,age,
following Paper I. Oxygen abundances in column 2
metallicity and the total mass in stars. The evo-
are derived from I[OII]+I[OIII] ratio using the semi-
I(Hβ) lution of the cluster is performed at every 0.1 Myr
empirical calibration of McCall et al. (1985). Note
interval, which is adequate to sample even the fast
thatthesampleregionshavemetallicitiesintherange
evolutionary phases of massive stars. Optical and
2–4Z , with the exception of the two high excitation
⊙ near infrared colors are computed and tabulated in
regions mentioned above. McCall et al. also provide
Paper II for three different scenarios of star forma-
relationbetweentheoxygenabundanceandT ,which
e tion at solar metallicity. As noted earlier, GEHRs
isusedtoderiveT givenincolumn3. Column4lists
e in the present sample have metallicities between 2–
the visualextinctionbasedontheBalmerdecrement.
4