The starburst phenomenon was viewed as increasingly important since the recognition that some galaxies have regions in which stars are forming so rapidly that a transient event must be seen. Such starbursts populate samples of galaxies selected either for UV or IR excess, and some were found from IRAS source identifications that must be quite heavily obscured at optical wavelengths. Many interpretations of the physical conditions in these objects and their stellar populations have relied on scaling from models of individual H II regions, and this certainly seems justified from the gross appearance of the optical spectra and IR spectral shapes. Collection of complementary UV, optical, and near-IR data is presented on a set of starbursts, with a preliminary analysis of models for more realistic internal structure

Keel, William C.

English

NASA Technical Reports Server

\Final Technical ReporL
NASA-Astrophysics Data Program grant NAG5-1386
3776- 
THE ROLE OF UV-OPTICAL OBSCURATION IN STARBURST GALAXIES
William C. Keel, Principal Investigator
University of Alabama
6 September 1991
(NA%A-CR-188771) THE R_LF OF UV-OPTICAL
U_DEUPATI_N IN STAK_UqST GALAXIFS Fin_|
Technical Repor _ (hlq_m_ Univ.) II p
CSCL
N91-31051
03_ Unclas
G3/90 0037753
https://ntrs.nasa.gov/search.jsp?R=19910021737 2020-03-19T17:22:32+00:00Z
INTRODUCTION
The starburst phenomenon has been viewed as increasingly important since the
recognition in the early 1980s that some galaxies have regions (either in their disks or
confined to a small nucleus) in which stars are forming so rapidly that we must be seeing
a transient event. Such starbursts populate samples of galaxies selected either for UV or
infrared excess, and some have been found from IRAS source identifications that must
be quite heavily obscured at optical wavelengths.
Many interpretations of the physical conditions in these objects and their stel-
lar populations have relied on scaling from models of individual H II regions, and this
certainly seems justified from the gross appearance of the optical spectra and IR spec-
tral shapes. However, there are certain lines of evidence that such a strMghtforward
interpretation may be misleading in these very large and luminous aggregates:
- Estimates of extinction obtained at various wavelengths disagree substantially, such
that IR indicators give higher extinctions than those at shorter wavelengths. This
indicates mixing of emitting and absorbing sources, and that there is no unique value of
extinction to be applied at a given wavelength independent of knowledge of the source
structure.
- Measurements of thermal emission confirm that many starbursts have a large amount
of associated dust, but strong optical emission Iine, and ultraviolet continua are ob-
served. Clearly a screen model for reddening is inappropriate, but the implications
of coextensive obscuring matter and ionizing star clusters have yet to be satisfactorily
worked out.
These observations showing that dust is present, but distributed in some complex
way intermingled with the emitting stars, so that simple scaling of H II region models
or analogs may give misleading results with regard to stellar ages or mass distributions
and total luminosities. This proposal involved collection of complementary UV, optical,
and near-infrared data on a set of starbursts, with a preliminary analysis of models for
more realistic internal structure.
UV AND OPTICAL DATA
A set of consistent spectral energy distributions has been generated for a sample
of starbursts in galactic nuclei and disks, using the IUE archives and new ground-based
optical and K-band data. The sample (Table 1) consists of the nuclei listed by Balzano
(1983) or Devereaux (1989) that had observations available in the IUE archives as of
early 1991.
IUE data handling: All IUE data were reprocessed using either the GEX Gaussian ex-
traction program (Urry and Reichert 1988) or the newer optimal extraction software at
the GSFC RDAF (Neill et al. 1991). GEX was used for those objects in which the UV
intensity profile along the IUE large aperture major axis is pointlike, while the opti-
mal procedure gives more reliable results when IUE resolves the UV light distribution.
Both the resulting integrated spectra and cross-cuts in multiple wavelength bands were
retained for analysis.
Optical imaging: Images in broad bands B,V,R,I, and narrow bands including redshifted
Ha emission were obtained with the 1.1-m telescope of Lowell Observatory. Absolute
flux calibrations were done both with BVRI standard stars and spectrophotometric
standards for the Ha filters, to allow absolute comparison with the UV results.
Near-infrared imaging: Images for most of the sample objects were obtained in the K
band (2.2#) during a January 1990 run with the 1.3-meter telescope at Kitt Peak. The
field size of 80 arcseconds was ample to allow accurate sky level measurements, so that
absolute intensities should be reliable, and comfortable larger than the IUE aperture
(Fig. 1).
gV-opticaI comparisor_: The images allow production of spectral energy distributions
covering exactly matched parts of each galaxy, so that, at the expense of possible added
3
light from older bulgestars in the redder bands,the resulting spectral shaperefers to one
spatially limited stellar population. The flux in eachbroad-band or emission-lineimage
was summed over the region coveredby the relevant IUE aperture, using a program
written by then-undergraduateM. Cooperunder support from this ADP grant. Errors
in UV-optical flux ratios are frequently dominated by pointing uncertainties rather than
intensity calibration uncertainties, so thesewere evaluatedfor eachobject by assessing
the dependenceof measuredoptical flux on assumedcenterlocation of the IUE aperture.
Even though the center of the IUE aperture is often well specifiedalong the "slit", the
pointing in the dispersion direction is uncertain at the few arcsecondlevel. In some
cases, the uncertainties in matching optical and UV apertures lead to uncertainties
4-10% in relative optical flux.
BROAD-BAND STARBURST SPECTRA
In addition to the observed continuum intensities from 1200 .& to 2.2#, corrected
for foreground Galactic reddening using the Seaton (1979) prescription, information is
available on the ionizing ultraviolet intensity intercepted by the gas in these systems.
The Ha intensity from the narrow-band images was corrected for contamination be
the neighboring IN II] forbidden lines using spectroscopically known line ratios, and
converted into a number of Lyman continuum photons using recombination theory.
The far-UV intensities thus inferred fall well above any extrapolation of the directly
observed stellar continuum longward of Lyman a. An example is shown in Fig. 2, the
spectral energy distributions of NGC 1614 and 7714. The intensity of ionizing radiation
is shown schematically as fiat between the H ° and He + ionization edges; the exact
level is sensitive to the spectral shape of the ionizing star clusters, since what we can
measure gives the total number of ionzing photons and not their total energy. The
ionizing continuum is almost ten times stronger than the starlight just longward of this.
This is a refinement of the spectral argument for selective obseuration that formed
the motivation of this study. It is a general property of starburst nuclei that the ionized
gas is illuminated by more stellar UV radiation than emerges along the direct line of
sight. Furthermore, the reddenings derived by techniques using optical, UV, and IR
indicators may disagree dramatically. These facts, along with an imaging demonstration
of emission-line material seen around optically invisible IR sources (see Keel 1989),
suggest a geometrical interpretation of obscuration in starbursts.
A SIMPLE MODEL: OBSERVABLES AND STARBURST STRUCTURE
A simple picture of star formation on distinct clouds illuminates many of the
issues in the structure of starbursts. The density and typical filling factor may be
estimated from recent CO interferometry (reported, for example, in IAU Symposium
146, "Dynamics of Galaxies and their Molecular Cloud Distributions", Paris 1990). The
amount of starlight absorbed by dust in these clouds and the surrounding medium may
be constrained by IRAS flux measures. Finally, the spatial extent and dust content
of the gas surrounding the star-forming zone are given by emission-line images, which
in many cases resolve the Strgmgren volume, and by observed Balmer emission-line
decrements.
These have been incorporated first into a two-dimensional model which allows
relatively quick examination of the impact of such factors as compactness of the cloud
configuration or typical shape of clouds on observables such as total IR or line emission,
or escaping stellar UV continuum, for a fixed total star-formation rate.
In these models, a set of molecular clouds is distributed in a Gaussian configu-
ration, and star clusters are formed near the cloud surfaces. Emergent rays are traced
until they either leave the grid (generally at negligible intensity) or intercept another
cloud. Each grid point not occupied by a cloud maintains a sum of UV intensities from
all clusters to whose radiation the point is exposed, so that a grid of local UV intensity
is maintained. This will be related to surface brightness in some recombination line;
?
5
in general the medium surrounding the clouds has measurabledust content, so that
the details need to be fine-tuned to match specific objects. A set of models has been
generated for fixed cloud number and rate of star formation, varying the size of the
whole configuration relative to the cloud size and the form factor of individual clouds
(circular vs. elliptical of various axial ratios).
Trends of global properties IR/blue ratio and relative emission-line intensity are
shown in Fig. 3 for this two-dimensional case. Because full three-dimensional treatments
still need to be run, these are best regarded as suggestive of scaling laws or applicable
to very flattened bursts such as have been suggested for M82. In the two-dimensional
limit, thermal IR emission from clouds heated by the young clusters increases for smaIler
configurations, while observed line emission decreases; note that the ratio of these may
vary by a factor 10 as the size of the starburst region changes by a factor 5.
The existing model is still being developed. Future extensions, besides adding a
third dimension to verify the scaling laws suggested from two dimensions, will include
realistic spectral energy distributions for star clusters and evolution of both the stars
and their coupling to molecular clouds.
Implications
These results begin to resolve some long-standing puzzles in understanding re-
gions of intense star formation. In particular, given the IR-based evidence for strong
extinction, why do we still see strong optical emission lines, and even continue to see
flat UV spectra? The answer is that we see some of the ionizing star clusters directly
(giving UV radiation with only slight reddening by the diffuse surrounding material),
and we see emission-line material powered by the many "searchlight beams" escaping
between more or less distinct molecular clouds from within the configuration, as well as
by the clusters we can see directly. Thus diagnostics often used for the age or stellar
population in H II regions, such as Balmer-line equivalent widths or UV/infrared flux
?
6
ratios, will be quite misleading in such an environment. The continuum and emission
lines refer to different fractions of the overall stellar populations.
In addition to the rangeof properties coveredby the objects in Table 1, statistical
analysiscan tell what the most typical diagnostic properties of starbursts are, and thus
what a representative role for selectiveobscuration effects is. As a start, Gallimore
(1990) has collected optical intensity and environmental data for a complete sample of
IRAS galaxies in a Master's thesis with partial support from this grant. This material
will be further used for analysisof the distribution of optical-IR ratios.
References
Balzano, V.A. 1983,Astrophys. J. 2698,602
Devereaux,N.A. 1989,Astrophys. J. 346,121
Gallimore, J.F. 1990,M.S. thesis, University of Alabama
Keel, W.C. 1989, St_y and Telescope 77, 18
Neill, D., Kinney, A. and Bohlin, R. 1991, IUE-RDAF memorandum.
Seaton, M. 1979, M.N.R.A.S. 187, 73P
Urry, C.M. and Reichert, G.A. 1988, NASA I17E Newsletter 34, 96
7
Galaxy
NGC 1614
NCC 2782
NCC 2798
NGC 3395
NGC 3396
NGC 3504
NGC 369O
NGC 3991
NCC 3995
NCC 6052
NGC 7714
Mkn 357
TABLE 1
Starburst nuclei - IUE and ground-baseddata
IUE Exposures
SWP 25483
SWP 23080,LWP 3416,LWR 5883
SWP 24699
SWP 25465
SWP 25485
SWP 5661, LWR 4906
Other images
BVRH_IK
BVRH_K
RHc_
BVRHe_IK
BVRH_IK
BVRH_IK
LWR 15387,15394,SWP13935,19341BVRHaIJK
SWP 19218
SWP 07212,LWR 12534
SWP 20119,LWR 1585
LWR 03499
SWP 18199
BVRH_IK
BVRH_IK
RH_
BVRH_IK
RH_
Figure 1. The IUE aperture (for observation LWR 3499) superimposed over a contour
display of the K-band image from Kitt Peak. Most of the intensity is contained inside the
IUE aperture, so that a reliable measurement of broadband spectral shape is possible.
n7714k[10"51,10'51]" NGC 7714 K lOOsec _1 no guide star
tO _0 30 44
4_
......... . . . . . ............................
I0 20 30
contoured from 50. to 2000., Inkerval = 50.
NOAO/[RAFv2.gEXPORTKEELeokPaFPi 10=54=09 06-Sep-gl
rFigure 2. Spectral energy distributions for the nuclei of NGC 1614 and 7714. The
Lyman-continuum flux is shown schematically as flat in F_,; its level will vary somewhat
depending on effective stellar temperature. The high level of ionizing radiation compared
to that seen directly from stars longward of Lyman a indicates a role for selective
extinction; the less of the starlight is seen along the direct line of sight than by the
whole volume of observable ionized gas.
2
1.5
1
.5
0
' I ' ' ' I ' ' ' I ' ' ' I
Inferred ionizing continuum
NGCV 14 (top)
NGC 1614 (bottom)
I
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I I I
200 400 600 800 1000
]k,nln
I0
- Figure 3. Behavior of far-infrared and emission-line luminosity of the 2-dimensional
starburst model, each in arbitrary units, as a function of nucleus size compared to a
typical cloud size The FIR luminosity declines for less compact configurations, while
the emission-line luminosity increases as more UV escapes to photoionize tha gas. The
range of emission-line intensities depicted here shows tha variation with cloud aspect
ratio (circular up to 5:1), while the FIR intensity is relatively insensitive to cloud shape.
_, .8
V
-P,4
.6
.4
.2
0
m
m
B
m
m
m
D
Line emission
FIR
m
u
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m
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m
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-,,,,I,,,,I,,,,I,,,,I,,,,I,,,,I,,,,
0 5 10 15 20 25 30 35
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11


The role of UV-optical obscuration in starburst galaxies

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