A computerized analysis of a starburst model is discussed. The model proposes that the absorption line equivalent width should scale with the level of star forming activity. Archival International Ultraviolet Explorer (IUE) data on IUE spectra of luminous blue galaxies were compared with previous IUE observations of extragalactic HII regions and low luminosity galaxies. The comparisons are summarized and causes for offsets are discussed

Gallagher, J. S.

English

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J_ o _ /_ A _ z-_ _ _A _ F
It
FINAL REPORT
NASA GRANT NAG 5-989
Archival Research on Absorption Lines
in Violently Star-Forming Galaxies
Principal Investigator
(NASA-CR-185960)
ABSORPTION LINES
GALAXIES (Lowell
Submitted: 22 September 1989
ARCHIVAL RESEARCH ON
IN VIOLENTLY STAR,-FORHING
Observatory) I p CSCL 03B
C3/'_O
N90-.1416]
https://ntrs.nasa.gov/search.jsp?R=19900004847 2020-03-19T23:32:24+00:00Z
FINAL TECHNICAL REPORT - NAG 5-989
IUE STUDY OF ULTRAVIOLET INTERSTELLAR ABSORPTION LINES IN
VIOLENTLY STAR-FORMING GALAXIES
INTRODUCTION
Recently a number of 9round-based studies have been
undertaken with the objective of revealin9 the nature of
objects responsible for metal absorption lines seen in the
ultraviolet spectra of hi9 h redshift quasars. These
investi9ations , includin 9 those by Co-lnvesti9ator D. York
and his collaborators, show that emission line 9alaxies are
found with the same redshifts as metal absorption lines
towards many quasars. We may then ask how the properties of
the intervenin9 emission line 9alaxies are linked to quasar
metal absorption line characteristics, includin9 ionization
levels, equivalent widths, and velocity structures.
Galaxies with the hi9 h equivalent widths of the [0 II]
3727 Angstrom emission line found towards some quasars must
either contain active 9alactic nuclei or excess populations
of massive OB stars. In the latter case the current star
formation rate is likely to be enhanced (a "starburst"
9alaxy) or the 9alaxy could be young. In this project we have
extended our exploration of the starburst model by usin9
archival IUE data to measure UV interstellar absorption line
properties of nearby starburst 9alaxy candidates for
comparison with metal absorption lines in quasars.
The project achieved only limited success due to the
modest signal-to-noise ratios of IUE spectra of mostly
apparently faint starburst 9ala×ies. A list of tar9et
9alaxies for this archival research project is 9iven in Table
1
TECHNIQUE
Absorption lines due to or with lar9e interstellar
components were measured in IUE SWP, LWP, and LWR spectra of
blue 9alaxies. Only about 1/2 of the objects in Table I had
spectra of sufficient quality to attempt absorption line
equivalent width determinations.
IUE spectra were analyzed on the Lowell Observatory Sun
4 computer with the NOAO IRAF software package. In addition
to limits imposed by si9nal-to-noise, the choice of continuum
is an important source of error. For this study an empirical
approach based on reasonable choices over large spectral
intervals was adopted, but eventually a more sophisticated
f
r
approach would be worthwhile. A sample spectrum with some
lines chosen for measurement is illustrated in Fi9ure 1. For
example, followin9 on the approach of R. W. O'Connell and
collaborators as well as some experiments that J. Galla9her
and D. York have carried out with their colleagues, a better
method mi9ht involve differencin9 observed spectra from those
predicted by theoretical stellar population models.
RESULTS
That blue 9alaxies can have very stron 9 interstellar
absorption lines is an established fact based on low
resolution IUE spectra of a number of nearby examples
(includin9 the studies by J. Gallagher in collaboration with
S. Lamb and D. Hunter) and a hi9h resolution IUE spectrum of
the hi9h surface bri9htness amorphous 9alaxy NGC 1705 (York
et al. 1989, Ap.J., in press). The issue lies in
understandin9 which physical process controls the absorption
equivalent widths of interstellar lines in these types of
objects.
For UV resonance lines absorption equivalent widths will
depend more stron91y on 9as velocity dispersions than ion
column densities. This occurs because resonance lines become
saturated and thus on the flat part of the curve of 9rowth
even for small column densities and therefore are insensitive
to column density. However, the equivalent width will
increase roughly linearly with the number of clouds havin 9
velocity Doppler shifts exceedin9 the intrinsic line width
that are found alon 9 the line of si9ht. This is exactly what
seems to be happenin9 in quasars, where high velocity
resolution spectra have shown that metal absorption features
break up into multiple components, usually spread over a
velocity ran9e of hundreds of km/s.
An issue, however, remains as to where these velocity
shifted absorption components ori9inate. The standard model
locates absorption features in 9alactic halos, while more
recently York and his collaborators have su99ested that
multiple velocity components occur in and around re9ions of
active star formation. This idea motivated the successful
9rond based optical search for violently star formin9
9alaxies associated with quasar absorption line systems by
York and his collaborators. In this model _uasar metal
absorption features often arise in objects havin 9 multiple
centers of star-formin 9 activity spread over large areas.
Such systems would have many of the properties expected of
youn9 9alaxies formin 9 in a universe dominated by cold dark
matter.
If star formation is responsible for _uasar absorption
line velocity spreads, then simple physical arguments also
predict that a relationship might exist between the level of
star formin9 activity and the velocity spread of interstellar
9as. Lar9e , ener9etic starbursts such as those found in the
central regions of some 9alaxies should produce wider
velocity spreads and therefore higher absorption line
eguivalent widths than are found in more modest star formin9
complexes, such as 9iant HII reBions.
In this project we undertook a test of this specific
model, that absorption line equivalent width should scale
with level of star formin9 activity, by analyzin9 archival
IUE spectra of luminous blue 9ala×ies to compare with
previous IUE observations of e×tra9alactic HII regions and
low luminosity 9ala×ies.
The comparison of measured equivalent widths of
ultraviolet absorption lines can be summarized as follows
(includin 9 results from luminous blue 9alaxies from this
study; other data from York et al 1989):
OBJECT CII -1335 Si IV - 1400
Low ionization, stron 9
line QSO absorber 1.3 0.4
Medium ionization, stron 9
line QSO absorber 1.2 1.8
Extragalactic HII
re9ions 1.9 2.5
Luminous blue
9alaxies 2.8 2.7
DISCUSSION
Interstellar absorption lines in luminous blue galaxies
are similar in equivalent width to those found in
extragalactic HII regions. This suggests that the scaling
between equivalent width and level of star forming activity
as measured by estimates of OB star luminosity is not linear.
We are currently exploring the implications of this result in
terms of velocities expected to be produced by OB stellar
clusters of varying sizes. Numerical models suggest that the
scaling between OB stellar population richness and expansion
velocities of the superbubbles they produce could be rather
flat, which would be consistent with our observational
results.
The offset in absorption equivalent widths measured in
the IUE spectra of galaxies and HII regions and those in
quasars could have two causes: (1) Stars may be contributing
to equivalent widths of "interstellar lines" in blue
gala×ies. (2) The IUE spectra of blue galaxies average over
a large regions in the target galaxies (set by the sizes of
OB associations with low obscuration---- 10 pc to _1 kpc) as
compared to that sampled by the lines of sight to quasars ( <
I pc).
PUBLICATIONS
We plan to submit a short paper reporting the results
from this study for publication in a refereed journal.
GALAXY
Mich 239
MK347
Haro 15
MK357
MK1027
MK7
MK8
Haro 1
N2537
MK702
MKI9
IC2458
MK25
N31_5
Haro 2
N3310
Haro 3
MK! 53
MK1267
MK170
MK171
POX 36
N4194
MK52
MK_09
MK213
MK54
MK59
POX124
MK66
Tol 89
N5430
MK487
MK691
MK496
MK499
MK309
MK325
TABLE 1
PROGRAMGALAXIES FOR IUE ARCHIVAL STUDIES
POSITION
00 22 19.0 -02 10 10
00 45 18.0 22 06 c)0
00 46 04.0 -12 59 25
01 19 56.6
02 ii 28.4
(37 _-- 19.0
07 2='_ 38. c)
07 33 39.4
08 09 43. 1
08 42 45.3
09 12 53.5
09 17 26.0
10 O0 22.0
22 54 33
04 56 24
72 40 00
72 14 00
35 21 15
46 08 33
16 16 46
59 58 53
64 27 c30
59 40 33
i0 04 18.c3 -29 41 00
I(_ 29 22.5 54 39 30
10 35 39.0 53 45 54
10 42 16.4
i0 46 03.8
10 50 28.6
ii _3 56.(3
ii 25 42.9
56 13 20
52 35 50
04 53 21
64 25 00
58 50 25
II 56 24.3 -18 45 03
12 ii 39.9
12 23 09.0
12 23 50.6
12 29 01.()
12 54 32.0
12 56 38.4
54 48 19
00 51 c)0
48 46 07
58 14 00
32 43 O0
35 06 54
13 04 44.9 -12 56 00
12 23 57.7 57 30 38
13 58 25.0 -32 50 0()
13 59 10.0 59 33 34
15 35 48.3
15 44 43.2
16 10 24.0
16 47 02.7
22 50 ¢39.9
23 25 11.0
55 25 -"7
18 (-)2_-_?°
52 35 00
48 47 43
24 27 51
°_ 19 00
COMMENTS
SWP7495, LWR6481
SWP27190, optical data
SWPI1259, optical data
SWP18199
SWP2i604, interactin_
SWPIII39,LWR 10572, optical data
SWP7286,LWRI0976, optical data
SWPI1269, optical data, underexp?
SWP22359,SWPI0473,SWPII270, MK86
SWP6706,LWR6730, 8-28A
SWP18237, interactins?
SWPI3396,LWRI0955 saturated?
SWP22785, LWP3174
LWR9894, opti_al data
SWP10471
SWPI3529,SWPI7323,LWRI3568,
optical data
SWP10483, MK35, optical data
SWP17314
LWPI769, type?
SWP6255, low lum obj
SWP6656-7-8, LWR5710, classic
starburst interact., opt. data
SWPI8113
SWP22784, LWP3412
SWP9341,LWR8097
SWPIO484,SWPI0495
LWRI0631
SWP3472, SWP9793, LWR3018
SWP5691-2,LWR3052, LWR4937, low lum
SWP25348
SWP24384
SWPI6971-2,LWPI554
7 SWP spectra, 6 LW spectra,
interactin9
SWP19697, optical data, low lum
SWP19291, LWRI5208
SWP17305
SWP26961
SWP24383
SWP7303,LWR9670, optical data
ORIG!NAL PAGE" IS
OF POOR QUALITY
LO
NOAO/IRAF V2.6BETA iue@louisXIV Thu 21:S9:18 30-Mar-89
[s023[*,I]]: INDEFs ap:O
J
1200
I000
800
600
400
200
I I I I f
I I
1250 1300 1350 1400 1450 1500 1550 1600
FIGURE I. IUE low resolution SWP spectrum of the starburst
nucleus system Haro 1. Wavelen9th in Angstroms is plotted
a9ainst relative intensity. Re9ions for equivalent width
measurements are marked. These data are typical of IUE
spectra of the intrinsically fainter luminous blue 9alaxies.
Once the data have been reprocessed as part of the program to
make an ultimate IUE archive, better S/N will be achievable
on absorption line equivalent widths.


Archival research on absorption lines in violently star-forming galaxies

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