Multiobject spectroscopy results of HII regions in nearby late-type spiral galaxies are given. Results include excitation measurements, log ((O III)/H beta), for 81 regions in M 101, 30 regions in NGC 2403, and 13 regions in M 51. Researchers conclude that late-type spirals can be classified into two distinct populations, examine possible causes of this division, and derive metallicity gradients for these galaxies. M 51 appears to have an anomalously shallow abundance gradient

Elston, Richard

Hill, J. M.

Zaritsky, Dennis

English

NASA Technical Reports Server

A Study of H 11 Regions in Spiral Galaxies Using Multiobject Spectroscopy 
Dennis Zaritskyl, Richard Elston1g2, and J.M. Hill1 
'Steward Observatory, University of Arizona, Tucson AZ, 85721 
'KPNO, Tucson AZ, 85721 
We pres m multiobject spectroscopy of H 11 regions in nearby'late-type spiral 
galaxies include excitation measurements, log ([O III]/H@I, for 81 regions in 
M 101, 30 regions in NGC 2403, and 13 regions in M 51. We conclude that late-type spirals 
can be classified into two distinct populations, examine possible causes of this division, 
and derive metallicity gradients for these galaxies. M 51 appears to have an anomalously 
shallow abundance gradient. 
Before the advent of multiobject spectroscopy only one galaxy, M 33, had excitations, 
log([O III]/HP), measured for more than 30 of its H I1 regions. In fact, excitation and 
metallicity gradients for most galaxies are derived from measurements of roughly 10 re- 
gions. Since the development of multiobject spectroscopy, we have presented excitation 
measurements for 45 regions in M 33 (Zaritsky, Elston, and Hill 1989a; Paper I) and Walsh 
and Roy (1989) have presented measurements for 49 regions in NGC 2997. In this paper, 
we present results from observations of 81 regions in M 101, 30 regions in NGC 2403, and 
13 regions in M 51. Multiobject spectroscopy provides the opportunity to observe a large 
number of regions in a fraction of the observing time previously required. 
The observations were made using the MX Spectrometer on the Steward Observatory 
90 in.(2.3 m) telescope on Kitt Peak. MX is a multiaperture spectrometer equipped with 
optical fibers that are attached to microprocessor controlled mobile probes. There are 32, 
4 arcsec aperture object-designated fibers. An 832 gpm grating in second order centered at 
M 4800 k provided a spectral scale of 0.7k per pixel. Typical exposure times were about 
1 hour. For more details of the observing procedure, data reduction, and error estimation 
see Paper I and Zaritsky, Elston, and Hill (1989b). 
In Figure la  we present the data for M 33 (from Paper I), M 51 (data from this study 
and from Searle (1971) and Smith (1975)), M 101, NGC 2403, and NGC 2997 (from Walsh 
and Roy 1989). It is apparent that these five galaxies can be sorted into two distinct groups 
with very little overlap. The demarcation between the two groups becomes more striking 
when one considers that the scatter in excitation measurements for a single galaxy at a 
given radius can be as large as 1 dex. We have used these data as the training sample with 
which to construct a classification scheme. The classification scheme was then applied to 
data from the literature for Sbc to Scd type galaxies for which at least five H I1 regions 
have been observed (from Searle 1971, Smith 1975, Pagel e t  aZ. 1979, Dufour e t  al. 1980, 
Edmunds and Pagel 1984, and McCall e t  aZ. 1985). The results are presented in Figure 
lb. The misclassification percentage (a regions is termed misclassified if its classification is 
different than that of the majority of the regions in its parent galaxy) is less than 9%, with 
many of the misclassified regions being less than lo away from the dividing line. Even if 
with additional data the division between the groups disappears, it is clear that there are 
significant global differences in excitation among late-type galaxies. 
169 
https://ntrs.nasa.gov/search.jsp?R=19910004846 2020-03-19T20:30:33+00:00Z
" *  
Figure la (left) - M 101 (open triangles), M33 (open squares), NGC 2403 (open circles), NGC 
2997 (solid circles), and M 51 (solid triangles). Dashed line indicates division between proposed 
populations. Figure lb (right) - Data from literature for NGC 300, NGC 628, NGC 2403, NGC 
3344, NGC 4736, NGC 5068, NGC 7793, M 33, and M 101 (open circles). Data from literature 
for NGC 1635, NGC 2903, NGC 2997 NGC 3184, NGC 3351, NGC 4254, NGC 4321, NGC 5055, 
NGC 6946, IC 342, M 51, and M 83 (solid circles). 
Excitation is a function of the dust content, the ionizing radiation field, and the 
abundance of the H I1 region. Searle (1971) argued against the first two parameters when 
discussing the cause of the excitation gradients in galaxies by citing the lack of radial 
variations in either Ha/HP (sensitive to dust), or in I(He I X5876)/I(HP) (sensitive to 
high energy photons). We examined whether variations in dust content or ionizing field 
could produce the global excitation differences between galaxies. There is no clear division 
between the distributions of either Ha/HP or I(He I X5876)/I(HP) for the two groups; 
therefore, we conclude that neither dust content nor ionizing field is the primary cause of 
the grouping in Figure 1. 
It has been suggested (McCall 1982) that metal abundance be plotted as a function 
of fractional effective radius or surface mass density instead of fractional isophotal radius. 
The reasoning is that both of these quantities provide better physical descriptions. In 
support of this, we find that when excitation is plotted against either of these quantities, 
the data fall more nearly on a single curve. However, there is still strong segregation 
between the groups and in the case of the relationship between excitation and fractional 
effective radius, more scatter than when excitation is plotted against fractional isophotal 
radius. 
We have applied the empirical calibration of log([O III]/HP) (Edmunds and Page1 
1984) to estimate the oxygen abundance of the observed H I1 regions. Empirical calibra- 
tions rely on a statistical relationship between the observed line ratios and abundance. 
Multiobject spectroscopy compliments this technique by providing a large number of mea- 
surements at a given radius, thereby allowing for an accurate statistical measurement of 
the metal abundance at that radius, provided the empirical calibration is correct. The 
abundance gradients we measure for M 33, M 51, M 101, NGC 2403, and NGC 2997 using 
this empirical calibration are -0.098f0.027, -0.042f0.010, -0.072f0.006, -0.089f0.033, 
-0.093 f 0.010 dex kpc-l, respectively. Previous measurements are -0.01 f 0.03 (using 
170 
data from Kwitter and Aller 1981), -0.081 f 0.027 (Tosi and Diaz 1985), -0.074 f 0.006 
(Evans 1986), -0.083 f 0.032 (Tosi and Diaz 1985), and 0.084 (Walsh and Roy 1989), 
for M 33, M 51, M101, NGC 2403, and NGC 2997 respectively. The excellent agreement 
(< la) between the gradients derived for M 33, M101, NGC 2403 and NGC 2997 imply 
that the empirical calibration for excitation when used in conjunction with measurements 
of over 30 H I1 regions produces results at least a8 accurate and precise as those obtained 
from a combination of the favored empirical calibration, that of ([0 111 + [0 III])/HP, and 
detailed modelling. The derived gradient for M 51 is in marginal agreement (< 20) with 
the hrevious determination, but the difference is due entirely to the additional regions, not 
to the abundance calculations. 
In conclusion, we have discussed briefly some of the preliminary results from multi- 
aperture spectroscopy of H 11 regions in some nearby late-type spiral galaxies, M 33, M 
51, M 101, NGC 2403, NGC 2997. We conclude that late-type spirals can be classified 
into two distinct groups based on the excitation of their H I1 regions at a given fractional 
isophotal radius. We also conclude that neither dust nor ionizing field is the prime cause 
of this segregation. 
We have also used an empirical calibration between excitation and abundance to mea- 
sure the abundance gradients in these five galaxies. The agreement between our results 
using the empirical calibration of log([O III]/HP) and those of previous authors using 
detailed models and the calibration of log(([O 111 + [0 III])/HP) demonstrates that excita- 
tion can be used to measure abundance gradients as accurately and precisely as previously, 
provided measurements for many H I1 regions are used. M 51 apparently has a shallower 
gradient than the other galaxies, which we propose might be a result of the interaction with 
NGC 5195. Future work will include further analysis of the cause of the two populations, 
analysis of azimuthal variations in excitation and abundance, and a corresponding study 
of earlier-type galaxies. 
DZ acknowledges support from a NSF Graduate Fellowship, and thanks the Minority 
Graduate Student Travel Fund at the U. of A. and the organizers of this meeting for travel 
support. 
References 
Dufour, R.J., Talbot, R.J. Jr., Jensen, E.B., Shields, G.A. 1980, Ap. J., 236, 119. 
Edmunds, M.G., and Pagel, B.E.J. 1984, M.N.R.A.S., 211,507. 
Evans, I.N. 1986, Ap. J., 309, 544. 
Kwitter, K.B., and Aller, L.H. 1981, M.N.R.A.S., 195, 939. 
McCall, M.L. 1982, Ph.D. dissertation, University of Texas. 
McCall, M.L., Rybski, P.M., and Shields, G.A. 1985, Ap. J. Suppl., 57, 1. 
Pagel, B.E.J., Edmunds, M.G., Blackwell, D.E., Chun, M.S., and Smith, G. 
1979, M.N.R.A.S., 189,95. 
Searle, L. 1971, Ap. J., 168, 327. 
Smith, H.E. 1975, Ap. J., 199, 591. 
Tosi, M., and Diaz, A.I. 1985, M.N.R.A.S., 217, 571. 
Walsh, J.R., and Roy, J.-R. 1989, Ap. J., 341, 722. 
Zaritsky, D., Elston, R., and Hill, J.M. 1989a, A. J., 97, 97; Paper I. 
Zaritsky, D., Elston, R., and Hill, J.M. 1989b, in preparation. 
171 


A study of HII regions in spiral galaxies using multiobject spectroscopy

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server