Results of a two year ADP study of gas in the Galactic halo are presented. This is partly a summary of 2 papers which were published in referred journals and partly a discussion of work currently underway

Massa, Derck

English

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NASA-CR-194374 /
FINAL REPORT FOR NAS5-30788
ADP STUDY OF THE STRUCTURE OF IUE HALO
J
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/
PI : Derck Massa
Applied Research Corporation
8201 Corporate Drive
Landover, MD 20785
We have extended the work of Savage and Massa (1987) on a sample
of halo stars which are mainly at Galactic latitudes less than 30 ° and
are very distant. This set of stars is ideal for studying the global
properties of the halo for three reasons, l) the long path lengths
allow the detection of less abundant ionic species such as N v, 2) the
low latitudes suppress the effects of peculiar vertical motions known
to exist exist in the halo (Danly 1989) and, 3) the long path lengths
tend to smooth out the clumpiness of the gas, allowing us to
concentrate on global gas properties.
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Savage and Massa (1987) demonstrated quiet clearly that the
effects of Galactic rotation influence the UV line data. Figure 1 is
from Savage and Massa and shows the average interstellar absorption
line velocity, <v> = (v_ + v+)/2, for the weak Si II line at 1808A and
the C IV 1550A doublet. Here, v_ and v+ are the LSR velocity of the
interstellar absorption at half absorption intensity on the negative
and positive velocity part of the line profile. The average
velocities for Si II and C IV are plotted against vr(d ) which is the
expected LSR radial velocity of absorption at the distance of the
various stars with the assumption that disk gas and halo gas corotate.
The points in Figure 1 are separated according to whether or not the
star is located near the disk (Izl < 0.5 kpc, solid circles) or in the
halo (Izl > 0.5 kpc, open circles). If interstellar absorption arises
from an optically thick slab of gas with a large scale height which is
corotating with the Galactic disk, then the mean absorption would
always be vr(d)/2 , indicated by line of slope 1/2 in the figure. Note
that the Si II data for the disk stars follow the line, demonstrating
the effects of galactic rotation, while the halo stars do not. This
behavior is most easily interpreted in terms of a small scale height
for the Si II. Thus, once Iz] exceeds ~ 0.5 kpc, there is very little
Si II and the line of sight simply runs out of absorbing gas. In
contrast, the data for C IV for the disk and halo stars both follow
the line to larger Ivr(d)]. since c IV has a larger scale height than
Si II, the effects of galactic rotation are even apparent in the data
for the halo stars. Comparison of these two plots clearly
demonstrates how Galactic rotation and the density distribution of a
particular ion are intimately linked. Notice too, that in those cases
where Ivr(d)I > i00 km/s it appears that even the C IV data do not
follow the average velocity expected if corotation were occurring.
The C IV data of Figure 1 suggest that the assumption of corotation
may become invalid at the larger Izl distances.
(NASA-CR-194374) ADP STUDY OF THE
STRUCTURE OF THE IUE HALO Final
geport, 2 Mar. 1990 - I Mar. 199Z
(Applied Research Corp.) 8 p
N94-12595
Unclas
G3190 0185607
https://ntrs.nasa.gov/search.jsp?R=19940008123 2020-06-16T20:52:26+00:00Z
During the period covered by this project, we extended the simple
global analysis outlined above, and performed a detailed analysis of
the density and kinematics of halo gas along two kinematically
interesting lines of sight. This resulted in 2 publications (Savage,
Massa and Sembach 1990, Sembach, Savage and Massa 1991). The basis of
these analyses was high quality IUE absorption profiles obtained by
adding several spectra taken at different locations in the IUE large
aperture to reduce the effects of fixed pattern noise. The
interstellar absorption profiles were converted into "r plots". These
are derived by taking the log of the residual intensity of the line
profile and they can be compared directly to the predictions of a
model for the kinematic and density structure of the absorbing gas
along the line of sight.
This technique and the models used to analyze the data, were
developed under this grant (details are given in the papers cited
above). Figures 2a and b show how changing the parameters in the
model affect the calculated profile for the line of sight toward HD
156359. The profiles are clearly very sensitive to the model
parameters. Figure 2c and compare the Si IV and C IV profiles in HD
156359 to the models.
Because only two lines of sight have been analyzed in detail, it
is premature to make strong conclusions concerning the global nature
of the halo gas. In order to extend our effort to more complex models
and to incorporate more lines of sight, it is no longer possible to
represent all of the data on a single graph. Therefore, if one wishes
to test more complicated models, different criteria must be adopted,
such as a measure of a X 2 statistic. Currently, the comparison
between the model and observed parameters is weighted by a combination
of the expected measurement AND DISTANCE uncertainties. The latter
effect can be quit large for certain lines of sight, we treat each
line from a given ion separately, since each represents an independent
set of observations. For example, if both components of the C IV 1550
doublet can be measured, this gives us two independent measurements
for the line of sight. The program uses the amoeba simplex algorithm
to minimize the squares of the weighted differences between the
observed and model quantities.
Clearly, there is a wealth of parameters which can be chosen as
variables, so we have begun with a very simple case. Figure 3a shows
the observed equivalent widths of C IV 1548, 1550 (about two thirds of
the sample had measurements for both components) versus those of a
model in which the mid-plane density and scale height were optimized
(corotation was assumed, and we adopted an RMS velocity dispersion of
26 km/s for the gas, in accordance with Jenkins 1978 results). Figure
3b shows similar results for a static model, in which a constant
velocity dispersion of 26 km/s was used. The effects of Galactic
rotation on the fits is clearly apparent. The RMS dispersion for the
two models are 0.21 and 0.29 dex for the dynamic and static models
respectively. The former is close to the expected measurement error,
-50% or 0.18 dex, while the latter represents an error nearly twice
this large.
The parameters derived from the optimization scheme differ
significantly from those derived by Savage and Massa (1987) from
essentially the same sample of stars. The Savage and Massa mid-plane
density and scale height were 7.0E-9 cm-' and 3.0 kpc, while the
optimization scheme gives 1.02E-8 and 5.62 for these same quantities.
It is interesting that both are larger, and that they combine to
produce a vertical column density of 1.8E14 cm -2 as opposed to 6.5E13
-- more than a 60% difference.
Figure 4 shows a plot of the observed half intensity velocities
versus those from the dynamic model. There is clearly unaccounted for
structure present in the data and, hence, additional information can
be extracted, whether this can be accounted for by employing
different density structures, such as a "transition region" near Izl z
1 kpc, or if it is necessary to invoke kinematical influences remains
to be seen, and improvements upon the preceding analysis are currently
being pursued under a new ADP grant.
REFERENCES
Danly, L. 1989, ApJ, 342, 785
Jenkins, E.B. 1978, ApJ, 220, 107
Savage, B.D. and Massa, D. 1987, ApJ, 314, 380
Savage, B.D., Massa, D. and Sembach, K. 1990, ApJ, 355, 114
Sembach, K., Savage, B.D. and Massa, D. 1991, ApJ, 372, 81
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Figure i. The average LSR interstellar absorption velocities of Si II
k1808 (Fig. la) and the mean of the C IV k1550 doublet (Fig. lb)
versus the expected LSR radial velocities for each star in the Savage
and Massa (1987) sample. The line has a slope of 1/2. Filled symbols
are for stars with Izl < 0.5 kpc, and open symbols are for stars withIzl > o.5 kpc.
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Figure 2 (From Sembach, Savage and Massa 1991). Figures 2a and b
illustrate how changing parameters in the halo model affect the
calculated profile for the line of sight toward HD 156359. Fig. 2a
shows the effect of changing the scale height of the gas from 0.3, to
1.0 to 3.0 kpc (altering the mid-plane density amounts to a simple
vertical translation). Fig. 2b shows the effect of changing the
velocity law in the halo from corotation, to a situation where the gas
begins to decouple from the rotation in the plane at JzJ = 1.5 kpc and
comes completely to rest at JzJ = 3.5 kpc (slow decoupllng) to a model
where the gas begins to decouple at JzJ = 1 kpc and comes to rest at
IzJ - 1.5 kpc (fast decoupling). Figures 2c and d compare the
observed Si IV and C IV profiles to the models. The lack of
absorption at large negative velocities in both profiles suggests that
some sort of a break-down of corotation is occurring in the halo,
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Log of equivolent widths in mA for I:he Stot;c model
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Figure 3. Log of the observed C IV equivalent widths in mA versus
those derived from models. The line in each figure has a slope . 1
(all the points would lie on the line if the model were perfect).
Figure 3a is the best fit for a model which includes Galactic
rotation, assumes corotation in the halo, and has been optimized to
determine the mid-plane density and scale height of the gas. Figure
3b shows the same data plotted against a model which ignors Galactic
rotation. 6
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Model half intensity velocities (kin/s)
Figure 4. The observed C IV half intensity velocities versus those
derived from a best fit model which includes Galactic rotation,
assumes corotation in the halo, and has been optimized to determine
the mid-plane density and scale height of the absorbing gas. The
crosses and triangles represent the negative and positive half power
points, respectively, and the line shown has slope = I. Notice that
the deviations about the line have a distinct "S-shaped" pattern which
is unaccounted for by the simple model.
Re.oort C;ocumen[ation Page
I. ReDO_ NO.
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The Density and Kinematical
Galactic Halo (ADP Study of
IUE Halo)
Structure of the
the Structure of
AUI_OtI|I
Dr. DerckMassa
Pe_orm*_ 0rgamzazJon Name _ AC4tt_
Applied Research Corporation
8201 Corporate Drive, Suite 1120
Landover, MD 20785
_onsor_ng _e_c'/ Na_e Ino _C_fe_
NASA/Goddard Space Flight Center
Code 684
Greenbelt, MD 20771
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Final
March 2, 1990-March i, 199
I_. S_on_rm 9 A_e_':-q ":C:e
15. SuCD,ementarV No[es
16. A0Stta¢:
Results of a two year ADP study of gas in the Galactic halo are presented.
This is partly a summary of 2 papers which were published in referred
journals and partly a discussion of work currently underway.
17. Key Wor=s (Sugges_,O _y Aut_onsl|
UV absorption, Interstellar Gas,
Galactic Halo
19. Secunw C:as._f. (of m,s re_ottJ
._0. Security Cta:_+l. to( :n,s ;_a;e)
_.. Pt,¢e
NASA FORM IK_| OCT


ADP study of the structure of the IUE halo

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