A Spectroscopic Study of UM673 A &B: on the size of Lyman- alpha Clouds

(1)

-37-A

SpectroscoPic StudY

of

UM673

A

&

B:

on

the

Size

of

LYman-c

Clouds

A.

Smette.,

J.

Sud,ei2,

P-A-

ShotetL,

CtB'

FoI1z3,1'8, Chofiee3'

R'J'

Weymonna',

R.E.

Wittirrms' and'

P'

Magoin2

l'uropean

_{Southero observatory,}

2lnstitut

d'Astropbysique, Université de Liège'

sMultiple

_t.ti,'o,'

_t"t."cope

Obserrratory aObservatories

of

Ca.rnegie

Institution

of

W""ni"gt",

sCuro

Tololo Inter-American Obserrratories

UM6?3isagravitationallylensedquasa'rataredshiftof2.T2T(Sr:rdejetoL'l987,Natrrre

32g, 695). The two images have

v

magnitudes of 17.0 and 19'1, and a're separated by 2t!2 on the sky. The redshifr of the

i.^iog

galaxl,is 0.ag3. The images a.re bright enough for absorption line specrroscopy, and

tn"

ô;*;td"hift

is high enough

foi

tUe

Lyc

forest

to

be accessible' This is therefore an ideal case for studying the size and structure of

Lyc

clouds by cross-correlating

the absorption spectra

in

the two images'

several

rp*t

orcopic obserratio* of

tho"

images were made

in

1987-1988' using

_T"^YYÏ'

the

CTIO

4-m. and

tu"

BSO 3.Sm telescopes,

with

spectral resolutions rangrng from 0'3 to 2 A

(F\MHM). Great care was taken to ensure

th"t th"

fainter image was not contarninated by light

from tbe brigbter

i-og"-

aaational

con-fi.r:mation of these results has been obtained recently by

s.

D,odoric o

et

dI-

1pii*t"

communication) using the loog-slit mode of

EMMI

on the NTT at La Siila. Sanple spectra of the f,wo images a're shown

in

Êg' 1'

40oo

40?j5

4o5o

4o?5

4loo

4125

4150

4t75

4200

0 r.5

I

.5 0

4250

42?5

4300

4325

4350 À(Â)

,^{'^Â)., I

r.r-.1 I ll t | | t l. .ll B(2

_l)

_{r | | t rr r rr I}

_{rr. I}

I r | | l-l I

(2)

MMT-r.75

-38-1.00 > r50 0.75 0.æ s 0!o

r.

Figure

2. -

correlation bebween the rest equirralent widths of correspouding

l.""ohrtion specrra

of

uM

673

A

&

B,

for the

21 single a.sd 26 blended Lya dashed crossac respectively).

The results of or:r analysis of these data can be summarized as follows:

1.

Almost all absorptisslines a.re cornrnon

to

both spectra- AU 68 lines present at

5o in

B are present

in Aithere

are only two anti-coincidences (present

in

A, not

in

B at

3o),

and these could be a

Mgtt

doublet.

2.

The velocily d.iferences between correspondi:rg lines in the two spectra are consistent with zero

(o:

t7

k"" s-l).

3.

Tbe equirralent wid,ths are strongly correlated in the two spectra (ûs- 2), essentially proving

that

the trro lines of sight intersect the s^"'e clouds'

4.

The cloud dia.m.eters are

in

the raûge

t2

<

d

<

160

/r;i

kpc (2o

li'"its).

or,

if

the anti-coincidences are Mg

u,

d

> 23lr*4

kpc'

or,

if

the clouds are isotbermal spheres, d

>

50

hff

kpc'

S. There is no correlation between velocity di-fference and equivalent width, hence no evidence

for ordered mass motions.

6.

The cloud

.tough.ess'

is less than 0.04 (o/mean) on scales from 3 to 2000

ât'

pc- An1' velocity gradients must also be very small'

Some heav-.r- element absorption systems are also present, and again there is a strong correla' tion in the equiralent widths of corresponding lines from different ions in the same systæm' This indicates

that

there a.re no strong variations, either

in

density

or

ionization' over

l-2

lrro{ kpc

in

these systems.

If

the two antlcoincidences

ia

the

Lyc

forest are

MgII'

then tbe distribution

of Mg

tI

varies over scales of f 6 hro{ kec.

A more complete account of this work has been submitted for publication iu

the

Astrophys-ical Journal.

lines

in

the 2 Â

lines (solid and