N87- 24355GRnUND-BASED FOLLOW tiP OF IRAS GALAXIES

M. DENNEFELD* - H. KAROJI

Institut d'Astrophysique, Paris

P. BOUCHET

ESO, Chile

L. BOTTINELLI - L. GOUGUENHEIM

Observatoire de Meudon, France

We have undertaken optical, near-infrared, radio-continuum and HI obser-

vations of the galaxies identified with IRAS sources in a few fields roughly of

the size of a sky survey plate. We present here results from two fields at

galactic latitude +27 ° and +43 ° over a total area of 100 square degrees (see also

Dennefeld et al. 1986).These regions contained 115 IRAS point sources, out of

which 26 were identified with stars and 81 with faint galaxies, 10 of which were

difficult to recognize on the Schmidt plates. A further 8 sources could not be

identified with any object down to the limit of the Palomar or ESO Sky Survey

Plates. As judged from the Cirrus Flags, at most 3 could be spurious sources. The

surface density of galaxies lies between 0.6 and 0.9 galaxies per square degree,

in accordance with other determinations (Helou, 1986). Our value is however of

little statistical significance, especially because the field at b : +27 ° seems to

contain a qroup of faint Qalaxies.

Spectroscopy was obtained with the ESO telescopes at a resolution of about

IOA. The vast majority of qalaxies have low excitation spectra dominated by low-

ionization lines. These spectra are typical of HII-reqion type galaxies, however

of much lower excitation (typically [OIII]/H_ _ I) than other starburst qalaxies

such as those described by Balzano 1983). Similar results have been found inde-

pendently by Allen, Roche and Norris (1985) and Elston, Cornell and Lebofsky

(1985). We stress here the importance of the reddeninQ as determined from the

H_/H_ ratio: the visual absorption A v ranges from 2 to 6 magnitudes and as a

consequence the corrected LIp/L R ratios are considerably reduced if those

reddeningsapply to the whole -gala_xy. Indeed 3-H, H-K colours can be reconcilied

with those of "normal" galaxies when such reddening corrections are applied. In

some cases, a substantial K-L excess remains, indicating a dust contribution even

at short wavelengths. The strong Na absorption line seen in some spectra is also

partly attributed to dust, in absence of any other late-type stellar feature.

Velocities up to 60000 km/s, resulting in large, infrared luminosities, have

been found. Several objects have LIR of a few 10 _" solar luminosities, simi-

lar to the ones of Arp 220 or NGC 62_0, which are thus not exceptional anymore.

If our numbers are representative, more than a thousand of such objects should

exist all over the sky. Star formation activity is believed to be the source for

this enormous IR emission. Evidence for this comes from the correlation between

IR and Ha luminosities (Dennefeld et al. 1986) or from the stronQ Balmer absorp-

tion lines seen in the spectra. Radio-continuum observations of these extreme

objects (Karoji et al. 1985) show that the star-formation activity is located in

* Visitinq Astronomer, European Southern Observatory, Chile.

Carol J. Lonsdale Persson (Editor)

Star Formation in Galaxies 605

M. DENNEFELD ET AL.

30

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F i g u r e l a - A CCD p i c t u r e i n r I G u n n l o f A 09111-1007 o b t a i n e d a t ESO. The e a s t e r n g a l a x y w i t h two companions i s - n o t t h e IRAS s o u r c e .

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I I I I I I I I I 1 0911 14 13 12 11 10 0 9 08 07

R I G H T ASCENSION

F i g u r e I b - A V L A map o f A 09111-1007 a t 20 cm. Peak b r i g h t n e s s i s 22 mJy/beam. T h i s f i g u r e i s t a k e n from K a r o j i e t a l . I 1 9 8 6 l .

GROUND-BASED FOLLOW UP OF IRAS GALAXIES

^0.gl 11-1007 EAST

I ' I '

>-

ml I i I , I ,5000 8000 7000

AOg111-1007 WEST

mml , ' i ' l ' , ' i I

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1_ I , I5000 5500 5000 5500 7000

WAVELENGTH

Figure Ic - Spectra obtained at ESO for the two galaxies seen in fig. la.

The high-excitation eastern galaxy [Sey2] is not an IRAS source.

The western one has a typical spectrum of IRAS galaxies with low

excitation, high reddening and interstellar absorption lines

607

M. DENNEFELD ET AL.

I I I I I I I I , i i I

v

50 --

7500

i I I i i' "I I I I

8000

radial velocity (km s-I)

"HI LINE PROFILE WITH A VEL_ITY RESOLUTION OF 2.6 KM S-1. RADIAL VELOCITIES

ARE GIVEN IN q_RMS OF HELIOCENTRIC OPTICAL REDSHIFT C Ai/_ O.

bi0

-i0

I I I I I

- i , , , , I4500 5000

I I

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I I5500

radial velocity (kin s -1)

HI LINE PROFILE WITH A VELOCIq_f RESOLUTION OF 10.6 KM S-1. RADIAL VELOCITIES

ARE GIVEN IN %_ERMS OF HELIOCENTRIC OPTICAL REDSHIFT C AA/_o"

NO_ THE ABSORPTION FEAqI/RE AT ABOUT 5140 KM S-I

Figure 2 - HI profiles of IRAS galaxies obtained at. Nangay

Upper image is A12488-2051, lower one is A09234-1146

608

GROUND-BASED FOLLOW UP OF IRAS GALAXIES

the central reqions (unresoived at 6 kpc diameter for our far-outlyinq qalaxies)

and that the radio/IR flux density ratio is typical of star-burst activity rather

than Seyfert 1 Indeed, very few Seyfert galaxies or Liners have been found in

our survey (see fig.1 For an illustration). It has been suqqested that interac-

tions between galaxies are more frequent within IRAS qalaxies than elsewhere(Lonsdale et al. 1984) and that this could represent the triqqerinq mechanism for

star formation. About 25_ of our qa!axies have neiohho,_rs within 2' but we still

need more analysis (velocities and imaqinq) to distJnquish interactions from sim-

pie clusterinq. At least the fuel for star formation is available: about ha_f t_objects have been detected in HI and have hydroqen masses in the ranqe ln--1 O-

Mo. Larqe central column densities are sometimes present (see fiq.2) as expec-ted for these hiqhly reddened objects. Rut it should he stressed that hioh star

formation rates are required (larqer than 1nO Mo per year) to explain the largeIR luminosities unless truncated mass functions are assumed (see the accomoanyinq

paper by Belfort, Mochkovitch and Dennefeld, this volume).

It seems therefore that the overall characteristics of faint IRAS galaxies

are now well established from the spectral point of view. Enough so to distin-

quish from spectra alone an IRAS candidate from another (see fiq.1) One particu-

larity is the hiqh reddeninq which no doubt explains why these fairly numerous

objects with strong Ha emission line were not discovered in the objective prism

surveys mostly conducted in the blue w

REFERENCES

Allen, D.A., Roche, P.F. and Norris, R.P. 1985, M.N.R.A.S. 21_.__3,67P.

8alzano, V.A. 1983, Ap.3 268, 602.Belfort, P., Mochkovitch,-_. and Dennefeld, M. 1986, this volume.

Dennefeld, M., Karoji, H. and Belfort, P. 1986 in "Star forminq dwarf qalaxies",

D. Kunth, T.X. Thuan and 3. Iran fhanh Van, editors, Editions Fronti6res,

France, p.351.

Etston, R., Corneil, M.E. and Lebofsky, M.3. 1985, Ap.3. 296, 106.

Hetou, G. 1986 in "Light on dark matter", F. Isra6i editor, 405.

Karoji, H., Dennefeld, N. and Ukita, N. 1986, Astron. Astrophys. 155, LS.

Lonsdale, C.3., Persson, S.E. and Matthews, K. 1984, AD.3. 28___7_7, 95.

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