Properties of the galaxies giving rise to MgII quasar absorption systems

0273—1177/91 $0.00+ .50 Copyright© 1991 COSPAR

Adv. SpaceRes. Vol. l1,No. 2,pp. (2)241—(2)244, 1991 Printed in Great Britain. All rights reserved.

PROPERTIES OF THE GALAXIES GIVING RISE TO MgII QUASAR ABSORPTION SYSTEMS J. Bergeron~and P. Boissé** *Institut d’Astrophysique

de Paris, 98 bis Boulevard Arago,

F-75014 Paris, France **Ecole Normale Supérieure, 24 rue Lhomond, F-75231 Paris Cedex 05, France

ABSTRACT We present a new imaging and spectroscopic survey aimed at identifying intervening galaxies responsible for low redahift (z 0.5) MgII absorption line systems in quasar spectra. Our observations and those already reported in the literature are used to build a sample of 17 absorption systems, which comprises 13 absorber identifications. Our results reveal no unsuspected class of absorbers and are fully compatible with the intervening galaxy hypothesis. In the quasar fields, we also have obtained spectroscopic observations of 13 other galaxies which are used as test sample. Our two samples roughly have the same [0111)s3727 rest equivalent width distribution than observed for z -~ 0.3 field galaxy surveys. Therefore, both the absorbers and the z 0.3 field galaxy population show an increased star formation activity as compared to the local field galaxy population. Comparison between the observed distribution of the absorber-quasar projected linear separations to the computed distribution of impact parameters favors a spherical geometry for the gaseous haloes. Furthermore, the similarity between the MgII halo sizes derived from absorption line system statistics and the observed sizes suggests that most, if not all, field galaxies have extended gaseous envelopes. These results imply a strong evolution from z 0 to 0.5 in halo shapes (or size) and star formation activity. INTRODUCTION The cosmological nature of sharp metal-rich absorption line systems in quasar spectra has been inferred from statistical arguments on the distribution of the absorption systems and from the clustering of CIV and MgII absorption redshifts on velocity scales as observed in individual galaxies and clusters of galaxies (Sargent et al., 1988a; Petitjean and Bergeron, 1990). This suggestion has been confirmed by the direct identification of z 0.5 absorbing galaxies (Bergeron 1988 and references therein, hereafter paper I, Bergeron and Boissé 1990). As suggested in paper I, a large fraction of the field galaxies should give rise to absorption line systems. Consequently, metal-rich systems could provide a galaxy sample representative of the whole field galaxy population. If this is confirmed for a broad range of redshifts and ionization levels, studies of metallic absorption line systems can then be used to derive properties of gaseous galactic components (such as their spatial extent, density, ionization level, chemical composition) and their evolution in redshift whatever the intrinsic luminosity of the absorber. Over the past five years, we have been undertaking an identification survey of objects responsible for MgII absorption line systems at z 0.5. The absorber candidates are searched for by deep broad-band imaging, followed by spectroscopy for the objects closest on the sky to the quasar. All the observations were made on the ESO 3.6m telescope with the faint object spectrograph and camera (EFOSC 1) at the Cassegrain focus. The redshifts of the selected MgII absorption systems range from 0.16 to 0.85 and there is a large spread in the quasar emission redshifts from 0.40 to 2.66. THE SAMPLE AND RESULTS We present results for a sample of 17 absorption systems which comprises our data and 4 cases studied by other groups. There are 13 absorbers identifications, i.e. a spatially resolved object close to the quasar image with a redshift equal to that of the MgII absorption system.The intervening absorbing galaxies typically lie (2)241

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at an angular separation in the range 5 to 12 arcsec, or a linear distance of 1.5 to 3.5 RH (the Holmberg radius RH 22 kpc with H 0 = 50 km s~ Mpc’). Ofthe four negative cases, there is one 21 cm absorption system, the lowest MgII redshift system of the sample and the remaining two are possibly uncertain MgII absorption systems. We also have obtained redshifts and magnitudes for a serendity sample of 13 field galaxies that we shall use as a test sample. We summarize below results discussed more eictensively in Bergeron and Boissé (1990).

The average redshift for the absorber sample is

< z > = 0.454 and 0.304 for the test sample. The magnitude detection limit of our broad-band r imaging is around 23. The absorbing galaxies span a moderate range of apparent magnitude, 19.1 < mr < 22.5, with a mean value mr 21.0. In average the test galaxies are brighter by 0.9 magnitude, which is a direct consequence of selecting preferentially somewhat brighter objects in addition to the absorber candidates for our long-slit spectroscopic observations. The spread in mr for the absorber sample is mainly due to the spread in redshift and not in intrinsic properties, as indicated by the narrow distribution of absolute magnitude, —22.2 < M(r) < —20.3. However, this is not the case for the test galaxies which show a large spread both in redshift and in absolute magnitude. The mean value of M(r) is —21.4 for the absorbers whereas in average the test galaxies are intrinsically fainter by 0.2 magnitude. Consequently, the apparent cut-off in the absolute magnitude distribution of the absorber sample should not result from an observational bias. To further investigate the existence of a low luminosity cut-off, we consider a rest frame parameter, the luminosity per unit frequency at the Balmer discontinuity, L,,3646. The overall spreads of the M(r) and LA3646 distributions are identical (one decade), although their shapes may be somewhat different.

Most absorbers have linear separations in the range 2 D/RH ~ 3, with an average value < D/RH > = 2.34 excluding the galaxy detected by Yanny et al. (1987) in the 0453—423 field (D/RH = 13.4), since we have found another likely candidate at much smaller angular separation. We have searched for the existence of a possible radius-luminosity scaling law for absorbing gaseous haloes using the sample of confirmed absorbers with measured D/RH and LA3646. The scatter of D values at a given L,~,3646is large as expected, since D only gives a lower bound to R and therefore, only the upper envelope of the D/RH, L,~3646relationship is meaningful. However, there may be an overall trend for larger separations at higher luminosities. The commonly adopted scaling law, /3 = 0.4, is compatible with the available data and may be written 424/LA3646 (erg cm2 (R/RH)

=

~_1)]O.4

.

(1)

3.2 [10

To investigate the dependence of D/RH on luminosity for a larger sample, we have then considered the absolute magnitude M(r) which allows to include absorbers observed by other groups. Our serendipity galaxy-quasar pairs can also be used to set severe upper limits on the sizes of gaseous envelopes. At present, the data are consistent with a unique halo radius value R -~ 4 RH or with scaling laws of index 0 < ~3 <0.4. New data on MgII absorption systems possibly associated with some of the close galaxy-quasar pairs, and on a very faint absorber candidate (if confirmed as absorber would have a magnitude M(r) = —17.4) will strongly constrain both the maximum size of gaseous envelopes and the radius-luminosity scaling law. The distribution of projected linear separations to the quasar sightline, D/RH, is directly related to the halo sizes and geometry. In order to extract this information we have considered the observed cumulative distribution which can be compared easily to the distribution expected for various assumptions. Two geometries have been considered: the commonly adopted spherical shape and also a linear geometry. Several examples of nearby galaxy-quasar pairs suggest that the latter may be appropriate for a number of cases (see Sargent et al. 1990 for S4 0248+43) because tidal interactions often generate elongated features such as tails or plumes. On the other hand, one may either consider a universal or luminosity dependent size; usually, authors have adopted the scaling law R o L~with /3 -~ 0.4. Looking at correlations between the observed separations and luminosities, we find this law to be consistent with our data although the latter do not strongly constrain the indes ~3 and still allow no dependence on luminosity at all. More details concerning the computation of the expected distributions are given in Bergeron and Boissé (1990). Two main conclusions can be drawn from this analysis: 1) the linear geometry is clearly ruled out which suggests that distant encounters, thus tidal interactions, should not contribute much to the total cross-section, 2) no strong dependence of the halo size on the luminosity is required and a satisfactory fit is obtained with a universal radius R 3.5 RH. Absorbers could also be thick discs, a possibility which has not been considered in our analysis. Probably, the distribution of separations alone could not allow to discriminate between spheres and thick discs (expected distributions being rather similar); on the other hand, deep and well sampled imaging data (some have already been acquired) will at least in some cases give the inclination of the absorbing galaxy and constrain the geometry in a more direct way. The case of the 1127-14 absorber already suggests a spherical halo shape since the intervening galaxy is seen nearly edge on.

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Our serendipity galaxy-quasar pairs of small impact parameters are also of particular ihterest to constrain the maximum size of gaseous haloes and to check whether all field galaxies do have absorbing gaseous envelopes whatever their morphological type. Unfortunately, the required data (either on the galaxy redshift or on the presence of an associated absorption system in the quasar spectrum at that redshift) is generally not available. We stress that observations of such cases are very important in that they would provide a completely independent means to check the consistency of the results derived from both the surveys of absorption systems and searches for absorbing galaxies. Most galaxies identified with MgII absorbers are fairly blue and show [Oil] emission. Only 23% of the absorbers have weak [011] emission with a rAest frame equivalent width Wr([OII] )~3727)< 10 A, whereas this percentage reaches 68% for ti e local (z -~ 0) galaxy survey of Peterson et al. (1986; see also Broadhurst et al., 1988). Considering the small number of objects in the absorber and the test galaxy samples, the weak difference in their wr([OIII A3727) distributions is not stastically significant, and we thus combine our two data sets to get a sample more representative of the field galaxy population. Our combined sample of 26 galaxies shows marked differences with the sample of Peterson et al. (1986): the fraction of galaxies with wr([OII] A3727) > 20 A rises from 15% at < z > = 0.05 to 48% at < z > = 0.38. To ascertain whether this difference is related to the nature of the absorbing galaxies or to an evolution of the properties of field galaxies, we compare our sample to galaxies at similar redshifts to avoid confusion with evolutionary effects. The two Durham faint galaxy surveys, in the magnitude ranges 20 < b, < 21.5 (Broadhurst et al., 1988) and 21 < b, < 22.5 (Colless et al., 1990) are well suited for such a comparison; they provide in total a sample of 273 galaxies at < z > = 0.31 out of which 51% have wr([OII] A3727) > 20 A. On the overall the wr([OII] A3727) distributions for our galaxy sample and that of the Durham surveys are similar. This suggests that galaxies with large gaseous absorbing envelopes have similar star formation activity than the general field galaxy population. One striking feature of the absorbing galaxies is the absence of [OIIj emission beyond the detected stellar component which implies that the density of the halo gas is much smaller than typical 3). interstellar gas densities (n > 1 cm DISCUSSION Aside from the presence of extended gaseous envelopes, the absorbing galaxies have many properties similar to those of blue field galaxies at comparable redshifts. This immediately raises the question: do all field galaxies at z 0.4 have extended gaseous envelopes? This could be answered by comparing the observed sizes of the absorbers to those estimated from the number per unit redshift of MgII systems given the space density of field galaxies at z -~ 0.4, as done in paper I, but using the most recent results on the local galaxy luminosity function and the evolutionary effects in the galaxy number-magnitude counts. For Friedmann models with zero cosmological constant, the number of absorption line systems per unit redshift is simply given by 5

=

,

(2)

1~—n0o~0(1 + z)’~(1 + 2q0z)° where n~,and a~,are the density per unit volume and the average cross-section of the absorbers at z = 0, and ~ represents a possible cosmological evolution with n(z) a(z) = n 6. To estimate this product we have used the luminosity function of local galaxies given by Efstathiou 0a,, (1+z) et a!. (1988) and de Lapparent et a!. (1989) with the M~magnitude of the adopted Schechter function M~(BT)= —21.1 (using H 0 = 50km s~ Mpc’ and q0 = 0), t. The a (gaseous)radius-(stellar)luminosity absorber and test galaxy samplesscaling constrain law,the R radius-luminosity ~ L~,and a low scaling luminosity law cut-off, giving 0 Xmjn ~ /3 = Lmtn/L 0.4. To estimate Xmin, we can directly compare our r absolute magnitudes, at < z > = 0.45, to B~.The absorber r magnitudes have a mean < M(r) > = —21.4 and span the range —20.3, —22.2. This leads to ~ = 0.5 and < L/L* > = < x > = 1.3. Taking the results of Sargent et a!. (1988b) for the MgII survey at z = 0.45 and with wr,min (MgII A2796) = 0.6 A, and assuming that all field galaxies brighter than Lmin have spherical gaseous haloes, we get for the halo radius of L~galaxies

R~=4.2 RH.

(3)

Although well above the observed separations this value is a lower limit since we have assumed gaseous spheres around all field galaxies, considered MgII samples with wr,mj,s(MgII A2796) = 0.6 A, and adopted the largest value of the volumic density ~ given for local galaxy samples. Can the excess of low-luminosity galaxies detected in the Durham surveys reduce or suppress the gap between the observed absorber size and R0? Assigning them a typical linear separation < D/RH > = 3.0 and introducing the excess of low-luminosity galaxies of a factor of two found in the Durham survey at z 0.3 (Colless et a!., 1990) we can marginally reconcile the values of D derived from our data and the estimate of R* inferred from the number density of MgII systems. Furthermore, we note that the faint field galaxies of the Durham survey and the absorbers have similar red and also blue absolute magnitudes.

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5 leads just about to From the above analysis, we conclude that using only assumptions which minimize R similar observed and predicted values for gaseous halo sizes, with R~ 3.0 RH. This strong!y suggests that most of the field galaxies, if not all, do have extended haloes of roughly spherical shape and that the spatial coverage factor of the MgII clouds within these clumpy haloes (Petitjean and Bergeron, 1990) is close to unity. Furthermore, the absorbing galaxies show a high level of star formation. Consequently, a strong recent evo!ution is required with a decrease in time of the ste!!ar formation activity and of the gaseous halo sizes (overall decrease or flattening). REFERENCES Bergeron, J.: 1988, IAU Symposium 130, eds J. Audouze, M.C. Pel!etan & A. Szalay. Kluwer Academic Publishers, p. 343 Bergeron, J., Boissé, P.: 1990, Asiron. Astrophys. , in press Broadhurst, T.J., Ellis, R.S., Shanks, T.: 1988, Monthly Notices Roy. Astron. Soc. 235, 827 Colless, M., Ellis, R.S., Taylor, K., Hook, R.N.: 1990, Monthly Notices Roy. Astron. Soc. 244, 408 de Lapparent, V., Geller, M.J., Huchra, J.P.: 1989, Astrophys. J. 343, 1 Efstathiou, G., Ellis, R.S., Peterson, B.A.: 1988, Monthly Notices Roy. Astron. Soc. 232, 431 Peterson, B.A., Ellis, R.S., Efstathiou, G., Shanks, T., Bean, A.J., Fong, R., Zen-Long, Z.: 1986, Monthly Notices Roy. Astron. Soc. 221, 233 Petitjean, P., Bergeron, J.: 1990, Astron. Astrophys. 231, 309 Sargent, W.L.W., Boksenberg, A., Steidel, C.C.: 1988a, Astrophys. J. Suppl. 68, 539 Sargent, W.L.W., Steidel, C.C., Boksenberg, A.: 1988b, Astrophys. J. 334, 22 Sargent, W.L.W., Steidel, C.C.: 1990, Astrophys. J. Letters 359, L37