85
OBSERVABLE MANIFESTATIONS OF ACTIVITY IN NORMAL SPIRAL GALAXIES
Giuseppe Gavazzi
Osservatorio di Brera, Milano, Italy
ABSTRACT: multifrequency observations of 874 spiral galaxies are analyzed with the aim of isolating few relevant parameters which govern their disk activity. It is shown that quite accurate predictions can be made if estimates of their old stellar component, of their present starfo rmation rate (SFR), and informations on their environment are available. 86 G. GAVAZZJ.
INTRODUCTION
High spatial resolution observations of several nearby galaxies became recently RV«il�bl"° at various frequencies. The most obvious example, yet anomalous in many respects, is M31 (see radio continuum map in Walterbos, 1986; 21 cm line map in Unwin, 1980; FIR maps in Walterbos and Schwering, 1987; CO map in Dame et al, 1991; Ha distribution in Pellet et al, 1978 and X-ray study in Trinchieri and Fabbiano, 1991), but other galaxies have been analyzed at several frequencies (see for example FIR-radio comparisons in Bicay and Helou, 1990 and in Fitt, et al, 1992). Any attempt to produce a realistic and sufficiently sophisticated model of galaxy evolution will have to face with this fast growing observational material spanning 10 orders of magnitude in fr equency. This is an ambitious and difficult task due to the complex relationships and feedback mechanisms which regulate the various manifestations of activity in galaxies. In fact dramatic galaxy-to-galaxy differences are observed, and even within galaxies, unexpected local peculiarities are found, which remain unexplained by most models. Actually, our understanding of galaxies is still in the pre-Darwinian phase, where a clear definition of species has still to be identified (see the controversy on the meaning of galaxy morphological classification which has been emphasized at this meeting). Obviously it is not the purpose of this paper to produce any detailed and universal explanation (we refer the reader to the work of Kennicutt, 1990 and of Mas-Hesse and Kunth, 1991 ). We will limit our investigation to a much simpler, although realistic approach. We will attempt to show that the available integrated over the galazy multifrequency data can constrain models of galaxy evolution. This "low resolution" approach is certainly simplistic, since it does not account for the infinite variety of small, yet important details mentioned above, but will unable us to use larger data samples. When we say "model of evolution" we mean a model which describes reasonably well the observed behavior of galaxies in the local Universe (z < 0.1), without any pretention to explore the early cosmological phases which gave birth to galaxies nor the dependence of galaxy properties on redshift.
In the remainder we will focus only on normal spiral galaxies. Consequently we will not account for any of the nuclear phenomena which originate in active galactic nuclei and in radio galaxies, considering only the low-level type of activity which takes place in quiescent disks and arms of galaxies. A schematic framework on which the following analysis is based is illustrated in Fig. 1, where, ordered as a function of the fr equency window used in the observations (vertical axis) some observable (circled) are connected by lines which represent causality relations and fe edback mechanisms. Neutral atomic hydrogen (21 cm line) is assumedto be the primordial reservoir of matter which "fuels"the star formationprocesses in galaxies via the collapse of molecularH2 clouds (see reviews on H2 in Young, 1990 and in Kenney, 1990). Stellar evolution provides the observed variety of stars (in the proper melange of types and with the proper life-times). The most abundant, slowly evolving, "boring", red stars are assumed to account for most of the mass in galaxies (therefore regulating the mass-dependent processes such as rotational properties etc. Dark matter might in fact do most of this job). They contribute to the nearIR emission. More interesting to us is the minority of young, massive, rapidly evolving stars (mostly observed in the UV spectrum). Their UV photons ionize the surrounding gas, making HIT regions visible (for example via emission of Balmer lines), heat the dust, producing strong far infrared flux OBSERVABLE MANIFES-TATIONS OF ACTIVITYIN GALAXIES 87
RADIO cm
RADIO mm
star FIR form
NIR cosmic RED rays
VISIBLE STARS
BLUE
UV
Fig. 1: Schematic model of the feedback processes which take place in galaxy disks and their observability. and contribute, with frequent Supernova explosions to inject relativistic particles in preexisting magnetic fields, producing synchrotron emission ( centimetric radio emission). The role of su pernova might be relevant also in the process of cloud collapse and the magnetic field structure should have some importantrole in determining the shape and size of the molecular clouds (thus regulating the Fffi temperature of the clouds themselves). The scheme, although quite naive, is already enough complicated forpretending to constraint it observationally. In the remainder we will focus on the simple question: is it possible to isolate few parameters which will enable us to make reliable predictions on the activity in disk galaxies?
1: THE SAMPLE
The sampleused in the following analysis is based on all CGCG (Zwicky et al, 1963-1968) galaxies (mp � 15. 7) in 9 nearby clusters of galaxies: A262 (Perseus supercluster), Cancer region, A1367, A1656 (Coma Supercluster), Virgo cluster (limited to mp � 14.5), A2147, A2151, A2197, A2199 (Hercules supercluster). In addition to the cluster galaxies, the sample includes all CGCG galaxies in the Coma wall between 11h3om 900 800 700 600 ffi 500 400 � 300 200 100 0 VEL m RADIOC. F.LR. Nl.R. B CCD HA BANDS Fig. 2: State of completion of the survey of 874 spiral galaxies (march 92). The database includes detections as well as upper limits (typically in the radio continuum and line and in the FIR). In addition to the photometric information, general properties have been determined or collected fromthe literature such as: UGC (Nilson, 1973), NGC and IC names (from RC3, de Vaucouleurs et al, 1991), accurate coordinates (with 2 arcsec accuracy), mor phological types (generally from the UGC or redetermined if large scale plate material was available), recessional velocities and other miscellaneous information. We tried to keep track of the references on each of these measurements. Individual aperture measurements are trans formedinto totalor isophotalmag nitudes and corrections are applied according to well defined criteria (RC3). Magnitudes have typical uncertainties of 0.1 mag. Distance dependent quanti ties are computed assuming 100 Km The transformed quantities constitute a Ho = .s-1Mpc- 1• database of 1758 entries, each with 70 fields. Since here we are interested only in disk galaxies we will fo cus on galaxies of morphological OBSERVABLE MANIFESTATIONS OF ACTIVITY INGALAXIES 89 type later than Sa (874 objects). Fig. 2 gives the present (march 1992) status of completion of the survey in some relevant bands. Except for the radio continuum and Fm (where magnitude limited samples were observed) the survey does not yet match any completeness criterion. NevertheleBB from Fig. 2 it appears that we are dealing with one of the largest samples of galaxies with homogeneous and accurate (integrated) photometrical measurements at many frequencies, which be used for statisticalpu rposes. The database has never been published can as such, since it is continuously evolving, but it can be made available in computer readable formupon request to the author and will be maintained and updated. 2: THE FIRST ORDER DEPENDENCE: GALAXY LUMINOSITY It has been stressed by many authors (e.g. Kennicutt, 1990, Gavazzi, 1991) that galaxies obey to a scaling law such that "bigger galaxies emit more of everything". Examples of such a law are given in Fig. 3 (a,b,c) where the luminosity in the Radio Continuum, 21 cm line (in solar masses) and FIR luminosities are plotted against absolute (photographic) magnitude. The relations shown are little more than trends, given the tremendous scatter of the data. Nevertheless these trends (indicating anal most direct proportionality) have important implica tions. The major implication is a negative one: one should never accomplish analyses based on luminosity-luminosity plots since these relations are dominated by the scaling law which hides other more interesting dependencies. 11 9 "' Q) a) (/) b) (/) 10 �c 8 a :J E -e L 9 00 7 a ._::, 0 0 0 "' 0 Ct: ..:'2- 6 8 � 0%0° i c;:: 0 �o � Q) o 0 I _J 06'�0 2 7 0 O> 5 oO 8 0 _J O> 0 4 _J 6 -22 -20 -18 -16 -14 -22 -20 -18 - 1 6 -14 M pg Mpg 6 � c) c 5 00 :J -e 0 4 ·" CJ .3 2 _J 2 O> 0 _J 1 -22 -20 -18 -16 -14 Mpg Fig. 3: Three examples of the nearly direct proportionality between luminosities in various bands and absolute magnitude. a) Fm, b) Neutral Hydrogen mass, c) radio continuum. 90 G. GAVAZ:ZJ. On the contrary, given the near proportionality observed, it is natural to produce normal ized quantities, such as radio (or FIR) "excess" over the light, obtained by dividing the radio (FIR) by the emitted in some t These flux flux op i.,al n:r _f:rhi !ll"ed band. a.dimensional qu=titias have the additional advantage of being distance independent. Gavazzi, Bosell and Kennicutt, .. (1991) illustrate that IR H band must be preferred to optical V band measurements, to perform a proper normalization, but either do a good job. Fig. 4 fu rther illustrates this point. 1.2 1.0 0.8 I I 0.6 ---, 0.4 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 B-V)o Fig. 4: The relation between the color index B-V and the infrared J-H for E+SO+S. Here J-H color indices are plotted versus B-V forour total sample ofE+SO+S. It is apparent that the interval in J-H is about half the interval in B-V. This indicates that galaxies have very similar IR properties, in spite of their variety of morphological types and of their star formation histories. This implies that galaxies of different types are dominated by the same type of cold stars (K giants), therefore measurements sensitive to low temperatures (typically few thousands degrees) are better probes of their total stellar content . Accordingly, we use Radio/H and FIR/H flux ratios in the following. The result of this section can be equivalently rephrased as: the overall galaxy luminosity LH is the first order parameter regulating many forms of activity (L..,)in galaxy disks: L.., L'H with 1 (1) The problem now is that of determining the second order parameter which "modulates" a. OBSERVABLE MANIFESTATIONS OF ACTIVITYIN GALAXIES 91 3: THE SECOND ORDER DEPENDENCE: THE PRESENT STAR FORMA TION RATE The second point we would like to discuss is what our integrated data tell us on the dependence of the radio continuum and FIR emission on the starcontent of disk galaxies. Let us takea look to typical "overall" spectra of two galaxies in our sample with extremely different nuclear properties (Fig. 5). radio FIR NIR Opt X-ray 2 • • 0 • 0 0 0 � .. , I ce 0 LL � -2 0 Q") 0 _J -4 -6 • u 6697 o 3C 264 • -8 6 8 10 12 14 16 18 20 Log Frequency (Hz) Fig. 5: Overall spectra (from radio to X-ray) of the irregular galaxy U6697 and of the radio galaxy 3C264. The data represent fluxes normalized to the H band flux of one Irregular (UGC 6697) and one radio galaxy (3C 264 NGC 3862). The difference in their optical colors is evident in V, B, = U compared to H. The FIR emissivity of UGC 6697 is much higher than that of the Elliptical. The 5 points in the radio of both galaxies are nicely fitted by a power law (of slope "-'-0.8): a firm signature of non-thermal synchrotron processes (notice that in the radio galaxy the same power law fits the X-ray measurement). However in the case of 3C 264 the emission originates in the nucleus, in UGC 6697 it is a genuine disk emission (see Gavazzi et al. at this meeting). Nothing is known on the radio continuum in the millimeter and submillimeterbands. The data in the FIR can be fittedby thermal laws at two temperatures (one around 25 K and one slightly hotter). The NIR measurements (J, H, K) and the visible data (V, B, U) can be fitted by thermal distributions around 3000 K and 6000 K respectively. They represent the cold and the relatively hotter stellar components. If one puts together many such spectra, grouped forexample in classes of constant B-V color index, one obtains diagrams like those illustrated in Fig. 6. The scatter is tremendous in each band: as we already noticed galaxies have large "individualities". However one can discern a tendency for decreasing FIR and radio continuum emission with increasing color index (going from blue to redder galaxies). This becomes much more convincing when averages are taken within color classes, as plotted in Fig. 7. Although qualitatively, the above argument illustrates that indeed the present color of the stellar population governs the emissivity in the FIR and 92 G. GAVAZZJ. 8 0 l l 8 0 CJ' 0 -1 -1 _J 11 -2 -2 lj -.3 '------'-----'------'------' -.3 '------'-----'-----'----' 8 10 12 14 16 8 10 12 14 16 Log Frequency (Hz) .3 c) I 2 - 8e � - :r: L:;:' a) b-v < 0.5 - 0 "'-- 0 i'8 i � o LL. 0 0 0 0 8 b) 0.5 < b-v < 0.7 CJ' 0. - 0 -1 ri 0 _J 0 � c) 0.7 < b-v < 0.9 -2 �I• 0 � -.3 8 10 12 14 16 Log Frequency (Hz) Fig. 6: Overall spectra of spiral galaxies grouped in classes of B-V color index. 2.0 1.5 . 1.0 / 0 .. . . ' ' � 0.5 :r: .o •. L:;:' • "'-- 0.0 LL. /\ CJ' 0 -0 5 _J .. . ·:,• -10 &.. ci \"' 0 -1.5 0 • -2.0 8 9 10 11 12 1.3 14 15 16 Log frequency (Hz) Fig. 7: Average overall spectra of galaxies of increasing B-V color: B-V <0.5 (open squares); 0.5 radio continuum bands. Further evidence is obtained by plotting the FIR/V and R.adio/V flux ratios as a function of U-B)o (Fig. 8 a and b respectively). 5 4 3 � 0 eO 0 0 > "--- 0:: 2 c;: 0 0 -1 -1.0 -0.5 0.0 0.5 1.0 1.5 U-B)o • E+SO+SOa 0 s 4 • �.3c 264 - ·�N6047 MB? �. e.:2 Jc JJB • • • • . I 0 f- 0 • 0 -1 I -1.0 -0.5 0.0 0.5 1.0 1.5 U-B)o Fig. 8: Relation between FIR/V (a) or R.adio/V (b) and U-B separately for Spirals andE+SOs. Some luminous radio galaxies are indicated. Jn these diagramswe have included allmorphologi calcl asses, fromE to Spirals to emphasize that in the radio continuum there is a difference between galaxies dominated by a diffuse disk (spirals) andgalaxies with strong nuclear sources (radio galaxies). While there is a clear trend 94 G. GAVAZZI with U-B)o for the former, E+SOs do not follow any trend, showing a four order of magnitude scatter (along with a small range in color) which must be attributed to their unrelated nuclear activity. The same is not true in the FIR/V vs. U-B)o diagram ; whJ.-h shows more continuity between the two morphological classes. In this plot the few points higher than normal are associated with interacting Spiral+Spiral systems. Color indices however are not the best available indicators of the star formation rate in galaxies, or at least not of the present star formation rate. This parameter is better quantified by the emissivity in the Ha line. o B-V • U-B 1.4 1.2 1.0 8 0.8 x 0 Q) 0.6 • -0 • c 0.4 0 • \._ 0.2 0 0.0 0 u -0.2 • -0.4 • -0.6 -0.8 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 Log H alpha E.W. Fig. 9: Relations between Ha E.W. and B-V or U-B color indices. Fig. 9 shows that color indices and Ha equivalent width (E.W.) are indeed non- linearly correlated (see Kennicutt and Kent , 1983). The strongest indication that both radio continuum and FIR emission are proportional to the present star formation rate is derived from Fig. 10, where the Radio/H andFIR/H flux ratios are plotted against Ha E.W. The radio correlates with Ha with a slope of one, while a slightly flatter slope is derived for FIR. This indicates that synchrotron emission can be taken as a direct indicator of the current star formation rate, or equivalently that sources of cosmic-ray electrons are identified with supernovae. This evidence was first discoveredby Lequeux, (1971) and further investigated by Klein, (1982); Condon et al, (1982); Kennicutt, (1983), Gavazzi and Jaffe, (1986) and Gavazzi, Boselli and Kennicutt, (1991 ). FIRemission, on the contrary, comes from two components: one associated with massive star formation, another with the general radiation field (see also Helou, 1986; Deveroux and Eales, 1989; Mas-Hesse, 1992 and Thuan at this meeting, who has shown clearly that the contribution from the general radiation field increases in galaxies of earlier type). In conclusion we have established that in second approximation in equation 1 is linearly related to the SFR for a radio continuum luminosity and with a somewhat milder slope for FIR luminosity. In other words, quite accurate predictions can be made on radio continuum and FIRlumino sities of disk galaxies given their H band luminosity (old stellar population) and estimates of the present SFR. OBSERVABLE MANIFESTATIONS OF ACTIVITYIN GALAXIES 95 • radio/H 0 FIR/H 4 3 2 0 0 0 0 :r: O@g 0 'G:' § o § o &I • � 0 0 8 0 ti- LL � . • >·. • Ql -1 • 0 0 I i •• ,:IJJl- li: _J � • . .. . , -2 • 1e• ...... • • . . -3 • -4 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 Log H alpha E.W. Fig. 10: The dependence of Radio/H and FIR/H on Ha E.W. 4: THE THIRD ORDER DEPENDENCE: THE INFLUENCE OF THE ENVI RONMENT It is not the scope of the present work to discuss the dependence of galaxy properties on the environment, which has been covered by Balkowski at this meeting. However we would like to point out briefly some evidence which has relevant implications on the subject we are discussing. We ask ourself the question: what fractionof galaxies surveyed in the radio continuum and FIR has a certain Radio/optical (FIR/optical) ratio and is this fr action similar in isolated and cluster galaxies? The question can be answered by constructing fractionallumino sity functions, a statistical tool which allows us to take into account detections as well as upper limits (see H el, 1981). umm The FIR/optical and Radio/optical luminosity functions are shown in Fig. 11 and 12 respectively (in this case we are forcedto use the optical photographic magnitude to performthe normalization, since only this measurement is available for all surveyed galaxies). The 9 panel figures allow us to compare the luminosity functions of each cluster (and for the 9 clusters put together) with that of the reference sample of isolated objects in the Coma Supercluster region (solid line). It is immediately apparent that the FIRlu minosity fu nctions arenot dissimilar one another, indicating that the FIR/optical ratio is strongly gaussian (in other words that FIR is almost proportional to optical, as we have already seen in Section 2). The only difference among cluster and isolated objects is that some clusters (typically: Coma, A2147 and Cancer) have a fr action of galaxies at high FIR/optical ratio (Log FIR/mp = 4.2) marginallylower than the corresponding isolated galaxies. In other words the high FIR/optical ratios are missing in 96 G. GAVAZZI I I l� t' �j�t �j�t ·1t ' :ii"' .N CJ> CJ> t'S � s : .... + N.. N.. r- "'r- � M :; � ,;.. ..,; N.. "' "' "'N "' "'N "' "' "'N N :'.] N I ,. ..; I " .. .. '"';< • E "S.,)"' N l'i: "' " : .... N "' ,; !;: ;;; 0 .. N u .. s .;.. s N.. "' "' "' "' "' N "' N "' "' "'N ,· N N ..; ..; ,· ..; I : .. ..' 1 .,J t"SS"' N l'i � : : "' : ... "" "'t>0 "' u ;:; N ;:; > � N.. � "' "'N "' "' N N "' "'N N � � ..; I I I (:,:) (ll).:1 !O'J (:,:) (l!M 101 (S) (ll).:I !OOJ Fig. 11: Fractional, differential distribution of FIR/mp in 9 clusters (filled squares) and in the reference sample of isolated galaxies (solid line). >rj qq· enel ...... !':' 2 I 2 2 �< Ul 1.5 1.6 1.5 � E f;; ...pi� I I I � = :;:! ;;:- - g -.s .5 -.s z � .a e: 0 .4 .8 1.2 I.I 0 .4 0 .4 z.a en 0 0 - 2 2 2 'Tl � > ,, 1.5 1.5 1.5 E I I = I I I -< ;;:- • �z .3 .5 I .5 .5 A282 0 .A2147 I 0 ! ! -.s ' I I I I I I I I I I • I I I I 01 -.5 .. .a -.5 .. .a 0 I I 1.2I u I 2 I u I z.a 0 .4 .8 1.2 I.I 2 I 2.4 I z.aI 0 1.2 I.I 2 u z.a � I I �iI I I I 1 l I �I I I I � 2 1.5 r 1.5 I t 1.5 E = l' 1 l ;;:- • .3 .5 5 jl - .A2 : I I I I I . I I I I I - I I -.s . : l,�.1�� 0 . a a.a 0 . . 1�,. Lo11.2 R (radlo/m.)I.I ,\2 2.4 J l . .8 Lo11.2 R (radlo/m.)I.I .12 2.4 a.aJ 0 . .a Lo&1.2 R (radlo/m.}LI 2 2.4 a.a co""' 98 G. GAVA'lll the cluster sample. This is not surprising, since it is expected that strong star formation rate is inhibited in clusters of galaxies (gas deficient objects are prevented from producing strong star formation). Alternatively we could argue that the ilu•t content of duster spirals might be reduced. Quite unexpectedly, the opposite is found from the radio luminosity function, where an excess of galaxies with strong Radio/optical ratio is found in some clusters with respect to the field (namely in Coma, A262, A2147 and A1367). This implies that either in these clusters the star formationrate is enhanced, or that some other parameter, namely the magnetic field strength, is enhanced in cluster galaxies. This can arise as a consequence of ram-pressure exerted on the fast moving galaxies by the intergalactic medium that permeates clusters of galaxies. IfFIR is taken as a direct measure of SFR we should conclude that the average SFR is quenched in clusters (see Fig. 11), therefore that the radio enhancement found in clusters is purely a consequence of magnetic field compression. However we have shown in Section 3 that this assumption is valid only in first approximation, therefore enhanced SFR in clusters cannot be ruled out on this basis. In fact evidence for enhanced star formation rate has been found in the Coma cluster by Bothun and Dressler, (1986), and similar evidences are found in three galaxies in A1367 (Gavazzi et al., this meeting). Therefore we are inclined to conclude that a combination of enhanced star formation rate and magnetic field amplification could produce the observed overabundance of strong Radio/optical ratios in cluster galaxies. 5: CONCLUSIONS We have analyzed how the SFR and the galaxy environment play an important role in determining the relationship between the luminosity of the old stellar component and those in the FIR and radio continuum in normal disk galaxies. Equation (1) can be once more rewritten as: L er(SFR,p,.i) radio,FIR ex:L H Where the dependence of on SFR is almost linear for the radio and slightly milder for a FIR and the dependence on the local galaxy density is positive for the radio and marginally negative for the FIR. We have also seen that the dependences on SFR and on are not Pgal totally independent. However we can say firmly that similar dependence on SFR is present in both the cluster and the reference samples. Once the database will be completed we will be able to quantify these relatioships on observable quantities: H band magnitudes, Her E.W., For the time being, we prefer to Pgal· conclude qualitatively that predictions on FIR and radio properties of disk galaxies can be made with reasonable accuracy once three fundamental parameters are known: an estimate of their old star content (H mag), of their present star formation rate ( Her or U-B) and of their environmental properties (membership to clusters). ACKNOWLEDGMENTS: I wish to thank Alessandro Boselli and Marco Scodeggio for their invaluable contribution to this work; Bianca Garilli for her precious help in reducing the ob servations and Tommaso Maccacaro for his criticism on the manuscript. A special thank is due to the telescope operators of the Arecibo, Loiano, KPNO, San Pedro Martir and Tirgo 0 bservatories. 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