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Optical properties of low z radio galaxies
New Astronomy Reviews 46 (2002) 353–356 www.elsevier.com / locate / newar
Optical properties of low z radio galaxies Riccardo Scarpa a , *, Federica Govoni b , Renato Falomo b , Giovanni Fasano b a
Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA b Padua Observatory, Vicolo dell’ Osservatorio 5, I-30122, Padova, Italy
Abstract The optical morphological and photometric properties of 79 low redshift radio galaxies are discussed. It is found that most radio galaxies are luminous bulge dominated systems similar to normal non-radio giant ellipticals. The average absolute magnitude of the sample is kMHOST (tot)l5 223.98, with a clear trend for FR I sources to be | 0.5 mag brighter than FR II galaxies. In about 40% of the objects we find an excess of light in the nucleus attributable to the presence of a nuclear point source. This contributes on average for |1–2% of the total flux from the host galaxy. Radio galaxies follow the same me 2 Re relationship of normal (non-active) elliptical galaxies. The distribution of ellipticity, the amount of twisting and shape of isophotes do not differ significantly from other ellipticals. These results support a scenario where radio emission is little related with the overall properties of the host galaxy. 2002 Published by Elsevier Science B.V. Keywords: Galaxies: Active; Galaxies: Elliptical; Radio continuum: Galaxies
1. Introduction Comparing radio-loud and radio-quiet ellipticals and galaxies hosting different kinds of radio sources (FR I–FR II), controversial results have been found. Here, optical R band observations for a sample of 79 radio galaxies in the redshift range 0.01 , z , 0.12 are analyzed. Selected sources are from 2 complete flux limited radio survey, the 2.7-GHz 2-Jy all-sky survey by Wall and Peacock (1985), and the Ekers et al. (1989) catalogue of southern radio galaxies with flux at 2.7 GHz.0.25 Jy. Observations were obtained at the ESO 2.2 m and 1.5 m, and at the NOT 2.5 m telescopes. We modeled the surface brightness profiles derived from 2-d isophotal fitting using a r 1 / 4 law plus, when required, a nuclear unresolved component and / or an exponential law to account for extra emission *Corresponding author. E-mail address: [email protected] (R. Scarpa).
in the outer regions (Fig. 1). This enable us to identify the presence of extra components and to quantify the deviations from the r 1 / 4 law. Total luminosity, morphology, and structural parameters were also derived.
2. Results • Radio galaxies are hosted by bulge dominated systems with average total absolute magnitude (extinction and k-corrected, and after removing the nuclear component) kMHOST l 5 2 23.9860.70. • Dividing radio galaxies according to their radio morphology we found that galaxies hosting FR I sources are systematically brighter than those hosting FR II sources (Fig. 2). In particular we found kMHOST l FRI 5 2 24.1060.71 and kMHOST l FRII 5 2 23.6260.73. • Accordingly with the well-known relationship
1387-6473 / 02 / $ – see front matter 2002 Published by Elsevier Science B.V. PII: S1387-6473( 01 )00207-X
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R. Scarpa et al. / New Astronomy Reviews 46 (2002) 353 – 356
Fig. 2. Top panel shows absolute R magnitude distribution in our sample. Bottom panel shows dividing radio galaxies according to their radio morphology we find that galaxies hosting FRI sources are systematically brighter than those hosting FRII sources. This difference is illustrated by comparing the two cumulative distributions.
•
Fig. 1. The peculiar radio galaxy 04301052 (3C120). Top panel shows isophotal contours. In this source we observed both a nuclear point source and an external disk. Spacing between isophotes is 0.5 mag. North is up and east to the left (1 pixel5 0.4030). Bottom panel shows radial profile which exemplifies the deviation from the de Vaucouleurs law often observed in our sample. The profile is fitted with three components: a de Vaucouleurs law (short dashed line), an exponential component (long dashed line), and a nuclear point source (dotted line). The solid line is the sum of the three components.
between effective radius and luminosity of elliptical galaxies, we also found FR I hosts are larger than FR II hosts: kr e l(FR I)518.3610.3 and kr e l(FR II)512.969.3 kpc. • Radio galaxies follow the same me 2 r e relation of
•
•
•
other non-radio ellipticals (Fig. 3). For our 79 radio galaxies we measure me 5 18.4(60.4) 1 2.6(60.3) 3 log r e . A substantial fraction ( | 40%) of objects show the presence of a nuclear point source with luminosity about few percent of the whole galaxy. The nuclear luminosity appears to be weakly correlated with the core radio power but unrelated to host luminosity (Figs. 4 and 5). The luminosity profile of several objects exhibit deviations from a r 1 / 4 law, in the form of light excess in the outer region (r . 10 Kpc). These light excess can be either disc components or extended halos. Radio galaxies have ellipticity indistinguishable from that of non-radio ellipticals (Figs. 6 and 7). Isophote deviations from ellipses are in general rather small and show a homogeneous distribution with no preference from disky or boxy shape; no correlation is found between radio power and boxiness of isophotes as suggested by previous studies. Apart from the different average luminosity and
R. Scarpa et al. / New Astronomy Reviews 46 (2002) 353 – 356
Fig. 3. Top panel shows the me 2 R e relation as derived measuring the radius of the isophote enclosing half total light. The best fit for FR I and FR II is shown. Bottom panel shows the me 2 R e relation as derived from fitting a de Vaucouleurs law. The best fit for radio galaxies and normal ellipticals (Hamabe and Kormendy, 1987) agree well.
Fig. 4. Distribution of nucleus-to-host-galaxy luminosity ratio observed in our sample for the source with a nuclear component. The average contribution of the nucleus to the total galaxy luminosity is less than 5%.
355
Fig. 5. The core radio power at 4.8GHz appears to be weakly correlated with the optical luminosity of the nucleus. This result agree with what was reported by Chiaberge et al. (1999) for a sample of radio galaxies spanning a much wider range of nuclear luminosity.
Fig. 6. Ellipticity distribution (at the effective radius) for our sample (panel a) and for a control sample of radio quiet ellipticals (Fasano and Vio, 1991; panel b). The two cumulative distributions (panel c) are indistinguishable.
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R. Scarpa et al. / New Astronomy Reviews 46 (2002) 353 – 356
size there are not other significant structural differences between galaxies hosting FR I and those hosting FR II radio sources. These results indicate that galaxies hosting radio sources have the same structure and properties of radio quiet ellipticals. The main difference is the presence of nuclear emission as found in several radio galaxies. Our results support the idea that all massive ellipticals may become radio loud at some time and that the radio activity phenomenon does not change significantly the structural and photometric properties of the host galaxies.
References
Fig. 7. Top panel shows amplitude of the c4 parameter, which measures the deviations of the isophotes from a perfect ellipse in the form of disky (c4 . 0) or boxy isophotes (c4 , 0). About 50% of our objects are boxy and 50% are disky. No differences were found between FR I and FR II galaxies. Bottom panel shows there is no correlation between radio power and isophotes shape.
Chiaberge, M., Capetti, A., Celotti, A., 1999. A&A 349, 77. Ekers, R.D., Wall, J.W., Shaver, P.A. et al., 1989. MNRAS 236, 737. Fasano, G., Vio, R., 1991. MNRAS 249, 629. Hamabe, M., Kormendy, J., 1987. In: T. de Zeeuw (Ed.), Structure and Dynamics of Elliptical Galaxies, IAU Symp. 127, p. 379. Wall, J.V., Peacock, J.A., 1985. MNRAS 216, 173.
Optical properties of low z radio galaxies Riccardo Scarpa a , *, Federica Govoni b , Renato Falomo b , Giovanni Fasano b a
Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA b Padua Observatory, Vicolo dell’ Osservatorio 5, I-30122, Padova, Italy
Abstract The optical morphological and photometric properties of 79 low redshift radio galaxies are discussed. It is found that most radio galaxies are luminous bulge dominated systems similar to normal non-radio giant ellipticals. The average absolute magnitude of the sample is kMHOST (tot)l5 223.98, with a clear trend for FR I sources to be | 0.5 mag brighter than FR II galaxies. In about 40% of the objects we find an excess of light in the nucleus attributable to the presence of a nuclear point source. This contributes on average for |1–2% of the total flux from the host galaxy. Radio galaxies follow the same me 2 Re relationship of normal (non-active) elliptical galaxies. The distribution of ellipticity, the amount of twisting and shape of isophotes do not differ significantly from other ellipticals. These results support a scenario where radio emission is little related with the overall properties of the host galaxy. 2002 Published by Elsevier Science B.V. Keywords: Galaxies: Active; Galaxies: Elliptical; Radio continuum: Galaxies
1. Introduction Comparing radio-loud and radio-quiet ellipticals and galaxies hosting different kinds of radio sources (FR I–FR II), controversial results have been found. Here, optical R band observations for a sample of 79 radio galaxies in the redshift range 0.01 , z , 0.12 are analyzed. Selected sources are from 2 complete flux limited radio survey, the 2.7-GHz 2-Jy all-sky survey by Wall and Peacock (1985), and the Ekers et al. (1989) catalogue of southern radio galaxies with flux at 2.7 GHz.0.25 Jy. Observations were obtained at the ESO 2.2 m and 1.5 m, and at the NOT 2.5 m telescopes. We modeled the surface brightness profiles derived from 2-d isophotal fitting using a r 1 / 4 law plus, when required, a nuclear unresolved component and / or an exponential law to account for extra emission *Corresponding author. E-mail address: [email protected] (R. Scarpa).
in the outer regions (Fig. 1). This enable us to identify the presence of extra components and to quantify the deviations from the r 1 / 4 law. Total luminosity, morphology, and structural parameters were also derived.
2. Results • Radio galaxies are hosted by bulge dominated systems with average total absolute magnitude (extinction and k-corrected, and after removing the nuclear component) kMHOST l 5 2 23.9860.70. • Dividing radio galaxies according to their radio morphology we found that galaxies hosting FR I sources are systematically brighter than those hosting FR II sources (Fig. 2). In particular we found kMHOST l FRI 5 2 24.1060.71 and kMHOST l FRII 5 2 23.6260.73. • Accordingly with the well-known relationship
1387-6473 / 02 / $ – see front matter 2002 Published by Elsevier Science B.V. PII: S1387-6473( 01 )00207-X
354
R. Scarpa et al. / New Astronomy Reviews 46 (2002) 353 – 356
Fig. 2. Top panel shows absolute R magnitude distribution in our sample. Bottom panel shows dividing radio galaxies according to their radio morphology we find that galaxies hosting FRI sources are systematically brighter than those hosting FRII sources. This difference is illustrated by comparing the two cumulative distributions.
•
Fig. 1. The peculiar radio galaxy 04301052 (3C120). Top panel shows isophotal contours. In this source we observed both a nuclear point source and an external disk. Spacing between isophotes is 0.5 mag. North is up and east to the left (1 pixel5 0.4030). Bottom panel shows radial profile which exemplifies the deviation from the de Vaucouleurs law often observed in our sample. The profile is fitted with three components: a de Vaucouleurs law (short dashed line), an exponential component (long dashed line), and a nuclear point source (dotted line). The solid line is the sum of the three components.
between effective radius and luminosity of elliptical galaxies, we also found FR I hosts are larger than FR II hosts: kr e l(FR I)518.3610.3 and kr e l(FR II)512.969.3 kpc. • Radio galaxies follow the same me 2 r e relation of
•
•
•
other non-radio ellipticals (Fig. 3). For our 79 radio galaxies we measure me 5 18.4(60.4) 1 2.6(60.3) 3 log r e . A substantial fraction ( | 40%) of objects show the presence of a nuclear point source with luminosity about few percent of the whole galaxy. The nuclear luminosity appears to be weakly correlated with the core radio power but unrelated to host luminosity (Figs. 4 and 5). The luminosity profile of several objects exhibit deviations from a r 1 / 4 law, in the form of light excess in the outer region (r . 10 Kpc). These light excess can be either disc components or extended halos. Radio galaxies have ellipticity indistinguishable from that of non-radio ellipticals (Figs. 6 and 7). Isophote deviations from ellipses are in general rather small and show a homogeneous distribution with no preference from disky or boxy shape; no correlation is found between radio power and boxiness of isophotes as suggested by previous studies. Apart from the different average luminosity and
R. Scarpa et al. / New Astronomy Reviews 46 (2002) 353 – 356
Fig. 3. Top panel shows the me 2 R e relation as derived measuring the radius of the isophote enclosing half total light. The best fit for FR I and FR II is shown. Bottom panel shows the me 2 R e relation as derived from fitting a de Vaucouleurs law. The best fit for radio galaxies and normal ellipticals (Hamabe and Kormendy, 1987) agree well.
Fig. 4. Distribution of nucleus-to-host-galaxy luminosity ratio observed in our sample for the source with a nuclear component. The average contribution of the nucleus to the total galaxy luminosity is less than 5%.
355
Fig. 5. The core radio power at 4.8GHz appears to be weakly correlated with the optical luminosity of the nucleus. This result agree with what was reported by Chiaberge et al. (1999) for a sample of radio galaxies spanning a much wider range of nuclear luminosity.
Fig. 6. Ellipticity distribution (at the effective radius) for our sample (panel a) and for a control sample of radio quiet ellipticals (Fasano and Vio, 1991; panel b). The two cumulative distributions (panel c) are indistinguishable.
356
R. Scarpa et al. / New Astronomy Reviews 46 (2002) 353 – 356
size there are not other significant structural differences between galaxies hosting FR I and those hosting FR II radio sources. These results indicate that galaxies hosting radio sources have the same structure and properties of radio quiet ellipticals. The main difference is the presence of nuclear emission as found in several radio galaxies. Our results support the idea that all massive ellipticals may become radio loud at some time and that the radio activity phenomenon does not change significantly the structural and photometric properties of the host galaxies.
References
Fig. 7. Top panel shows amplitude of the c4 parameter, which measures the deviations of the isophotes from a perfect ellipse in the form of disky (c4 . 0) or boxy isophotes (c4 , 0). About 50% of our objects are boxy and 50% are disky. No differences were found between FR I and FR II galaxies. Bottom panel shows there is no correlation between radio power and isophotes shape.
Chiaberge, M., Capetti, A., Celotti, A., 1999. A&A 349, 77. Ekers, R.D., Wall, J.W., Shaver, P.A. et al., 1989. MNRAS 236, 737. Fasano, G., Vio, R., 1991. MNRAS 249, 629. Hamabe, M., Kormendy, J., 1987. In: T. de Zeeuw (Ed.), Structure and Dynamics of Elliptical Galaxies, IAU Symp. 127, p. 379. Wall, J.V., Peacock, J.A., 1985. MNRAS 216, 173.