Paschen Lines and the Reddening of the Radio Galaxy 3C 109

THE ASTRONOMICAL JOURNAL, 118 : 666È669, 1999 August ( 1999. The American Astronomical Society. All rights reserved. Printed in U.S.A.

PASCHEN LINES AND THE REDDENING OF THE RADIO GALAXY 3C 109 RICHARD J. RUDY,1 R. C. PUETTER,1,2 AND S. MAZUK1 Received 1999 March 18 ; accepted 1999 May 5

ABSTRACT This paper presents spectrophotometric observations from 0.8 to 2.5 km of the broad emission line radio galaxy 3C 109. Three Paschen lines plus Ha are measured and combined with a published value for Hb to estimate the reddening for the continuum source and the region producing the broad emission lines. A key factor in this determination is the foreground reddening due to the Galaxy. The recent COBE/IRAS maps of Schlegel, Finkbeiner, & Davis indicate a Galactic extinction of E(B[V ) \ 0.57, more than twice the value inferred from the H I measurements. If the former value is adopted, the in situ reddening is E(B[V ) \ 0.77. This value implies, per the analysis of Goodrich & Cohen, that 3C 109 is an object of extremes, displaying a dereddened UV-optical continuum that is extraordinarily blue ( f D l l1.5) and harboring dust grains that are more efficient polarizers than any found in the Milky Way. Both extremes hint at some additional factor or mechanism, and a few possibilities are discussed. Regardless, the signiÐcant amount of internal reddening indicated by infrared spectroscopy is consistent with results from previous optical polarimetry and spectropolarimetry, optical and X-ray spectroscopy, and infrared photometry, in demonstrating that 3C 109 is an obscured quasarÈa luminous object whose nuclear radiations from the soft X-ray to the near-infrared are absorbed, scattered, polarized, and reradiated further into the infrared by a circumnuclear envelope of dust. Key words : dust, extinction È galaxies : individual (3C 109) È galaxies : nuclei È infrared radiation È quasars : emission lines È radio emission lines 1.

INTRODUCTION

& Cohen (1992, hereafter GC). They were able to Ðt the polarization throughout the optical region with two Serkowski laws, one representing the small interstellar polarization within our Galaxy with P \ 1%, and a max second, much larger component (P \ 7.7%) originating max within 3C 109. The high luminosity in the infrared was found to persist to 60 km by IRAS (Golombek, Miley, & Neugebauer 1988 ; Hes, Barthel, & Hoekstra 1995). Additional conÐrmation both of the presence of dust and of a large intrinsic luminosity was provided by the ROSAT observations of Allen & Fabian (1992). These data showed evidence of absorption in the soft X-ray region in excess of what could be attributed to dust in the Milky Way (indicating that it was due to dust within 3C 109) and an X-ray luminosity comparable to 3C 273. The latter made 3C 109 one of the most luminous X-ray sources out to a redshift of 0.5. While the presence of dust in the nucleus of 3C 109 is Ðrmly established, one ““ loose end ÏÏ was noted by GC. They found that the reddening deduced from the Ha/Hb ratio [E(B[V ) [ 0.86] implied an intrinsic spectral shape bluer than is generally observed in quasars (i.e., its dereddened spectral index was a [ 1.0 ^ 0.3, where f P la). While a smaller reddening (and thus a more typical lcontinuum) was possible if the intrinsic Balmer decrement were larger than expected, this raised a di†erent problemÈin order to produce the strong polarization with the smaller extinction, the polarizing efficiency of the dust grains within 3C 109 would have to be greater than those seen in the Milky Way. GC noted the importance of an improved value for the reddening and wrote that ““ infrared spectroscopy . . . may provide a better estimate.ÏÏ Accordingly, we present infrared spectra of 3C 109. Fortuitously, its redshift falls within a narrow range that allows us to observe Ha, Pad, Pab, and a portion of Paa, with the same spectrograph. These simultaneous measurements of relative line strengths are used to derive a reÐned value for the reddening.

The emission-line radio galaxy 3C 109 (z \ 0.3066) was one of the Ðrst objects in which it was recognized how signiÐcantly circumnuclear dust can alter our perception of an active galactic nucleus. At visible wavelengths, 3C 109 appears as a faint, 17.6th magnitude galaxy with a point nucleus (a so-called N galaxy). However, early optical studies revealed a large Balmer decrement and a red continuum (Yee & Oke 1978 ; Grandi & Osterbrock 1978) that hinted at the presence of obscuring dust. In a polarimetric survey of broad-line radio galaxies, Rudy et al. (1983) found a very strong (D8%) optical polarization that was constant over two epochs of observation. This constancy suggested that polarization was not intrinsic to the emission process (such behavior is generally characterized by extreme ““ blazar ÏÏ variability) but arose through some reprocessing of that continuum. In a follow-on study, Rudy et al. (1984, hereafter RSST) obtained additional polarimetric observations in several discrete Ðlters and found that the large polarization included not just the continuum, but the strong Ha line as well. They concluded that the polarization arose from the passage of the nuclear light through aligned dust grains in a manner identical to that which produces the interstellar polarization within the Galaxy. The presence of dust was further indicated by their infrared photometry, which showed that 3C 109 was a powerful source of infrared emission to at least 20 km. They noted that in the absence of the absorbing dust, 3C 109 would appear as a bright quasar rather than a faint N galaxy. This notion was put on a Ðrm basis by the spectropolarimetric measurements of Goodrich ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ 1 Space and Environment Technology Center, M2/266, The Aerospace Corporation, P.O. Box 92957, Los Angeles, CA 90009 ; richard.j.rudy=aero.org, steve.mazuk=aero.org. 2 Center for Astrophysics and Space Sciences, C-0111, University of California, San Diego, La Jolla, CA 92093 ; rpuetter=ucsd.edu.

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REDDENING OF 3C 109 2.

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OBSERVATIONS

The infrared spectrophotometry of 3C 109 was acquired on the nights of 1998 September 30 and October 1 (UT) with the 3 m Shane reÑector of Lick Observatory. The instrument used was the Aerospace near-infrared imaging spectrograph. The spectrograph uses two separate channels to provide nearly continuous coverage between 0.8 and 2.5 km. A common slit and Ðeld lens are followed by a beam splitter that reÑects the light shortward of 1.38 km to the ““ blue ÏÏ channel and transmits the longer wavelengths to the ““ red.ÏÏ The crossover point is chosen to fall within a deep telluric H O vapor absorption band. Each channel has its 2 own collimator, di†raction grating, camera, and detector array. The collimators and cameras use refractive, antireÑection-coated elements. The detectors are twoquadrant NICMOS3 devices providing 256 channels in the spectral dimension and 128 in the spatial. A 600 line mm~1 grating blazed at 1.0 km operates in the blue channel, while a 300 line mm~1 blazed at 2.0 km services the red. Each channel has a nearly constant dispersionÈ14 Ó for the blue channel and 36 Ó for the red, when using the 2A slit width employed for the 3C 109 observations. The cameras reduce the image to provide a scale of 1A pixel~1 at the arrays, providing for a 2@ slit length at the telescope focus. Each channel has approximately 24 mm of free spectral range to be covered by a 10 mm array. To accomplish this, the arrays are mounted to translational stages with a single degree of freedom in the spectral direction. These stages are moved to cover the full spectral range using externally mounted, linear actuators capable of positioning to 1 km accuracy. The spectrum from each of the channels is, in fact, a composite of six separate spectra. There are three widely separated (but slightly overlapping) positions, and for each of these, two spectra are acquired, which are spaced by 2.5 pixels. This Ðne spacing provides both oversampling of the spectrum and protection against bad pixels. To provide the wavelength calibration necessary to register component spectra, features from emission-line lamps (helium and argon), telluric absorption features, and OH lines from the night sky are employed. The observations were reduced in the normal way, that is, by dividing by the spectrum comparison star to remove the instrumental response and most of the e†ects of atmospheric absorption. The comparison star on both nights was the bright star HR 1319, an F5 V. It was observed immediately after 3C 109 and was at an identical air mass on both nights. The intrinsic spectrum of HR 1319 was removed from this ratio by using the model for an F5 V from Kurucz (1991). An absolute Ñux scale was determined by normalizing to the K magnitude for HR 1319 of 5.31. The latter was obtained from the V magnitude tabulated in the Bright Star Catalogue (Hoffleit & Jaschek 1982) and the V [K color appropriate to this spectral type (Koornneef 1983). This procedure yielded the spectrum shown in Figure 1. With regard to the uncertainty in the absolute level of the spectrum, we compared the levels for the two nights of observation and found that they di†ered by less than 5%. As a check on the systematic errors, we compared our results with those of GC and RSST. GC found a Ñux of 320 ] 10~15 ergs cm~2 s~1 for the Ha complex, a third greater than our value. The agreement with the JHK photometry of RSST is much better : when the spectrum is averaged over the bandpass of each of the Ðlters, the di†erence

FIG. 1.ÈNear-infrared spectrum of 3C 109. Data are a composite of two nights of observation. The wavelength scale is in the observed frame of reference ; the emission lines are seen at a redshift of 0.3066. The gaps in the data at 1.4 and 1.9 km are due to atmospheric absorption ; the one at 2.2 km is a region where no data were acquired. [S III] j9532 was the only feature detected from the narrow-line region ; its width provides a gauge of the instrumental spectral resolution. The three open squares denote the JHK photometry of RSST.

in the photometry between the two sets of data is less than 6% for each of the three Ðlters (see Fig. 1). We are unsure as to whether the discrepancies among the di†erent data sets are instrumental in origin or perhaps indicate variations in 3C 109, but we believe that for these observations, the error in the absolute level is ¹20%. Note that the Ñuxes for the emission lines given in Table 1 reÑect only the uncertainties in the relative line strengths (i.e., in the boundaries of the line and placement of the continuum) and not that of the absolute level. Because the red wing of Paa was severely a†ected by atmospheric absorption, we measured the blue portion to the line center and doubled the value to produce the entry in Table 1. This is the reason for the very large error. 3.

RESULTS AND DISCUSSION

The principal goal in obtaining these observations of 3C 109 was to better quantify the reddening of the nucleus. In addition to the Paschen lines and Ha, we use the Hb value of GC (scaled to our Ñux for Ha) for this purpose. Here we distinguish between the region that gives rise to the broad emission lines, and that manifests the same extinction and polarization as the continuum source, and the more distant and comparatively unobscured zone that produces the narrow emission lines. To determine the reddening of the former, we must Ðrst correct for the contribution from TABLE 1 OBSERVED EMISSION-LINE FLUXES FOR 3C 109

Line

Flux (10~15 ergs cm~2 s~1)

Haa,b . . . . . . . . . . . . . . [S III] j9532 . . . . . . Pada . . . . . . . . . . . . . . . Paba . . . . . . . . . . . . . . . Paaa . . . . . . . . . . . . . . .

240 ^ 15 5.4 ^ 0.7 18 ^ 4 49 ^ 3 120 ^ 25

a Includes the narrow component. b Includes [N II] jj6548, 6584.

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RUDY, PUETTER, & MAZUK

the latter, as well as for the reddening of the Milky Way. To estimate the Galactic reddening, we use the recent COBE/ IRAS infrared maps of Schlegel, Finkbeiner, & Davis (1998), which indicate E(B[V ) \ 0.57. Note that this is very di†erent from the value of E(B[V ) \ 0.27 ^ 0.03 derived from the H I measurements of Burstein & Heiles (1982), and employed by both RSST and GC. The new method should be the more accurate since it observes the dust directly and its results are supported by X-ray observations (Allen et al. 1997). Even so, we checked for possible Ñaws in the extinction map of Schlegel et al. (1998) in the direction of 3C 109 by averaging the reddening over a 1 deg2 region centered on 3C 109. This value was nearly identical to the value for the pixels immediately surrounding the source, ruling out a map error local to 3C 109. To correct the observed line Ñuxes for the contribution from the narrow-line region, we estimate the strengths of the narrow components of Hb, Pad, Pab, and Paa relative to Ha from the case B values of Storey & Hummer (1995) for an N \ 104 cm~3, T \ 104 K gas. Their absolute e found from thee ratio of the narrow component strengths are of Ha to the total Ha ] [N II] complex, as measured by GC, and our Ñux for this complex. These values were also adjusted for reddening from the Milky Way, and the possibility of in situ reddening was also considered. With regard to this possibility, the spectropolarimetry and spectroscopy of GC have large uncertainties but deÐnitely indicate that this reddening of the narrow lines is much smaller than that of the broad emission lines and the nonstellar continuum. Once again, we varied this value to investigate the e†ects on the reddening calculated for the broad-line region. Because the contribution from the narrow lines is so small, varying the reddening from E(B[V ) \ 0 to E(B[V ) \ 0.5 makes only a 0.03 mag di†erence in the reddening derived for the broad emission lines. After these adjustments have been made, the resulting broad-line values, relative to Pab, are Hb/Ha/Pad/Pab/Paa \ 1.56/11.0/0.40/1.00/1.95. These are the values that we believe would be observed from outside the Galaxy with a proper accounting for the narrow-line components. Because case B conditions are not satisÐed within the broad emission line region of 3C 109 (i.e., the optical depths in the subordinate lines are no longer small), the prediction of the intrinsic hydrogen line ratios, which is necessary to compute the reddening, is not straightforward. We assume the intrinsic ratios of the broad hydrogen lines in 3C 109 match the ““ standard ÏÏ QSO line ratios of Hubbard & Puetter (1985). These were largely drawn from published observations or, in the case of Pad and Pab, derived from models that reproduced the observed ratios of the other lines. Referenced to Pab, they are Hb/Ha/Pad/Pab/Paa \ 7.5/25/0.4/1.0/2.0. A least-squares Ðt to the observations, with each line weighted inversely by the square of its uncertainty, yields E(B[V ) \ 0.77 (see Fig. 2). To investigate how this value depends on the various parameters, we considered a number of alternate cases. 1. A Ðt where each line is weighted equally : E(B[V ) \ 0.75. 2. The assumption that the intrinsic hydrogen line ratios could be described by the case B values of Storey & Hummer (1995) for N \ 1010 cm~3 and T \ 104 K : e e E(B[V ) \ 0.67. 3. Using the standard QSO line ratios but excluding Hb (the line we did not measure) : E(B[V ) \ 0.78.

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FIG. 2.ÈReddening of the broad emission line region of 3C 109 as indicated by the hydrogen lines. The abscissa axis represents the extinction di†erential, as determined from the Savage & Mathis (1979) reddening curve, between Pab and the other hydrogen lines in the rest frame. The ordinate axis denotes the ratio of the observed line strengths to the case B values for an N \ 108 cm~3, T \ 104 K gas. This particular parame eterization permits a linear Ðt eto the data points to determine the reddening. All data have been corrected for contributions from the narrow emission line region (see text) and a Galactic reddening of E(B[V ) \ 0.57 as derived from the COBE/IRAS maps of Schlegel et al. (1998). E(B[V ) \ 0.77 represents the best-Ðt value weighting each line by the square of the ratio of the line Ñux to its respective error.

4. Using the old, smaller value for the Galactic reddening employed by RSST and GC : E(B[V ) \ 0.98. Folding these considerations together with the formal errors in the Ðts and allowing for uncertainty in the intrinsic line ratios, our best values for the reddening and its error are E(B[V ) \ 0.77 ^ 0.09. The value E(B[V ) \ 0.77 implies a spectral index a D 1.5 (see Fig. 3). This is signiÐcantly higher than the value associated with classical accretion disks (a \ 1 ), 3 exceeds the a ¹ 0.5 that is commonly observed in quasars

FIG. 3.ÈCombined optical and infrared spectrum of 3C 109 dereddened by Galactic and total extinction. The optical data are from Goodrich & Cohen (1992) ; the infrared data are from this study. All data are shown in the rest frame. The bottom spectrum is corrected for only for the Galactic reddening of E(B[V ) \ 0.57. The dash-dotted line is a power law corresponding to f D l~1.0. The upper curve is corrected both for Galacl intrinsic reddening of E(B[V ) \ 0.77. In this case the tic reddening and an power law (solid line) is f D l1.5. l

No. 2, 1999

REDDENING OF 3C 109

(Sanders et al. 1989 ; Barvainis 1990), and approaches the a \ 2 Rayleigh-Jeans value. GC have considered the implications of this, and we can add little to their discussion other than to note that the spectral index is very sensitive to reddening. However, the internal reddening would have to be E(B[V ) \ 0.5 to bring the index into agreement with other QSOs. With regard to the polarization of 3C 109, if the maximum value (as a function of wavelength) measured by GC (7.7%) is divided by our value for the extinction, the result is P /E(B[V ) \ 10%. Allowing for errors, this max equals or slightly exceeds the largest values (9%) observed for the Galaxy (Serkowski, Mathewson, & Ford 1975). However, given that the Galactic maximum represents the extreme for a range of values observed for a large sample of stars, the dust grains of 3C 109 may certainly be more efficient polarizers than their Galactic counterparts. It is interesting to compare and contrast 3C 109 with another heavily reddened broad-line radio galaxy, 3C 445. This object also has a large Ha/Hb ratio and a strong Paa feature that indicates a reddening of E(B[V ) \ 1.0 (Rudy & Tokunaga 1982). This is slightly greater than the intrinsic reddening of 3C 109, yet the polarization for this object is only about 1.5% (Rudy et al. 1983). While strong polarization does not always accompany high extinction, this example helps to underscore the exceptional conditions that must exist within 3C 109, conditions that include a large column density of strongly dichroic grains, a well-ordered magnetic Ðeld to align them, and, probably, a preferred line of sight. The extreme nature of both the polarization and the continuum suggests that some process in addition to interstellar extinction may be operating in 3C 109. GC considered the possibility that scattering by an envelope of dust could be returning light to the line of sight as well as removing it. Since scattering is expected to be more pronounced in the blue, such a mechanism might produce a continuum that is bluer than expected. GC looked for this but found no evidence for an enhancement of Ñux at shorter wavelengths. Because the continuum and broad-line region are similarly polarized, any mechanism returning continuum Ñux to the observer should do the same for the broadline emission. To look for additional hydrogen line emission in the blue, we included Hc in our reddening determination. We estimated its strength from the GC spectrum and found it to be approximately 25% of Hb. With it, the derived

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reddening is E(B[V ) \ 0.82, ruling out an enhancement at Hc. Any attempt to explain the slope of the continuum using a peculiar reddening law can also be excluded since the reddening is estimated from the line emission, which is a†ected in an identical manner as the continuum. In principle, abnormal intrinsic hydrogen line ratios could lead to an overestimate of the reddening. We have tried to guard against this e†ect by including lines from both the Balmer and Paschen series in the reddening determination. As an example of the motivation for this approach, we note that a large intrinsic Ha/Hb ratio can result from radiative transfer e†ects but appears to be caused by high extinction. However, the intrinsic Paa/Ha ratio formed under such conditions is small. This is opposite of the ratio that would be produced by reddening, removing the uncertainty that would arise if only the Ha and Hb were observed. We are unaware of any conditions that can produce an intrinsic hydrogen line spectrum whose Balmer and Paschen ratios mimic the e†ects of reddening. In short, we can identify no mechanism that can reconcile a more typical continuum shape with the observations but point out that UV observations, primarily of the continuum, but also of Lya, could be extremely helpful in better understanding this system. In summary, we have presented infrared spectrophotometry that indicates the presence a moderate amount of internal reddening in the emission-line radio galaxy 3C 109. This observational result is consistent with a host of other measurements that indicate 3C 109 is an object with a very blue intrinsic continuum and a quasar-like bolometric luminosity, but whose appearance at optical wavelengths is altered signiÐcantly by dust extinction. Moreover, the magnitude of this extinction, when weighed against the attendant polarization, suggests that the dust grains that produce both in 3C 109 may be spectacularly efficient in polarizing its nuclear light. We are grateful to K. Baker and W. Earthman, the telescope operators at Lick Observatory, for their help in acquiring these data. An anonymous referee is thanked for an important suggestion. This work was supported by the Independent Research and Development program of the Aerospace Corporation. R. C. P. acknowledges support from NASA.

REFERENCES Allen, S. W., & Fabian, A. C. 1992, MNRAS, 258, 29P Rudy, R. J., Schmidt, G. D., Stockman, H. S., & Moore, R. L. 1983, ApJ, Allen, S. W., Fabian, A. C., Idesawa, E., Inoue, H., Kii, T., & Otani, C. 1997, 271, 59 MNRAS, 286, 765 Rudy, R. J., Schmidt, G. D., Stockman, H. S., & Tokunaga, A. T. 1984, ApJ, Barvainis, R. 1990, ApJ, 353, 419 278, 530 (RSST) Burstein, D., & Heiles, C. 1982, AJ, 87, 1165 Rudy, R. J., & Tokunaga, A. T. 1982, ApJ, 256, L1 Golombek, D., Miley, G. K., & Neugebauer, G. 1988, AJ, 95, 26 Sanders, D. B., Phinney, E. S., Neugebauer, G., Soifer, B. T., & Mathews, Goodrich, R. W., & Cohen, M. H. 1992, ApJ, 391, 623 (GC) K. 1989, ApJ, 347, 29 Grandi, S. A., & Osterbrock, D. E. 1978, ApJ, 220, 783 Savage, B. D., & Mathis, J. S. 1979, ARA&A, 17, 73 Hes, R., Barthel, P. D., & Hoekstra, H. 1995, A&A, 303, 8 Schlegel, D. J., Finkbeiner, D. P., & Davis, M. 1998, ApJ, 500, 525 Hoffleit, D., & Jaschek, C. 1982, The Bright Star Catalogue (4th rev. ed. ; Serkowski, K., Mathewson, D. S., & Ford, V. L. 1975, ApJ, 196, 261 New Haven : Yale Univ. Obs.) Storey, P. J., & Hummer, D. G. 1995, MNRAS, 272, 41 Hubbard, E. N., & Puetter, R. C. 1985, ApJ, 290, 394 Yee, H. C. K., & Oke, J. B. 1978, ApJ, 226, 753 Koornneef, J. 1983, A&A, 128, 84 Kurucz, R. L. 1991, in Precision Photometry, ed. A. G. D. Philip, A. R. Upgren, & K. A. Janes (Schenectady : L. Davis), 27