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Register a new userOn the nature of optical nuclei in FR I radio-galaxies from ACS/HST imaging polarimetry
حول طبيعة النوويات البصرية في FR I راديو-المجرات من تصوير المنظومات المصغرة الضوئية / هوست
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Alessandro Capetti
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نحصلنا على التصوير البصري المشع باستخدام ACS / HRC على HST من 9 أقرب المجرات الإذاعية في كتالوج 3C مع شكل FR I. وتم العثور على المصادر النووية التي تم رؤيتها في الصور المباشرة من هذه المجرات كانت مشدودة بشكل كبير مع مستويات في النطاق ~2-11٪ مع قيمة وسيطة 7٪. نحن نناقش الآليات المختلفة التي تنتج الإشعاع المشدود ونحتسب أن التفسير الوحيد الآمن هو مصدر سينكروتروني للنووي. يعزز هذا الأفكار من التشابه مع خصائص التشديد لكائنات BL Lac، ويوفر دعماً إضافياً أيضاً لنموذج FRI / BL Lac الموحد. يؤكد هذا على الاقتراحات السابقة بأن الآلية الرئيسية للإشعاع الصوتي الضعيف في AGN هي مرتبطة بالإشعاع الغير حراري الذي ينتجه مؤشر الطائرات. بالإضافة إلى التشديد النووي (والجيوت البصرية على نطاق واسع)، يتم الكشف عن التشديد مع القرص الدقيق النووي الغباري، على الأرجح بسبب الانتقال الثنائي؛ وتكون القوافي التشديد على طول القرص كما يتوقع عندما يتم امتداد المغناطيس المسؤول عن تحديد الحبوب بواسطة الدوران الاختلافي. نحن فحصنا الإمكانية للكشف عن التشديد المشع للإشعاع الطائرة الغير متطابقة في FR I، المتوقع في مصادرنا في إطار نموذج FR I / BL Lac الموحد. لم نجد هذا الأثر في أي من المجرات، ولكن نتائجنا لا تؤكد بشكل واضح على أن الطائرة الغير متطابقة تكون موجودة حقاً في FR I.
We obtained optical imaging polarimetry with the ACS/HRC aboard the HST of the 9 closest radio-galaxies in the 3C catalogue with an FR I morphology. The nuclear sources seen in direct HST images in these galaxies are found to be highly polarized with levels in the range ~2-11 % with a median value of 7 %. We discuss the different mechanisms that produce polarized emission and conclude that the only viable interpretation is a synchrotron origin for the optical nuclei. This idea is strengthened by the analogy with the polarization properties of BL Lac objects, providing also further support to the FRI/BL Lac unified model. This confirms previous suggestions that the dominant emission mechanism in low luminosity radio-loud AGN is related to non-thermal radiation produced by the base of their jets. In addition to the nuclear polarization (and to the large scale optical jets), polarization is detected co-spatially with the dusty circumnuclear disks, likely due to dichroic transmission; the polarization vectors are tangential to the disks as expected when the magnetic field responsible for the grains alignment is stretched by differential rotation. We explored the possibility to detect the polarimetric signature of a misaligned radiation beam in FR I, expected in our sources in the frame of the FR I/ BL Lac unification. We did not find this effect in any of the galaxies, but our the results are not conclusive on whether a misaligned beam is indeed present in FR I.
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