The Origin Of The Balmer Reduction Of The Sloan Sky Survey Active Galactic Nucleus

Exploring the nature and origin of gas and dust in the central region of active galactic nuclei is a very important topic in astrophysics.In this paper,the so-far largest 3785 broad-line AGN sample for Balmer-decrement research selected from the database of galaxies and quasars observed spectroscopically in SDSS DR7 is used to study the origin of Balmer decrement,with the main research including the its distributions in BLR and NLR and its relation with the inclination of the dusty torus and accretion rate of the black hole.Applying statistical analysis,the main results are as follows:(1)The mean value of the Balmer decrement for BLR is H?b/H?b=3.776-0.852+0.099.while it is H?n/H?n=5.023-1.392 +1.927 for the NLR.The relation of the Balmer decrement between the BLR and the NLR is weak with the correlation coefficient being-0.21.The larger mean value of the NLR Balmer decrement than the BLR implies that the NLR is subject to more reddening by dust than the BLR or has a larger intrinsic value.(2)The BLR/NLR decrement has a strong correlation with the inclination traced by H?n/H?b(correlation coefficient being 0.51,-0.35 respectively).As the inclination angle increases,the BLR decrement increases,suggesting that BLR is related to the covering factor of the torus.(3)BLR decrement has a weak negative correlation with the accretion rate M or LBol/LEdd,while there is a weak positive correlation for the NLR.At a fixed inclination,the BLR decrement is smaller for AGNs with larger M.It implies that in addition to the inclination,M has an effect on the BLR decrement at a fixed inclination angle.As the M increases,changes in the torus would lead to smaller BLR Balmer decrement values and the lower temperature or high density in NLRs would lead to higher NLR decrement.(4)We also find that there exists a correlation between BLR/NLR decrement and L5100.It supports receding torus model.It is found that the inclination tracer H?n/H?b has a strong negative correlation with the color of W1-W2.The correlation between the BLR decrement and the inclination or W1-W2 shows that the BLR reddening is driven by the dusty torus.It is consistent with the AGN orientation-based unified model.In conclusion,for broad-line AGN sample of SDSS,the BLR Balmer decrement is dependent on the dusty torus,while the NLR Balmer decrement is on the physical conditions of NLR.The accretion process of black holes would affect the torus and NLR,resulting in a weak correlation with the BLR/NLR decrement.