Active Galactic Nuclei Alessandro Marconi Department of Physics and Astronomy University of Florence Challenges in UV Astronomy, ESO Garching, 7-11 October 2013 Outline A brief introduction to Active Galactic Nuclei (or ... accreting supermassive black holes) Coevolution of black holes and host galaxies Open questions: seeking answers in near-UV spectroscopy A. Marconi Challenges in UV Astronomy, ESO Garching 2 Outline A brief introduction to Active Galactic Nuclei (or ... accreting supermassive black holes) Coevolution of black holes and host galaxies Open questions: seeking answers in near-UV spectroscopy A. Marconi Challenges in UV Astronomy, ESO Garching 3 AGN in a nutshell An Active Galactic Nucleus (AGN) is a galaxy nucleus with indications non-stellar activity (L~1011-1015 L☉ , non-stellar continuum, prominent emission lines, jets of relativistic material, strong and fast variability) Many observational classes: Quasars, Seyfert galaxies, radio gal., etc. Radio MicrowavesMicroonde Infrarosso Infrared Visibile Visible UV raggiX-rays X [Å] 47 6000 4000 2000 46.0 46 45.5 Nucleo AGN Attivo 45 14.8 15.0 ) [erg/s] SpiralSpirale [Å] L 44 6000 4000 2000 45.0 log( 44.5 43 44.0 43.5 42 EllipticalEllittica 43.0 14.8 15.0 41 10 12 14 16 18 log() [Hz] 472 ZHENG ET AL. Vol. 475 corresponding wavelength range. The sum of the normal- lar, yield signi–cant *s2 because of the very high S/N level. ized spectra was smoother than the template at shorter Therefore, the selected windows at long wavelengths are wavelengths where the S/N level was lower, and it serves as narrow. The region near 912Aé was excluded because of an the –nal composite spectrum. apparent local depression in Ñux (discussed below). The Figure 3 displays the number of merged spectra as a region longward of 2200Aé was not –tted because of the function of wavelength. For some objects, multiple obser- strong Fe II emission seen there. vations taken with the same grating exist. In such cases, we Since the propagated error arrays in our calculations merged these spectra into a single spectrum, which counts contain correlated errors that are not taken into account, as one inFigure 3. Near 1200 Aé the number of merged we experimented with two di†erent methods of assigning spectra reaches a peak of D100, and at very short wave- errors and evaluating the goodness of our –ts. The –rst lengths it is only 2 or 3.Figure 4 displays the S/N level of directly uses the propagated error array. The second uses the composite spectrum after binning by 10 pixels. The the rms spectrum. An advantage of using the rms spectrum spectral region near 1200Aé has the highest S/N level of for the error bars is that it avoids giving excessive weight to D130 perAé ,A and the region near 350é has S/N B 8 per Aé . the longer wavelength portions of the spectrum where the The actual S/N level may be slightly lower as the Ñat-–eld number of merging spectra is large. To calculate the rms correction of the FOS instrument limits the S/N in any spectrum, each normalized spectrum was di†erenced with given spectrum to D50 or less. But we do not expect a respect to the composite. The squared di†erences were signi–cant e†ect as any Ñat-–eld irregularities should be summed, with equal weights, and the sum was divided by smeared out because of the di†erent redshifts. the number of spectra contributing to each pixel. The square root of the result gives the rms deviation, shown in 3. RESULTS Figure 6.The dispersion is D10% around 2500 Aé , and D30% around 400 Aé . 3.1. Continuum A –t with a single power-law continuum using the propa- The composite HST quasar spectrum, as shown in gated error array yields s2\13, 684 for 1504 data points Figure 5, shows a change in slope of the continuumUV-Optical between and two free parameters. In contrast, a brokenSpectra power law of Quasars 350 and 3000Aé that can be approximated as a broken gives s2\1196 for 1504 points and four free parameters, power law. We used the IRAF task spec–t (Kriss 1994) to –t with the power-law index a \[1.00 ^ 0.01 above 1038 Aé the shape of the continuum and the emission lines. With and a \[2.02 ^ 0.02 below. We conclude that a broken spec–t, complex line and continuumProminent models can be –tted broad to power (FWHM law is a good –t to>1000 the continuum (exceptkm/s, for the up to ~10000 km/s) emission lines data using a nonlinear s2-minimization technique. For our region between 900 and 1200Aé where the continuum shape –ts we binned the composite spectrum by 3 pixels (0.3 Aé ). changes gradually) and that a single power law can be The following wavelengthStrong windows for the blue continuum continuumexcluded at much greater than the 5 p con–dence level. –tting were selected: 350È450, 500È640, 720È750, 800È825, Fits using the rms spectrum for the errors give similar 930È950, 1100È1130, 1450È1470, 1975È2010, and 2150È results. The –tted power-law index a \[0.99 ^ 0.01 above 2200Aé .A Above 900é any small deviations from a power law D1052Aé and a \[1.96 ^ 0.02 below. The power-law that are caused by weak emission features, Fe II in particu- index in the region with j[1052Aé derived554 with the rms VANDEN BERK ET AL. Vol. 122 ed by the power law; this region is better Ðtted by a separate power law with an index ofa \[2.45 (a \ 0.45; Fig. 5), which was determined usingl the wavelengthj ranges 6005È 6035A and 7160È7180A . The abrupt change in the contin- uum slope is discussed in ° 5. As a comparison, we have also measured the power-law indices for the median composite, which are a \[0.46 l (a \[1.54) anda \[1.58 (a \[0.42) for the respec- tivej wavelength regionsl (Fig. 3).j The index found for the Lya-to-Hb region is almost indistinguishable from that found for the geometric mean composite. The indices for both spectra redward of Hb are signiÐcantly di†erent, however, and are a result of the di†erent combining pro- cesses. The geometric mean should give a better estimate of FIG. 5.ÈComposite FOS spectrum of 101 quasars, binned to 2Aé . Prominent emission lines and the Lyman limit are labeled, and two possible emission features are marked. The continuum –tting windows are marked with the bars near the bottom. the average index, but comparison with mean or median Composite HST Quasar composite spectra from other studies is probably reason- spectrum (Zheng+1997) able in the Lya-to-Hb region, given the small di†erence in the indices measured for our composite spectra. The contin- FIG. 5.ÈComposite quasar spectrum generated using the geometric uum blueward of the Lya emission line is heavily absorbed mean of the inputComposite spectra. Power-law ÐSDSSts to the estimated Quasar continuum Ñux are shown. The geometric mean is a better estimator than the arithmetic by Lya forest absorption, as seen in Figure 3. However, mean (or median)spectrum for power-law (Vanden distributions. TheBerk+2001) resolution of the input because the strength of the Lya forest is a strong function of spectra is B1800 in the observed frame, which gives a wavelength redshift and a large range of redshifts was used in construc- resolution of about 1A in the rest frame. ting the sample, no conclusions can be drawn about the A. Marconi Challenges in UV Astronomy, ESO Garching 5 absorption or the continuum in that region. 4.2. Emission and Absorption L ines The high S/N and relatively high resolution (1A ) of the composite allows us to locate and identify weak emission The geometric mean of the Ñux density values was calcu- features and resolve some lines that are often blended in lated in each wavelength bin to form the geometric mean other composites. It is also possible that our sample composite quasar spectrum, shown in Figure 5 on a includes a higher fraction of spectra with narrower line pro- logarithmic scale. The median and geometric mean com- Ðles, which could also help in distinguishing emission fea- posites are quite similar, but there are subtle di†erences in tures. For example, close lines that are clearly distinguished both the continuum slopes and the emission-line proÐles, in the spectrum include Ha/[N II](jj6548, 6563, 6583), discussed further in the following sections, which justify the Si III]/C III](jj1892, 1908), the [S II](jj6716, 6730) doublet, construction of both composite spectra. and Hc/[O III](jj4340, 4363). Emission-line features above the continuum were identiÐed manually in the median spec- 4. CONTINUUM, EMISSION, AND ABSORPTION FEATURES trum. Including the broad Fe II and Fe III complexes, 85 emission features were detected. The endpoints of line posi- 4.1. T he Continuum tionsj andj were estimated to be where the Ñux density The geometric mean spectrum is shown on a log-log scale was indistinguishablelo hi from the local ““ continuum.ÏÏ The in Figure 5, in which a single power law will appear as a local continuum is not necessarily the same as the power- straight line. The problem of Ðtting the quasar continuum is law continuum estimated in ° 4.1, since the emission lines complicated by the fact that there are essentially no may appear to lie on top of other emission lines or broad emission-lineÈfree regions in the spectrum. Our approach is Fe II emission features.