- Email: [email protected]
Host galaxies of luminous high redshift quasars
New Astronomy Reviews 50 (2006) 806–808 www.elsevier.com/locate/newastrev
Host galaxies of luminous high redshift quasars Malte Schramm a
a,*
, Lutz Wisotzki a, Knud Jahnke
b
Astrophysikalisches Institut Potsdam, An der Sternwarte 16, D-14482 Potsdam, Germany b Max-Planck-Institute for Astronomy, Ko¨nigstuhl 17, D-69117 Heidelberg, Germany Available online 25 July 2006
Abstract We have observed a sample of 7 luminous QSOs in the redshift range z = 1.8–2.9 selected from the Hamburg/ESO survey (HES). The aim of this study is to detect the galaxies that host the quasar and to estimate luminosities, colours, and stellar masses of the host galaxies. We were able to resolve the host galaxy around HE 2355–5457 and HE 2348–1444, both located at z = 2.9, in two bands. These data will shed light on the link between host galaxy and nuclear components of massive galaxies at high-redshifts. 2006 Elsevier B.V. All rights reserved. Keywords: Quasars: general; Galaxies: active; Galaxies: high redshift
Contents 1. 2. 3. 4. 5.
Data sample and observations . . . . . . . . . Data analysis. . . . . . . . . . . . . . . . . . . . . . Host galaxy magnitudes and colours . . . . . Stellar masses and virial black hole masses. Conclusion . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . .
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1. Data sample and observations We have obtained images of 7 luminous high redshift quasars with the ESO–VLT during several nights between May and July 2002, using the near-infrared camera ISAAC with its long-wavelength (LW) arm. We focused on the highest level of nuclear luminosities, MB 28, which would allow us to target the most massive ellipticals (McLure et al., 1999). We defined two redshift windows *
Corresponding author. E-mail address: [email protected] (M. Schramm).
1387-6473/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.newar.2006.06.025
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of 1.8 < z < 2.0 and 2.6 < z < 3.0 to avoid contamination from emission lines and to sample the stellar continuum ˚ break with the standard NIR filters J, above the 4000 A H and Ks. These filters always correspond approximately to rest-frame B and V. Observing each QSO in two different filters allows us to determine rest-frame B V colours for the quasar hosts and thus constrain their stellar population. Observations were photometric, and we had seeing conditions ranging from 0.600 to 0.800 . The data were reduced using the Eclipse software package provided by ESO, in particular the ISAAC reduction pipeline recipe lwjitter.
M. Schramm et al. / New Astronomy Reviews 50 (2006) 806–808
2. Data analysis Resolving the faint hosts of high redshift QSOs is still a challenge because the bright nucleus outshines the underlying galaxy. Besides good seeing, an excellent control of the point spread function (PSF) is required to successfully resolve the host galaxy. We used two different strategies to decompose our quasar images. The first was the simple but very successful PSF subtraction method, where we used bright unsaturated field stars to construct and subtract a rescaled PSF from the QSO. For the second approach we employed the decomposition tool PAMDAI by Kuhlbrodt et al. (2004) which uses a fully analytic description of the PSF. Unfortunately the LW arm of the ISAAC instrument shows a strong distortion pattern over the field which even varied between objects and between filters. So PSF construction proved to be a difficult task, with only few suitable stars available that were sufficiently close to a quasar. 3. Host galaxy magnitudes and colours For our two-dimensional analysis we assumed an elliptical morphology and plausible scale lengths for the host galaxy of 3, 5 and 10 kpc (adopting a standard cosmology with H0 = 70 km s1 Mpc1, Xm = 0.3 and XK = 0.7). Two quasars of the sample, HE 2355–5457 at z = 2.904 and HE 2348–1444 at z = 2.933, show extended emission coming from the host galaxy in both observed bands. We have a positive detection for the Ks band as well as (although weaker) for the H band of HE 2355–5457. This is shown in Fig. 1 where we present an example from our two-
807
dimensional modelling using a scale length of 10 kpc. We find that for the Ks band more than 8% and for the H band more than 6% of the total flux corresponds to the host galaxy. These numbers do not depend much on the assumed scale length. Results from our peak subtraction analysis are shown in Fig. 2 where we plot surface brightness profiles of the quasar, two different PSF stars and profiles of the residuals after subtraction. We find good agreement between the results from two-dimensional modelling and peak subtraction. We determined an averaged colour of (B V) = 0.23 ± 0.30 and an absolute magnitude of MV = 25.94 ± 0.30 for the peak subtraction analysis. For the the two-dimensional modelling we find (B V) = 0.04 ± 0.30 and MV = 25.82 ± 0.30. In order to compute the differential K corrections we assumed stellar populations of 0.1–1 Gyr; however, the K corrections are extremely small as Ks and H very nearly reproduce restframe V and B, respectively, at z . 2.9. For HE 2348 1444 we determined (B V) = 0.18 ± 0.30 and MV = 26.39 ± 0.30 using PAMDAI, while for the peak subtraction we find (B V) = 0.08 ± 0.30 and MV = 26.51 ± 0.30. The measured B V colours are rather blue and indicate relatively young stellar populations, with luminosity-weighted ages of a few hundreds of Myr, dominating the rest-frame visible light. For three further QSOs we could only estimate upper limits for the host galaxy magnitudes in the longer wavelength band. Finally, two QSOs in the sample did not have sufficiently close PSF stars to correct for the spatial PSF variations, and for these objects it was not even possible to determine meaningful upper limits on the host luminosities.
Fig. 1. Results of the two-dimensional model fits of HE 2355–5457. Top panel shows (from left to right) H band image of QSO composite, subtracted nucleus model, the host model assuming a scale length of 10 kpc and zero ellipticity and in the last frame subtracted nucleus and host model. The bottom panel shows the equivalent for the Ks band.
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M. Schramm et al. / New Astronomy Reviews 50 (2006) 806–808
H band
K s band
Fig. 2. Comparison of the radial surface brightness profiles of the H band and Ks band of HE 2355–5457 with two PSF stars. Each plot shows the azimuthally averaged profile of the QSO (solid line), first PSF star (dotted line) and second PSF star (dashed line) and the radial profiles of the residuals after subtraction of a rescaled PSF from the QSO image. The dotted line with open circles shows the profile after subtraction of the first PSF star and the dashed line with triangles shows the profile after subtraction of the second PSF star.
4. Stellar masses and virial black hole masses The derived B V colours give us the possibility to estimate the stellar masses for HE 2355–5457 and HE 2348– 1444. We used the conversion from colour to mass-to-light ratio from Bell et al. (2003) and derived 4–8 · 1011Mx for HE 2355–5457, and 8–12 · 1011Mx for HE 2348–1444. Using archival optical spectra, we measured C IV line widths to estimate black hole masses. We employed the formula by Vestergaard (2002) 2 ˚ 0:7 FWHMðC ivÞ kLk ð1350 AÞ 6 M bh ¼ 1:6:10 ð1Þ 1000 km s1 1044 erg s1 yielding Mbh = 3.1 · 1010Mx for HE 2355–5457 and Mbh = 1.3 · 1010Mx for HE 2348–1444. 5. Conclusion We were able to resolve the host galaxies of HE 2355– 5457 and HE 2348–1444, both at z 2.9, in H and Ks band
imaging using two-dimensional modelling and peak subtraction analysis. We derived B V rest-frame colours for both objects indicating significantly blue stellar populations, similar to what is seen for low-redshift QSO hosts Jahnke et al. (2004). The rest frame colours allowed us to estimate the stellar masses to be of order 1012Mx for the host galaxies. Using C IV line width measurements, we derived virial black hole masses of Mbh 3 · 1010Mx which are within the upper limits predicted by Vestergaard (2004).
References Bell, E.F., McIntosh, D.H., Katz, N., et al., 2003. ApJS 149, 289. Jahnke, K., Kuhlbrodt, B., Wisotzki, L., 2004. MNRAS 352, 399. Kuhlbrodt, B., Wisotzki, L., Jahnke, K., 2004. MNRAS 349, 1027. McLure, R.J., Kukula, M.J., Dunlop, J.S., et al., 1999. MNRAS 308, 377. Vestergaard, M., 2002. ApJ 571, 733. Vestergaard, M., 2004. ApJ 601, 676.
Host galaxies of luminous high redshift quasars Malte Schramm a
a,*
, Lutz Wisotzki a, Knud Jahnke
b
Astrophysikalisches Institut Potsdam, An der Sternwarte 16, D-14482 Potsdam, Germany b Max-Planck-Institute for Astronomy, Ko¨nigstuhl 17, D-69117 Heidelberg, Germany Available online 25 July 2006
Abstract We have observed a sample of 7 luminous QSOs in the redshift range z = 1.8–2.9 selected from the Hamburg/ESO survey (HES). The aim of this study is to detect the galaxies that host the quasar and to estimate luminosities, colours, and stellar masses of the host galaxies. We were able to resolve the host galaxy around HE 2355–5457 and HE 2348–1444, both located at z = 2.9, in two bands. These data will shed light on the link between host galaxy and nuclear components of massive galaxies at high-redshifts. 2006 Elsevier B.V. All rights reserved. Keywords: Quasars: general; Galaxies: active; Galaxies: high redshift
Contents 1. 2. 3. 4. 5.
Data sample and observations . . . . . . . . . Data analysis. . . . . . . . . . . . . . . . . . . . . . Host galaxy magnitudes and colours . . . . . Stellar masses and virial black hole masses. Conclusion . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . .
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. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
. . . . . .
1. Data sample and observations We have obtained images of 7 luminous high redshift quasars with the ESO–VLT during several nights between May and July 2002, using the near-infrared camera ISAAC with its long-wavelength (LW) arm. We focused on the highest level of nuclear luminosities, MB 28, which would allow us to target the most massive ellipticals (McLure et al., 1999). We defined two redshift windows *
Corresponding author. E-mail address: [email protected] (M. Schramm).
1387-6473/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.newar.2006.06.025
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806 807 807 808 808 808
of 1.8 < z < 2.0 and 2.6 < z < 3.0 to avoid contamination from emission lines and to sample the stellar continuum ˚ break with the standard NIR filters J, above the 4000 A H and Ks. These filters always correspond approximately to rest-frame B and V. Observing each QSO in two different filters allows us to determine rest-frame B V colours for the quasar hosts and thus constrain their stellar population. Observations were photometric, and we had seeing conditions ranging from 0.600 to 0.800 . The data were reduced using the Eclipse software package provided by ESO, in particular the ISAAC reduction pipeline recipe lwjitter.
M. Schramm et al. / New Astronomy Reviews 50 (2006) 806–808
2. Data analysis Resolving the faint hosts of high redshift QSOs is still a challenge because the bright nucleus outshines the underlying galaxy. Besides good seeing, an excellent control of the point spread function (PSF) is required to successfully resolve the host galaxy. We used two different strategies to decompose our quasar images. The first was the simple but very successful PSF subtraction method, where we used bright unsaturated field stars to construct and subtract a rescaled PSF from the QSO. For the second approach we employed the decomposition tool PAMDAI by Kuhlbrodt et al. (2004) which uses a fully analytic description of the PSF. Unfortunately the LW arm of the ISAAC instrument shows a strong distortion pattern over the field which even varied between objects and between filters. So PSF construction proved to be a difficult task, with only few suitable stars available that were sufficiently close to a quasar. 3. Host galaxy magnitudes and colours For our two-dimensional analysis we assumed an elliptical morphology and plausible scale lengths for the host galaxy of 3, 5 and 10 kpc (adopting a standard cosmology with H0 = 70 km s1 Mpc1, Xm = 0.3 and XK = 0.7). Two quasars of the sample, HE 2355–5457 at z = 2.904 and HE 2348–1444 at z = 2.933, show extended emission coming from the host galaxy in both observed bands. We have a positive detection for the Ks band as well as (although weaker) for the H band of HE 2355–5457. This is shown in Fig. 1 where we present an example from our two-
807
dimensional modelling using a scale length of 10 kpc. We find that for the Ks band more than 8% and for the H band more than 6% of the total flux corresponds to the host galaxy. These numbers do not depend much on the assumed scale length. Results from our peak subtraction analysis are shown in Fig. 2 where we plot surface brightness profiles of the quasar, two different PSF stars and profiles of the residuals after subtraction. We find good agreement between the results from two-dimensional modelling and peak subtraction. We determined an averaged colour of (B V) = 0.23 ± 0.30 and an absolute magnitude of MV = 25.94 ± 0.30 for the peak subtraction analysis. For the the two-dimensional modelling we find (B V) = 0.04 ± 0.30 and MV = 25.82 ± 0.30. In order to compute the differential K corrections we assumed stellar populations of 0.1–1 Gyr; however, the K corrections are extremely small as Ks and H very nearly reproduce restframe V and B, respectively, at z . 2.9. For HE 2348 1444 we determined (B V) = 0.18 ± 0.30 and MV = 26.39 ± 0.30 using PAMDAI, while for the peak subtraction we find (B V) = 0.08 ± 0.30 and MV = 26.51 ± 0.30. The measured B V colours are rather blue and indicate relatively young stellar populations, with luminosity-weighted ages of a few hundreds of Myr, dominating the rest-frame visible light. For three further QSOs we could only estimate upper limits for the host galaxy magnitudes in the longer wavelength band. Finally, two QSOs in the sample did not have sufficiently close PSF stars to correct for the spatial PSF variations, and for these objects it was not even possible to determine meaningful upper limits on the host luminosities.
Fig. 1. Results of the two-dimensional model fits of HE 2355–5457. Top panel shows (from left to right) H band image of QSO composite, subtracted nucleus model, the host model assuming a scale length of 10 kpc and zero ellipticity and in the last frame subtracted nucleus and host model. The bottom panel shows the equivalent for the Ks band.
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M. Schramm et al. / New Astronomy Reviews 50 (2006) 806–808
H band
K s band
Fig. 2. Comparison of the radial surface brightness profiles of the H band and Ks band of HE 2355–5457 with two PSF stars. Each plot shows the azimuthally averaged profile of the QSO (solid line), first PSF star (dotted line) and second PSF star (dashed line) and the radial profiles of the residuals after subtraction of a rescaled PSF from the QSO image. The dotted line with open circles shows the profile after subtraction of the first PSF star and the dashed line with triangles shows the profile after subtraction of the second PSF star.
4. Stellar masses and virial black hole masses The derived B V colours give us the possibility to estimate the stellar masses for HE 2355–5457 and HE 2348– 1444. We used the conversion from colour to mass-to-light ratio from Bell et al. (2003) and derived 4–8 · 1011Mx for HE 2355–5457, and 8–12 · 1011Mx for HE 2348–1444. Using archival optical spectra, we measured C IV line widths to estimate black hole masses. We employed the formula by Vestergaard (2002) 2 ˚ 0:7 FWHMðC ivÞ kLk ð1350 AÞ 6 M bh ¼ 1:6:10 ð1Þ 1000 km s1 1044 erg s1 yielding Mbh = 3.1 · 1010Mx for HE 2355–5457 and Mbh = 1.3 · 1010Mx for HE 2348–1444. 5. Conclusion We were able to resolve the host galaxies of HE 2355– 5457 and HE 2348–1444, both at z 2.9, in H and Ks band
imaging using two-dimensional modelling and peak subtraction analysis. We derived B V rest-frame colours for both objects indicating significantly blue stellar populations, similar to what is seen for low-redshift QSO hosts Jahnke et al. (2004). The rest frame colours allowed us to estimate the stellar masses to be of order 1012Mx for the host galaxies. Using C IV line width measurements, we derived virial black hole masses of Mbh 3 · 1010Mx which are within the upper limits predicted by Vestergaard (2004).
References Bell, E.F., McIntosh, D.H., Katz, N., et al., 2003. ApJS 149, 289. Jahnke, K., Kuhlbrodt, B., Wisotzki, L., 2004. MNRAS 352, 399. Kuhlbrodt, B., Wisotzki, L., Jahnke, K., 2004. MNRAS 349, 1027. McLure, R.J., Kukula, M.J., Dunlop, J.S., et al., 1999. MNRAS 308, 377. Vestergaard, M., 2002. ApJ 571, 733. Vestergaard, M., 2004. ApJ 601, 676.