PhD project at the APC in Paris
We offer a PhD project as described below. The thesis will be conducted at the François
Arago Centre (http://www.apc.univ-paris7.fr/FACe), a space data and analysis centre at the
APC (http://www.apc.univ-paris7.fr/APC_CS/) in Paris, France.
Interested students should contact Volker Beckmann (Beckmann@apc.univ-paris7.fr) in order
to get further information. The applications for the PhD position have to be directed to the
1
Ecole Doctorale Astronomy & Astrophysics (ED 127 ). Based on the recommendation of
Volker Beckmann, the ED will determine the best candidate for the announced project and
will decide whether to fund the position (in addition external funding has been requested, but
the decision will not be available before the deadline for students’ application on May 19,
2013).
More information about the application process can be found under
http://ecole-doctorale.obspm.fr/-Candidater-a-un-contrat-doctoral-ED-127Application dead line: May 19, 2013
st
Start date for the PhD project: October 1 , 2013
Duration of the PhD project: 3 years
In order to derive the required recommendation from Volker Beckmann, you should contact
rd
him before May 3 , providing a CV and contact information of 2 references.
PhD thesis project: The Absorbed Universe
A large fraction of the emission from active galactic nuclei (AGN) in our Universe is absorbed
close to the central engine. Much of this absorbed flux is re-emitted in the infrared domain. In
addition there are studies which imply that the absorption of AGN was on average larger at
higher redshifts, thus, there has been an evolution. Without a precise knowledge of the
redshift evolution of absorption we are not able to determine the accretion history of the
Universe, which in turn has also implications on the star and galaxy formation because of
feedback processes between AGN and surrounding galaxy. At the same time, the amount of
intrinsic absorption might also depend on other aspects, such as the luminosity of the AGN.
For an in-depth introduction to the topic, see e.g. Beckmann & Shrader (2012).
Absorption is an important consideration when studying the distribution of AGN in space
through luminosity functions. With X-ray observations satellite missions like e.g. Chandra and
XMM-Newton we pick up also absorbed sources which we might miss in the optical domain,
except for extreme cases, with a hydrogen column density in excess of NH > 1E24 1/cm**2.
At those column densities it becomes practically impossible for most photons below 10 keV to
pass through the absorbing material. Another way to look for absorbed sources is to use the
sub-millimeter range. Using observations at 850 mu from the SCUBA array, Almaini et al.
(1999) pointed out that in considering the observed X-ray background intensity, that an
increasing fraction of the higher redshift sources has to be obscured.
It is important to note that any assessment of the net accretion power in the Universe must
carefully account for the effects of absorption. A large fraction of the cosmic infrared
background has to come from obscured AGN. Almaini et al. (1999) estimated that there might
be three times as many obscured AGN, i.e. with NH > 1E22 1/cm*2 than unobscured ones at
larger redshift. The same ratio should also apply for the comparison between Seyfert 2 and
Seyfert 1 galaxies. In the local Universe this ratio appears to be different. Seyfert 2 galaxies
are more difficult to assess, as their weaker optical emission lines can be hard to detect. One
way to tackle the problem is to study the luminosity function of a specific emission line, for
example of the [OIII] line at 500.7 nm. The ratio between type 1 and type 2 objects in the local
Universe (z < 0.15) seems to be a function of the luminosity of the objects, with the more
luminous objects being preferentially of type 1 (Beckmann & Shrader 2012). The numbers of
1
2
See http://ecole-doctorale.obspm.fr/-3-Information-for-foreignSee e.g. http://www.amazon.com/dp/3527410783
Seyfert 1 and Seyfert 2 galaxies are comparable at low luminosity, while at high luminosity,
Seyfert 1 galaxies outnumber Seyfert 2 galaxies by a factor of 2-4 (Hao et al. 2005). Thus, it
appears that we observe an evolution of AGN absorption, with a correspondingly higher
fraction of absorbed objects at redshifts z ~ 2.
Recently, Fanidakis et al. (2012) have put these observations into a theoretical framework.
Their model predicts that the comoving space density of AGN peaks at z ~ 3, similar to
cosmic star formation history. However, when taking into account obscuration, the space
density of faint AGN peaks at lower redshift (z<2) than that of bright AGN (z ~ 2-3). This
implies that the cosmic evolution of AGN is shaped in part by obscuration. Indeed, it appears
that absorption plays a major role and has to be taken into account when studying the
evolution of AGN. The evolution of absorption can be observed directly in X-ray data.
But studies of AGN at different redshift bins using X-ray observations have not given a fully
conclusive picture yet. La Franca et al. (2005) found more absorbed AGN at low luminosities
and at high redshifts using X-ray data from XMM-Newton, while other studies could not
confirm this trend (Ueda et al. 2003; Treister & Urry 2005). La Franca et al. also find that a
combination of effects (the fraction of absorbed AGN decreases with the intrinsic X-ray
luminosity and increases with the redshift) can be explained by a luminosity-dependent
density evolution model. They further show that the luminosity function of AGNs with low
luminosities, such as those presented here, peaks at z~ 0.7, while high-luminosity AGNs
peak at z~ 2. But also bright quasars which are significantly absorbed have been found. For
example, Polletta et al. (2006,2008) found heavily absorbed and bright quasars at z = 1.3-3
using infrared observations. Using hard X-ray data, e.g. Beckmann et al. (2006,2009) showed
that the fraction of AGN with mild absorption (NH > 1e22) is about 50%, but that heavily
absorbed (Compton thick) sources are rare in the local Universe, only about 5% of the AGN
belong to this class.
We can summarise what we know about the absorption in AGN as follows. First the heavy
super massive black holes are generated, and they dominate the high redshift Universe. The
fainter AGN and the type 2 sources are the main class of objects accreting since z ~ 1.5. The
fraction of absorbed sources increases with redshift, at least to redshift z ~ 2 but potentially
even further back in time. While the type 1 sources dominate optically selected samples, Xray surveys pick up both, type 1 and type 2. It seems apparent that the absorbed sources
outnumber the unabsorbed ones.
Still, this rough outline has yet to be confirmed through observations, plus we have to get a
handle on the exact numbers and the evolution if we want to understand the data of the
upcoming deep survey missions, like Euclid and LSST. The proposed PhD thesis is going to
investigate the absorption evolution in the X-rays and infrared, thereby providing important
information about AGN, the history of accretion in the Universe, and the potential of Euclid.
The intrinsic absorption of AGN can be measured in different ways. X-ray spectra can be
used to measure the hydrogen column density in the line of sight directly. Infrared data show
much of the reprocessed absorbed X-ray flux. A combination of both techniques, based on
large and deep sky surveys, is the most promising tool in order to understand the absorption
history. In this PhD project, both ways are going to be explored in order to measure the
average absorption of AGN at different redshifts. The hard X-ray surveys, ideal to get an
unobstructed view on the AGN population in the Universe, cover the whole sky, but compared
to the "pencil beam" surveys of Chandra and XMM-Newton the hard X-ray surveys are very
shallow, probing the Universe at redshift z<0.1. Progress is now coming from the NuSTAR
mission, which has been launched in June 2012. Data will become public in summer 2013
and the PhD student can start with investigating these data after becoming familiar with X-ray
analysis techniques and investigation of so far unpublished X-ray data from XMM-Newton
and Chandra. In addition, the imprint of absorption on the infrared emission of AGN will be
studied. Near infrared data will be used (such as the 2MASS, SDSS, or CFHT ground based
sky survey) to estimate the absorption of faint extragalactic X-ray sources (which are mainly
AGN). The results and the experience gained by the student will be used in order to provide a
prediction of the expected AGN source population as it will be visible to the near infrared
survey of Euclid. This approved ESA medium size mission is foreseen for launch in 2020. The
next years will be crucial to predict what we will be able to measure with Euclid. At the same
time, simulated Euclid data become available as the instruments are being build. The PhD
student will use simulated Euclid NIS data in order to determine to what precision we will be
able to measure absorption in AGN up to which redshift.
The results from the student's studies on the absorption history described earlier, will then
feed back into the Euclid simulations.
References:
Almaini, Lawrence, Boyle 1999, MNRAS, 305, L59
Beckmann, Soldi, Shrader, Gehrels, Produit 2006, ApJ, 652, 126
Beckmann, Soldi, Ricci, et al. 2009, A&A, 505, 417
2
Beckmann & Shrader 2012, "Active Galactic Nuclei", text book, Wiley-VCH
Fanidakis, Baugh, Benson, et al. 2012, MNRAS, 419, 2797
Hao, Strauss, Fan, et al. 2005, AJ, 129, 1795
La Franca, Fiore, Comastri, et al. 2005, ApJ, 635, 864
Polletta et al. 2006, ApJ, 642, 673
Polletta et al. 2008, ApJ, 675, 960
Treister & Urry 2005, ApJ, 630, 115
Ueda, Akiyama, Ohta & Miyaji 2003, ApJ, 598, 886
2
See e.g. http://www.amazon.com/dp/3527410783