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Morphologies and Environments of the Brightest X-ray
Selected AGN
1 Introduction/Motivation
What role is played by the environments of AGN in determining their properties? And how
do these relate to their host properties? Can we quantify the star formation in AGN hosts
and relate it to either larger obscuration or higher fueling rates (AGN luminosity)?
Galaxy interactions are an important mechanism in galaxy formation and nuclear fueling,
because they create non-axisymmetric disk perturbations and are an eфcient way to transfer
angular momentum and to drive gas inwards. Thus they have been related to enhanced star
formation, to the formation of the spheroidal components and of supermassive black holes in
the galaxy centers and to the triggering of galactic nuclear activity. Given that the presence
of a supermassive black hole is not a crucial discriminator between AGN and non-AGN
galaxies (e.g. Richstone et al. 1998, Magorrian et al. 1998, Taniguchi 2002), it follows
that the triggering process is much more important in initiating nuclear activity, ansd it
also crucially depends on the particular characteristics of the host galaxy (for instance the
stability of disks is dependent on the bulge-to-disk ratio). Even though the dynamical eects
by non-axisymmetric structures (such as bars) and galactic interactions do not give a simple
triggering mechanism for AGNs, it seems reasonable and consistent with many observational
results, that most AGNs in the local universe are triggered by minor (Seyferts) or major
(quasars) galactic mergers.
It is intriguing that IR-selected (redder colors, higher-luminosity) AGN are often found
in equal-mass merging systems and often show massive starbursts, while optically-selected
(bluer colors, lower-luminosity) AGN are found primarily in weakly interacting systems and
might not have enhanced star formation (quantify and qualify this statement). If this is
true it could suggest that the AGN fueling is relatively long-lived and steady compared with
the tidal disturbances and accompanying starbursts. Any such conclusions are plagued by
selection eects. Optically selected AGN samples pick up the brightest AGN and are biased
by orientation eects (thus missing maybe up to 80% of obscured AGN, eg. Webster et al.
1995). IR samples select redder (more obscured) AGN and are supposedly less aected by
orientation eects. This was shown initially by IRAS in the mid and far-IR wavelengths
(e.g. Beichman et al. 1986, Low et al. 1988, Cutri et al. 1994, Hines et al. 1995 and 1999,
etc) for extremely reddened objects, and subsequently also in the n-IR (e.g. 2MASS Cutri
et al. 2001 and 2002). The above imply a large missing population of red QSOs, which is
consistent with theoretical predictions for the X-ray background (eg. Comastri et al. 1995).
It is only hard X-ray samples that are selecting AGN regardless of optical obscuration, eg.
pick up objects that in the optical are dominated by their host light, and thus can oer
unbiased views of the nature of galactic nuclear activity, its possible triggering mechanisms
and its relationship to the host galaxies and the star formation activity.
Here I suggest to use a volume limited sample of X-ray selected AGN (a) to study their
host galaxy properties, their environments and how these relate to the properties of the
active nucleus, (b) to compare them with AGN samples selected in other wavelengths, (c)
to compare them to the properties of a \control" sample of lower Lx objects and address
the questions of AGN vs starburst dominance. This sample has the advantage of being
unaected by evolutionary eects.
Alternatively/Complementary study of a magnitude limited sample of X-ray selected
AGN, spanning a larger redshift range (up to 1) to address the same questions but having
a better handle in the study of their morphological properties. (Since this will cover a
range of redshifts and give more objects for study than above, it would be possible to study
(in a limited way) the evolutionary eects on interactions/host morphology and SF/AGN
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contributions)
2 Samples and Observations
Size of samples:
Starting with the source lists from the z-band detection images on the two GOODS elds,
we can select:
(a) A volume-limited sample: Among the objects with available redshifts (photometric
or spectroscopic) we have 9 objects (4S, 5N) with Lx>10 42 and z<0.6. For 7 of them,
z mag =19-22. A control sample with Lx<10 42 and z<0.6, contains many dozen objects with
z mag =17.5-25.5
(b) A Magnitude limited sample: Among the objects with availablke redshifts we select
about 20 with z mag <21 and Lx>10 42 , spanning a redshift range up to z=1. Many more
are selected in the same mag and redshift ranges to form a control sample with Lx<10 42
Observations to be sought after (to be completed with proposed instruments:
- How many of the selected objects have resolved spectroscopic information? If not, slit
spectroscopy would be useful!
- Two dimensional integral eld spectroscopy to study the host properties (star forma-
tion) vs. the nuclear (AGN) properties.
3 Work using ACS images:
Broad directions
(a) Characterize the nuclei and host galaxies, (b) Estimate the relative contributions
of the AGN, starburst and stellar components, (c) Quantify morphological signatures of
interactions and characterize their immediate environments.
In some detail:
Classify them according to:
- Nuclear type: Their AGN classication and type will be based on spectroscopic infor-
mation (when available), Lx and hardness ratios, and SED tting.
- Host type: Detailed prole decompositions (after tting and subtracting the AGN light)
with 2-D (GalFit) and 1-D (Ellipse) tting.
Properties to be measured:
- AGN luminosities and colors (through PSF tting)
- Host luminosities, colors (through detailed tting and AGN subtraction) and color
gradients.
- Light concentration (nuclear/host measures and various concentration indices)
- Clumpiness measures, mapping star forming regions (color gradients, uxes)
- Interaction features (tidal, bars, etc)
- Survey their immediate environments (ideally redshifts, spectral and detailed photo-
metric information also for the companions)
(Most of these studies have to be repeated in many dierent bands, including n-IR when
available. For instance, to spot star formation triggered by mergers it is best to observe in
blue lters, but to spot multiple nuclei one has to use the reddest lters)
Products
Tables: Listing all the above properties in meaningful groupings for both target and
control samples.
Images: (a) Original, PSF/nucleus subtracted, plus isophotal contours (both observed
and tted, to indicate the deviations due to tidal features). (b) Light prole ts for nuclear
and host components.
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Spectra: If available (in particular in case of 2D spectra). Ionization maps, velocity maps
(if 2D spectroscopy available).
4 Discussion
Intercompare these properties: e.g. optical colors might be related to obscuration (and
maybe Nh), or/and stellar populations, interaction stage, etc. Asymmetries and clumpiness
to interaction stage and/or star formation events. Dierent AGN types/luminosities might
reside in dierent host types, or be related to dierent optical obscuration, degrees of star
formation, interaction stages, types of environments.
Furthermore, relate the optical and X-ray properties of the target and the control sam-
ples. For instance the latter (low Lx) may contain intrinsically low-Lx AGN that might be
revealed by the light prole tting. Or the hardness ratios and Lx uxes might be related
to the nuclear/star formation contributions revealed by the optical properties. And so on.
Compare with IR-selected (e.g. 2MASS AGN samples as in Hutchings et al. 2003,
Marble et al. 2003), and optically-selected (and line-emission?) samples, in the same range
of redshifts/luminosities.
5 Desired Results..
Find some kind of interaction sequence that correlates with starburst and AGN properties:
Declining starburst/AGN contribution with time after initial interaction or/and declining
fueling rate. Obscured AGN more likely to be found in messy systems (or some other type
of hosts). AGN luminosity scales with host luminosity. etc..
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