introduction and motivation - dartmouth...

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Introduction and Motivation

My research focuses on exploring these questions at z ~ 2 – 3.

This last two days at this conference, we’ve focused on two large questions regarding the role that AGNs play in galaxy evolution:


Wall et al. (2005) Star formation rate density and QSO space density

Why z ~ 2 – 3 ?

At this redshift range, both star formation and AGN activity were at a peak in the universe.

What can we say about the AGNs that are hosted by galaxies undergoing the bulk of the total star formation at this epoch?

What types of galaxies host AGNs at this redshift range?

How does the presence of an AGN influence gas kinematics and star formation?

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BX

BM

LBG

MD

Steidel et al. (2004)

The objects that comprise the UV-selected AGN sample were initially selected using UGR colors to be in the redshift range of z ~ 1.0 – 3.5 (LBGs and BX/BM objects).


LBG

BX

BM


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Rest-frame UV spectra for the LBGs has been used to examine gas kinematics for these objects.

The LBGs have strong star-formation (SFRs ~ 10 – 1000 Msun yr-1), with ubiquitous outflows ( >100 km s-1, Shapley et al. 2003, Steidel et al. 2010)



The Lyman-Break galaxy samples acts as an ideal high-redshift non-active control sample in order to decouple the effects of an AGN and star-formation.

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UV-Selected AGNs at z ~ 2 – 3

Multi-wavelength data set: - Optical magnitudes (Steidel et al. 2003, 2004, Reddy et al. 2006) -  Near-IR coverage in J and K - Spitzer IRAC coverage in the [3.6], [4.5], [5.8] and [8.0] µm bands for 11 objects. - Rest-frame UV Spectra (Steidel et al. 2002) - Rest-frame optical spectra for a subset of the objects. (Erb et al. 2006)

Q2343-BX333 z = 2.397

Redshift 2 3 4

AGNs non-AGNs

The AGNs were selected by virtue of strong UV emission features.

The sample consists of 33 “UV-selected” z~2-3 Type II AGN, expanded from an initial sample of 16 Type II AGN at z~3 (Steidel et al. 2002)

Based on clustering analysis in Steidel et al. (2002), the UV-selected narrow-line AGNs are though to be hosted by the equivalent of LBGs.

Rest-Frame UV Composite Spectrum

-  The rest-frame UV contains many emission and absorption features that are useful in tracing gas kinematics. -  LRIS spectra for the objects in our AGN sample were shifted to the rest-frame and co-added to create a high signal-to-noise composite spectrum. 7

Hainline et al. (2011)

The UV composite spectrum shows a much redder slope when compared to the non-AGN LBG composite from Shapley et al. (2003).

Absorption line strength is inversely correlated to Ly-α EW, which can be understood if the escape of Ly-α photons is at least partially modified by the covering fraction of neutral gas in the ISM. 8

Rest-Frame UV Composite Spectrum Hainline et al. (2011)

(Non-AGN composite spectrum from Shapley et al. 2003)

The Si IV λ1393 feature has a -180 km s-1 offset in the non-AGN spectrum.

The feature has a -845 ± 171 km s-1 offset in the AGN spectrum. 9

Rest-Frame UV Composite Spectrum Hainline et al. (2011)

Host Galaxy Stellar Populations

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In collaboration with Alice Shapley, Jenny Greene, Charles Steidel, Naveen Reddy, and Dawn Erb

locally

High- redshift

Log(Mstar / Msun)

Local AGNs are preferentially found in massive, bulge dominated galaxies.

Recent results suggest that active galaxies at higher-redshift reside in massive galaxies with intermediate colors.

Kauffmann et al. (2003)

Log(Mstar / Msun)

R-K

Kriek et al. (2007)

U -

B (A

B)

Rosario et al. (2011)

Log(Mstar / Msun)

0.5

1.0

1.5 2.0 < z < 2.8 z ~ 2.3

AGNs AGNs


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-  Large R-K colors, as compared to the R-K colors from a sample of z~2-3 non active galaxies, imply large stellar masses, older stellar ages, and/or more dust extinction for the AGN hosts.

-  The AGN fraction in LBGs is around ~3%.

-  In order to understand the origin of this difference in R-K color, and see what types of galaxies host AGNs at high-redshift, we performed stellar population synthesis modeling on the UV-selected AGNs.

R-K


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Assef et al. (2010)

Dust emission

AGN

Stellar Model Total

We used stellar population synthesis modeling to understand the host galaxy populations of the UV-selected AGNs.

The 11 IRAC AGNs were modeled using both a stellar population model and an AGN template. observed wavelength (µm)


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Constant Star formation


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We can infer AGN properties for the IRAC AGNs from our dual-component modeling:

- Accretion Rate M = Lbol /εradc2

<M>= 0.3 M⊙ yr −1

- Black Hole Masses Using the Haring and Rix (2004) and Merloni et al. (2010) relations,

<log(MBH/M⊙)> = 8.36

- Eddington Ratios λEdd = Lbol/Ledd

Median λEdd = 0.03

The UV-selected AGNs are accreting at significantly sub-Eddington rates, indicating that they must have done much of their growth in the past.


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The IRAC AGNs span the full range of power-law slopes, and are a representative subsample of the entire population of UV-selected AGNs.

We apply the results from modeling the IRAC AGNs with dual-component modeling to the UV-selected AGNs without IRAC coverage.

Ste

rn e

t al.

(200

5)

non-AGN LBGs

Donley et al. (2012)

AGNs


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Chabrier IMF

AGN Sample

For constant star-formation, the average host galaxy properties:

<E(B-V)> = 0.22 <Age> = 1548 Myr <SFR> = 63 Msun/yr <log(Mass/Msun)> = 10.85


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UV selected AGN Sample

For constant star-formation, the average host galaxy specific star formation rate:

<sSFR> = 0.85 Gyr-1 Mainieri et al. (2011)


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AGN Sample non-AGN Sample

UGR-selected, same redshift range, no UV emission features, and fit in the same way as the non-IRAC AGNs.

Mass-Matched non-AGN Sample

For each AGN, we chose six non-AGNs with the same stellar mass to form a comparison sample.

The z~2-3 star-forming galaxies show a blue sequence, and the AGNs exist in similarly-colored galaxies as the non-AGNs matched in stellar mass.


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AGN Sample non-AGN Sample Mass-Matched non-AGN Sample

The AGN host galaxies have indistinguishable properties to those of a mass-matched non-active sample.

0

Log(M*/Msun) 10 11 12

0.1

0.2

0.3

Obs

erve

d A

GN

Fra

ctio

n


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The UV continuum of the AGN UV composite is predominantly starlight, and the red slope is due to high levels of extinction.


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We created composite UV spectra after separating the AGN spectra into two bins by mass. The EW for the AGN emission lines are shown to be strongly dependent on stellar mass.


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The EW and the extinction-corrected line luminosities for the AGN emission lines are larger in higher mass host galaxies.


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We can explain the mass segregation of the UV-selected AGNs if, at z~2-3: 1.  There is a constant Eddington ratio distribution 2.  MBH is proportional to M∗

We estimated the CIV luminosity for galaxies below 1010 Msun under the assumption that emission line luminosity traces AGN luminosity, and scales linearly with stellar mass.

Our results indicate that CIV would be too weak to be detected in lower mass galaxies.

The segregation of UV-selected AGNs in high-mass hosts suggests that MBH and M∗ are already correlated at z > 2, during the epoch when both bulges and black holes are actively growing.

Conclusions & Summary

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•  The rest-frame UV composite spectrum for our AGN sample shows several emission lines characteristic of AGNs, as well as interstellar absorption features detected in star-forming LBGs.

•  The UV continuum slope of the composite spectrum is significantly redder than that of a sample of non-AGN UV-selected star-forming galaxies.

•  Blueshifted SiIV absorption provides evidence for outflowing highly ionized gas in these objects at speeds of ~103 km s-1, quantitatively different from what is seen in the outflows of non-AGN LBGs.

•  The host galaxies for the UV-selected AGNs have high masses, older stellar ages, and higher SFRs on average than what is measured for the full sample of non-active star-forming galaxies, but similar U-V colors, SFRs, stellar ages, and E(B-V) values to those derived for a mass-matched non-AGN control sample.

•  We estimated that CIV emission would not be detectable in galaxies below 1010 Msun, assuming a constant Eddington ratio and a correlation between black hole mass and stellar mass at z > 2. Alternatively, the low-mass galaxies lack supermassive BHs, or are radiating at significantly lower Eddington ratios.

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