The Role of Galaxy Mergers Among High
Redshift ULIRGsJeyhan S. Kartaltepe
Hubble Fellow - NOAO
Hubble Fellows Symposium2012 March 7
The Role of Galaxy Mergers Among High
Redshift ULIRGsJeyhan S. Kartaltepe
Hubble Fellow - NOAO
Hubble Fellows Symposium2012 March 7
z < 0.3 z~2z~2
What are LIRGs/ULIRGs?LIR(8-1000 mm) > 1011 L LIRGs
LIR > 1012 L ULIRGs, LIR > 1013 L HyLIRGs!!Rare locally …
PAH Complex
DustStellar ContinuumArp 220
Kennicutt et al. 2003
High Redshift (U)LIRGs
ULIRGsLIRGs
Comoving Infrared Energy Density
Low Lum
Le Floc’h et al. 2005
LIRGs dominate IR energy density (and cosmic star formation rate) at z > 0.7 Le Floc’h et al. 2005
ULIRGs as important or dominate by z~2 Caputi et al. 2007; Magnelli
et al. 2009, 2011; Bethermin et al. 2011; Murphy et al. 2011
Important role at the peak of galaxy assembly
MotivationWhat drives star formation and AGN activity
and how does this change as a function of redshift?What is the role of galaxy mergers?Are mergers necessary for the extreme
luminosities of (U)LIRGs?What was the role of ‘cold flow’ gas
accretion in the early universe?How does galaxy morphology correlate with
luminosity and therefore star formation rate?
Properties of Local (U)LIRGs
ULIRGs: IRAS 1 Jy Sample, med(z) = 0.145 (Veilleux, Kim, & Sanders 2002) > 99% are major mergers of gas rich spirals
LIRGs: RBGS, med(z) =0.0082 (Sanders et al 2003, Ishida 2004)
log(LIR) > 11.5 strongly interacting major mergers (65%) doubles (18%) minor interactions (18%)
log(LIR) < 11.5 strongly interacting major mergers (36%) doubles (23%), minor interactions (15%) high luminosity end of normal starforming disks (26%)
Fraction of mergers increases systematically with LIR!
Properties of Local (U)LIRGsFraction of (U)LIRGs with an AGN increases with
LIR Veilleux et al. 1995, 1999; Tran et al. 2001; Yuan
et al. 2010< 20% for LIR < 1011 L
> 50% for LIR > 1012.3 L
Large fraction of composites Mix of SF, AGN, shocks Difficult to disentangle
Yuan et al. 2010
The Merger Scenario Mergers among
ULIRGs are ubiquitous
High AGN fraction among ULIRGs
Leads to the merger scenario (Sanders et al. 1988) Evolutionary
connection: Gas-rich mergers LIRG ULIRG QSO
Eventually form “red and dead” elliptical
Hopkins et al. 2008
The Role of Cold FlowsTheoretical simulations suggest that the high SFRs
of (U)LIRGs can be sustained at high redshift (z~2) by ‘cold flows’ (e.g., Dekel et al. 2009, Davé et al. 2009) Accretion of cold gas along filaments Minor mergers
Observationally supported by apparent lack of mergers among high redshift (U)LIRGs (e.g., Genzel et al. 2006; Forster-Schreiber et al. 2009) Mix of results at high redshift so far High redshift mergers are hard to see Cold flows have yet to be observed!
High Redshift (U)LIRGsAre mergers necessary to produce these extreme
luminosities?Studies at high redshift have been limited so far
Small samples Uncertain luminosities Lack of rest-frame optical imaging
Most previous studies have been based on Spitzer 24 mm selection Pre-Herschel, longer wavelength data has only been
available for small numbers of objects (e.g., Spitzer 70/160 mm, Submillimeter)
Previous High Redshift (z~2) Studies
24 mm / color selected samples e.g., Dasyra et al. 2008; Melbourne et al. 2009; Bussmann et
al. 2009, 2011; Zamojski et al. 2010 Wide range of results
Submillimeter Galaxies (SMGs) Morphology (e.g., Conselice et al. 2003; Swinbank et al. 2010) Kinematics (e.g., Tacconi et al. 2008) Tend to find high fractions of mergers
IFU Kinematics of massive star forming galaxies (SMGs/BzKs, etc.) e.g., Genzel et al. 2008; Forster-Schreiber et al. 2009 Mixed results – 1/3 mergers, 1/3 rotation dominated, 1/3
dispersion dominated
GOODS-HerschelHerschel coverage of both GOODS fields (PI: D. Elbaz)
Deepest Herschel data taken! Full GOODS-N field: 1.6 mJy (@ 100 mm) Central part of GOODS-S: 0.6 mJy (@ 100 mm)
Small area coverage but very deepWell suited for identifying high
redshift (z > 1) sources100-500 mm coverage is closer
to the peak of emission Better for constraining LIR
and Tdust
GOODS-N GOODS-S
Identifying Merging Galaxies
Easy in the local universe!
Nearby Examples: The
Mice & NGC 520
Identifying Merging Galaxies
Easy in the local universe!
At high redshift surface brightness dimming becomes a problem…
Hibbard & Vacca 1997
Identifying Merging Galaxies
And band shifting becomes important!Rest-frame UV versus Rest-frame optical
ACS I band
WFC3 H band
CANDELSCosmic Assembly Near-infrared Deep Extragalactic
Legacy Survey (PIs: S. Faber & H. Ferguson)Multi-cycle HST Treasury ProgramWFC3 NIR imaging of portions of 5 deep fields
GOODS-N, GOODS-S, UDS, COSMOS, EGS
UDS Mosaic
GOODS-Herschel & CANDELS
A match made in heaven!
Rest-frame optical imaging for z~2 galaxies
Ideal for probing structure of z~2 ULIRGs
GOODS-Herschel ULIRG Sample
• 52 ULIRGs with CANDELS imaging
• First complete, FIR selected sample of ULIRGs at z~2!
• Additional 70 LIRGs over this z range
<z> ~ 2.2
<log(LIR)> ~ 12.26
• Focus on all ULIRGs with z = 1.5 - 3.0 in GOODS-S
Comparison SampleSelected 260 comparison galaxies (5 for each ULIRG)
Not Herschel detected less luminous z~2 populationMatched to redshift and H magnitudeRandomized and classified
ULIRGs + comparison Visual classification scheme Classified by me + 3-5 other
classifiers Analyzed agreement Eventually compare to
quantitative merger selections
Visual Classification Scheme
Main Morphologi
cal Class
Interaction Class
Results
Merger+Int+IrrKartaltepe et al. 2012
Comparison with z ~ 1 COSMOS 70 mm
277 sources 0.8 < z < 1.2 Relatively shallow, but large
area coverage log(LIR) = 11.3 - 12.9
GOODS-Herschel North+South 100/160 mm Smaller area, significantly
deeper 293 sources w/ 0.8 < z < 1.2 Log(LIR) = 10.6 – 12.4
Classified using ACS imaging and same scheme
”Main Sequence” and Starburst Galaxies
Correlation between star formation rate (SFR) and stellar mass True locally (e.g., Brinchmann et al. 2004) and at high
redshift (e.g., Noeske et al. 2007; Daddi et al. 2007) Evolves with redshift (increase in gas fraction?)
Galaxies elevated above this relation Starbursts
Noeske et al. 2007
”Main Sequence” and Starburst Galaxies
Elbaz et al. 2011
Rodighiero et al. 2011
What is the role of mergers among starbursts and “main sequence” galaxies at z~2?
Are z~2 (U)LIRGs Starbursts?Main Sequence (MS)
from Elbaz et al. 2011
MS x3
Kartaltepe et al. 2012
Are z~2 (U)LIRGs Starbursts?
Starbursts
Starbursts
Kartaltepe et al. 2012
Starbursts
Starbursts
Kartaltepe et al. 2012
Are z~2 (U)LIRGs Starbursts?Disks:
med(sSFR/sSFRMS) = 2.4
Starbursts
Starbursts
Kartaltepe et al. 2012
Are z~2 (U)LIRGs Starbursts?Interactions &
Mergers:med(sSFR/sSFRMS) =
3.8
Starbursts
Kartaltepe et al. 2012
What are z~2 “Main Sequence” Galaxies?
“Main Sequence”Non-interacting disks: 57%Interactions & Mergers: 24%
Starbursts
Kartaltepe et al. 2012
Are z~2 Starbursts Mergers?
StarburstsNon-interacting disks: 42%Interactions & Mergers: 50%
ConclusionsMorphologies of z~2 ULIRGs span a wide
range‘Disks’ make up a significant fraction (many
irregular)40% non-interacting, 60% total
~45% are mergers or interactionsAdditional ~25% are irregular (minor mergers?)Comparable to fractions at z~1, slightly lowerMore likely to be interacting pairs than advanced
mergers z~2 ULIRGs are more like z~0 LIRGs
ConclusionsMost (U)LIRGs on the main sequence are
non-interacting disks (57%)BUT significant (24%) number are
mergers/interactionsSignificant enhancement of SFR for a short
time during the mergerHalf of starbursts are clear
mergers/interactions Large fraction of “Irregular Disks”
Enhanced role of minor mergers?
Future Directions(aka: Next Year’s Talk)
The results presented were small numbers! Need rest of CANDELS fields + expanded
Herschel coverage for firm conclusions/statistics
Herschel+CANDELS – PI: M. Dickinson (OT2)Coming soon!
NIR spectroscopic follow-upStarburst-AGN emission line diagnosticsWhat is the role/contribution of the AGN?Large international Subaru FMOS program of
COSMOS
Starburst Montage
Fiber Multi-Object Spectrograph (FMOS)Low-res (R~600) Hi-res (R~2200) In low-res mode,
can cover 0.9-1.8 mm at once
400 fibers30’ diameter FOV
Ideal for COSMOS!
AGN Fraction
5% at low LIR
>70% for ULIRGs
100% of HyLIRGs
log (LIR)
AGN
Frac
tion
AGN Fraction increases systematically with LIR (as it does locally)!
Kartaltepe et al. 2010a
Spectroscopic AGN Selection
Spectroscopic selection (e.g., Baldwin et al. 1981, Veilleux & Osterbrock 1987, Kauffmann et al. 2003, Kewley et al. 2006, Yuan et al. 2010)
Classes: SF, AGN dominated and compositesDAGN measure separates relative contribution
Kewley et al. 2006 Yuan et al. 2010
Spectroscopic AGN Selection
At low redshift, we can classify 283 galaxies from COSMOS 70 mm sample using standard line diagnostics 0 < z < 0.5 (where Ha still observable, 38% of sample at these
redshifts) log(LIR) = 8.8 – 12.1, <log(LIR)> = 10.7
AGN
HII
Comp.
Beyond z>0.5, lose Ha and [NII] lines, but have [OIII] and Hb out to z~1
Color as a proxy for [NII]/Ha? (e.g., Yan et al. 2010)
Appears to work well for normal galaxies locally…
At higher redshifts….
Yan et al. 2010
Applied to 70 mm Sample
500 galaxies with [OIII] and Hb
0 < z < 1 (~40% of sample in this range)
log(LIR) = 8.5 – 12.5, <log(LIR)> = 11.1
30 ULIRGs, 256 LIRGs70 mm selected
galaxies just aren’t red!
AGN
Mass-Excitation
Method of Juneau et al. 2011 uses mass instead
Applied to 70 mm Sample
Same 500 galaxies with O[III] and Hb
Appears to work better for infrared selected galaxies
X-ray mostly in AGN region
Trend with LIR
AGN
What about higher redshifts?
Current and future surveys with NIR spectrographs Until now, only small samples possible with longslit
spectographs Several multi-object spectrographs now online or will
be soon MOIRCS, FMOS, LUCIFER, FLAMINGOS2, MOSFIRE, etc.
Will become possible to use BPT type analysis at z = 1 - 3 for large samples
COSMOS FMOS survey will cover 1000’s of IR selected galaxies and AGN over the next few years
Sample Spectra
Preliminary FMOS Results
~60 (U)LIRGs at 1.0 < z < 1.6 with coverage of all four lines
Preliminary FMOS Results
Same sources on Mass-Excitation Diagram
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