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HST/NICMOS Imaging of Bright High-redshift 24 μm Selected Galaxies: Merging Properties

Zamojski, Michel and Yan, Lin and Dasyra, Kalliopi and Sajina, Anna and Surace, Jason and Heckman, Tim and Helou, George (2011) HST/NICMOS Imaging of Bright High-redshift 24 μm Selected Galaxies: Merging Properties. Astrophysical Journal, 730 (2). Art. No. 125. ISSN 0004-637X.

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We present new results on the physical nature of infrared-luminous sources at 0.5 < z < 2.8 as revealed by HST/NICMOS imaging and Infrared Spectrograph mid-infrared spectroscopy. Our sample consists of 134 galaxies selected at 24 μm with a flux of S(24 μm)>0.9 mJy. We find many (~60%) of our sources to possess an important bulge and/or central point source component, most of which reveal additional underlying structures after subtraction of a best-fit Sérsic (or Sérsic+PSF) profile. Based on visual inspection of the NIC2 images and their residuals, we estimate that ~80% of all our sources are mergers. We calculate lower and upper limits on the merger fraction to be 62% and 91%, respectively. At z < 1.5, we observe objects in early (pre-coalescence) merging stages to be mostly disk and star formation dominated, while we find mergers to be mainly bulge dominated and active galactic nucleus (AGN)-starburst composites during coalescence and then AGN dominated in late stages. This is analogous to what is observed in local ULIRGs. At z ≥ 1.5, we find a dramatic rise in the number of objects in pre-coalescence phases of merging, despite an increase in the preponderance of AGN signatures in their mid-IR spectra and luminosities above 10^(12.5) L_☉. We further find the majority of mergers at those redshifts to retain a disk-dominated profile during coalescence. We conclude that, albeit still driven by mergers, these high-z ULIRGs are substantially different in nature from their local counterparts and speculate that this is likely due to their higher gas content. Finally, we observe obscured (τ_(9.7) μm>3.36) quasars to live in faint and compact hosts and show that these are likely high-redshift analogs of local dense-core mergers. We find late-stage mergers to possess predominantly unobscured AGN spectra, but do not observe other morphological classes to carry any specific combination of τ_(9.7) μm and polycyclic aromatic hydrocarbon (PAH) equivalent width. This suggests a high degree of variation in the PAH emission and silicate absorption properties of these mergers, and possibly throughout the merging process itself.

Item Type:Article
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URLURL TypeDescription
Yan, Lin0000-0003-1710-9339
Sajina, Anna0000-0002-1917-1200
Surace, Jason0000-0001-7291-0087
Helou, George0000-0003-3367-3415
Additional Information:© 2011 American Astronomical Society. Received 2010 July 12; accepted 2010 December 30; published 2011 March 14. We are grateful to M. Elitzer and L. Armus for insightful discussions about various aspects of mid-IR spectroscopy as well as to J. Lotz on aspects of automated classification. We are very much obliged toward J. Kartaltepe for kindly sharing her results ahead of publication and for providing us with helpful information. We thank P. Hopkins for providing us data from his simulations. We are grateful to V. Fadeyev and C. Peng for their help with their improved NICMOS pipeline and GALFIT softwares, respectively. We would like to extend our thanks to the referee for his/her comments that helped improve this paper. M.Z. acknowledges support from the HST GO grant 11142. Facilities: HST (NICMOS), Spitzer (IRS, MIPS)
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
NASAGO 11142
Subject Keywords:galaxies: evolution; galaxies: high-redshift; infrared: galaxies
Issue or Number:2
Classification Code:PACS: 98.54.Ep; 98.62.Qz; 95.30.Ky; 98.65.Fz; 98.54.Cm; 98.62.Js
Record Number:CaltechAUTHORS:20110415-095543716
Persistent URL:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:23343
Deposited By: Jason Perez
Deposited On:18 Apr 2011 16:02
Last Modified:04 Nov 2019 02:13

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