PHYSICAL PROPERTIES OF A PILOT SAMPLE OF SPECTROSCOPIC CLOSE PAIR GALAXIES ATz∼ 2

Abstract

We use Hubble Space Telescope Wide-field Camera 3 rest-frame optical imaging to select a pilot sample of starforming galaxies in the redshift range z = 2.00-2.65 whose multi-component morphologies are consistent with expectations for major mergers. We follow up this sample of major merger candidates with Keck/NIRSPEC longslit spectroscopy obtained in excellent seeing conditions (FWHM ∼0.5 arcsec) to obtain Hα-based redshifts of each of the morphological components in order to distinguish spectroscopic pairs from false pairs created by projection along the line of sight. Of the six candidate pairs observed, companions (estimated mass ratios 5:1 and 7:1) are detected for two galaxies down to a 3s limiting emission-line flux of 10 17 -erg s −1 cm −2 . This detection rate is consistent with a ∼50% false-pair fraction at such angular separations (1-2 arcsec) and with recent claims that the star formation rate (SFR) can differ by an order of magnitude between the components in such mergers. The two spectroscopic pairs identified have a total SFR, SFR surface densities, and stellar masses consistent on average with the overall z 2 star-forming galaxy population.

Imaging Spectroscopy of Ionized Gaseous Nebulae around Optically Faint AGNs at Redshift z ∼ 2

“…In Law et al (2012b), we found that the apparent pair fraction (i.e., the number of systems whose rest-optical morphologies show a companion within 2″ of uncertain physical association) for z∼2 star-forming galaxies was 23 6 7 -+ %, consistent with the 33% observed here. Similarly, in Law et al (2015), we demonstrated using deep spectroscopic observations that ∼50% of such apparent pairs in the galaxy sample had spectroscopic redshifts consistent with physical association. We would therefore expect 1.5 genuine spectroscopic pairs in our sample of 12 AGNs, consistent with the two such systems observed.…”

Order out of Randomness: Self-Organization Processes in Astrophysics

“…Our thick disk (which does not have arms, nor do other thick disks (they are nearly but not quite ubiquitous) formed 11-12 Gyrs ago, at z = 2 − 2.5 equivalent time, which is very close to the peak of the star formation rate for the local universe as a whole (Madau and Dickinson 2014) and probably also for the Milky Way (though we expect to know more about this topic when the Gaia data are fully in and analyzed), but it is likely that the Milky Way spiral pattern requires more than one mechanism. The oldest spiral reported so far (in a sea of dwarf irregular structures imaged by HST in its deep field) is labeled as Q2343-BX442 (Law et al 2015) and is of the "companion driven" variety, being seen at a redshift that corresponds to an age near 11 Gyrs, and evolution tends to move galaxies from flocculent to grand design (Francis and Anderson 2009).…”

Spectroscopically Identified Emission Line Galaxy Pairs in the WISP survey

“…The uncertainties in the merger rate measurement at z ≥ 1 are caused by various factors, including sample selection, pair morphology and merging timescales (e.g. Law et al 2015;López-Sanjuan et al 2015;Snyder et al 2017;Mantha et al 2018;Duncan et al 2019). At z > 0.5, pair identification becomes more difficult, mostly related to the increasing uncertainties in photometric redshifts and declining resolution for morphology identifications.…”

“…False pair identification is high (>50%) from morphological identification alone, due to chance sky alignments (e.g. Patton & Atfield 2008;Chou et al 2012;Law et al 2012Law et al , 2015. Groundbased spectroscopy is only available for small samples (e.g.…”

“…Groundbased spectroscopy is only available for small samples (e.g. Law et al 2015), or limited to local galaxies (z < 0.2), as large separations are required for multi-object spectrographs (e.g. Ellison et al 2008).…”