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Register a new userThe Rest-Frame FUV Morphologies of Star-Forming Galaxies at z ~ 1.5 and z ~ 4
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We apply a new approach to quantifying galaxy morphology and identifying galaxy mergers to the rest-frame far-ultraviolet images of 82 z ~ 4 Lyman break galaxies (LBGs) and 55 1.2 < z < 1.8 emission-line galaxies in the GOODS and Ultra Deep Fields. We compare the distributions of the Gini coefficient (G), second-order moment of the brightest 20% of galaxy light (M20), and concentration (C) for high-redshift and low-redshift galaxies with average signal to noise per pixel > 2.5 and Petrosian radii > 0.3 arcsec. Ten of the 82 LBGs have M20 >= -1.1 and possess bright double or multiple nuclei, implying a major-merger fraction of star-forming galaxies ~ 10-25% at M_{FUV} < -20, depending on our incompleteness corrections. Galaxies with bulge-like morphologies (G >= 0.55, M20 < -1.6) make up ~ 30% of the z ~ 4 LBG sample, while the remaining ~ 50% have G and M20 values higher than expected for smooth bulges and disks and may be star-forming disks, minor-mergers or post-mergers. The star-forming z ~ 1.5 galaxy sample has a morphological distribution which is similar to the UDF z ~ 4 LBGs, with an identical fraction of major-merger candidates but fewer spheroids. The observed morphological distributions are roughly consistent with current hierarchical model predictions for the major-merger rates and minor-merger induced starbursts at z ~ 1.5 and ~4. We also examine the rest-frame FUV-NUV and FUV-B colors as a function of morphology and find no strong correlations at either epoch.
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We use the high angular resolution in the near-infrared of the WFC3 on HST to determine YHVz color-color selection criteria to identify and characterize 1.5<z<3.5 galaxies in the HUDF09 and ERS (GOODS-South) fields. The WFC3 NIR images reveal galaxies at these redshifts that were undetected in the rest-frame UV HUDF/GOODS images, as well as true centers and regular disks in galaxies classified as highly irregular in rest-frame UV light. Across the 1.5<z<2.15 redshift range, regular disks are unveiled in the WFC3 images of ~25% of both intermediate and high mass galaxies, i.e., above 10^10 Msun. Meanwhile, galaxies maintaining diffuse and/or irregular morphologies in the rest-frame optical light---i.e., not yet dynamically settled---at these epochs are almost entirely restricted to masses below 10^11 Msun. In contrast at 2.25 < z < 3.5 these diffuse and/or irregular structures overwhelmingly dominate the morphological mix in both the intermediate and high mass regimes, while no regular disks, and only a small fraction (25%) of smooth spheroids, are evident above 10^11 Msun. Strikingly, by 1.5 < z < 2.25 roughly 2 out of every 3 galaxies at the highest masses are spheroids. In our small sample, the fraction of star-forming galaxies at these mass scales decreases concurrently from ~60% to ~5%. If confirmed, this indicates that z~2 is the epoch of both the morphological transformation and quenching of star-formation which assemble the first substantial population of massive ellipticals.
We have examined in detail the morphologies of seven z~1.5 passively evolving luminous red galaxies using high resolution HST NICMOS and ACS imaging data. Almost all of these galaxies appear to be relaxed systems, with smooth morphologies at both rest-frame UV and visible wavelengths. Previous results from spectral synthesis modeling favor a single burst of star formation more than 1 Gyr before the observed epoch. The prevalence of old stellar populations, however, does not correlate exclusively with early-type morphologies as it does in the local universe; the light profiles for some of these galaxies appear to be dominated by massive exponential disks. This evidence for massive old disks, along with the apparent uniformity of stellar age across the disk, suggests formation by a mechanism better described as a form of monolithic collapse than as a hierarchical merger. These galaxies could not have undergone a single major merging event since the bulk of their stars were formed, more than 1 Gyr earlier. There is at least one case, however, that appears to be undergoing a dry merger, which may be an example of the process that converts these unusual galaxies into the familiar spheroids that dominate galaxies comprising old stellar populations at the present epoch.
A new set of color selection criteria (VJL) analogous with the BzK method is designed to select both star-forming galaxies (SFGs) and passively-evolving galaxies (PEGs) at 2.3<z<3.5 by using rest-frame UV--optical (V-J vs. J-L) colors. The criteria are thoroughly tested with theoretical stellar population synthesis models and real galaxies with spectroscopic redshifts to evaluate their efficiency and contamination. We apply the well-tested VJL criteria to the HST/WFC3 Early Release Science field and study the physical properties of selected galaxies. The redshift distribution of selected SFGs peaks at z~2.7, slightly lower than that of Lyman Break Galaxies at z~3. Comparing the observed mid-infrared fluxes of selected galaxies with the prediction of pure stellar emission, we find that our VJL method is effective at selecting massive dusty SFGs that are missed by the Lyman Break Technique. About half of the star formation in massive (M_{star}>10^{10}M_{Sun}) galaxies at 2.3<z<3.5 is contributed by dusty (extinction E(B-V)>0.4) SFGs, which however, only account for ~20% of the number density of massive SFGs. We also use the mid-infrared fluxes to clean our PEG sample, and find that galaxy size can be used as a secondary criterion to effectively eliminate the contamination of dusty SFGs. The redshift distribution of the cleaned PEG sample peaks at z~2.5. We find 6 PEG candidates at z>3 and discuss possible methods to distinguish them from dusty contamination. We conclude that at least part of our candidates are real PEGs at z~3, implying that this type of galaxies began to form their stars at z>5. We measure the integrated stellar mass density of PEGs at z~2.5 and set constraints on it at z>3. We find that the integrated stellar mass density grows by at least about factor of 10 in 1 Gyr at 3<z<5 and by another factor of 10 in next 3.5 Gyr (1<z<3).
We present results from Subaru/FMOS near-infrared (NIR) spectroscopy of 118 star-forming galaxies at $zsim1.5$ in the Subaru Deep Field. These galaxies are selected as [OII]$lambda$3727 emitters at $zapprox$ 1.47 and 1.62 from narrow-band imaging. We detect H$alpha$ emission line in 115 galaxies, [OIII]$lambda$5007 emission line in 45 galaxies, and H$beta$, [NII]$lambda$6584, and [SII]$lambdalambda$6716,6731 in 13, 16, and 6 galaxies, respectively. Including the [OII] emission line, we use the six strong nebular emission lines in the individual and composite rest-frame optical spectra to investigate physical conditions of the interstellar medium in star-forming galaxies at $zsim$1.5. We find a tight correlation between H$alpha$ and [OII], which suggests that [OII] can be a good star formation rate (SFR) indicator for galaxies at $zsim1.5$. The line ratios of H$alpha$/[OII] are consistent with those of local galaxies. We also find that [OII] emitters have strong [OIII] emission lines. The [OIII]/[OII] ratios are larger than normal star-forming galaxies in the local Universe, suggesting a higher ionization parameter. Less massive galaxies have larger [OIII]/[OII] ratios. With evidence that the electron density is consistent with local galaxies, the high ionization of galaxies at high redshifts may be attributed to a harder radiation field by a young stellar population and/or an increase in the number of ionizing photons from each massive star.
We present an analysis of the gas dynamics of star-forming galaxies at z~1.5 using data from the KMOS Galaxy Evolution Survey (KGES). We quantify the morphology of the galaxies using $HST$ CANDELS imaging parametrically and non-parametrically. We combine the H$alpha$ dynamics from KMOS with the high-resolution imaging to derive the relation between stellar mass (M$_{*}$) and stellar specific angular momentum (j$_{*}$). We show that high-redshift star-forming galaxies at z~1.5 follow a power-law trend in specific stellar angular momentum with stellar mass similar to that of local late-type galaxies of the form j$_*$$propto$M$_*^{0.53 pm 0.10}$. The highest specific angular momentum galaxies are mostly disc-like, although generally, both peculiar morphologies and disc-like systems are found across the sequence of specific angular momentum at a fixed stellar mass. We explore the scatter within the j$_{*}$-M$_{*}$ plane and its correlation with both the integrated dynamical properties of a galaxy (e.g. velocity dispersion, Toomre Q$_{rm g}$, H$alpha$ star formation rate surface density $Sigma_{rm SFR}$) and its parameterised rest-frame UV/optical morphology (e.g. Sersic index, bulge to total ratio, Clumpiness, Asymmetry and Concentration). We establish that the position in the j$_{*}$-M$_{*}$ plane is strongly correlated with the star-formation surface density and the Clumpiness of the stellar light distribution. Galaxies with peculiar rest-frame UV/optical morphologies have comparable specific angular momentum to disc-dominated galaxies of the same stellar mass, but are clumpier and have higher star-formation rate surface densities. We propose that the peculiar morphologies in high--redshift systems are driven by higher star formation rate surface densities and higher gas fractions leading to a more clumpy inter-stellar medium.
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