High Star Formation Rate Density Research Articles

The Imprint of Clump Formation at High Redshift. II. The Chemistry of the Bulge

In Paper I, we showed that clumps in high-redshift galaxies, having a high star formation rate density (ΣSFR), produce disks with two tracks in the [Fe/H]–[α/Fe] chemical space, similar to that of the Milky Way’s (MW’s) thin+thick disks. Here we investigate the effect of clumps on the bulge’s chemistry. The chemistry of the MW’s bulge is comprised of a single track with two density peaks separated by a trough. We show that the bulge chemistry of an N-body + smoothed particle hydrodynamics clumpy simulation also has a single track. Star formation within the bulge is itself in the high-ΣSFR clumpy mode, which ensures that the bulge’s chemical track follows that of the thick disk at low [Fe/H] and then extends to high [Fe/H], where it peaks. The peak at low metallicity instead is comprised of a mixture of in situ stars and stars accreted via clumps. As a result, the trough between the peaks occurs at the end of the thick disk track. We find that the high-metallicity peak dominates near the mid-plane and declines in relative importance with height, as in the MW. The bulge is already rapidly rotating by the end of the clump epoch, with higher rotation at low [α/Fe]. Thus clumpy star formation is able to simultaneously explain the chemodynamic trends of the MW’s bulge, thin+thick disks, and the splash.

The Lyman Alpha Reference Sample

Aims. We use Hubble Space Telescope data of 45 nearby star-forming galaxies to investigate properties of Lyman-alpha (Lyα) halos, Lyα morphology, and the star-forming characteristics of galaxies. We study how the morphology of Lyα emission is related to other Lyα observables. Furthermore, we study the interdependencies of Lyα morphological quantities. Methods. We studied the spatial extent of Lyα using surface brightness profiles in the following two ways (i) using circular apertures and (ii) within faint Lyα isophotes. We also measured the average intensity and the size of the regions with a high star formation rate density. The morphology of the galaxies was quantified by computing centroid position, axis ratio, and position angle in the Lyα, ultraviolet continuum, and I band maps. Results. We found that galaxies with more extended star-forming regions possess larger Lyα halos. Furthermore, galaxies with more elongated Lyα morphology are also more extended in Lyα. Our data suggest that Lyα bright galaxies appear rounder in their Lyα morphology, and there is less of a contribution from their Lyα halo to their overall luminosity. We compared our results with studies at high redshift and found that whilst the Lyα extent in the inner regions of the galaxies in our sample are similar to the high-z Lyα emitters (LAEs), Lyα halos are more extended in high-z LAEs. Conclusions. Our analysis suggests that the Lyα morphology affects the measurement of other observable quantities concerning Lyα emission, and some of the conclusions drawn from high redshift LAEs might be biased towards galaxies with specific Lyα shapes. In particular, faint Lyα emitters have larger Lyα scale lengths and halo fractions. This implies that faint Lyα emitters are harder to detect at high redshift than previously believed.

ALMA 200 pc Imaging of a z ∼ 7 Quasar Reveals a Compact, Disk-like Host Galaxy

Abstract We present 0.″035 resolution (∼200 pc) imaging of the 158 μm [C ii] line and the underlying dust continuum of the z = 6.9 quasar J234833.34–305410.0. The 18 hour Atacama Large Millimeter/submillimeter Array observations reveal extremely compact emission (diameter ∼1 kpc) that is consistent with a simple, almost face-on, rotation–supported disk with a significant velocity dispersion of ∼160 km s−1. The gas mass in just the central 200 pc is ∼4 × 109 M ⊙, about a factor of two higher than that of the central supermassive black hole. Consequently we do not resolve the black hole’s sphere of influence, and find no kinematic signature of the central supermassive black hole. Kinematic modeling of the [C ii] line shows that the dynamical mass at large radii is consistent with the gas mass, leaving little room for a significant mass contribution by stars and/or dark matter. The Toomre–Q parameter is less than unity throughout the disk, and thus is conducive to star formation, consistent with the high-infrared luminosity of the system. The dust in the central region is optically thick, at a temperature >132 K. Using standard scaling relations of dust heating by star formation, this implies an unprecedented high star formation rate density of >104 M ⊙ yr−1 kpc−2. Such a high number can still be explained with the Eddington limit for star formation under certain assumptions, but could also imply that the central supermassive black hole contributes to the heating of the dust in the central 200 pc.

Metals and a search for molecules in the distant Universe: Magellan mike observations of sub-DLAs at 2 < z < 3

ABSTRACT We present abundance measurements of the elements Zn, S, O, C, Si, and Fe for four sub-DLAs at redshifts ranging from z = 2.173 to 2.635 using observations from the MIKE spectrograph on the Magellan telescope to constrain the chemical enrichment and star formation of gas-rich galaxies. Using weakly depleted elements O, S, and or Zn, we find the metallicities after the photoionization corrections to be [S/H] = −0.50 ± 0.11, [O/H] > −0.84, [O/H] = −1.27 ± 0.12, and [Zn/H] = +0.40 ± 0.12 for the absorbers at z = 2.173, 2.236, 2.539, and 2.635, respectively. Moreover, we are able to put constraints on the electron densities using the fine structure lines of C ii⋆ and Si ii⋆ for two of the sub-DLAs. We find that these values are much higher than the median values found in DLAs in the literature. Furthermore, we estimate the cooling rate lc = 1.20 × 10−26 erg s−1 per H atom for an absorber at z = 2.173, suggesting higher star formation rate density in this sub-DLA than the typical star formation rate density for DLAs at similar redshifts. We also study the metallicity versus velocity dispersion relation for our absorbers. Most of the absorbers follow the trend one can expect from the mass versus metallicity relation for sub-DLAs in the literature. Finally, we are able to put limits on the molecular column density from the non-detections of various strong lines of CO molecules. We estimate 3σ upper limits of logN(CO, J = 0) < 13.87, logN(CO, J = 0) < 13.17, and logN(CO, J = 0) < 13.08, respectively, from the non-detections of absorption from the J = 0 level in the CO AX 0–0, 1–0, and 2–0 bands near 1544, 1510, and 1478 Å.

The imprint of clump formation at high redshift – I. A disc α-abundance dichotomy

The disc structure of the Milky Way is marked by a chemical dichotomy, with high-alpha and low-alpha abundance sequences, traditionally identified with the geometric thick and thin discs. This identification is aided by the old ages of the high-alpha stars, and lower average ages of the low-alpha ones. Recent large scale surveys such as APOGEE have provided a wealth of data on this chemical structure, including showing that an identification of chemical and geometric thick discs is not exact, but the origin of the chemical dichotomy has remained unclear. Here we demonstrate that a dichotomy arises naturally if the early gas-rich disc fragments, leading to some fraction of the star formation occurring in clumps of the type observed in high-redshift galaxies. These clumps have high star formation rate density. They, therefore, enrich rapidly, moving from the low-alpha to the high-alpha sequence, while more distributed star formation produces the low-alpha sequence. We demonstrate that this model produces a chemically-defined thick disc that has many of the properties of the Milky Way's thick disc. Because clump formation is common in high redshift galaxies, we predict that chemical bimodalities are common in massive galaxies.

Parameters of the brightest star formation regions in the two principal spiral arms of NGC 628

We study the photometric properties, chemical abundances and sizes of star formation regions in the two principal arms of the galaxy NGC 628 (M74). Galaxy Evolution Explorer (GALEX) ultraviolet, optical UBVRI and Hα surface photometry data are used, including those obtained with the 1.5-m telescope of the Maidanak Observatory. The 30 brightest star formation regions in ultraviolet light located in the spiral arms of NGC 628 are identified and studied. We find that the star formation regions in one (longer) arm are systematically brighter and larger than the regions in the other (shorter) arm. However, both luminosity and size distribution functions have approximately the same slopes for the samples of star formation regions in both arms. The star formation regions in the longer arm have a higher star formation rate density than the regions in the shorter arm. The regions in the shorter arm show higher N/O ratio at a higher oxygen abundance, but they have lower ultraviolet and Hα luminosities. These findings can be explained if we assume that star formation regions in the shorter arm had a higher star formation rate in the past, but it is now lower than that in the opposite arm. Results of stellar evolutionary synthesis show that the brightest regions in the longer arm are slightly younger than those in the shorter arm (3.5 ± 2.2 Myr versus 6.0 ± 1.1 Myr). Our results demonstrate that there is a difference in inner structure and parameters of the interstellar medium between the spiral arms of NGC 628, one of which is long and hosts a regular chain of bright star formation complexes, while the other, shorter, one does not.

A variation of the fraction of stars that form in bound clusters within M83

Abstract Recent observations, as well as theoretical studies, have suggested that stellar cluster formation may depend on local and global environmental properties. In particular, the fraction of stars that form within long-lived bound clusters (Γ) may depend on environment, with indications that it may be higher in the more extreme environments of high star formation rate density galaxies. How Γ varies has important implications on the use of clusters to determine the star formation histories of galaxies as well as our understanding of the star formation process itself. Previous studies have estimated Γ over full galaxies, making it difficult to discern the physical cause of the reported variations. Here, we use existing star cluster catalogues and Hubble Space Telescope Wide Field Camera 3 V and I images of the grand design, face-on spiral galaxy M83 in order to see if and how Γ varies within a single galaxy. We find that Γ decreases strongly as a function of galactocentric radius, by a factor of ∼5 over the inner ∼6 kpc, in agreement with recent theoretical predictions and decreasing trends observed in the gas surface density of the galaxy.

MOIRCS Deep Survey. VII. NIR Morphologies of Star-Forming Galaxies at Redshift z 1

Abstract We investigated rest-frame near-infrared (NIR) morphologies of a sample of 139 galaxies with $M_{\mathrm{s}}$$\geq$ 1 $\times$ 10$^{10}\ M_{\odot}$ at z$=$ 0.8–1.2 in the GOODS-North field using our deep NIR imaging data (MOIRCS Deep Survey, MODS). We focused on Luminous Infrared Galaxies (LIRGs), which dominate a high star formation rate (SFR) density at z$\sim $ 1, in a sample identified by cross-correlating with the Spitzer/MIPS 24 $\mu $m source catalog. We performed two-dimensional light profile fimonospaceings of z$\sim $ 1 galaxies in the $K_{\mathrm{s}}$-band (rest-frame $J$-band) with a single-component Sérsic model. We found that at z$\sim $ 1, $\sim $ 90% of the LIRGs have low Sérsic indices ($n$$\lt$ 2.5, similar to disk-like galaxies) in the $K_{\mathrm{s}}$-band, and that those disk-like LIRGs consist of $\sim $ 60% of the whole disk-like sample above $M_{\mathrm{s}}$$\geq$ 3 $\times$ 10$^{10}\ M_{\odot}$. The z$\sim $ 1 disk-like LIRGs are comparable to or $\sim $ 20% scer at the maximum in size compared to local disk-like galaxies in the same stellar mass range. When we examined rest-frame UV–optical morphologies using the HST/ACS images, the rest-frame $B$-band sizes of the z$\sim $ 1 disk-like galaxies were found to be comparable to those of the local disk-like galaxies, as reported by previous studies on the size evolution of disk-like galaxies in the rest-frame optical band. By measuring color gradients (galaxy sizes as a function of wavelength) of the z$\sim $ 1 and local disk-like galaxies, we found that the z$\sim $ 1 disk-like galaxies have a 3–5 times steeper color gradient than the local ones. Our results indicate that (i) more than a half of the relatively massive disk-like galaxies at z$\sim $ 1 are in violent star-formation epochs observed as LIRGs, and also (ii) that most of those LIRGs are constructing their fundamental disk structure vigorously. The high SFR density in the universe at z$\sim $ 1 may be dominated by such star formation in the disk region in massive galaxies.

The Discovery of Primeval Large-Scale Structures with Forming Clusters at Redshift 6

We report the discovery of primeval large-scale structures (LSSs) including two protoclusters in a forming phase at z = 5.7. We carried out extensive deep narrowband imaging in the 1 deg2 sky of the Subaru/XMM-Newton Deep Field and obtained a cosmic map of 515 Lyα emitters (LAEs) in a volume with a transverse dimension of 180 Mpc × 180 Mpc and a depth of ~40 Mpc in comoving units. This cosmic map shows filamentary LSSs, including clusters and surrounding 10-40 Mpc scale voids, similar to the present-day LSSs. Our spectroscopic follow-up observations identify overdense regions in which two dense clumps of LAEs with a sphere of 1 Mpc diameter in physical units are included. These clumps show about 130 times higher star formation rate density, mainly due to a large overdensity, ~80, of LAEs. These clumps would be clusters in a formation phase involving a burst of galaxy formation.