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Register a new userEnvironmental Effects on the Star Formation Activity at z~0.9 in the COSMOS Field
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We investigated the fraction of [OII] emitters in galaxies at z~0.9 as a function of the local galaxy density in the Hubble Space Telescope (HST) COSMOS 2 square degree field. [OII] emitters are selected by the narrow-band excess technique with the NB711-band imaging data taken with Suprime-Cam on the Subaru telescope. We carefully selected 614 photo-z selected galaxies with M_U3500 < -19.31 at z=0.901-0.920, which includes 195 [OII] emitters, to directly compare results with our previous study at z~1.2. We found that the fraction is almost constant at 0.3 Mpc^-2 < Sigma_10th < 10 Mpc^-2. We also checked the fraction of galaxies with blue rest-frame colors of NUV-R < 2 in our photo-z selected sample, and found that the fraction of blue galaxies does not significantly depend on the local density. On the other hand, the semi-analytic model of galaxy formation predicted that the fraction of star-forming galaxies at z~0.9 decreases with increasing the projected galaxy density even if the effects of the projection and the photo-z error in our analysis were taken into account. The fraction of [OII] emitters decreases from ~60% at z~1.2 to ~30% at z~0.9 independent of the galaxy environment. The decrease of the [OII] emitter fraction could be explained mainly by the rapid decrease of the star formation activity in the universe from z~1.2 to z~0.9.
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Combining the catalogue of galaxy morphologies in the COSMOS field and the sample of H$alpha$ emitters at redshifts $z=0.4$ and $z=0.84$ of the HiZELS survey, we selected $sim$ 220 star-forming bulgeless systems (Sersic index $n leq 1.5$) at both epochs. We present their star formation properties and we investigate their contribution to the star formation rate function (SFRF) and global star formation rate density (SFRD) at $z < 1$. For comparison, we also analyse H$alpha$ emitters with more structurally evolved morphologies that we split into two classes according to their Sersic index $n$: intermediate ($ 1.5 < n leq 3 $) and bulge-dominated ($n > 3$). At both redshifts the SFRF is dominated by the contribution of bulgeless galaxies and we show that they account for more than 60% of the cosmic SFRD at $z < 1$. The decrease of the SFRD with redshift is common to the three morphological types but it is stronger for bulge-dominated systems. Star-forming bulgeless systems are mostly located in regions of low to intermediate galaxy densities ($Sigma sim 1 - 4$ Mpc$^{-2}$) typical of field-like and filament-like environments and their specific star formation rates (sSFRs) do not appear to vary strongly with local galaxy density. Only few bulgeless galaxies in our sample have high (sSFR $>$ 10$^{-9}$ yr$^{-1}$) and these are mainly low-mass systems. Above $M_* sim 10^{10}$ M$_{odot}$ bulgeless are evolving at a normal rate (10$^{-9}$ yr$^{-1} <$ sSFR $<$10$^{-10}$ yr$^{-1}$) and in the absence of an external trigger (i.e. mergers/strong interactions) they might not be able to develop a central classical bulge.
We study the star formation rates (SFRs) of galaxies as a function of local galaxy density at 0.6<z<0.9. We used a low-dispersion prism in IMACS on the 6.5-m Baade (Magellan I) telescope to obtain spectra and measured redshifts to a precision of sigma_z/(1+z)=1% for galaxies with z<23.3 AB mag. We utilized a stellar mass-limited sample of 977 galaxies above M>1.8x10^{10} Msun to conduct our main analysis. With three different SFR indicators, (1) Spitzer MIPS 24-micron imaging, (2) SED fitting, and (3) [OII]3727 emission, we find the median specific SFR (SSFR) and SFR to decline from the low-density field to the cores of groups and a rich cluster. For the SED and [OII] based SFRs, the decline in SSFR is roughly an order of magnitude while for the MIPS based SFRs, the decline is a factor of ~4. We find approximately the same magnitude of decline in SSFR even after removing the sample of galaxies near the cluster. Galaxies in groups and a cluster at these redshifts therefore have lower star formation (SF) activity than galaxies in the field, as is the case at z~0. We investigated whether the decline in SFR with increasing density is caused by a change in the proportion of quiescent and star forming galaxies (SFGs) or by a decline in the SFRs of SFGs. Using the rest-frame U-V and V-J colors to distinguish quiescent galaxies from SFGs we find the fraction of quiescent galaxies increases from ~32% to 79% from low to high density. In addition, we find the SSFRs of SFGs, selected based on U-V and V-J colors, to decline with increasing density by factors of ~5-6 for the SED and [OII] based SFRs. The MIPS based SSFRs for SFGs decline with a shallower slope. The order of magnitude decline in the SSFR-density relation at 0.6<z<0.9 is therefore driven by both a combination of declining SFRs of SFGs as well as a changing mix of SFGs and quiescent galaxies [ABRIDGED].
We present a multi-wavelength, UV-to-radio analysis for a sample of massive (M$_{ast}$ $sim$ 10$^{10}$ M$_odot$) IRAC- and MIPS 24$mu$m-detected Lyman Break Galaxies (LBGs) with spectroscopic redshifts z$sim$3 in the GOODS-North field (L$_{rm UV}$$>1.8times$L$^{ast}_{z=3}$). For LBGs without individual 24$mu$m detections, we employ stacking techniques at 24$mu$m, 1.1mm and 1.4GHz, to construct the average UV-to-radio spectral energy distribution and find it to be consistent with that of a Luminous Infrared Galaxy (LIRG) with L$rm_{IR}$=4.5$^{+1.1}_{-2.3}$$times 10^{11}$ L$_{odot}$ and a specific star formation rate (SSFR) of 4.3 Gyr$^{-1}$ that corresponds to a mass doubling time $sim$230 Myrs. On the other hand, when considering the 24$mu$m-detected LBGs we find among them galaxies with L$rm_{IR}> 10^{12}$ L$_{odot}$, indicating that the space density of $zsim$3 UV-selected Ultra-luminous Infrared Galaxies (ULIRGs) is $sim$(1.5$pm$0.5)$times 10^{-5}$ Mpc$^{-3}$. We compare measurements of star formation rates (SFRs) from data at different wavelengths and find that there is tight correlation (Kendalls $tau >$ 99.7%) and excellent agreement between the values derived from dust-corrected UV, mid-IR, mm and radio data for the whole range of L$rm_{IR}$ up to L$rm_{IR}$ $sim$ 10$^{13}$ L$_{odot}$. This range is greater than that for which the correlation is known to hold at z$sim$2, possibly due to the lack of significant contribution from PAHs to the 24$mu$m flux at $zsim$3. The fact that this agreement is observed for galaxies with L$rm_{IR}$ $>$ 10$^{12}$ L$_{odot}$ suggests that star-formation in UV-selected ULIRGs, as well as the bulk of star-formation activity at this redshift, is not embedded in optically thick regions as seen in local ULIRGs and submillimeter-selected galaxies at $z=2$.
We investigate the relationship between environment and the galaxy main sequence (the relationship between stellar mass and star formation rate) and also the relationship between environment and radio luminosity (P$_{rm 1.4GHz}$) to shed new light on the effects of the environments on galaxies. We use the VLA-COSMOS 3 GHz catalogue that consists of star-forming galaxies (SFGs) and quiescent galaxies (AGN) in three different environments (field, filament, cluster) and for three different galaxy types (satellite, central, isolated). We perform for the first time a comparative analysis of the distribution of SFGs with respect to the main sequence (MS) consensus region from the literature, taking into account galaxy environment and using radio observations at 0.1 $leq$ z $leq$ 1.2. Our results corroborate that SFR is declining with cosmic time which is consistent with the literature. We find that the slope of the MS for different $z$ and M$_{*}$ bins is shallower than the MS consensus with a gradual evolution towards higher redshift bins, irrespective of environments. We see no SFR trends on both environments and galaxy type given the large errors. In addition, we note that the environment does not seem to be the cause of the flattening of MS at high stellar masses for our sample.
We present an infrared study of a z=0.872 cluster, SpARCS J161314+564930, with the primary aim of distinguishing the dynamical histories of spectroscopically confirmed star-forming members to assess the role of cluster environment. We utilize deep MIPS imaging and a mass-limited sample of 85 spectroscopic members to identify 16 24um-bright sources within the cluster, and measure their 24um star formation rates (SFRs) down to ~6 Msolar/year. Based on their line-of-sight velocities and stellar ages, MIPS cluster members appear to be an infalling population that was recently accreted from the field with minimal environmental dependency on their star formation. However, we identify a double-sequenced distribution of star-forming galaxies amongst the members, with one branch exhibiting declining specific SFRs with mass. The members along this sub-main sequence contain spectral features suggestive of passive galaxies. Using caustic diagrams, we kinematically identify these galaxies as a virialized and/or backsplash population. Moreover, we find a mix of dynamical histories at all projected radii, indicating that standard definitions of environment (i.e., radius and density) are contaminated with recently accreted interlopers, which could contribute to a lack of environmental trends for star-forming galaxies. A cleaner narrative of their dynamical past begins to unfold when using a proxy for accretion histories through profiles of constant (r/r_200)x(Delta v/sigma_v); galaxies accreted at earlier times possess lower values of (r/r_200)x(Delta v/sigma_v) with minimal contamination from the distinct infalling population. Therefore, adopting a time-averaged definition for density (as traced by accretion histories) rather than an instantaneous density yields a depressed specific SFR within the dynamical cluster core.
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