94 research outputs found

    Z-FIRE: ISM properties of the z = 2.095 COSMOS Cluster

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    We investigate the ISM properties of 13 star-forming galaxies within the z~2 COSMOS cluster. We show that the cluster members have [NII]/Ha and [OIII]/Hb emission-line ratios similar to z~2 field galaxies, yet systematically different emission-line ratios (by ~0.17 dex) from the majority of local star-forming galaxies. We find no statistically significant difference in the [NII]/Ha and [OIII]/Hb line ratios or ISM pressures among the z~2 cluster galaxies and field galaxies at the same redshift. We show that our cluster galaxies have significantly larger ionization parameters (by up to an order of magnitude) than local star-forming galaxies. We hypothesize that these high ionization parameters may be associated with large specific star formation rates (i.e. a large star formation rate per unit stellar mass). If this hypothesis is correct, then this relationship would have important implications for the geometry and/or the mass of stars contained within individual star clusters as a function of redshift.Comment: 11 pages, 5 figures, accepted for publication in Ap

    ZFIRE: Using Hα\alpha equivalent widths to investigate the in situ initial mass function at z~2

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    We use the ZFIRE survey (http://zfire.swinburne.edu.au) to investigate the high mass slope of the initial mass function (IMF) for a mass-complete (log10(M∗_*/M⊙_\odot)~9.3) sample of 102 star-forming galaxies at z~2 using their Hα\alpha equivalent widths (Hα\alpha-EW) and rest-frame optical colours. We compare dust-corrected Hα\alpha-EW distributions with predictions of star-formation histories (SFH) from PEGASE.2 and Starburst99 synthetic stellar population models. We find an excess of high Hα\alpha-EW galaxies that are up to 0.3--0.5 dex above the model-predicted Salpeter IMF locus and the Hα\alpha-EW distribution is much broader (10--500 \AA) than can easily be explained by a simple monotonic SFH with a standard Salpeter-slope IMF. Though this discrepancy is somewhat alleviated when it is assumed that there is no relative attenuation difference between stars and nebular lines, the result is robust against observational biases, and no single IMF (i.e. non-Salpeter slope) can reproduce the data. We show using both spectral stacking and Monte Carlo simulations that starbursts cannot explain the EW distribution. We investigate other physical mechanisms including models with variations in stellar rotation, binary star evolution, metallicity, and the IMF upper-mass cutoff. IMF variations and/or highly rotating extreme metal poor stars (Z~0.1Z⊙_\odot) with binary interactions are the most plausible explanations for our data. If the IMF varies, then the highest Hα\alpha-EWs would require very shallow slopes (Γ\Gamma>-1.0) with no one slope able to reproduce the data. Thus, the IMF would have to vary stochastically. We conclude that the stellar populations at z~2 show distinct differences from local populations and there is no simple physical model to explain the large variation in Hα\alpha-EWs at z~2.Comment: Accepted to MNRAS. 43 pages, 27 Figures. Survey website: http://zfire.swinburne.edu.au

    ZFIRE: The Evolution of the Stellar Mass Tully-Fisher Relation to Redshift 2.0 < Z < 2.5 with MOSFIRE

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    Using observations made with MOSFIRE on Keck I as part of the ZFIRE survey, we present the stellar mass Tully-Fisher relation at 2.0 < z < 2.5. The sample was drawn from a stellar mass limited, Ks-band selected catalog from ZFOURGE over the CANDELS area in the COSMOS field. We model the shear of the Halpha emission line to derive rotational velocities at 2.2X the scale radius of an exponential disk (V2.2). We correct for the blurring effect of a two-dimensional PSF and the fact that the MOSFIRE PSF is better approximated by a Moffat than a Gaussian, which is more typically assumed for natural seeing. We find for the Tully-Fisher relation at 2.0 < z < 2.5 that logV2.2 =(2.18 +/- 0.051)+(0.193 +/- 0.108)(logM/Msun - 10) and infer an evolution of the zeropoint of Delta M/Msun = -0.25 +/- 0.16 dex or Delta M/Msun = -0.39 +/- 0.21 dex compared to z = 0 when adopting a fixed slope of 0.29 or 1/4.5, respectively. We also derive the alternative kinematic estimator S0.5, with a best-fit relation logS0.5 =(2.06 +/- 0.032)+(0.211 +/- 0.086)(logM/Msun - 10), and infer an evolution of Delta M/Msun= -0.45 +/- 0.13 dex compared to z < 1.2 if we adopt a fixed slope. We investigate and review various systematics, ranging from PSF effects, projection effects, systematics related to stellar mass derivation, selection biases and slope. We find that discrepancies between the various literature values are reduced when taking these into account. Our observations correspond well with the gradual evolution predicted by semi-analytic models.Comment: 21 pages, 14 figures, 1 appendix. Accepted for publication by Apj, February 28, 201

    A massive, quiescent galaxy at redshift of z=3.717

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    In the early Universe finding massive galaxies that have stopped forming stars present an observational challenge as their rest-frame ultraviolet emission is negligible and they can only be reliably identified by extremely deep near-infrared surveys. These have revealed the presence of massive, quiescent early-type galaxies appearing in the universe as early as z∌\sim2, an epoch 3 Gyr after the Big Bang. Their age and formation processes have now been explained by an improved generation of galaxy formation models where they form rapidly at z∌\sim3-4, consistent with the typical masses and ages derived from their observations. Deeper surveys have now reported evidence for populations of massive, quiescent galaxies at even higher redshifts and earlier times, however the evidence for their existence, and redshift, has relied entirely on coarsely sampled photometry. These early massive, quiescent galaxies are not predicted by the latest generation of theoretical models. Here, we report the spectroscopic confirmation of one of these galaxies at redshift z=3.717 with a stellar mass of 1.7×\times1011^{11} M⊙_\odot whose absorption line spectrum shows no current star-formation and which has a derived age of nearly half the age of the Universe at this redshift. The observations demonstrates that the galaxy must have quickly formed the majority of its stars within the first billion years of cosmic history in an extreme and short starburst. This ancestral event is similar to those starting to be found by sub-mm wavelength surveys pointing to a possible connection between these two populations. Early formation of such massive systems is likely to require significant revisions to our picture of early galaxy assembly.Comment: 6 pages, 7 figures. This is the final preprint corresponding closely to the published version. Uploaded 6 months after publication in accordance with Nature polic

    ZFIRE: A KECK/MOSFIRE Spectroscopic Survey of Galaxies in Rich Environments at z~2

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    We present an overview and the first data release of ZFIRE, a spectroscopic redshift survey of star-forming galaxies that utilizes the MOSFIRE instrument on Keck-I to study galaxy properties in rich environments at 1.5<z<2.51.5<z<2.5. ZFIRE measures accurate spectroscopic redshifts and basic galaxy properties derived from multiple emission lines. The galaxies are selected from a stellar mass limited sample based on deep near infra-red imaging (KAB<25\mathrm{K_{AB}<25}) and precise photometric redshifts from the ZFOURGE and UKIDSS surveys as well as grism redshifts from 3DHST. Between 2013--2015 ZFIRE has observed the COSMOS and UDS legacy fields over 13 nights and has obtained 211 galaxy redshifts over 1.57<z<2.661.57<z<2.66 from a combination of nebular emission lines (such as \Halpha, \NII, \Hbeta, \OII, \OIII, \SII) observed at 1--2\micron. Based on our medium-band NIR photometry, we are able to spectrophotometrically flux calibrate our spectra to \around10\% accuracy. ZFIRE reaches 5σ5\sigma emission line flux limits of \around3×10−18 erg/s/cm2\mathrm{3\times10^{-18}~erg/s/cm^2} with a resolving power of R=3500R=3500 and reaches masses down to \around109^{9}\msol. We confirm that the primary input survey, ZFOURGE, has produced photometric redshifts for star-forming galaxies (including highly attenuated ones) accurate to Δz/(1+zspec)=0.015\Delta z/(1+z\mathrm{_{spec})}=0.015 with 0.7%0.7\% outliers. We measure a slight redshift bias of <0.001<0.001, and we note that the redshift bias tends to be larger at higher masses. We also examine the role of redshift on the derivation of rest-frame colours and stellar population parameters from SED fitting techniques. The ZFIRE survey extends spectroscopically-confirmed z∌2z\sim 2 samples across a richer range of environments, here we make available the first public release of the data for use by the community.\footnote{\url{http://zfire.swinburne.edu.au}}Comment: Published in ApJ. Data available at http://zfire.swinburne.edu.au, Code for figures at https://github.com/themiyan/zfire_survey, 31 pages, 24 figure

    ZFOURGE: Using Composite Spectral Energy Distributions to Characterize Galaxy Populations at 1<z<4

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    We investigate the properties of galaxies as they shut off star formation over the 4 billion years surrounding peak cosmic star formation. To do this we categorize ∌7000\sim7000 galaxies from 1<z<41<z<4 into 9090 groups based on the shape of their spectral energy distributions (SEDs) and build composite SEDs with R∌50R\sim 50 resolution. These composite SEDs show a variety of spectral shapes and also show trends in parameters such as color, mass, star formation rate, and emission line equivalent width. Using emission line equivalent widths and strength of the 4000\AA\ break, D(4000)D(4000), we categorize the composite SEDs into five classes: extreme emission line, star-forming, transitioning, post-starburst, and quiescent galaxies. The transitioning population of galaxies show modest Hα\alpha emission (EWREST∌40EW_{\rm REST}\sim40\AA) compared to more typical star-forming composite SEDs at log⁥10(M/M⊙)∌10.5\log_{10}(M/M_\odot)\sim10.5 (EWREST∌80EW_{\rm REST}\sim80\AA). Together with their smaller sizes (3 kpc vs. 4 kpc) and higher S\'ersic indices (2.7 vs. 1.5), this indicates that morphological changes initiate before the cessation of star formation. The transitional group shows a strong increase of over one dex in number density from z∌3z\sim3 to z∌1z\sim1, similar to the growth in the quiescent population, while post-starburst galaxies become rarer at zâ‰Č1.5z\lesssim1.5. We calculate average quenching timescales of 1.6 Gyr at z∌1.5z\sim1.5 and 0.9 Gyr at z∌2.5z\sim2.5 and conclude that a fast quenching mechanism producing post-starbursts dominated the quenching of galaxies at early times, while a slower process has become more common since z∌2z\sim2.Comment: Accepted for publication in The Astrophysical Journa

    Z-FIRE: ISM PROPERTIES OF THE z=2.095 COSMOS CLUSTER

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    We investigate the ISM properties of 13 star-forming galaxies within the z~ 2 COSMOS cluster. We show that the cluster members have [N ii]/Halpha and [O iii]/Hbeta emission-line ratios similar to z~ 2 field galaxies, yet systematically different emission-line ratios (by ~0.17 dex) from the majority of local star-forming galaxies. We find no statistically significant difference in the [N ii]/Halpha and [O iii]/Hbeta line ratios or ISM pressures among the z~ 2 cluster galaxies and field galaxies at the same redshift. We show that our cluster galaxies have significantly larger ionization parameters (by up to an order of magnitude) than local star-forming galaxies. We hypothesize that these high ionization parameters may be associated with large specific star formation rates (SFRs; i.e., a large SFR per unit stellar mass). If this hypothesis is correct, then this relationship would have important implications for the geometry and/or the mass of stars contained within individual star clusters as a function of redshift

    Beyond UVJ: Color Selection of Galaxies in the JWST Era

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    We present a new rest-frame color-color selection method using "synthetic us−gsu_s-g_s and gs−isg_s-i_s'', (ugi)s(ugi)_s colors to identify star-forming and quiescent galaxies. Our method is similar to the widely-used U−VU-V versus V−JV-J (UVJUVJ) diagram. However, UVJUVJ suffers known systematics. Spectroscopic campaigns have shown that UVJUVJ-selected quiescent samples at z≳3z \gtrsim 3 include ∌10−30%\sim 10-30\% contamination from galaxies with dust-obscured star formation and strong emission lines. Moreover, at z>3z>3, UVJUVJ colors are extrapolated because the rest-frame J-band shifts beyond the coverage of the deepest bandpasses at <5 Όm< 5~\mu m (typically SpitzerSpitzer/IRAC 4.5 ÎŒm\mu m or future JWSTJWST/NIRCam observations). We demonstrate that (ugi)s(ugi)_s offers improvements to UVJUVJ at z>3z>3, and can be applied to galaxies in the JWSTJWST era. We apply (ugi)s(ugi)_s selection to galaxies at 0.5<z<60.5<z<6 from the (observed) 3D-HST and UltraVISTA catalogs, and to the (simulated) JAGUAR catalogs. We show that extrapolation can affect (V−J)0(V-J)_0 color by up to 1 magnitude, but changes (us−is)0(u_s-i_s)_0 color by ≀\leq 0.2 mag, even at z≃6z\simeq 6. While (ugi)s(ugi)_s-selected quiescent samples are comparable to UVJUVJ in completeness (both achieve ∌\sim85-90% at z=3−3.5z=3-3.5), (ugi)s(ugi)_s reduces contamination in quiescent samples by nearly a factor of two, from ≃\simeq35% to ≃\simeq17% at z=3z=3, and from ≃\simeq 60% to ≃\simeq 33% at z=6z=6. This leads to improvements in the true-to-false-positive ratio (TP/FP), where we find TP/FP ≳\gtrsim 2.2 for (ugi)s(ugi)_s at z≃3.5−6z \simeq 3.5 - 6, compared to TP/FP << 1 for UVJUVJ-selected samples. This indicates that contaminants will outnumber true quiescent galaxies in UVJUVJ at these redshifts, while (ugi)s(ugi)_s will provide higher-fidelity samples.Comment: Submitted to Ap
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