arXiv subjects

Naveen Reddy

Publications and source records attributed to Naveen Reddy.

At least 37 records · Page 2Linked to original sources

arXiv2021

Variation of the Nebular Dust Attenuation Curve with the Properties of Local Star-forming Galaxies

We use a sample of $78,340$ star-forming galaxies at $z\simeq 0.04-0.1$ from the SDSS DR8 survey to calculate the average nebular dust attenuation curve and its variation with the physical properties of galaxies. Using the first four low-order Balmer emission lines (H$α$, H$β$, H$γ$, H$δ$) detected in the composite spectrum of all galaxies in the sample, we derive a nebular attenuation curve in the range of $0.41\,μ$m to $0.66\,μ$m that has a similar shape and normalization to that of the Galactic extinction curve (Milky Way curve), the SMC curve and the nebular attenuation curve derived recently for typical star-forming galaxies at $z\sim2$. We divide the galaxies into bins of stellar mass, gas-phase metallicity, and specific star-formation rate, and derive the nebular attenuation curve in each of these bins. This analysis indicates that there is very little variation in the shape of the nebular dust attenuation curve with the properties used to bin the galaxies, and suggests a near universal shape of the nebular dust attenuation curve at least among the galaxies and the range of properties considered in our sample.

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arXiv2020

The MOSDEF Survey: An Improved Voronoi Binning Technique on Spatially Resolved Stellar Populations at z~2

We use a sample of 350 star-forming galaxies at $1.25<z<2.66$ from the MOSFIRE Deep Evolution Field survey to demonstrate an improved Voronoi binning technique that we use to study the properties of resolved stellar populations in $z\sim2$ galaxies. Stellar population and dust maps are constructed from the high-resolution CANDELS/3D-HST multi-band imaging. Rather than constructing the layout of resolved elements (i.e., Voronoi bins) from the S/N distribution of the $H_{160}$-band alone, we introduce a modified Voronoi binning method that additionally incorporates the S/N distribution of several resolved filters. The SED-derived resolved E(B-V)$_{\text{stars}}$, stellar population ages, SFRs, and stellar masses that are inferred from the Voronoi bins constructed from multiple filters are generally consistent with the properties inferred from the integrated photometry within the uncertainties, with the exception of the inferred E(B-V)$_{\text{stars}}$ from our $z\sim1.5$ sample due to their UV slopes being unconstrained by the resolved photometry. The results from our multi-filter Voronoi binning technique are compared to those derived from a "traditional" single-filter Voronoi binning approach. We find that single-filter binning produces inferred E(B-V)$_{\text{stars}}$ that are systematically redder by 0.02 mag on average, but could differ by up to 0.20 mag, and could be attributed to poorly constrained resolved photometry covering the UV slope. Overall, we advocate that our methodology produces more reliable SED-derived parameters due to the best-fit resolved SEDs being better constrained at all resolved wavelengths--particularly those covering the UV slope.

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arXiv2020

The MOSDEF Survey: the Variation of the Dust Attenuation Curve with Metallicity

We derive the UV-optical stellar dust attenuation curve of galaxies at z=1.4-2.6 as a function of gas-phase metallicity. We use a sample of 218 star-forming galaxies, excluding those with very young or heavily obscured star formation, from the MOSFIRE Deep Evolution Field (MOSDEF) survey with H$α$, H$β$, and [NII]$λ6585$ spectroscopic measurements. We constrain the shape of the attenuation curve by comparing the average flux densities of galaxies sorted into bins of dust obscuration using Balmer decrements, i.e., H$α$-to-H$β$ luminosities. The average attenuation curve for the high-metallicity sample (12+log(O/H)>8.5, corresponding to $M_*\gtrsim10^{10.4}\,M_{\odot}$) has a shallow slope, identical to that of the Calzetti local starburst curve, and a significant UV 2175A extinction bump that is $\sim 0.5\times$ the strength of the Milky Way bump. On the other hand, the average attenuation curve of the low-metallicity sample (12+log(O/H) $\sim 8.2-8.5$) has a steeper slope similar to that of the SMC curve, only consistent with the Calzetti slope at the $3σ$ level. The UV bump is not detected in the low-metallicity curve, indicating the relative lack of the small dust grains causing the bump at low metallicities. Furthermore, we find that on average the nebular reddening (E(B-V)) is a factor of 2 times larger than that of the stellar continuum for galaxies with low metallicities, while the nebular and stellar reddening are similar for galaxies with higher metallicities. The latter is likely due to a high surface density of dusty clouds embedding the star forming regions but also reddening the continuum in the high-metallicity galaxies.

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arXiv2019

The MOSDEF survey: a census of AGN-driven ionized outflows at $z = 1.4-3.8$

Using data from the MOSFIRE Deep Evolution Field (MOSDEF) survey, we present a census of AGN-driven ionized outflows in a sample of 159 AGNs at $1.4 \le z \le 3.8$. The sample spans AGN bolometric luminosities of $10^{44-47} \mathrm{~erg~s}^{-1}$ and includes both quiescent and star-forming galaxies extending across three orders of magnitude in stellar mass. We identify and characterize outflows from the \hbeta, [OIII], \halpha ~and [NII] emission line spectra. We detect outflows in $17\%$ of the AGNs, seven times more often than in a mass-matched sample of inactive galaxies in MOSDEF. The outflows are fast and galaxy-wide, with velocities of $\sim 400-3500 ~\mathrm{km~s}^{-1}$ and spatial extents of $0.3-11.0$ kpc. The incidence of outflows among AGNs is independent of the stellar mass of the host galaxy, with outflows detected in both star-forming and quiescent galaxies. This suggests that outflows exist across different phases in galaxy evolution. We investigate relations between outflow kinematic, spatial, and energetic properties and both AGN and host galaxy properties. Our results show that AGN-driven outflows are widespread in galaxies along the star-forming main sequence. The mass-loading factors of the outflows are typically $0.1-1$ and increase with AGN luminosity, capable of exceeding unity at $L_\mathrm{AGN} \gtrsim 10^{46.3} \mathrm{~erg~s}^{-1}$. In these more luminous sources the ionized outflow alone is likely sufficient to regulate star formation, and when combined with outflowing neutral and molecular gas may be able to quench star formation in their host galaxies.

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arXiv2019

Two sub-millimetre bright protoclusters bounding the epoch of peak star-formation activity

We present James Clerk Maxwell Telescope Submillimetre Common-User Bolometer Array 2 (SCUBA-2) 850 & 450 $μ$m observations ($σ_{850}\sim0.5$ mJy, $σ_{450}\sim5$ mJy) of the HS1549+19 and HS1700+64 survey fields containing two of the largest known galaxy over-densities at $z=2.85$ and $2.30$, respectively. We detect 56 sub-millimetre galaxies (SMGs) with SNR $> 4$ over $\sim 50$ arcmin$^2$ at 850 $μ$m with flux densities of 3 - 17 mJy. The number counts indicate over-densities in the $3$-arcmin diameter core region ($\sim 1.5$ Mpc at $z=2.5$) of $6^{+4}_{-2}\times$ (HS1549) and $4^{+6}_{-2}\times$ (HS1700) compared to blank field surveys. Within these core regions, we spectroscopically confirm that approximately one third of the SMGs lie at the protocluster redshifts for both HS1549 and HS1700. We use statistical identifications of other SMGs in the wider fields to constrain an additional four candidate protocluster members in each system. We combine multi wavelength estimates of the star-formation rates (SFRs) from Lyman-break dropout- and narrowband-selected galaxies, and the SCUBA-2 SMGs, to estimate total SFRs of 12,$500\pm2800$ M$_\odot$ yr$^{-1}$ ($4900\pm1200$ M$_\odot$ yr$^{-1}$) in HS1549 (HS1700), and SFR densities (SFRDs) within the central 1.5-Mpc diameter of each protocluster to be $3000\pm900$ M$_\odot$ yr$^{-1}$ Mpc$^{-3}$ ($1300\pm400$ M$_\odot$ yr$^{-1}$ Mpc$^{-3}$) in the HS1549 (HS1700) protocluster, $\sim10^4\times$ larger than the global SFRDs found at their respective epochs, due to the concentration of star-forming galaxies in the small volume of the dense cluster cores. Our results suggest centrally concentrated starbursts within protoclusters may be a relatively common scenario for the build up of mass in rich clusters assembling at $z\gtrsim2$.

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arXiv2019

UV Diagnostics of Galaxies from the Peak of Star-Formation to the Epoch of Reionization

The rest-frame UV emission from massive stars contains a wealth of information about the physical nature and conditions of star formation in galaxies. Using studies of the rest-frame UV, the past decade has witnessed the beginning of knowledge about the existence and properties of galaxies during the first few billion years after the Big Bang. This period of history corresponds to the formation of the first stars, the rapid formation of galaxy stellar populations, the reionization of the IGM, the production and dissemination of heavy elements, and the formation of the first black holes. Massive stars in these galaxies drive all of these events, and their light dominates the spectral energy distributions of galaxies. As we look to the 2020s, fundamental questions remain about the nature of these stellar populations and their evolution, from just before the peak of the cosmic star formation density (z~3), up to the epoch of reionization (z > 6). This next decade will provide transformative gains both in our ability to identify star-forming galaxies and accreting supermassive black holes at these early epochs with imaging surveys in the rest-frame UV (e.g., LSST, WFIRST). Ground-based, rest-frame UV spectroscopy on >20 m-class telescopes (e.g., GMT/TMT) offers the ability to investigate the astrophysical conditions in galaxies at the earliest cosmic times. This includes studies of the evolution in galaxy stellar populations, gas ionization (temperature, pressure), metallicity, and interstellar (and circumgalactic) gas kinematics and covering fractions. In this white paper, we describe the scientific prospects and the requirements for research in this area.

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arXiv2018

A census of galaxy constituents in a Coma Progenitor observed at z>3

We present a detailed census of galaxies in and around PC217.96+32.3, a spectroscopically confirmed Coma analog at z=3.78. Diverse galaxy types identified in the field include Lya emitters (LAEs), massive star-forming galaxies, and ultra-massive galaxies (log (Mstar/Msun)>= 11) which may have already halted their star formation. The sky distribution of the star-forming galaxies suggests the presence of a significant overdensity (delta_g=8+/-2), which is spatially offset from the previously confirmed members by 3-4 Mpc to the west. Candidate quiescent and post-starburst galaxies are also found in large excess (a factor of ~ 8-15 higher surface density than the field) although their redshifts are less certain. We estimate that the total enclosed mass traced by star-forming galaxy candidates is roughly comparable to that of PC217.96+32.3 traced by the LAEs. We speculate that the true extent of PC217.96+32.3 may be larger than previously known, a half of which is missed by our LAE selection. Alternatively, the newly discovered overdensity may belong to another Coma progenitor not associated with PC217.96+32.3. Expectations from theory suggest that both scenarios are equally unlikely (<1%), particularly in the cosmic volume probed in our survey. If confirmed as a single structure, its total mass will be well in excess of Coma, making this a singularly large cosmic structure rarely seen even in large cosmological simulations. Finally, we find that the protocluster galaxies follow the same SFR-M_star scaling relation as the field galaxies, suggesting that the environmental effect at z~4 is a subtle one at best for normal star-forming galaxies.

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arXiv2018

Discovery of a Very Large (~20 kpc) Galaxy at z=3.72

We report the discovery and spectroscopic confirmation of a very large star-forming Lyman Break galaxy, G6025, at z_spec=3.721+/-0.003. In the rest-frame ~2100A, G6025 subtends ~24 kpc in physical extent when measured from the 1.5-sigma isophote, in agreement with the parametric size measurements which yield the half-light radius of 4.9+/-0.5 kpc and the semi-major axis of 12.5+/-0.1 kpc. G6025 is also very UV-luminous (~5L*(z~4}) and young (~140+/-60 Myr). Despite its unusual size and luminosity, the stellar population parameters and dust reddening (M_star~M*(z~4)$, and E(B-V)=0.18+/-0.05) estimated from the integrated light, are similar to those of smaller galaxies at comparable redshifts. The ground-based morphology and spectroscopy show two dominant components, both located off-center, embedded in more diffuse emission. We speculate that G6025 may be a scaled-up version of chain galaxies seen in deep HST imaging, or alternatively, a nearly equal-mass merger involving two super-L* galaxies in its early stage. G6025 lies close to but not within a known massive protocluster at z=3.78. We find four companions within 6 Mpc from G6025, two of which lie within 1.6 Mpc. While the limited sensitivity of the existing spectroscopy does not allow us to robustly characterize the local environment of G6025, it likely resides in a locally overdense environment. The luminosity, size, and youth of G6025 make it uniquely suited to study the early formation of massive galaxies in the universe.

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arXiv2018

THE MOSDEF SURVEY: The Nature of Mid-Infrared Excess Galaxies and a Comparison of IR and UV Star Formation Tracers at z~2

We present an analysis using the MOSFIRE Deep Evolution Field (MOSDEF) survey on the nature of "MIR-excess" galaxies, which have star formation rates (SFR) inferred from mid-infrared (MIR) data that is substantially elevated relative to that estimated from dust-corrected UV data. We use a sample of $\sim$200 galaxies and AGN at $1.40<z<2.61$ with 24 $μ$m detections (rest-frame 8$μ$m) from MIPS/\textit{Spitzer}. We find that the identification of MIR-excess galaxies strongly depends on the methodologies used to estimate IR luminosity ($\rm L_{IR}$) and to correct the UV light for dust attenuation. We find that extrapolations of the SFR from the observed 24 $μ$m flux, using luminosity-dependent templates based on local galaxies, substantially overestimate $\rm L_{IR}$ in $z\sim2$ galaxies. By including \textit{Herschel} observations and using a stellar mass-dependent, luminosity-independent $\rm L_{IR}$, we obtain more reliable estimates of the SFR and a lower fraction of MIR-excess galaxies. Once stellar mass selection biases are taken into account, we identify $\sim24\%$ of our galaxies as MIR-excess. However, $\rm SFR_{Hα}$ is not elevated in MIR-excess galaxies compared to MIR-normal galaxies, indicating that the intrinsic fraction of MIR-excess may be lower. Using X-ray, IR, and optically-selected AGN in MOSDEF, we do not find a higher prevalence for AGN in MIR-excess galaxies relative to MIR-normal galaxies. A stacking analysis of X-ray undetected galaxies does not reveal a harder spectrum in MIR-excess galaxies relative to MIR-normal galaxies. Our analysis indicates that AGN activity does not contribute substantially to the MIR excess and instead implies that it is likely due to the enhanced PAH emission.

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arXiv2017

The MOSDEF Survey: Broad Emission Lines at z=1.4-3.8

We present results from the MOSFIRE Deep Evolution Field (MOSDEF) survey on broad flux from the nebular emission lines H$α$, [NII], [OIII], H$β$, and [SII]. The sample consists of 127 star-forming galaxies at $1.37 < z < 2.61$ and 84 galaxies at $2.95 < z < 3.80$. We decompose the emission lines using narrow ($\text{FWHM} < 275 \ \text{km s}^{-1}$) and broad ($\text{FWHM} > 300 \ \text{km s}^{-1}$) Gaussian components for individual galaxies and stacks. Broad emission is detected at $>3σ$ in $<10$% of galaxies and the broad flux accounts for 10-70% of the total flux. We find a slight increase in broad to narrow flux ratio with mass but note that we cannot reliably detect broad emission with $\text{FWHM} < 275 \ \text{km s}^{-1}$, which may be significant at low masses. Notably, there is a correlation between higher signal-to-noise (S/N) spectra and a broad component detection indicating a S/N dependence in our ability to detect broad flux. When placed on the N2-BPT diagram ([OIII]/H$β$ vs. [NII]/H$α$) the broad components of the stacks are shifted towards higher [OIII]/H$β$ and [NII]/$α$ ratios compared to the narrow component. We compare the location of the broad components to shock models and find that the broad component could be due to shocks, but we do not rule out other possibilities such as the presence of an AGN. We estimate the mass loading factor (mass outflow rate/star formation rate) assuming the broad component is a photoionized outflow and find that the mass loading factor increases as a function of mass which agrees with previous studies. We show that adding emission from shocked gas to $z\sim0$ SDSS spectra shifts galaxies towards the location of $z\sim2$ galaxies on several emission line diagnostic diagrams.

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arXiv2017

The MOSDEF survey: the prevalence and properties of galaxy-wide AGN-driven outflows at $z\sim 2$

Using observations from the first two years of the MOSFIRE Deep Evolution Field (MOSDEF) survey, we study 13 AGN-driven outflows detected from a sample of 67 X-ray, IR and/or optically-selected AGN at $z \sim 2$. The AGN have bolometric luminosities of $\sim10^{44}-10^{46} ~\mathrm{erg~s^{-1}}$, including both quasars and moderate-luminosity AGN. We detect blueshifted, ionized gas outflows in the H$β$ , [OIII], H$α$ ~and/or [NII] emission lines of $19\%$ of the AGN, while only 1.8\% of the MOSDEF galaxies have similarly-detected outflows. The outflow velocities span $\sim$300 to 1000 km s$^{-1}$. Eight of the 13 outflows are spatially extended on similar scales as the host galaxies, with spatial extents of 2.5 to 11.0 kpc. Outflows are detected uniformly across the star-forming main sequence, showing little trend with the host galaxy SFR. Line ratio diagnostics indicate that the outflowing gas is photoionized by the AGN. We do not find evidence for positive AGN feedback, in either our small MOSDEF sample or a much larger SDSS sample, using the BPT diagram. Given that a galaxy with an AGN is ten times more likely to have a detected outflow, the outflowing gas is photoionzed by the AGN, and estimates of the mass and energy outflow rates indicate that stellar feedback is insufficient to drive at least some of these outflows, they are very likely to be AGN-driven. The outflows have mass-loading factors of the order of unity, suggesting that they help regulate star formation in their host galaxies, though they may be insufficient to fully quench it.

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arXiv2017

The Diversity of Diffuse Ly$α$ Nebulae around Star-Forming Galaxies at High Redshift

We report the detection of diffuse Ly$α$ emission, or Ly$α$ halos (LAHs), around star-forming galaxies at $z\approx3.78$ and $2.66$ in the NOAO Deep Wide-Field Survey Boötes field. Our samples consist of a total of $\sim$1400 galaxies, within two separate regions containing spectroscopically confirmed galaxy overdensities. They provide a unique opportunity to investigate how the LAH characteristics vary with host galaxy large-scale environment and physical properties. We stack Ly$α$ images of different samples defined by these properties and measure their median LAH sizes by decomposing the stacked Ly$α$ radial profile into a compact galaxy-like and an extended halo-like component. We find that the exponential scale-length of LAHs depends on UV continuum and Ly$α$ luminosities, but not on Ly$α$ equivalent widths or galaxy overdensity parameters. The full samples, which are dominated by low UV-continuum luminosity Ly$α$ emitters ($M_{\rm UV} \gtrsim -21$), exhibit LAH sizes of 5$\,-\,6\,$kpc. However, the most UV- or Ly$α$-luminous galaxies have more extended halos with scale-lengths of 7$\,-\,9\,$kpc. The stacked Ly$α$ radial profiles decline more steeply than recent theoretical predictions that include the contributions from gravitational cooling of infalling gas and from low-level star formation in satellites. On the other hand, the LAH extent matches what one would expect for photons produced in the galaxy and then resonantly scattered by gas in an outflowing envelope. The observed trends of LAH sizes with host galaxy properties suggest that the physical conditions of the circumgalactic medium (covering fraction, HI column density, and outflow velocity) change with halo mass and/or star-formation rates.

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arXiv2017

The MOSDEF Survey: Metallicity Dependence of the PAH Emission at High Redshift and Implications for 24 micron-inferred IR Luminosities and Star Formation Rates at z~2

We present results on the variation of 7.7 micron Polycyclic Aromatic Hydrocarbon (PAH) emission in galaxies spanning a wide range in metallicity at z ~ 2. For this analysis, we use rest-frame optical spectra of 476 galaxies at 1.37 < z < 2.61 from the MOSFIRE Deep Evolution Field (MOSDEF) survey to infer metallicities and ionization states. Spitzer/MIPS 24 micron and Herschel/PACS 100 and 160 micron observations are used to derive rest-frame 7.7 micron luminosities (L(7.7)) and total IR luminosities (L(IR)), respectively. We find significant trends between the ratio of L(7.7) to L(IR) (and to dust-corrected SFR) and both metallicity and [OIII]/[OII] (O32) emission-line ratio. The latter is an empirical proxy for the ionization parameter. These trends indicate a paucity of PAH emission in low metallicity environments with harder and more intense radiation fields. Additionally, L(7.7)/L(IR) is significantly lower in the youngest quartile of our sample (ages of 500 Myr) compared to older galaxies, which may be a result of the delayed production of PAHs by AGB stars. The relative strength of L(7.7) to L(IR) is also lower by a factor of ~ 2 for galaxies with masses $M_* < 10^{10}M_{\odot}$, compared to the more massive ones. We demonstrate that commonly-used conversions of L(7.7) (or 24 micron flux density; f(24)) to L(IR) underestimate the IR luminosity by more than a factor of 2 at $M_*$ ~ $10^{9.6-10.0} M_{\odot}$. We adopt a mass-dependent conversion of L(7.7) to L(IR) with L(7.7)/L(IR)= 0.09 and 0.22 for $M_* < 10^{10}$ and $> 10^{10} M_{\odot}$, respectively. Based on the new scaling, the SFR-$M_*$ relation has a shallower slope than previously derived. Our results also suggest a higher IR luminosity density at z ~ 2 than previously measured, corresponding to a ~ 30% increase in the SFR density.

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arXiv2016

The MOSDEF survey: AGN multi-wavelength identification, selection biases and host galaxy properties

We present results from the MOSFIRE Deep Evolution Field (MOSDEF) survey on the identification, selection biases, and host galaxy properties of 55 X-ray, IR and optically-selected active galactic nuclei (AGN) at $1.4 < z < 3.8$. We obtain rest-frame optical spectra of galaxies and AGN and use the BPT diagram to identify optical AGN. We examine the uniqueness and overlap of the AGN identified at different wavelengths. There is a strong bias against identifying AGN at any wavelength in low mass galaxies, and an additional bias against identifying IR AGN in the most massive galaxies. AGN hosts span a wide range of star formation rate (SFR), similar to inactive galaxies once stellar mass selection effects are accounted for. However, we find (at $\sim 2-3σ$ significance) that IR AGN are in less dusty galaxies with relatively higher SFR and optical AGN in dusty galaxies with relatively lower SFR. X-ray AGN selection does not display a bias with host galaxy SFR. These results are consistent with those from larger studies at lower redshifts. Within star-forming galaxies, once selection biases are accounted for, we find AGN in galaxies with similar physical properties as inactive galaxies, with no evidence for AGN activity in particular types of galaxies. This is consistent with AGN being fueled stochastically in any star-forming host galaxy. We do not detect a significant correlation between SFR and AGN luminosity for individual AGN hosts, which may indicate the timescale difference between the growth of galaxies and their supermassive black holes.

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arXiv2016

The Bursty Star Formation Histories of Low-mass Galaxies at $0.4<z<1$ Revealed by Star Formation Rates Measured from H$β$ and FUV

We investigate the burstiness of star formation histories (SFHs) of galaxies at $0.4<z<1$ by using the ratio of star formation rates (SFRs) measured from H$β$ and FUV (1500 Å) (H$β$--to--FUV ratio). Our sample contains 164 galaxies down to stellar mass (M*) of $10^{8.5} M_\odot$ in the CANDELS GOODS-N region, where Team Keck Redshift Survey DEIMOS spectroscopy and HST/WFC3 F275W images from CANDELS and Hubble Deep UV Legacy Survey are available. When the {\it ratio} of H$β$- and FUV-derived SFRs is measured, dust extinction correction is negligible (except for very dusty galaxies) with the Calzetti attenuation curve. The H$β$--to--FUV ratio of our sample increases with M* and SFR. The median ratio is $\sim$0.7 at M*$\sim10^{8.5} M_\odot$ (or SFR$\sim 0.5 M_\odot/yr$) and increases to $\sim$1 at M*$\sim10^{10} M_\odot$ (or SFR $\sim 10 M_\odot/yr$). At M*$<10^{9.5} M_\odot$, our median H$β$--to--FUV ratio is lower than that of local galaxies at the same M*, implying a redshift evolution. Bursty SFH on a timescale of a few tens of megayears on galactic scales provides a plausible explanation of our results, and the importance of the burstiness increases as M* decreases. Due to sample selection effects, our H$β$--to--FUV ratio may be an upper limit of the true value of a complete sample, which strengthens our conclusions. Other models, e.g., non-universal initial mass function or stochastic star formation on star cluster scales, are unable to plausibly explain our results.

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arXiv2016

Spectroscopic Confirmation of a Protocluster at z=3.786

We present new observations of the field containing the z=3.786 protocluster, PC217.96+32.3. We confirm that it is one of the largest and most overdense high-redshift structures known. Such structures are rare even in the largest cosmological simulations. We used the Mayall/MOSAIC1.1 imaging camera to image a 1.2x0.6 deg area (~150x75 comoving Mpc) surrounding the protocluster's core and discovered 165 candidate Lyman Alpha emitting galaxies (LAEs) and 788 candidate Lyman Break galaxies (LBGs). There are at least 2 overdense regions traced by the LAEs, the largest of which shows an areal overdensity in its core (i.e., within a radius of 2.5 comoving Mpc) of 14+/-7 relative to the average LAE spatial density in the imaged field. Further, the average LAE spatial density in the imaged field is twice that derived by other field LAE surveys. Spectroscopy with Keck/DEIMOS yielded redshifts for 164 galaxies (79 LAEs and 85 LBGs); 65 lie at a redshift of 3.785+/-0.010. The velocity dispersion of galaxies near the core is 350+/-40 km/s, a value robust to selection effects. The overdensities are likely to collapse into systems with present-day masses of >10^{15} solar masses and >6x10^{14} solar masses. The low velocity dispersion may suggest a dynamically young protocluster. We find a weak trend between narrow-band (Lyman Alpha) luminosity and environmental density: the Lyman Alpha luminosity is enhanced on average by 1.35X within the protocluster core. There is no evidence that the Lyman Alpha equivalent width depends on environment. These suggest that star-formation and/or AGN activity is enhanced in the higher density regions of the structure. PC217.96+32.3 is a Coma cluster analog, witnessed in the process of formation.

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arXiv2015

A Critical Assessment of Stellar Mass Measurement Methods

In this paper we perform a comprehensive study of the main sources of random and systematic errors in stellar mass measurement for galaxies using their Spectral Energy Distributions (SEDs). We use mock galaxy catalogs with simulated multi-waveband photometry (from U-band to mid-infrared) and known redshift, stellar mass, age and extinction for individual galaxies. Given different parameters affecting stellar mass measurement (photometric S/N ratios, SED fitting errors, systematic effects, the inherent degeneracies and correlated errors), we formulated different simulated galaxy catalogs to quantify these effects individually. We studied the sensitivity of stellar mass estimates to the codes/methods used, population synthesis models, star formation histories, nebular emission line contributions, photometric uncertainties, extinction and age. For each simulated galaxy, the difference between the input stellar masses and those estimated using different simulation catalogs, $Δ\log(M)$, was calculated and used to identify the most fundamental parameters affecting stellar masses. We measured different components of the error budget, with the results listed as follows: (1). no significant bias was found among different codes/methods, with all having comparable scatter; (2). A source of error is found to be due to photometric uncertainties and low resolution in age and extinction grids; (3). The median of stellar masses among different methods provides a stable measure of the mass associated with any given galaxy; (4). The deviations in stellar mass strongly correlate with those in age, with a weaker correlation with extinction; (5). the scatter in the stellar masses due to free parameters are quantified, with the sensitivity of the stellar mass to both the population synthesis codes and inclusion of nebular emission lines studied.

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arXiv2015

The Relation Between SFR and Stellar Mass for Galaxies at 3.5 $\le z\le$ 6.5 in CANDELS

Distant star-forming galaxies show a correlation between their star formation rates (SFR) and stellar masses, and this has deep implications for galaxy formation. Here, we present a study on the evolution of the slope and scatter of the SFR-stellar mass relation for galaxies at $3.5\leq z\leq 6.5$ using multi-wavelength photometry in GOODS-S from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) and Spitzer Extended Deep Survey. We describe an updated, Bayesian spectral-energy distribution fitting method that incorporates effects of nebular line emission, star formation histories that are constant or rising with time, and different dust attenuation prescriptions (starburst and Small Magellanic Cloud). From $z$=6.5 to $z$=3.5 star-forming galaxies in CANDELS follow a nearly unevolving correlation between stellar mass and SFR that follows SFR $\sim$ $M_\star^a$ with $a = 0.54 \pm 0.16$ at $z\sim 6$ and $0.70 \pm 0.21$ at $z\sim 4$. This evolution requires a star formation history that increases with decreasing redshift (on average, the SFRs of individual galaxies rise with time). The observed scatter in the SFR-stellar mass relation is tight, $σ(\log \mathrm{SFR}/\mathrm{M}_\odot$ yr$^{-1})< 0.3\ - $ 0.4 dex, for galaxies with $\log M_\star/\mathrm{M}_\odot > 9$ dex. Assuming that the SFR is tied to the net gas inflow rate (SFR $\sim$ $\dot{M}_\mathrm{gas}$), then the scatter in the gas inflow rate is also smaller than 0.3$-$0.4 dex for star-forming galaxies in these stellar mass and redshift ranges, at least when averaged over the timescale of star formation. We further show that the implied star formation history of objects selected on the basis of their co-moving number densities is consistent with the evolution in the SFR-stellar mass relation.

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