arXiv subjects

Robin Ciardullo

Publications and source records attributed to Robin Ciardullo.

At least 73 records · Page 4Linked to original sources

arXiv2021

The $z \sim 2$ $\rm{[O\ III]}$ Luminosity Function of Grism-selected Emission-line Galaxies

Upcoming missions such as Euclid and the Nancy Grace Roman Space Telescope (Roman) will use emission-line selected galaxies to address a variety of questions in cosmology and galaxy evolution in the $z>1$ universe. The optimal observing strategy for these programs relies upon knowing the number of galaxies that will be found and the bias of the galaxy population. Here we measure the $\rm{[O\ III]}\ λ5007$ luminosity function for a vetted sample of 1951 $m_{\rm J+JH+H} < 26$ galaxies with unambiguous redshifts between $1.90 < z < 2.35$, which were selected using HST/WFC3 G141 grism frames made available by the 3D-HST program. These systems are directly analogous to the galaxies that will be identified by the Euclid and Roman missions, which will utilize grism spectroscopy to find $\rm{[O\ III]}\ λ5007$-emitting galaxies at $0.8 \lesssim z \lesssim 2.7$ and $1.7 \lesssim z \lesssim 2.8$, respectively. We interpret our results in the context of the expected number counts for these upcoming missions. Finally, we combine our dust-corrected $\rm{[O\ III]}$ luminosities with rest-frame ultraviolet star formation rates to present a new calibration of the SFR density associated with $1.90 < z < 2.35$ $\rm{[O\ III]}$-emitting galaxies. We find that these grism-selected galaxies contain roughly half of the total star formation activity at $z\sim2$.

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arXiv2021

AGN and Star Formation at Cosmic Noon: Comparison of Data to Theoretical Models

In theoretical models of galaxy evolution, AGN and star formation (SF) activity are closely linked and AGN feedback is routinely invoked to regulate galaxy growth. In order to constrain such models, we compare the hydrodynamical simulations IllustrisTNG and SIMBA, and the semi-analytical model SAG to the empirical results on AGN and SF at cosmic noon ($0.75 < z < 2.25$) reported in Florez et al. (2020). The empirical results are based on a large mass-complete sample drawn from 93,307 galaxies with and without high X-ray luminosity AGN ($L_X \gtrsim 10^{44}$ erg s$^{-1}$), selected from a 11.8 deg$^2$ area ($\sim 0.18$ Gpc$^3$ comoving volume at $z=0.75-2.25$). The main results of our comparisons are: (i) SAG and IllustrisTNG both qualitatively reproduce the empirical result that galaxies with high X-ray luminosity AGN have higher mean SFR, at a given stellar mass, than galaxies without such AGN. SAG, however, strongly over-produces the number density of high X-ray luminosity AGN by a factor of 10 to 100, while IllustrisTNG shows a lack of high X-ray luminosity AGN at high stellar mass ($M* > 10^{11} \ M_{\odot}$) at $z \sim 2$. (ii) In SIMBA, the mean SFR of galaxies with high X-ray luminosity AGN is lower than the SFR of galaxies without such AGN. Contrary to the data, many high X-ray luminosity AGN in SIMBA have quenched SF, suggesting that AGN feedback, or other feedback modes in galaxies with such AGN, might be too efficient in SIMBA.

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arXiv2021

Correcting correlation functions for redshift-dependent interloper contamination

The construction of catalogues of a particular type of galaxy can be complicated by interlopers contaminating the sample. In spectroscopic galaxy surveys this can be due to the misclassification of an emission line; for example in the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) low redshift [OII] emitters may make up a few percent of the observed Ly$α$ emitter (LAE) sample. The presence of contaminants affects the measured correlation functions and power spectra. Previous attempts to deal with this using the cross-correlation function have assumed sources at a fixed redshift, or not modelled evolution within the adopted redshift bins. However, in spectroscopic surveys like HETDEX, where the contamination fraction is likely to be redshift dependent, the observed clustering of misclassified sources will appear to evolve strongly due to projection effects, even if their true clustering does not. We present a practical method for accounting for the presence of contaminants with redshift-dependent contamination fractions and projected clustering. We show using mock catalogues that our method, unlike existing approaches, yields unbiased clustering measurements from the upcoming HETDEX survey in scenarios with redshift-dependent contamination fractions within the redshift bins used. We show our method returns auto-correlation functions with systematic biases much smaller than the statistical noise for samples with at least as high as 7 per cent contamination. We also present and test a method for fitting for the redshift-dependent interloper fraction using the LAE-[OII] galaxy cross-correlation function, which gives less biased results than assuming a single interloper fraction for the whole sample.

astro-ph.CO↗

arXiv2021

Detection of Lyman Continuum from 3.0 < z < 3.5 Galaxies in the HETDEX Survey

Questions as to what drove the bulk reionization of the Universe, how that reionization proceeded, and how the hard ionizing radiation reached the intergalactic medium remain open and debated. Observations probing that epoch are severely hampered by the increasing amounts of neutral gas with increasing redshift, so a small, but growing number of experiments are targeting star forming galaxies ($z\sim3$) as proxies. However, these studies, while providing fantastic detail, are time intensive, contain relatively few targets, and can suffer from selection biases. As a complementary alternative, we investigate whether stacking the already vast (and growing) numbers of low-resolution ($Δλ/ λ= 800$) Lyman-$α$ Emitting (LAE) galaxy spectra from the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) can be used to measure ionizing photons (restframe 880-910Å) escaping their galaxy hosts. As a blind survey, HETDEX avoids the biases from continuum selected galaxies and its planned 540 square degree coverage promotes the statistical power of large numbers. In this paper, we confirm the feasibility of Lyman continuum detection by carefully selecting a sample of \lyccount\ high redshift ($z\sim$3) LAEs from a subset of HETDEX observations, stacking their spectra and measuring a $\gtrsim$3$σ$ detection of $0.10 μ$Jy restframe Lyman continuum emission, uncorrected for attenuation in the intergalactic medium, over the full sample stack ($3.0 < z < 3.5$ and $-22.0 \lesssim M_{\text{UV}} \lesssim -19.0$).

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arXiv2021

The Shape and Scatter of The Galaxy Main Sequence for Massive Galaxies at Cosmic Noon

We present the main sequence for all galaxies and star-forming galaxies for a sample of 28,469 massive ($M_\star \ge 10^{11}$M$_\odot$) galaxies at cosmic noon ($1.5 < z < 3.0$), uniformly selected from a 17.5 deg$^2$ area (0.33 Gpc$^3$ comoving volume at these redshifts). Our large sample allows for a novel approach to investigating the galaxy main sequence that has not been accessible to previous studies. We measure the main sequence in small mass bins in the SFR-M$_{\star}$ plane without assuming a functional form for the main sequence. With a large sample of galaxies in each mass bin, we isolate star-forming galaxies by locating the transition between the star-forming and green valley populations in the SFR-M$_{\star}$ plane. This approach eliminates the need for arbitrarily defined fixed cutoffs when isolating the star-forming galaxy population, which often biases measurements of the scatter around the star-forming galaxy main sequence. We find that the main sequence for all galaxies becomes increasingly flat towards present day at the high-mass end, while the star-forming galaxy main sequence does not. We attribute this difference to the increasing fraction of the collective green valley and quiescent galaxy population from $z=3.0$ to $z=1.5$. Additionally, we measure the total scatter around the star-forming galaxy main sequence and find that it is $\sim0.5-1.0$ dex with little evolution as a function of mass or redshift. We discuss the implications that these results have for pinpointing the physical processes driving massive galaxy evolution.

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arXiv2021

HETDEX [OIII] Emitters I: A spectroscopically selected low-redshift population of low-mass, low-metallicity galaxies

We assemble a sample of 17 low metallicity (7.45 < log(O/H)+12 < 8.12) galaxies with z < 0.1 found spectroscopically, without photometric pre-selection, in early data from the Hobby Eberly Telescope Dark Energy Experiment (HETDEX). Star forming galaxies that occupy the lowest mass and metallicity end of the mass-metallicity relation tend to be under sampled in continuum-based surveys as their spectra are typically dominated by emission from newly forming stars. We search for galaxies with high [OIII]$λ$5007 / [OII]$λ$3727, implying highly ionized nebular emission often indicative of low metallicity systems. With the Second Generation Low Resolution Spectrograph on the Hobby Eberly Telescope we acquired follow-up spectra, with higher resolution and broader wavelength coverage, of each low-metallicity candidate in order to confirm the redshift, measure the H$α$ and [NII] line strengths and, in many cases, obtain deeper spectra of the blue lines. We find our galaxies are consistent with the mass-metallicity relation of typical low mass galaxies. However, galaxies in our sample tend to have similar specific star formation rates (sSFRs) as the incredibly rare "blueberry" galaxies found in (Yang et. al. 2017). We illustrate the power of spectroscopic surveys for finding low mass and metallicity galaxies and reveal that we find a sample of galaxies that are a hybrid between the properties of typical dwarf galaxies and the more extreme blueberry galaxies.

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arXiv2021

Towards Precision Cosmology With Improved PNLF Distances Using VLT-MUSE I. Methodology and Tests

The [O III ] 5007 Planetary Nebula Luminosity Function (PNLF) is an established distance indicator that has been used for more than 30 years to measure the distances of galaxies out to ~15 Mpc. With the advent of the Multi-Unit Spectroscopic Explorer on the Very Large Telescope (MUSE) as an efficient wide-field integral field spectrograph, the PNLF method is due for a renaissance, as the spatial and spectral information contained in the instrument's datacubes provides many advantages over classical narrow-band imaging. Here we use archival MUSE data to explore the potential of a novel differential emission-line filter (DELF) technique to produce spectrophotometry that is more accurate and more sensitive than other methods. We show that DELF analyses are superior to classical techniques in high surface brightness regions of galaxies and we validate the method both through simulations and via the analysis of data from two early-type galaxies (NGC 1380 and NGC 474) and one late-type spiral (NGC 628). We demonstrate that with adaptive optics support or under excellent seeing conditions, the technique is capable of producing precision (< 0.05 mag) [O III ] photometry out to distances of 40 Mpc while providing discrimination between planetary nebulae and other emission-line objects such as H II regions, supernova remnants, and background galaxies. These capabilities enable us to use MUSE to measure precise PNLF distances beyond the reach of Cepheids and the tip of the red giant branch method, and become an additional tool for constraining the local value of the Hubble constant.

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arXiv2021

The NEWFIRM HETDEX Survey: Photometric Catalog and a Conservative Sample of Massive Quiescent Galaxies at z=3-5 over 17.5 deg^2 in the SHELA Field

We present the results of a deep K_s-band (2.1 um) imaging survey of the Spitzer/HETDEX Exploratory Large Area (SHELA) field using the NEWFIRM near-infrared (NIR) camera on the KPNO Mayall 4-m telescope. This NEWFIRM HETDEX Survey (NHS) reaches a 5-sigma depth of 22.4 AB mag (2"-diameter apertures corrected to total), is ~50% and 90% complete at K~22.65 and K~22.15, respectively, and covers 22 deg^2 of the 24 deg^2 SHELA Spitzer/ IRAC footprint (within ``Stripe 82' of the Sloan Digital Sky Survey). We present a K_s-band-selected catalog which includes deep ugriz imaging from the Dark Energy Camera and 3.6 and 4.5 um imaging from Spitzer/IRAC, with forced-photometry of 1.7 million sources across 17.5 deg^2. The large area and moderate depth of this catalog enables the study of the most massive galaxies at high redshift, and minimizes uncertainties associated with counting statistics and cosmic variance. As a demonstration, we derive stellar masses (M*) and star-formation rates (SFRs) for candidate galaxies at 3 < z < 5, and select a conservative sample of nine candidate massive (M* > 10^11 Msol) quiescent galaxies, which have measured SFRs significantly below the main-sequence at this redshift. Five are ultra-massive with M* > 10^12, though uncertainties in IRAC blending, gravitational lensing, or AGN emission could result in true masses which are lower. Simulations predict that these galaxies should be extremely rare, thus we discuss what physical processes in models could be altered to allow the formation of such massive quiescent galaxies at such early times.

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arXiv2021

Stellar, Gas, and Dust Emission of Star Forming Galaxies out to z~2

While dust is a major player in galaxy evolution, its relationship with gas and stellar radiation in the early universe is still not well understood. We combine 3D-HST emission line fluxes with far-UV through far-IR photometry in a sample of 669 emission-line galaxies (ELGs) between 1.2 < z < 1.9 and use the MCSED spectral energy distribution fitting code to constrain the galaxies' physical parameters, such as their star formation rates (SFRs), stellar masses, and dust masses. We find that the assumption of energy balance between dust attenuation and emission is likely unreasonable in many cases. We highlight a relationship between the mass-specific star formation rate (sSFR), stellar mass, and dust mass, although its exact form is still unclear. Finally, a stacking of H$α$ and H$β$ fluxes shows that nebular attenuation increases with stellar mass and SFR for IR-bright ELGs.

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arXiv2021

The HETDEX Survey: The Ly$α$ Escape Fraction from 3D-HST Emission Line Galaxies at $z \sim 2$

We measure the Ly$α$ escape fraction of 935 [OIII]-emitting galaxies between $1.9 < z < 2.35$ by comparing stacked spectra from the Hubble Space Telescope/WFC3's near-IR grism to corresponding stacks from the Hobby Eberly Telescope Dark Energy Experiment's Internal Data Release 2. By measuring the stacks' H$β$ to Ly$α$ ratios, we determine the Ly$α$ escape fraction as a function of stellar mass, star formation rate, internal reddening, size, and [OIII]/H$β$ ratio. We show that the escape fraction of Ly$α$ correlates with a number of parameters, such as galaxy size, star formation rate, and nebular excitation. However, we also demonstrate that most of these relations are indirect, and the primary variables that control the escape of Ly$α$ are likely stellar mass and internal extinction. Overall, the escape of Ly$α$ declines from $\gtrsim 18\%$ in galaxies with $\log M/M_{\odot} \lesssim 9$ to $\lesssim 1\%$ for systems with $\log M/M_{\odot} \gtrsim 10$, with the sample's mean escape fraction being $6.0^{+0.6\%}_{-0.5\%}$.

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arXiv2021

Measuring Stellar Masses of Emission Line Galaxies at 1.2<z<1.9

The accurate measurement of stellar masses over a wide range of galaxy properties is essential for better constraining models of galaxy evolution. Emission line galaxies (ELGs) tend to have better redshift estimates than continuum-selected objects and have been shown to span a large range of physical properties, including stellar mass. Using data from the 3D-HST Treasury program, we construct a carefully vetted sample of 4350 ELGs at redshifts 1.16<z<1.90. We combine the 3D-HST emission line fluxes with far-UV through near-IR photometry and use the MCSED spectral energy distribution fitting code to constrain the galaxies' physical parameters, such as their star formation rate (SFRs) and stellar masses. Our sample is consistent with the z~2 mass-metallicity relation. More importantly, we show there is a simple but tight correlation between stellar mass and absolute magnitude in a near-IR filter that will be particularly useful in quickly calculating accurate stellar masses for millions of galaxies in upcoming missions such as Euclid and the Nancy Grace Roman Space Telescope.

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arXiv2021

The Stars of the HETDEX Survey. I. Radial Velocities and Metal-Poor Stars from Low-Resolution Stellar Spectra

The Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) is an unbiased, massively multiplexed spectroscopic survey, designed to measure the expansion history of the universe through low-resolution ($R\sim750$) spectra of Lyman-Alpha Emitters. In its search for these galaxies, HETDEX will also observe a few 10$^{5}$ stars. In this paper, we present the first stellar value-added catalog within the internal second data release of the HETDEX Survey (HDR2). The new catalog contains 120,571 low-resolution spectra for 98,736 unique stars between $10 < G < 22$ spread across the HETDEX footprint at relatively high ($b\sim60^\circ$) Galactic latitudes. With these spectra, we measure radial velocities (RVs) for $\sim$42,000 unique FGK-type stars in the catalog and show that the HETDEX spectra are sufficient to constrain these RVs with a 1$σ$ precision of 28.0 km/s and bias of 3.5 km/s with respect to the LAMOST surveys and 1$σ$ precision of 27.5 km/s and bias of 14.0 km/s compared to the SEGUE survey. Since these RVs are for faint ($G\geq16$) stars, they will be complementary to Gaia. Using t-Distributed Stochastic Neighbor Embedding (t-SNE), we also demonstrate that the HETDEX spectra can be used to determine a star's T${\rm{eff}}$, and log g and its [Fe/H]. With the t-SNE projection of the FGK-type stars with HETDEX spectra we also identify 416 new candidate metal-poor ([Fe/H] $< -1$~dex) stars for future study. These encouraging results illustrate the utility of future low-resolution stellar spectroscopic surveys.

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arXiv2021

The RR Lyrae Delay-Time Distribution: A Novel Perspective on Models of Old Stellar Populations

The delay-time distribution (DTD) is the occurrence rate of a class of objects as a function of time after a hypothetical burst of star formation. DTDs are mainly used as a statistical test of stellar evolution scenarios for supernova progenitors, but they can be applied to many other classes of astronomical objects. We calculate the first DTD for RR Lyrae variables using 29,810 RR Lyrae from the OGLE-IV survey and a map of the stellar-age distribution (SAD) in the Large Magellanic Cloud (LMC). We find that $\sim 46\%$ of the OGLE-IV RR Lyrae are associated with delay-times older than 8 Gyr (main-sequence progenitor masses less than 1 M$_{\odot}$), and consistent with existing constraints on their ages, but surprisingly about $51\%$ of RR Lyrae appear have delay times $1.2-8$ Gyr (main-sequence masses between $1 - 2$ M$_{\odot}$ at LMC metallicity). This intermediate-age signal also persists outside the Bar-region where crowding is less of a concern, and we verified that without this signal, the spatial distribution of the OGLE-IV RR Lyrae is inconsistent with the SAD map of the LMC. Since an intermediate-age RR Lyrae channel is in tension with the lack of RR Lyrae in intermediate-age clusters (noting issues with small-number statistics), and the age-metallicity constraints of LMC stars, our DTD result possibly indicates that systematic uncertainties may still exist in SAD measurements of old-stellar populations, perhaps stemming from the construction methodology or the stellar evolution models used. We described tests to further investigate this issue.

astro-ph.SR↗

arXiv2021

Two Luminous Post-AGB Stars in the Galactic Globular Cluster M19

We report the discovery of a luminous "yellow" post-asymptotic-giant-branch (PAGB) star in the globular cluster (GC) M19 (NGC 6273), identified during our uBVI survey of Galactic GCs. The uBVI photometric system is optimized to detect stars with large Balmer discontinuities, indicating very low surface gravities and high luminosities. The spectral-energy distribution (SED) of the star is consistent with an effective temperature of about 6250 K and a surface gravity of $\log g=0.5$. We use Gaia data to show that the star's proper motion and radial velocity are consistent with cluster membership. One aim of our program is to test yellow PAGB stars as candidate Population II standard candles for determining extragalactic distances. We derive a visual absolute magnitude of $M_V=-3.39\pm0.09$ for the M19 star. This is in close agreement with the $M_V$ values found for yellow PAGB stars in the GCs omega Cen, NGC 5986, and M79, indicating a very narrow luminosity function. These objects are four magnitudes brighter than RR Lyrae variables, and they can largely avoid the issues of interstellar extinction that are a problem for Population I distance indicators. We also identified a second luminous PAGB object in M19, this one a hotter "UV-bright" star. Its SED is consistent with an effective temperature of about 11,750 K and $\log g=2.0$. The two objects have nearly identical bolometric luminosities, $\log L/L_\odot=3.24$ and 3.22, respectively.

astro-ph.SR↗

arXiv2020

Investigating The Growing Population of Massive Quiescent Galaxies at Cosmic Noon

We explore the buildup of quiescent galaxies using a sample of 28,469 massive ($M_\star \ge 10^{11}$M$_\odot$) galaxies at redshifts $1.5<z<3.0$, drawn from a 17.5 deg$^2$ area (0.33 Gpc$^3$ comoving volume at these redshifts). This allows for a robust study of the quiescent fraction as a function of mass at $1.5<z<3.0$ with a sample $\sim$40 times larger at log($M_{\star}$/$\rm M_{\odot}$)$\ge11.5$ than previous studies. We derive the quiescent fraction using three methods: specific star-formation rate, distance from the main sequence, and UVJ color-color selection. All three methods give similar values at $1.5<z<2.0$, however the results differ by up to a factor of two at $2.0<z<3.0$. At redshifts $1.5 < z < 3.0$ the quiescent fraction increases as a function of stellar mass. By $z=2$, only 3.3 Gyr after the Big Bang, the universe has quenched $\sim$25% of $M_\star = 10^{11}$M$_\odot$ galaxies and $\sim$45% of $M_\star = 10^{12}$M$_\odot$ galaxies. We discuss physical mechanisms across a range of epochs and environments that could explain our results. We compare our results with predictions from hydrodynamical simulations SIMBA and IllustrisTNG and semi-analytic models (SAMs) SAG, SAGE, and Galacticus. The quiescent fraction from IllustrisTNG is higher than our empirical result by a factor of $2-5$, while those from SIMBA and the three SAMs are lower by a factor of $1.5-10$ at $1.5<z<3.0$.

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arXiv2020

Cosmological 3D HI Gas Map with HETDEX Ly$α$ Emitters and eBOSS QSOs at $z=2$: IGM-Galaxy/QSO Connection and a $\sim$ 40-Mpc Scale Giant HII Bubble Candidate

We present cosmological ($30-400$ Mpc) distributions of neutral hydrogen (HI) in the inter-galactic medium (IGM) traced by Ly$α$ Emitters (LAEs) and QSOs at $z=2.1-2.5$, selected with the data of the on-going Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) and the eBOSS survey. We investigate spatial correlations of LAEs and QSOs with HI tomography maps reconstructed from HI Ly$α$ forest absorption in the spectra of background galaxies and QSOs obtained by the CLAMATO survey and this study, respectively. In the cosmological volume far from QSOs, we find that LAEs reside in regions of strong HI absorption, i.e. HI rich, which is consistent with results of previous galaxy-background QSO pair studies. Moreover, there is an anisotropy in the HI-distribution plot of transverse and line-of-sight distances; on average the HI absorption peak is blueshifted by $\sim 200$ km s$^{-1}$ from the LAE Ly$α$ redshift, reproducing the known average velocity offset between the Ly$α$ emission redshift and the galaxy systemic redshift. We have identified a $\sim$ 40-Mpc scale volume of HI underdensity that is a candidate for a giant HII bubble, where six QSOs and an LAE overdensity exist at $\left < z \right > =2.16$. The coincidence of the QSO and LAE overdensities with the HI underdensity indicates that the ionizing photon radiation of the QSOs has created a highly ionized volume of multiple proximity zones in a matter overdensity. Our results suggest an evolutionary picture where HI gas in an overdensity of galaxies becomes highly photoionized when QSOs emerge in the galaxies.

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arXiv2020

Exploring AGN and Star Formation Activity of Massive Galaxies at Cosmic Noon

We investigate the relation between AGN and star formation (SF) activity at $0.5 < z < 3$ by analyzing 898 galaxies with X-ray luminous AGN ($L_X > 10^{44}$ erg s$^{-1}$) and a large comparison sample of $\sim 320,000$ galaxies without X-ray luminous AGN. Our samples are selected from a large (11.8 deg$^2$) area in Stripe 82 that has multi-wavelength (X-ray to far-IR) data. The enormous comoving volume ($\sim 0.3$ Gpc$^3$) at $0.5 < z < 3$ minimizes the effects of cosmic variance and captures a large number of massive galaxies ($\sim 30,000$ galaxies with $M_* > 10^{11} \ M_{\odot}$) and X-ray luminous AGN. While many galaxy studies discard AGN hosts, we fit the SED of galaxies with and without X-ray luminous AGN with Code Investigating GALaxy Emission (CIGALE) and include AGN emission templates. We find that without this inclusion, stellar masses and star formation rates (SFRs) in AGN host galaxies can be overestimated, on average, by factors of up to $\sim 5$ and $\sim 10$, respectively. The average SFR of galaxies with X-ray luminous AGN is higher by a factor of $\sim 3$ to $10$ compared to galaxies without X-ray luminous AGN at fixed stellar mass and redshift, suggesting that high SFRs and high AGN X-ray luminosities may be fueled by common mechanisms. The vast majority ($> 95 \%$) of galaxies with X-ray luminous AGN at $z=0.5-3$ do not show quenched SF: this suggests that if AGN feedback quenches SF, the associated quenching process takes a significant time to act and the quenched phase sets in after the highly luminous phases of AGN activity.

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arXiv2020

MCSED: A flexible spectral energy distribution fitting code and its application to $z \sim 2$ emission-line galaxies

We present MCSED, a new spectral energy distribution (SED)-fitting code, which mates flexible stellar evolution calculations with the Markov Chain Monte Carlo algorithms of the software package emcee. MCSED takes broad, intermediate, and narrow-band photometry, emission-line fluxes, and/or absorption line spectral indices, and returns probability distributions and co-variance plots for all model parameters. MCSED includes a variety of dust attenuation curves with parameters for varying the UV slopes and bump strengths, a prescription for continuum and PAH emission from dust, models for continuum and line emission from ionized gas, options for fixed and variable stellar metallicity, and a selection of star formation rate (SFR) histories. The code is well-suited for exploring parameter inter-dependencies in sets of galaxies with known redshifts, for which there is multi-band photometry and/or spectroscopy. We apply MCSED to a sample of $\sim2000$ $1.90<z<2.35$ galaxies in the five CANDELS fields, which were selected via their strong [O III] $\lambda5007$ emission, and explore the systematic behavior of their SEDs. We find the galaxies become redder with stellar mass, due to both increasing internal attenuation and a greater population of older stars. The slope of the UV extinction curve also changes with stellar mass, and at least some galaxies exhibit an extinction excess at 2175 Angstroms. Finally, we demonstrate that below $M\lesssim10^9\,M_{\odot}$), the shape of the star-forming galaxy main sequence is highly dependent on the galaxies' assumed SFR history, as calculations which assume a constant SFR produce stellar masses that are $\sim1$ dex smaller than those found using more realistic SFR histories.

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