arXiv subjects

Sandro Tacchella

Publications and source records attributed to Sandro Tacchella.

At least 325 records · Page 18Linked to original sources

arXiv2019

Model-independent constraints on the hydrogen-ionizing emissivity at z>6

Modelling reionization often requires significant assumptions about the properties of ionizing sources. Here, we infer the total output of hydrogen-ionizing photons (the ionizing emissivity, $\dot{N}_\textrm{ion}$) at $z=4-14$ from current reionization constraints, being maximally agnostic to the properties of ionizing sources. We use a Bayesian analysis to fit for a non-parametric form of $\dot{N}_\textrm{ion}$, allowing us to flexibly explore the entire prior volume. We infer a declining $\dot{N}_\textrm{ion}$ with redshift at $z>6$, which can be used as a benchmark for reionization models. Model-independent reionization constraints from the CMB optical depth and Ly$α$ and Ly$β$ forest dark pixel fraction produce $\dot{N}_\textrm{ion}$ evolution ($d\log_{10}\dot{N}_\textrm{ion}/dz|_{z=6\rightarrow8} = -0.31\pm0.35$ dex) consistent with the declining UV luminosity density of galaxies, assuming constant ionizing photon escape fraction and efficiency. Including measurements from Ly$α$ damping of galaxies and quasars produces a more rapid decline: $d\log_{10}\dot{N}_\textrm{ion}/dz|_{z=6\rightarrow8} =-0.44\pm0.22$ dex, steeper than the declining galaxy luminosity density (if extrapolated beyond $M_\mathrm{UV} \lesssim -13$), and constrains the mid-point of reionization to $z = 6.93\pm0.14$.

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arXiv2019

Beyond UVJ: More Efficient Selection of Quiescent Galaxies With UV / Mid-IR Fluxes

The UVJ color-color diagram is a popular and efficient method to distinguish between quiescent and star-forming galaxies through their rest-frame $U-V$ vs. $V-J$ colors. Here we explore the information content of this color-color space using the Bayesian inference machine Prospector. We fit the same physical model to two datasets: (i) UVJ fluxes alone, and (ii) full UV-mid IR (MIR) broadband SEDs from the 3D-HST survey. Notably this model uses both nonparametric SFHs and a flexible dust attenuation curve, both of which have the potential to `break' the typical correlations observed in UVJ color-color space. Instead, these fits confirm observed trends between UVJ colors and observed galaxy properties, including specific star formation rate (sSFR), dust attenuation, stellar age, and stellar metallicity. They also demonstrate that UVJ colors do not, on their own, constrain stellar age or metallicity; the observed trends in the UVJ diagram are instead driven by galaxy scaling relationships and thus will evolve with cosmological time. We also show that UVJ colors 'saturate' below $\log(\mathrm{sSFR/yr}^{-1})\lesssim -10.5$, i.e. changing sSFR no longer produces substantial changes in UVJ colors. We show that far-UV and/or MIR fluxes continue to correlate with sSFR down to low sSFRs and can be used in color-color diagrams to efficiently target galaxies with much lower levels of ongoing star formation. We provide selection criteria in these new color-color spaces as a function of desired sample sSFR.

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arXiv2019

Morphology and star formation in IllustrisTNG: the build-up of spheroids and discs

Using the IllustrisTNG simulations, we investigate the connection between galaxy morphology and star formation in central galaxies with stellar masses in the range $10^9-10^{11.5}~\mathrm{M}_{\odot}$. We quantify galaxy morphology by a kinematical decomposition of the stellar component into a spheroidal and a disc component (spheroid-to-total ratio, S/T) and by the concentration of the stellar mass density profile ($C_{82}$). S/T is correlated with stellar mass and star-formation activity, while $C_{82}$ correlates only with stellar mass. Overall, we find good agreement with observational estimates for both S/T and $C_{82}$. Low and high mass galaxies are dominated by random stellar motion, while only intermediate-mass galaxies ($M_{\star}\approx10^{10}-10^{10.5}~\mathrm{M}_{\odot}$) are dominated by ordered rotation. Whereas higher-mass galaxies are typical spheroids with high concentrations, lower-mass galaxies have low concentration, pointing to different formation channels. Although we find a correlation between S/T and star-formation activity, in the TNG model galaxies do not necessarily change their morphology when they transition through the green valley or when they cease their star formation, this depending on galaxy stellar mass and morphological estimator. Instead, the morphology (S/T and $C_{82}$) is generally set during the star-forming phase of galaxies. The apparent correlation between S/T and star formation arises because earlier-forming galaxies had, on average, a higher S/T at a given stellar mass. Furthermore, we show that mergers drive in-situ bulge formation in intermediate-mass galaxies and are responsible for the recent spheroidal mass assembly in the massive galaxies with $M_{\star}>10^{11}~\mathrm{M}_{\odot}$. In particular, these massive galaxies assemble about half of the spheroidal mass while star-forming and the other half through mergers while quiescent.

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arXiv2019

Atomic and molecular gas in IllustrisTNG galaxies at low redshift

We have recently developed a post-processing framework to estimate the abundance of atomic and molecular hydrogen (HI and H2, respectively) in galaxies in large-volume cosmological simulations. Here we compare the HI and H2 content of IllustrisTNG galaxies to observations. We mostly restrict this comparison to $z \approx 0$ and consider six observational metrics: the overall abundance of HI and H2, their mass functions, gas fractions as a function of stellar mass, the correlation between H2 and star formation rate, the spatial distribution of gas, and the correlation between gas content and morphology. We find generally good agreement between simulations and observations, particularly for the gas fractions and the HI mass-size relation. The H2 mass correlates with star formation rate as expected, revealing an almost constant depletion time that evolves up to z = 2 as observed. However, we also discover a number of tensions with varying degrees of significance, including an overestimate of the total neutral gas abundance at z = 0 by about a factor of two and a possible excess of satellites with no or very little neutral gas. These conclusions are robust to the modelling of the HI/H2 transition. In terms of their neutral gas properties, the IllustrisTNG simulations represent an enormous improvement over the original Illustris run. All data used in this paper are publicly available as part of the IllustrisTNG data release.

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arXiv2019

High redshift JWST predictions from IllustrisTNG: Dust modelling and galaxy luminosity functions

The James Webb Space Telescop (JWST) promises to revolutionise our understanding of the early Universe, and contrasting its upcoming observations with predictions of the $Λ$CDM model requires detailed theoretical forecasts. Here, we exploit the large dynamic range of the IllustrisTNG simulation suite, TNG50, TNG100, and TNG300, to derive multi-band galaxy luminosity functions from $z=2$ to $z=10$. We put particular emphasis on the exploration of different dust attenuation models to determine galaxy luminosity functions for the rest-frame ultraviolet (UV), and apparent wide NIRCam bands. Our most detailed dust model is based on continuum Monte Carlo radiative transfer calculations employing observationally calibrated dust properties. This calibration results in constraints on the redshift evolution of the dust attenuation normalisation and dust-to-metal ratios yielding a stronger redshift evolution of the attenuation normalisation compared to most previous theoretical studies. Overall we find good agreement between the rest-frame UV luminosity functions and observational data for all redshifts, also beyond the regimes used for the dust-model calibrations. Furthermore, we also recover the observed high redshift ($z=4-6$) UV luminosity versus stellar mass relation, the H$α$ versus star formation rate relation, and the H$α$ luminosity function at $z=2$. The bright end ($M_{\rm UV}>-19.5$) cumulative galaxy number densities are consistent with observational data. For the F200W NIRCam band, we predict that JWST will detect $\sim 80$ ($\sim 200$) galaxies with a signal-to-noise ratio of $10$ ($\sim 5$) within the NIRCam field of view, $2.2\times2.2 \,{\rm arcmin}^{2}$, for a total exposure time of $10^5{\rm s}$ in the redshift range $z=8 \pm 0.5$. These numbers drop to $\sim 10$ ($\sim 40$) for an exposure time of $10^4{\rm s}$.

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arXiv2019

Kiloparsec Scale Properties of Star-Formation Driven Outflows at z~2.3 in the SINS/zC-SINF AO Survey

We investigate the relationship between star formation activity and outflow properties on kiloparsec scales in a sample of 28 star forming galaxies at $z\sim$ 2-2.6, using adaptive optics assisted integral field observations from SINFONI on the VLT. The narrow and broad components of the H$α$ emission are used to simultaneously determine the local star formation rate surface density ($Σ_{\rm SFR}$), and the outflow velocity $v_{\rm out}$ and mass outflow rate $\dot{M}_{\rm out}$, respectively. We find clear evidence for faster outflows with larger mass loading factors at higher $Σ_{\rm SFR}$. The outflow velocities scale as $v_{\rm out}$ $\propto$ $Σ_{\rm SFR}^{0.34 \pm 0.10}$, which suggests that the outflows may be driven by a combination of mechanical energy released by supernova explosions and stellar winds, as well as radiation pressure acting on dust grains. The majority of the outflowing material does not have sufficient velocity to escape from the galaxy halos, but will likely be re-accreted and contribute to the chemical enrichment of the galaxies. In the highest $Σ_{\rm SFR}$ regions the outflow component contains an average of $\sim$45% of the H$α$ flux, while in the lower $Σ_{\rm SFR}$ regions only $\sim$10% of the H$α$ flux is associated with outflows. The mass loading factor, $η$ = $\dot{M}_{\rm out}$/SFR, is positively correlated with $Σ_{\rm SFR}$ but is relatively low even at the highest $Σ_{\rm SFR}$: $η\lesssim$ 0.5 $\times$ (380 cm$^{-3}$/n$_e$). This may be in tension with the $η$ $\gtrsim$ 1 required by cosmological simulations, unless a significant fraction of the outflowing mass is in other gas phases and has sufficient velocity to escape the galaxy halos.

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arXiv2019

Stochastic modeling of star-formation histories I: the scatter of the star-forming main sequence

We present a framework for modelling the star-formation histories of galaxies as a stochastic process. We define this stochastic process through a power spectrum density with a functional form of a broken power-law. Star-formation histories are correlated on short timescales, the strength of this correlation described by a power-law slope, $α$, and they decorrelate to resemble white noise over a timescale that is proportional to the timescale of the break in the power spectrum density, $τ_{\rm break}$. We use this framework to explore the properties of the stochastic process that, we assume, gives rise to the log-normal scatter about the relationship between star-formation rate and stellar mass, the so-called galaxy star-forming main sequence. Specifically, we show how the measurements of the normalisation and width ($σ_{\rm MS}$) of the main sequence, measured in several passbands that probe different timescales, give a constraint on the parameters of the underlying power spectrum density. We first derive these results analytically for a simplified case where we model observations by averaging over the recent star-formation history. We then run numerical simulations to find results for more realistic observational cases. As a proof of concept, we use observational estimates of the main sequence scatter at $z\sim0$ and $M_{\star}\approx10^{10}~M_{\odot}$ measured in H$α$, UV+IR and the u-band, and show that combination of these point to $τ_{\rm break}=178^{+104}_{-66}$ Myr, when assuming $α=2$. This implies that star-formation histories of galaxies lose "memory" of their previous activity on a timescale of $\sim200$ Myr, highlighting the importance of baryonic effects that act over the dynamical timescales of galaxies.

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arXiv2019

The MUSE Atlas of Disks (MAD): Resolving Star Formation Rates and Gas Metallicities on < 100pc Scales

We study the physical properties of the ionized gas in local disks using the sample of 38 nearby $\sim10^{8.5-11.2}$M$_\odot$ Star-Forming Main Sequence (SFMS) galaxies observed so far as part of the MUSE Atlas of Disks (MAD). Specifically, we use all strong emission lines in the MUSE wavelength range 4650-9300 Å to investigate the resolved ionized gas properties on $\sim$100 pc scales. This spatial resolution enables us to disentangle HII regions from the Diffuse Ionized Gas (DIG) in the computation of gas metallicities and Star Formation Rates (SFRs) of star forming regions. The gas metallicities generally decrease with radius. The metallicity of the HII regions is on average $\sim$0.1 dex higher than that of the DIG, but the metallicity radial gradient in both components is similar. The mean metallicities within the inner galaxy cores correlate with the total stellar mass of the galaxies. On our <100 pc scales, we find two correlations previously reported at kpc scales: a spatially resolved Mass-Metallicity Relation (RMZR) and a spatially resolved SFMS (RSFMS). We find no secondary dependency of the RMZR with the SFR density. We find that both resolved relations have a local origin, as they do not depend on the total stellar mass. The observational results of this paper are consistent with the inside-out scenario for the growth of galactic disks.

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arXiv2019

A new class of flares from accreting supermassive black holes

Accreting supermassive black holes (SMBHs) can exhibit variable emission across the electromagnetic spectrum and over a broad range of timescales. The variability of active galactic nuclei (AGNs) in the ultraviolet and optical is usually at the few tens of per cent level over timescales of hours to weeks. Recently, rare, more dramatic changes to the emission from accreting SMBHs have been observed, including tidal disruption events, 'changing look' AGNs and other extreme variability objects. The physics behind the 're-ignition', enhancement and 'shut-down' of accretion onto SMBHs is not entirely understood. Here we present a rapid increase in ultraviolet-optical emission in the centre of a nearby galaxy, marking the onset of sudden increased accretion onto a SMBH. The optical spectrum of this flare, dubbed AT 2017bgt, exhibits a mix of emission features. Some are typical of luminous, unobscured AGNs, but others are likely driven by Bowen fluorescence - robustly linked here with high-velocity gas in the vicinity of the accreting SMBH. The spectral features and increased ultraviolet flux show little evolution over a period of at least 14 months. This disfavours the tidal disruption of a star as their origin, and instead suggests a longer-term event of intensified accretion. Together with two other recently reported events with similar properties, we define a new class of SMBH-related flares. This has important implications for the classification of different types of enhanced accretion onto SMBHs.

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arXiv2019

The optical morphologies of galaxies in the IllustrisTNG simulation: a comparison to Pan-STARRS observations

We have generated synthetic images of $\sim$27,000 galaxies from the IllustrisTNG and the original Illustris hydrodynamic cosmological simulations, designed to match Pan-STARRS observations of $\log_{10}(M_{\ast}/{\rm M}_{\odot}) \approx 9.8$-$11.3$ galaxies at $z \approx 0.05$. Most of our synthetic images were created with the SKIRT radiative transfer code, including the effects of dust attenuation and scattering, and performing the radiative transfer directly on the Voronoi mesh used by the simulations themselves. We have analysed both our synthetic and real Pan-STARRS images with the newly developed $\tt{statmorph}$ code, which calculates non-parametric morphological diagnostics -- including the Gini-$M_{20}$ and concentration-asymmetry-smoothness (CAS) statistics -- and performs two-dimensional Sérsic fits. Overall, we find that the optical morphologies of IllustrisTNG galaxies are in good agreement with observations, and represent a substantial improvement compared to the original Illustris simulation. In particular, the locus of the Gini-$M_{20}$ diagram is consistent with that inferred from observations, while the median trends with stellar mass of all the morphological, size and shape parameters considered in this work lie within the $\sim$1$σ$ scatter of the observational trends. However, the IllustrisTNG model has some difficulty with more stringent tests, such as producing a strong morphology-colour relation. This results in a somewhat higher fraction of red discs and blue spheroids compared to observations. Similarly, the morphology-size relation is problematic: while observations show that discs tend to be larger than spheroids at a fixed stellar mass, such a trend is not present in IllustrisTNG.

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arXiv2018

A redshift-independent efficiency model: star formation and stellar masses in dark matter halos at z>4

We explore the connection between the UV luminosity functions (LFs) of high-$z$ galaxies and the distribution of stellar masses and star-formation histories (SFHs) in their host dark matter halos. We provide a baseline for a redshift-independent star-formation efficiency model to which observations and models can be compared. Our model assigns a star-formation rate (SFR) to each dark matter halo based on the growth rate of the halo and a redshift-independent star-formation efficiency. The dark matter halo accretion rate is obtained from a high-resolution $N$-body simulation in order to capture the stochasticity in accretion histories and to obtain spatial information for the distribution of galaxies. The halo mass dependence of the star-formation efficiency is calibrated at $z=4$ by requiring a match to the observed UV LF at this redshift. The model then correctly predicts the observed UV LF at $z=5-10$. We present predictions for the UV luminosity and stellar mass functions, \textit{JWST} number counts, and SFHs. In particular, we find a stellar-to-halo mass relation at $z=4-10$ that scales with halo mass at $M_{\rm h}<10^{11}~M_{\odot}$ as $M_{\star}\propto M_{\rm h}^2$, with a normalization that is higher than the relation inferred at $z=0$. The average SFRs increase as a function of time to $z=4$, although there is significant scatter around the average: about 6\% of the $z=4$ galaxies show no significant mass growth. Using these SFHs, we present redshift-dependent UV-to-SFR conversion factors, mass return fractions, and mass-to-light ratios for different intial mass functions and metallicities, finding that current estimates of the cosmic SFR density at $z\sim10$ may be overestimated by $\sim0.1-0.2~\mathrm{dex}$.

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arXiv2018

The MUSE Hubble Ultra Deep Field Survey XI. Constraining the low-mass end of the stellar mass - star formation rate relation at $z<1$

Star-forming galaxies have been found to follow a relatively tight relation between stellar mass ($M_{*}$) and star formation rate (SFR), dubbed the `star formation sequence'. A turnover in the sequence has been observed, where galaxies with $M_{*} < 10^{10} {\rm M}_{\odot}$ follow a steeper relation than their higher mass counterparts, suggesting that the low-mass slope is (nearly) linear. In this paper, we characterise the properties of the low-mass end of the star formation sequence between $7 \leq \log M_{*}[{\rm M}_{\odot}] \leq 10.5$ at redshift $0.11 < z < 0.91$. We use the deepest MUSE observations of the Hubble Ultra Deep Field and the Hubble Deep Field South to construct a sample of 179 star-forming galaxies with high signal-to-noise emission lines. Dust-corrected SFRs are determined from H$β$ $λ4861$ and H$α$ $λ6563$. We model the star formation sequence with a Gaussian distribution around a hyperplane between $\log M_{*}$, $\log {\rm SFR}$, and $\log (1+z)$, to simultaneously constrain the slope, redshift evolution, and intrinsic scatter. We find a sub-linear slope for the low-mass regime where $\log {\rm SFR}[{\rm M}_{\odot}/{\rm yr}] = 0.83^{+0.07}_{-0.06} \log M_{*}[{\rm M}_{\odot}] + 1.74^{+0.66}_{-0.68} \log (1+z)$, increasing with redshift. We recover an intrinsic scatter in the relation of $σ_{\rm intr} = 0.44^{+0.05}_{-0.04}$ dex, larger than typically found at higher masses. As both hydrodynamical simulations and (semi-)analytical models typically favour a steeper slope in the low-mass regime, our results provide new constraints on the feedback processes which operate preferentially in low-mass halos.

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arXiv2018

Stellar Mass Profiles of Quiescent Galaxies in Different Environments at $z\sim0$

We present the stellar mass profiles of 147 isolated quiescent galaxies in very low-density environments (i.e., void regions) in the local Universe ($0.01<z<0.06$) from the Sloan Digital Sky Survey. These galaxies have stellar masses between $ 9.8\lesssim \log(M_{\ast}/M_{\odot})\lesssim11.2$ and they represent $\sim15\%$ of the whole galaxy population in the void regions down to $M_{r} = -19$. We do not find any isolated quiescent galaxies with $\log(M_{\ast}/M_{\odot})\gtrsim11.2$. We compare the stellar mass profiles of these isolated quiescent galaxies with the profiles of stellar mass-matched samples of the quiescent galaxies in group and cluster environments. We find that, at fixed mass, quiescent galaxies in voids have similar central ($1$ kpc) mass densities ($Σ_1$) and central velocity dispersions ($σ_1$) compared to their counterparts in groups and clusters. We show that quiescent galaxies in voids have at most $10-25\%$ smaller half-mass (and half-light) sizes compared to quiescent galaxies in groups and clusters. We conclude that for the intermediate stellar mass range of $10^{10}-10^{11}M_{\odot}$ in the local Universe, environmental mechanisms have no significant additional effect on the mass profiles of the quiescent galaxies.

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arXiv2018

The JWST Extragalactic Mock Catalog: Modeling galaxy populations from the UV through the near-IR over thirteen billion years of cosmic history

We present an original phenomenological model to describe the evolution of galaxy number counts, morphologies, and spectral energy distributions across a wide range of redshifts (0.2 6]. Our model follows observed mass and luminosity functions of both star-forming and quiescent galaxies, and reproduces the redshift evolution of colors, sizes, star-formation and chemical properties of the observed galaxy population. Unlike other existing approaches, our model includes a self-consistent treatment of stellar and photoionized gas emission and dust attenuation based on the BEAGLE tool. The mock galaxy catalogs generated with our new model can be used to simulate and optimize extragalactic surveys with future facilities such as the James Webb Space Telescope (JWST), and to enable critical assessments of analysis procedures, interpretation tools, and measurement systematics for both photometric and spectroscopic data. As a first application of this work, we make predictions for the upcoming JWST Advanced Deep Extragalactic Survey (JADES), a joint program of the JWST/NIRCam and NIRSpec Guaranteed Time Observations teams. We show that JADES will detect, with NIRCam imaging, thousands of galaxies at z>6, and tens at z>10 at m_AB<30 (5-sigma) within the 236 arcmin^2 of the survey. The JADES data will enable accurate constraints on the evolution of the UV luminosity function at z>8, and resolve the current debate about the rate of evolution of galaxies at z>8. Ready to use mock catalogs and software to generate new realizations are publicly available as the JAdes extraGalactic Ultradeep Artificial Realizations (JAGUAR) package.

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arXiv2017

On the evolution of the central density of quiescent galaxies

We investigate the origin of the evolution of the population-averaged central stellar mass density ($Σ_1$) of quiescent galaxies (QGs) by probing the relation between stellar age and $Σ_1$ at $z\sim0$. We use the Zurich ENvironmental Study (ZENS), which is a survey of galaxy groups with a large fraction of satellite galaxies. QGs shape a narrow locus in the $Σ_1-M_{\star}$ plane, which we refer to as $Σ_1$ ridgeline. Colors of ($B-I$) and ($I-J$) are used to divide QGs into three age categories: young ($<2~\mathrm{Gyr}$), intermediate ($2-4~\mathrm{Gyr}$), and old ($>4~\mathrm{Gyr}$). At fixed stellar mass, old QGs on the $Σ_1$ ridgeline have higher $Σ_1$ than young QGs. This shows that galaxies landing on the $Σ_1$ ridgeline at later epochs arrive with lower $Σ_1$, which drives the zeropoint of the ridgeline down with time. We compare the present-day zeropoint of the oldest population at $z=0$ with the zeropoint of the quiescent population 4 Gyr back in time, at $z=0.37$. These zeropoints are identical, showing that the intrinsic evolution of individual galaxies after they arrive on the $Σ_1$ ridgeline must be negligible, or must evolve parallel to the ridgeline during this interval. The observed evolution of the global zeropoint of 0.07 dex over the last 4 Gyr is thus largely due to the continuous addition of newly quenched galaxies with lower $Σ_1$ at later times ("progenitor bias"). While these results refer to the satellite-rich ZENS sample as a whole, our work suggests a similar age-$Σ_1$ trend for central galaxies.

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arXiv2017

The Origins of UV-optical Color Gradients in Star-forming Galaxies at z ~ 2: Predominant Dust Gradients But Negligible sSFR Gradients

The rest-frame UV-optical (i.e., $NUV-B$) color is sensitive to both low-level recent star formation (specific star formation rate - sSFR) and dust. In this Letter, we extend our previous work on the origins of $NUV-B$ color gradients in star-forming galaxies (SFGs) at $z\sim1$ to those at $z\sim2$. We use a sample of 1335 large (semi-major axis radius $R_{\rm SMA}>0.''18$) SFGs with extended UV emission out to $2R_{\rm SMA}$ in the mass range $M_{\ast} = 10^{9}-10^{11}M_{\odot}$ at $1.5<z<2.8$ in the CANDELS/GOODS-S and UDS fields. We show that these SFGs generally have negative $NUV-B$ color gradients (redder centres), and their color gradients strongly increase with galaxy mass. We also show that the global rest-frame $FUV-NUV$ color is approximately linear with $A_{\rm V}$, which is derived by modeling the observed integrated FUV to NIR spectral energy distributions of the galaxies. Applying this integrated calibration to our spatially-resolved data, we find a negative dust gradient (more dust extinguished in the centers), which steadily becomes steeper with galaxy mass. We further find that the $NUV-B$ color gradients become nearly zero after correcting for dust gradients regardless of galaxy mass. This indicates that the sSFR gradients are negligible and dust reddening is likely the principal cause of negative UV-optical color gradients in these SFGs. Our findings support that the buildup of the stellar mass in SFGs at the Cosmic Noon is self-similar inside $2R_{\rm SMA}$.

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arXiv2017

Stacking the Cosmic Web in Fluorescent Lyman alpha Emission with MUSE

Cosmological simulations suggest that most of the matter in the Universe is distributed along filaments connecting galaxies. Illuminated by the cosmic UV background (UVB), these structures are expected to glow in fluorescent Lyman alpha emission with a Surface Brightness (SB) that is well below current observational limits for individual detections. Here, we perform a stacking analysis of the deepest MUSE/VLT data using three-dimensional regions (subcubes) with orientations determined by the position of neighbouring Lyman alpha galaxies (LAEs) at 3<z<4. Our method should increase the probability of detecting filamentary Lyman alpha emission, provided that these structures are Lyman Limit Systems (LLSs). By stacking 390 oriented subcubes we reach a 2 sigma sensitivity level of SB ~ 0.44e-20 erg/s/cm^2/arcsec^2 in an aperture of 1 arcsec^2 x 6.25 Angstrom, which is three times below the expected fluorescent Lyman alpha signal from the Haardt-Madau 2012 (HM12) UVB at z~3.5. No detectable emission is found on intergalactic scales, implying that at least two thirds of our subcubes do not contain oriented LLSs for a HM12 UVB. On the other hand, significant emission is detected in the circum-galactic medium (CGM) of galaxies in the direction of the neighbours. The signal is stronger for galaxies with a larger number of neighbours and appears to be independent of any other galaxy properties such as luminosity, redshift and neighbour distance. We estimate that preferentially oriented satellite galaxies cannot contribute significantly to this signal, suggesting instead that gas densities in the CGM are typically larger in the direction of neighbouring galaxies on cosmological scales.

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arXiv2017

Connection Between Stellar Mass Distributions Within Galaxies and Quenching Since z =2

We study the history from $z\sim2$ to $z\sim0$ of the stellar mass assembly of quiescent and star-forming galaxies in a spatially resolved fashion. For this purpose we use multi-wavelength imaging data from the Hubble Space Telescope (HST) over the GOODS fields and the Sloan Digital Sky Survey (SDSS) for the local population. We present the radial stellar mass surface density profiles of galaxies with $M_{\ast}>10^{10} M_{\odot}$, corrected for mass-to-light ratio ($M_{\ast}/L$) variations, and derive the half-mass radius ($R_{m}$), central stellar mass surface density within 1 kpc ($Σ_{1}$) and surface density at $R_{m}$ ($Σ_{m}$) for star-forming and quiescent galaxies and study their evolution with redshift. At fixed stellar mass, the half-mass sizes of quiescent galaxies increase from $z\sim2$ to $z\sim0$ by a factor of $\sim3-5$, whereas the half-mass sizes of star-forming galaxies increase only slightly, by a factor of $\sim2$. The central densities $Σ_{1}$ of quiescent galaxies decline slightly (by a factor of $\lesssim1.7$) from $z\sim2$ to $z\sim0$, while for star-forming galaxies $Σ_{1}$ increases with time, at fixed mass. We show that the central density $Σ_{1}$ has a tighter correlation with specific star-formation rate (sSFR) than $Σ_{m}$ and for all masses and redshifts galaxies with higher central density are more prone to be quenched. Reaching a high central density ($Σ_{1} \gtrsim 10^{10} M_{\odot} \mathrm{kpc}^2$) seems to be a prerequisite for the cessation of star formation, though a causal link between high $Σ_{1}$ and quenching is difficult to prove and their correlation can have a different origin.

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