Population Synthesis Models Research Articles

Binary pathways to SLSNe-I: SN 2017gci

ABSTRACT Some hydrogen-poor superluminous supernovae (SLSNe) exhibit bumps in the tails of their light curves associated with hydrogen features in their late time spectra. Here we use the explosion parameters of one such SLSN – SN 2017gci – to search the stellar models of the Binary Population And Spectral Synthesis (BPASS) code for potential progenitors. We find good matches for a 30 M⊙ progenitor star in a binary system and no matches from single-star models. Common envelope and mass transfer after the giant branch, combined with increased mass loss from strong stellar winds immediately before death, allow the progenitor to lose its hydrogen envelope decades before the explosion. This results in a hydrogen poor SLSN and allows for delayed interaction of the ejecta with the lost stellar material.

Fingerprints of stellar populations in the near-infrared: an optimized set of spectral indices in the JHK bands 0

ABSTRACT Stellar population studies provide unique clues to constrain galaxy formation models. So far, detailed studies based on absorption line strengths have mainly focused on the optical spectral range although many diagnostic features are present in other spectral windows. In particular, the near-infrared (NIR) can provide a wealth of information about stars, such as evolved giants, that have less evident optical signatures. Due to significant advances in NIR instrumentation and extension of spectral libraries and stellar population synthesis (SPS) models to this domain, it is now possible to perform in-depth studies of spectral features in the NIR to a high level of precision. In this work, taking advantage of state-of-the-art SPS models covering the NIR spectral range, we introduce a new set of NIR indices constructed to be maximally sensitive to the main stellar population parameters, namely age, metallicity, and initial mass function (IMF). We fully characterize the new indices against these parameters as well as their sensitivity to individual elemental abundance variations, velocity dispersion broadening, wavelength shifts, signal-to-noise ratio, and flux calibration. We also present, for the first time, a method to ensure that the analysis of spectral indices is not affected by sky contamination, which is a major challenge when dealing with NIR spectroscopy. Moreover, we discuss two main applications: (i) the ability of some NIR spectral indices to constrain the shape of the low-mass IMF and (ii) current issues in the analysis of NIR spectral indices for future developments of SPS modelling.

Systematic uncertainties in models of the cosmic dawn

ABSTRACT Models of the reionization and reheating of the intergalactic medium (IGM) at redshifts z ≳ 6 continue to grow more sophisticated in anticipation of near-future 21-cm, cosmic microwave background, and galaxy survey measurements. However, there are many potential sources of systematic uncertainty in models that could bias and/or degrade upcoming constraints if left unaccounted for. In this work, we examine three commonly ignored sources of uncertainty in models for the mean reionization and thermal histories of the IGM: the underlying cosmology, halo mass function (HMF), and choice of stellar population synthesis (SPS) model. We find that cosmological uncertainties affect the Thomson scattering optical depth at the few per cent level and the amplitude of the global 21-cm signal at the ∼5–10 mK level. The differences brought about by choice of HMF and SPS models are more dramatic, comparable to the 1σ error bar on τe and an ∼20 mK effect on the global 21-cm signal amplitude. Finally, we jointly fit galaxy luminosity functions and global 21-cm signals for all HMF/SPS combinations and find that (i) doing so requires additional free parameters to compensate for modelling systematics and (ii) the spread in constraints on parameters of interest for different HMF and SPS choices, assuming 5 mK noise in the global signal, is comparable to those obtained when adopting the 'true' HMF and SPS with ≳20 mK errors. Our work highlights the need for dedicated efforts to reduce modelling uncertainties in order to enable precision inference with future data sets.

The final orbital separation in common envelope evolution

In the majority of population synthesis calculations of close binary stars, the common envelope (CE) phase is modeled using a standard prescription based upon the conservation of energy, known as the alpha prescription. In this prescription, the orbital separation of the secondary and giant core at the end of the CE phase is taken to be the orbital separation when the envelope becomes unbound. However, recent observations of planetary nebulae with binary cores (BPNe), believed to be the immediate products of CE evolution, indicate orbital periods that are significantly shorter than predicted by population synthesis models using the alpha prescription. We argue that unbinding the envelope provides a necessary, but not sufficient, condition to escape a merger during CE evolution. The spiral-in of the secondary must also be halted. This requires the additional dynamical constraint that the frictional torque on the secondary be reduced to approximately zero. In this paper, we undertake a preliminary examination of the effect of adding this dynamical constraint in population synthesis calculations of BPNe. We assume that the frictional torque will be sufficiently reduced when the secondary enters a region within the giant where the mass-radius profile is flat. We crudely estimate the location of this region as a function of the core mass based upon existing stellar models of AGB stars between 1 and 7 M⊙. We calculate a theoretical orbital period distribution of BPNe using a population synthesis code that incorporates this dynamical constraint along with the alpha prescription.

IMF-induced intrinsic uncertainties on measuring galaxy distances based on the number of giant stars: the case of the ultradiffuse galaxy NGC 1052-DF2

ABSTRACT The surface brightness fluctuation (SBF) technique is one of the distance measurement methods that has been applied on the low surface brightness (LSB) galaxy NGC 1052-DF2 yielding a distance of about 20 Mpc implying it to be a dark matter deficient galaxy. We assume the number of giant stars above a given luminosity threshold to represent the SBF magnitude. The SBF magnitude depends on the distance, but this is degenerate with the star formation history (SFH). Using a stellar population synthesis model, we calculate the number of giant stars for stellar populations with different galaxy-wide stellar initial mass functions (gwIMFs), ages, metallicities, and SFHs. If the gwIMF is the invariant canonical IMF, the 1σ (3σ) uncertainty in colour allows a distance as low as 12 Mpc (8 Mpc). If instead the true underlying gwIMF is the integrated galaxy-wide IMF (IGIMF) then overestimating distances for low-mass galaxies would be a natural result, allowing NGC 1052-DF2 to have a distance of 11 Mpc within the 1σ colour uncertainty. Finally, we show that our main conclusion on the existence of a bias in the SBF distance estimation is not much affected by changing the luminosity lower limit for counting giant stars.

Astraeus I: the interplay between galaxy formation and reionization

ABSTRACT We introduce a new self-consistent model of galaxy evolution and reionization, astraeus (seminumerical rAdiative tranSfer coupling of galaxy formaTion and Reionization in N-body dArk mattEr simUlationS), which couples a state-of-the-art N-body simulation with the semi-analytical galaxy evolution delphi and the seminumerical reionization scheme cifog. astraeus includes all the key processes of galaxy formation and evolution (including accretion, mergers, supernova, and radiative feedback) and follows the time and spatial evolution of the ionized regions in the intergalactic medium (IGM). Importantly, it explores different radiative feedback models that cover the physically plausible parameter space, ranging from a weak and delayed to a strong and immediate reduction of gas mass available for star formation. From our simulation suite that covers the different radiative feedback prescriptions and ionization topologies, we find that radiative feedback continuously reduces star formation in galaxies with $M_\mathrm{ h}\lesssim 10^{9.5}\, {\rm \rm M_\odot }$ upon local reionization; larger mass haloes are unaffected even for the strongest and immediate radiative feedback cases during reionization. For this reason, the ionization topologies of different radiative feedback scenarios differ only on scales smaller than 1–2 comoving Mpc, and significant deviations are found only when physical parameters (e.g. the escape fraction of ionizing photons) are altered based on galactic properties. Finally, we find that observables (the ultraviolet luminosity function, stellar mass function, reionization histories and ionization topologies) are hardly affected by the choice of the used stellar population synthesis models that model either single stars or binaries.

Mapping the inhomogeneous Universe with standard sirens: degeneracy between inhomogeneity and modified gravity theories

ABSTRACT The detection of gravitational waves (GWs) and an accompanying electromagnetic (E/M) counterpart have been suggested as a future probe for cosmology and theories of gravity. In this paper, we present calculations of the luminosity distance of sources taking into account inhomogeneities in the matter distribution that are predicted in numerical simulations of structure formation. In addition, we show that inhomogeneities resulting from clustering of matter can mimic certain classes of modified gravity theories, or other effects that dampen GW amplitudes, and deviations larger than $\delta \nu \sim \mathcal {O}(0.1)\ (99{{\ \rm per\ cent}}\ \rm {C.L.})$ to the extra friction term ν, from zero, would be necessary to distinguish them. For these, we assume mock GWs sources, with known redshift, based on binary population synthesis models, between redshifts z = 0 and z = 5. We show that future GW detectors, like Einstein Telescope or Cosmic Explorer, will be needed for strong constraints on the inhomogeneity parameters and breaking the degeneracy between modified gravity effects and matter anisotropies by measuring ν at $5 {{\ \rm per\ cent}}$ and $1 {{\ \rm per\ cent}}$ level with 100 and 350 events, respectively.

Host Galaxy Mass Combined with Local Stellar Age Improve Type Ia Supernovae Distances

Abstract Type Ia supernovae (SNe Ia) are standardizable candles, but for over a decade there has been a debate on how to properly account for their correlations with host galaxy properties. Using the Bayesian hierarchical model UNITY, we simultaneously fit for the SN Ia light curve and host galaxy standardization parameters on a set of 103 Sloan Digital Sky Survey II SNe Ia. We investigate the influences of host stellar mass, along with both localized (r <3 kpc) and host-integrated average stellar ages, derived from stellar population synthesis modeling. We find that the standardization for the light-curve shape (α) is correlated with host galaxy standardization terms (γ i ) requiring simultaneous fitting. In addition, we find that these correlations themselves are dependent on host galaxy stellar mass that includes a shift in the color term (β) of 0.8 mag, only significant at 1.2σ due to the small sample. We find a linear host mass standardization term at the 3.7σ level, that by itself does not significantly improve the precision of an individual SN Ia distance. However, a standardization that uses both stellar mass and average local stellar age is found to be significant at >3σ in the two-dimensional posterior space. In addition, the unexplained scatter of SNe Ia absolute magnitude post standardization, is reduced from to 0.109 ± 0.017 mag, or ∼10%. We do not see similar improvements when using global ages. This combination is consistent with either metallicity or line-of-sight dust affecting the observed luminosity of SNe Ia.

The delay time distribution of Type-Ia supernovae in galaxy clusters: the impact of extended star-formation histories

ABSTRACT The delay time distribution (DTD) of Type-Ia supernovae (SNe Ia) is important for understanding chemical evolution, SN Ia progenitors, and SN Ia physics. Past estimates of the DTD in galaxy clusters have been deduced from SN Ia rates measured in cluster samples observed at various redshifts, corresponding to different time intervals after a presumed initial brief burst of star formation. A recent analysis of a cluster sample at z = 1.13–1.75 confirmed indications from previous studies of lower redshift clusters, that the DTD has a power-law form, DTD(t) = R1(t/Gyr)α, with amplitude R1, at delay $t=1\,\rm Gyr$, several times higher than measured in field-galaxy environments. This implied that SNe Ia are somehow produced in larger numbers by the stellar populations in clusters. This conclusion, however, could have been affected by the implicit assumption that the stars were formed in a single brief starburst at high z. Here, we re-derive the DTD from the cluster SN Ia data, but relax the single-burst assumption. Instead, we allow for a range of star-formation histories and dust extinctions for each cluster. Via MCMC modelling, we simultaneously fit, using stellar population synthesis models and DTD models, the integrated galaxy-light photometry in several bands, and the SN Ia numbers discovered in each cluster. With these more-realistic assumptions, we find a best-fitting DTD with power-law index $\alpha =-1.09_{-0.12}^{+0.15}$, and amplitude $R_1=0.41_{-0.10}^{+0.12}\times 10^{-12}\,{\rm yr}^{-1}\, {\rm M}_\odot ^{-1}$. We confirm a cluster-environment DTD with a larger amplitude than the field-galaxy DTD, by a factor ∼2–3 (at 3.8σ). Cluster and field DTDs have consistent slopes of α ≈ −1.1.

Hidden in plain sight: a double-lined white dwarf binary 26 pc away and a distant cousin

ABSTRACT We present high-resolution spectroscopy of two nearby white dwarfs with inconsistent spectroscopic and parallax distances. The first one, PG 1632+177, is a 13th magnitude white dwarf only 25.6 pc away. Previous spectroscopic observations failed to detect any radial velocity changes in this star. Here, we show that PG 1632+177 is a 2.05-d period double-lined spectroscopic binary (SB2) containing a low-mass He-core white dwarf with a more-massive, likely CO-core white dwarf companion. After L 870−2, PG 1632+177 becomes the second closest SB2 white dwarf currently known. Our second target, WD 1534+503, is also an SB2 system with an orbital period of 0.71 d. For each system, we constrain the atmospheric parameters of both components through a composite model-atmosphere analysis. We also present a new set of non-local thermodynamic equilibrium (NLTE) synthetic spectra appropriate for modelling high-resolution observations of cool white dwarfs, and show that NLTE effects in the core of the H α line increase with decreasing effective temperature. We discuss the orbital period and mass distribution of SB2 and eclipsing double white dwarfs with orbital constraints, and demonstrate that the observed population is consistent with the predicted period distribution from the binary population synthesis models. The latter predict more massive CO + CO white dwarf binaries at short (<1 d) periods, as well as binaries with several day orbital periods; such systems are still waiting to be discovered in large numbers.

Giant planet migration during the disc dispersal phase

Context. Transition discs are expected to be a natural outcome of the interplay between photoevaporation and giant planet formation. Massive planets reduce the inflow of material from the outer to the inner disc, therefore triggering an earlier onset of disc dispersal due to photoevaporation through a process known as Planet-Induced PhotoEvaporation. In this case, a cavity is formed as material inside the planetary orbit is removed by photoevaporation, leaving only the outer disc to drive the migration of the giant planet. Aims. We investigate the impact of photoevaporation on giant planet migration and focus specifically on the case of transition discs with an evacuated cavity inside the planet location. This is important for determining under what circumstances photoevaporation is efficient at halting the migration of giant planets, thus affecting the final orbital distribution of a population of planets. Methods. For this purpose, we use 2D FARGO simulations to model the migration of giant planets in a range of primordial and transition discs subject to photoevaporation. The results are then compared to the standard prescriptions used to calculate the migration tracks of planets in 1D planet population synthesis models. Results. The FARGO simulations show that once the disc inside the planet location is depleted of gas, planet migration ceases. This contradicts the results obtained by the impulse approximation, which predicts the accelerated inward migration of planets in discs that have been cleared inside the planetary orbit. Conclusions. These results suggest that the impulse approximation may not be suitable for planets embedded in transition discs. A better approximation that could be used in 1D models would involve halting planet migration once the material inside the planetary orbit is depleted of gas and the surface density at the 3:2 mean motion resonance location in the outer disc reaches a threshold value of 0.01 g cm‒2.

On the Thermally Pulsing Asymptotic Giant Branch Contribution to the Light of Nearby Disk Galaxies

Abstract The study of the luminosity contribution from thermally pulsing asymptotic giant branch (TP-AGB) stars to the stellar populations of galaxies is crucial to determine their physical parameters (e.g., stellar mass and age). We use a sample of 84 nearby disk galaxies to explore diverse stellar population synthesis models with different luminosity contributions from TP-AGB stars. We fit the models to optical and near-infrared (NIR) photometry, on a pixel-by-pixel basis. The statistics of the fits show a preference for a low-luminosity contribution (i.e., high mass-to-light ratio in the NIR) from TP-AGB stars. Nevertheless, for 30%–40% of the pixels in our sample a high-luminosity contribution (hence low mass-to-light ratio in the NIR) from TP-AGB stars is favored. According to our findings, the mean TP-AGB star luminosity contribution in nearby disk galaxies may vary with Hubble type. This may be a consequence of the variation of the TP-AGB mass-loss rate with metallicity, if metal-poor stars begin losing mass earlier than metal-rich stars, because of a pre-dust wind that precedes the dust-driven wind.

A Census of Blue Stragglers in Gaia DR2 Open Clusters as a Test of Population Synthesis and Mass Transfer Physics

Abstract We use photometry and proper motions from Gaia DR2 to determine the blue straggler star (BSS) populations of 16 old (1–10 Gyr), nearby (d < 3500 pc) open clusters. We find that the fractional number of BSS compared to red giant branch stars increases with age, starting near zero at 1 Gyr and flattening to ∼0.35 by 4 Gyr. Fitting stellar evolutionary tracks to these BSSs, we find that their mass distribution peaks at a few tenths of a solar mass above the main-sequence turnoff. BSSs more than 0.5 M ⊙ above the turnoff make up only ∼25% of the sample, and BSSs more than 1.0 M ⊙ above the turnoff are rare. We compare this to Compact Object Synthesis and Monte Carlo Investigation Code population synthesis models of BSSs formed via mass transfer. We find that standard population synthesis assumptions dramatically under-produce the number of BSS in old open clusters. We also find that these models overproduce high-mass BSSs relative to lower-mass BSSs. The expected number of BSSs formed through dynamics do not fully account for this discrepancy. We conclude that in order to explain the observed BSS populations from Roche lobe overflow, mass transfer from giant donors must be more stable than assumed in canonical mass-transfer prescriptions, and including nonconservative mass transfer is important in producing realistic BSS masses. Even with these modifications, it is difficult to achieve the large number of BSSs observed in the oldest open clusters. We discuss some additional physics that may explain the large number of observed blue stragglers among old stellar populations.

Weighing Milky Way satellites with LISA

ABSTRACT White dwarf stars are a well-established tool for studying Galactic stellar populations. Two white dwarfs in a tight binary system offer us an additional messenger – gravitational waves – for exploring the Milky Way and its immediate surroundings. Gravitational waves produced by double white dwarf (DWD) binaries can be detected by the future Laser Interferometer Space Antenna (LISA). Numerous and widespread DWDs have the potential to probe shapes, masses, and formation histories of the stellar populations in the Galactic neighbourhood. In this work we outline a method for estimating the total stellar mass of Milky Way satellite galaxies based on the number of DWDs detected by LISA. To constrain the mass we perform a Bayesian inference using binary population synthesis models and considering the number of detected DWDs associated with the satellite and the measured distance to the satellite as the only inputs. Based on a fiducial binary population synthesis model we find that for large satellites the stellar masses can be recovered to within (1) a factor 2 if the star formation history (SFH) is known and (2) an order of magnitude when marginalizing over different SFH models. For smaller satellites we can place upper limits on their stellar mass. Gravitational wave observations can provide mass measurements for large satellites that are comparable, and in some cases more precise, than standard electromagnetic observations.

The Gaia DR2 halo white dwarf population: the luminosity function, mass distribution, and its star formation history

ABSTRACT We analyse the volume-limited nearly complete 100 pc sample of 95 halo white dwarf candidates identified by the second data release of Gaia. Based on a detailed population synthesis model, we apply a method that relies on Gaia astrometry and photometry to accurately derive the individual white dwarf parameters. This method is tested with 25 white dwarfs of our sample for which we took optical spectra and performed spectroscopic analysis. We build and analyse the halo white dwarf luminosity function, for which we find for the first time possible evidences of the cut-off, leading to an age estimate of ${\simeq}12\pm 0.5$ Gyr. The mass distribution of the sample peaks at $0.589\, \mathrm{ M}_{\odot }$, with $71{{\ \rm per\ cent}}$ of the white dwarf masses below $0.6\, \mathrm{ M}_{\odot }$ and just two massive white dwarfs of more than $0.8\, \mathrm{ M}_{\odot }$. From the age distribution, we find three white dwarfs with total ages above 12 Gyr, of which J1312−4728 is the oldest white dwarf known with an age of $12.41\pm 0.22\,$ Gyr. We prove that the star formation history is mainly characterized by a burst of star formation that occurred from 10 to 12 Gyr in the past, but extended up to 8 Gyr. We also find that the peak of the star formation history is centred at around 11 Gyr, which is compatible with the current age of the Gaia-Enceladus encounter. Finally, $13{{\ \rm per\ cent}}$ of our halo sample is contaminated by high-speed young objects (total age <7 Gyr). The origin of these white dwarfs is unclear but their age distribution may be compatible with the encounter with the Sagittarius galaxy.

It has to be cool: Supergiant progenitors of binary black hole mergers from common-envelope evolution

Common-envelope (CE) evolution in massive binary systems is thought to be one of the most promising channels for the formation of compact binary mergers. In the case of merging binary black holes (BBHs), the essential CE phase takes place at a stage when the first BH is already formed and the companion star expands as a supergiant. We aim to decipher the kinds of BH binaries with supergiant companions that could potentially evolve through and survive a CE phase. To this end, we compute envelope binding energies from detailed massive stellar models at different evolutionary stages and metallicities. We make multiple physically extreme choices of assumptions that favor easier CE ejection as well as account for recent advancements in mass-transfer stability criteria. We find that even with the most optimistic assumptions, a successful CE ejection in BH binaries is only possible if the donor is a massive convective-envelope giant, namely a red supergiant (RSG). The same is true for neutron-star binaries with massive companions. In other words, pre-CE progenitors of BBH mergers are BH binaries with RSG companions. We find that because of its influence on the radial expansion of massive giants, metallicity has an indirect but a very strong effect on the chemical profile, density structure, and the binding energies of RSG envelopes. Our results suggest that merger rates from population-synthesis models could be severely overestimated, especially at low metallicity. Additionally, the lack of observed RSGs with luminosities above log(L/L⊙) ≈ 5.6 − 5.8, corresponding to stars withM ≳ 40 M⊙, puts into question the viability of the CE channel for the formation of the most massive BBH mergers. Either such RSGs elude detection due to very short lifetimes, or they do not exist and the CE channel can only produce BBH systems with total mass ≲50 M⊙. Finally, we discuss an alternative CE scenario in which a partial envelope ejection is followed by a phase of possibly long and stable mass transfer.

Powderday: Dust Radiative Transfer for Galaxy Simulations

Abstract We present powderday (available at https://github.com/dnarayanan/powderday), a flexible, fast, open-source dust radiative transfer package designed to interface with both idealized and cosmological galaxy formation simulations. powderday builds on fsps stellar population synthesis models, and hyperion dust radiative transfer, and employs yt to interface between different software packages. We include our stellar population synthesis modeling on the fly, allowing significant flexibility in the assumed stellar physics and nebular line emission. The dust content follows either simple observationally motivated prescriptions (i.e., constant dust-to-metals ratios, or dust-to-gas ratios that vary with metallicity), direct modeling from galaxy formation simulations that include dust physics, as well as a novel approach that includes the dust content via learning-based algorithms from the simba cosmological galaxy formation simulation. Active galactic nuclei (AGNs) can additionally be included via a range of prescriptions. The output of these models are broadband (912 Å–1 mm) spectral energy distributions (SEDs), as well as filter-convolved monochromatic images. powderday is designed to eliminate last-mile efforts by researchers that employ different hydrodynamic galaxy formation models and seamlessly interfaces with gizmo, arepo, gasoline, changa, and enzo. We demonstrate the capabilities of the code via three applications: a model for the star formation rate–infrared luminosity relation in galaxies (including the impact of AGNs), the impact of circumstellar dust around AGB stars on the mid-infrared emission from galaxy SEDs, and the impact of galaxy inclination angle on dust attenuation laws.

The Metallicity Dependence of the High-mass X-Ray Binary Luminosity Function

Abstract In this work, we present detailed constraints on the metallicity dependence of the high-mass X-ray binary (HMXB) X-ray luminosity function (XLF). We analyze ≈5 Ms of Chandra data for 55 actively star-forming galaxies at D ≲ 30 Mpc, with gas-phase metallicities spanning ≈ 7–9.2. Within the galactic footprints, our sample contains a total of 1311 X-ray point sources, of which ≈49% are expected to be HMXBs, with the remaining sources likely to be low-mass X-ray binaries (LMXBs; ≈22%) and unrelated background sources (≈29%). We construct a model that successfully characterizes the average HMXB XLF over the full metallicity range. We demonstrate that the SFR-normalized HMXB XLF shows clear trends with metallicity, showing steadily increasing numbers of luminous and ultraluminous X-ray sources ( (erg s−1) = 38–40.5) with declining metallicity. However, we find that the low-luminosity ( (erg s−1) = 36–38) HMXB XLF appears to show a nearly constant SFR scaling and slope with metallicity. Our model provides a revised scaling relation of integrated L X/SFR versus , and a new characterization of its SFR-dependent stochastic scatter. The general trend of this relation is broadly consistent with past studies based on integrated galaxy emission; however, our model suggests that this relation is driven primarily by the high-luminosity end of the HMXB XLF. Our results have implications for binary population synthesis models, the nature of super-Eddington accreting objects (e.g., ultraluminous X-ray sources), recent efforts to identify active galactic nucleus candidates in dwarf galaxies, and the X-ray radiation fields in the early universe during the epoch of cosmic heating at z ≳ 10.

Spatially-resolved stellar population properties of the M51–NGC 5195 system from multi-wavelength photometric data

Using multi-band photometric images of M51 and its companion NGC 5195 from ultraviolet to optical and infrared, we investigate spatially resolved stellar population properties of this interacting system with stellar population synthesis models. The observed infrared excess (IRX) is used to constrain dust extinction. Stellar mass is also inferred from the model fitting. By fitting observed spectral energy distributions (SEDs) with synthetical ones, we derive two-dimensional distributions of stellar age, metallicity, dust extinction and stellar mass. In M51, two grand-designed spiral arms extending from the bulge show young age, rich metallicity and abundant dust. The inter-arm regions are filled with older, metal-poorer and less dusty stellar populations. Except for the spiral arm extending from M51 into NGC 5195, the stellar population properties of NGC 5195 are quite featureless. NGC 5195 is much older than M51, and its core is very dusty with AV up to 1.67 mag and dense in stellar mass surface density. The close encounters might drive the dust in the spiral arm of M51 into the center of NGC 5195.

Sengi: A small, fast, interactive viewer for spectral outputs from stellar population synthesis models

We present Sengi, https://christopherlovell.github.io/sengi , an online tool for viewing the spectral outputs of stellar population synthesis (SPS) codes. Typical SPS codes require significant disk space or computing resources to produce spectra for simple stellar populations with arbitrary parameters. This makes it difficult to present their results in an interactive, web-friendly format. Sengi uses Non-negative Matrix Factorisation (NMF) and bilinear interpolation to estimate output spectra for arbitrary values of stellar age and metallicity. The reduced disk requirements and computational expense allows the result to be served as a client-based Javascript application. In this paper we present the method for generating grids of spectra, fitting those grids with NMF, bilinear interpolation across the fitted coefficients, and finally provide estimates of the prediction and interpolation errors.