Continuous Star Formation Research Articles

Exploring simulated early star formation in the context of the ultrafaint dwarf galaxies

Ultrafaint dwarf galaxies (UFDs) are typically assumed to have simple, stellar populations with star formation ending at reionization. Yet as the observations of these galaxies continue to improve, their star formation histories (SFHs) are revealed to be more complicated than previously thought. In this paper, we study how star formation, chemical enrichment, and mixing proceed in small, dark matter halos at early times using a high-resolution, cosmological, hydrodynamical simulation. The goals are to inform the future use of analytic models and to explore observable properties of the simulated halos in the context of UFD data. Specifically, we look at analytic approaches that might inform metal enrichment within and beyond small galaxies in the early Universe. We find that simple assumptions for modeling the extent of supernova-driven winds agree with the simulation on average whereas inhomogeneous mixing and gas flows have a large effect on the spread in simulated stellar metallicities. In the context of the UFDs, this work demonstrates that simulations can form halos with a complex SFH and a large spread in the metallicity distribution function within a few hundred Myr in the early Universe. In particular, bursty and continuous star formation are seen in the simulation and both scenarios have been argued from the data. Spreads in the simulated metallicities, however remain too narrow and too metal-rich when compared to the UFDs. Future work is needed to help reduce these discrepancies and advance our interpretation of the data.

Dust masses of z > 5 galaxies from SED fitting and ALMA upper limits

We aim at constraining the dust mass in high-redshift ($z\gtrsim 5$) galaxies using the upper limits obtained by ALMA in combination with the rest-frame UV--optical spectral energy distributions (SEDs). For SED fitting, because of degeneracy between dust extinction and stellar age, we focus on two extremes: continuous star formation (Model A) and instantaneous star formation (Model B). We apply these models to Himiko (as a representative UV-bright object) and a composite SED of $z>5$ Lyman break galaxies (LBGs). For Himiko, Model A requires a significant dust extinction, which leads to a high dust temperature $>70$ K for consistency with the ALMA upper limit. This high dust temperature puts a strong upper limit on the total dust mass $M_\mathrm{d}\lesssim 2\times 10^6$ M$_{\odot}$, and the dust mass produced per supernova (SN) $m_\mathrm{d,SN}\lesssim 0.1$ M$_{\odot}$. Such a low $m_\mathrm{d,SN}$ suggests significant loss of dust by reverse shock destruction or outflow, and implies that SNe are not the dominant source of dust at high $z$. Model B allows $M_\mathrm{d}\sim 2\times 10^7$ M$_{\odot}$ and $m_\mathrm{d,SN}\sim 0.3$ M$_{\odot}$.} We could distinguish between Models A and B if we observe Himiko at {wavelength $<$ 1.2 mm by ALMA. For the LBG sample, we obtain $M_\mathrm{d}\lesssim 2\times 10^6$ M$_{\odot}$ for a typical LBG at $z>5$, but this only puts an upper limit for $m_\mathrm{d,SN}$ as $\sim 2$ M$_{\odot}$. This clarifies the importance of observing UV-bright objects (like Himiko) to constrain the dust production by SNe.

From K giants to G dwarfs: stellar lifetime effects on metallicity distributions derived from red giants

We examine the biases inherent to chemical abundance distributions when targets are selected from the red giant branch (RGB), using simulated giant branches created from isochrones. We find that even when stars are chosen from the entire colour range of RGB stars and over a broad range of magnitudes, the relative numbers of stars of different ages and metallicities, integrated over all stellar types, are not accurately represented in the giant branch sample. The result is that metallicity distribution functions derived from RGB star samples require a correction before they can be fit by chemical evolution models. We derive simple correction factors for over- and under-represented populations for the limiting cases of single-age populations with a broad range of metallicities and of continuous star formation at constant metallicity; an important general conclusion is that intermediate-age populations (~1-4Gyr) are over-represented in RGB samples. We apply our models to the case of the Large Magellanic Cloud bar and show that the observed metallicity distribution underestimates the true number of metal-poor stars by more than 25%; as a result, the inferred importance of gas flows in chemical evolution models could potentially be overestimated. The age- and metallicity-dependences of RGB lifetimes require careful modelling if they are not to lead to spurious conclusions about the chemical enrichment history of galaxies.

N/O abundance ratios in gamma-ray burst and supernova host galaxies at z &lt; 4. Comparison with AGN, starburst and H ii regions

The distribution of the N/O element abundance ratios calculated by the detailed modelling of different galaxy spectra at z < 4 is investigated. Supernova (SN) and long gamma-ray-burst (LGRB) host galaxies cover different redshift domains. N/O ratios in SN hosts increase due to secondary N production towards low z (0.01) accompanying the growing trend of active galaxies [active galactic nucleus (AGN), low-ionization nuclear emission-line region (LINER)]. N/O ratios in LGRB hosts decrease rapidly between z > 1 and z ∼ 0.1 following the N/H trend and reach the characteristic N/O ratios calculated for the H ii regions in local and nearby galaxies. The few short-period gamma-ray-burst (SGRB) hosts included in the galaxy sample show N/H ≤ 0.04 solar and O/H solar. They seem to continue the low bound N/H trend of SN hosts at z < 0.3. The distribution of N/O as a function of metallicity for SN and LGRB hosts is compared with star chemical evolution models. The results show that several LGRB hosts can be explained by star multibursting models when 12+log(O/H) < 8.5, while some objects follow the trend of continuous star formation models. N/O in SN hosts at log(O/H)+12 < 8.5 are not well explained by stellar chemical evolution models calculated for starburst galaxies. At 12+log(O/H) > 8.5 many different objects are nested close to O/H solar with N/O ranging between the maximum corresponding to starburst galaxies and AGN and the minimum corresponding to H ii regions and SGRB.

The challenging task of determining star formation rates: the case of a massive stellar burst in the brightest cluster galaxy of Phoenix galaxy cluster

Star formation in galaxies at the center of cooling-flow galaxy clusters is an important phenomenon in the context of formation and evolution of massive galaxies in the Universe. Yet, star formation rates (SFRs) in such systems continue to be elusive. We use our Bayesian-motivated spectral energy distribution (SED)-fitting code, BAYESCOOL, to estimate the plausible SFR values in the brightest cluster galaxy of a massive, X-ray luminous galaxy cluster, Phoenix. Previous studies of Phoenix have resulted in the highest measurement of SFR for any galaxy, with the estimates reaching up to 1000 solar masses/yr. However, a very small number of models have been considered in those studies. BAYESCOOL allows us to probe a large parameter space. We consider two models for star formation history, instantaneous bursts and continuous star formation, a wide range of ages for the old and the young stellar population, along with other discrete parameters, such as the initial mass function, metallicities, internal extinction and extinction law. We find that in the absence of any prior except that the maximum cooling rate < 3000 solar masses/yr, the SFR lies in the range (2230-2890) solar masses/yr. If we impose an observational prior on the internal extinction, E(B-V) < 0.6, the best-fit SFR lies in (454-494) solar masses/yr, and we consider this as the most probable range of SFR values for Phoenix. The SFR dependence on the extinction is a reflection of the standard age-extinction degeneracy, which can be overcome by using a prior on one of the two quantities in question.

The history of the Galactic bulge

AbstractPlanetary nebulae form in stellar populations with ages from 1 to 10 Gyr, and can be used to trace their star formation histories. Here we apply this to the Galactic bulge, where there are indications both for an old origin and for younger stars. We use new stellar models, which have significant different evolutionary speeds during the post-AGB phase. We apply these new models to a sample of 32 planetary nebulae with HST imaging and VLT spectroscopy. The results show evidence for an old starburst, followed by continuous star formation until at least 2Gyr ago. This agrees very well with recent analysis of colour-magnitude diagrams of the bulge. We show that the new models can also explain the [OIII] luminosity functions, and predict the uniform luminosity cut-off both in spiral galaxies and old elliptical galaxies.

Q1549-C25: A CLEAN SOURCE OF LYMAN-CONTINUUM EMISSION AT z = 3.15∗

ABSTRACT We present observations of Q1549-C25, an ∼ L * star-forming galaxy at z = 3.15 for which Lyman-continuum (LyC) radiation is significantly detected in deep Keck/LRIS spectroscopy. We find no evidence of contamination from a lower-redshift interloper close to the line of sight in the high signal-to-noise spectrum of Q1549-C25. Furthermore, the morphology of Q1549-C25 in V 606, J 125, and H 160 Hubble Space Telescope (HST) imaging reveals that the object consists of a single, isolated component within 1″. In combination, these data indicate Q1549-C25 as a clean spectroscopic detection of LyC radiation, only the second such object discovered to date at z ∼ 3. We model the spectral energy distribution of Q1549-C25, finding evidence of negligible dust extinction, an age (assuming continuous star formation) of ∼1 Gyr, and a stellar mass of M * = 7.9 × 10 9 M ⊙ . Although it is not possible to derive strong constraints on the absolute escape fraction of LyC emission, f esc(LyC), from a single object, we use simulations of intergalactic and circumgalactic absorption to infer f esc ( LyC ) ≥ 0.51 at 95% confidence. The combination of deep Keck/LRIS spectroscopy and Hubble Space Telescope imaging is required to assemble a larger sample of objects like Q1549-C25, and obtain robust constraints on the average f esc(LyC) at z ∼ 3 and beyond.

Modelling reionization in a bursty universe

We present semi-analytic models of the epoch of reionization focusing on the differences between continuous and bursty star formation (SF). Our model utilizes physically motivated analytic fits to 1D radiative transfer simulations of HII regions around dark matter halos in a representative cosmic volume. Constraining our simulations with observed and extrapolated UV luminosity functions of high redshift galaxies, we find that for a fixed halo mass, stellar populations forming in bursty models produce larger HII regions which leave behind long-lived relic HII regions which are able to maintain partial ionization in the intergalactic medium (IGM) in a manner similar to an early X-ray background. The overall effect is a significant increase in the optical depth of the IGM, $\tau_e$, and a milder increase of the redshift of reionization. To produce $\tau_e =0.066$ observed by Planck and complete reionization by redshift $z_{\rm re}\sim 6$, models with bursty SF require an escape fraction $f_{\rm esc}\sim 2\%-10\%$ that is 2-10 times lower than $f_{\rm esc}\sim 17\%$ found assuming continuous SF and is consistent with upper limits on $f_{\rm esc}$ from observations at $z=0$ and $z\sim 1.3-6$. The ionizing photon budget needed to reproduce the observed $\tau_e$ and $z_{\rm re}$ depends on the period and duty cycle of the bursts of SF and the temperature of the neutral IGM. These results suggest that the tension between observed and predicted ionizing photon budget for reionization can be alleviated if reionization is driven by short bursts of SF, perhaps relating to the formation of Population~III stars and compact star clusters such as proto-globular clusters.

CONSTRAINTS ON PHOTOIONIZATION FEEDBACK FROM NUMBER COUNTS OF ULTRA-FAINT HIGH-REDSHIFT GALAXIES IN THE FRONTIER FIELDS

ABSTRACT We exploit a sample of ultra-faint high-redshift galaxies (demagnified Hubble Space Telescope, HST, H 160 magnitude &gt; 30) in the Frontier Fields clusters A2744 and M0416 to constrain a theoretical model for the UV luminosity function in the presence of photoionization feedback. The objects have been selected on the basis of accurate photometric redshifts computed from multi-band photometry including seven HST bands and deep K s and IRAC observations. Magnification is computed on an object-by-object basis from all available lensing models of the two clusters. We take into account source detection completeness as a function of luminosity and size, magnification effects, and systematics in the lens modeling of the clusters under investigation. We find that our sample of high-z galaxies constrain the cutoff halo circular velocity below which star formation is suppressed by photoionization feedback to v c cut &lt; 50 km s−1. This circular velocity corresponds to a halo mass of ≈5.6 × 109 M ⊙ and ≈2.3 × 109 M ⊙ at z = 5 and 10, respectively: higher-mass halos can thus sustain continuous star formation activity without being quenched by external ionizing flux. More stringent constraints are prevented by the uncertainty in the modeling of the cluster lens, as embodied by systematic differences among the lens models available.

ATLASGAL-selected massive clumps in the inner Galaxy

The processes leading to the birth of high-mass stars are poorly understood. We characterise here a sample of 430 massive clumps from the ATLASGAL survey, which are representative of different evolutionary stages. To establish a census of molecular tracers of their evolution we performed an unbiased spectral line survey covering the 3-mm atmospheric window between 84-117 GHz with the IRAM 30m. A smaller sample of 128 clumps has been observed in the SiO (5-4) transition with the APEX telescope to complement the SiO (2-1) line and probe the excitation conditions of the emitting gas, which is the main focus of the current study. We report a high detection rate of >75% of the SiO (2-1) line and a >90% detection rate from the dedicated follow-ups in the (5-4) transition. The SiO (2-1) line with broad line profiles and high detection rates, is a powerful probe of star formation activity, while the ubiquitous detection of SiO in all evolutionary stages suggests a continuous star formation process in massive clumps. We find a large fraction of infrared-quiet clumps to exhibit SiO emission, the majority of them only showing a low-velocity component (FWHM~5-6 km/s) centred at the rest velocity of the clump. In the current picture, where this is attributed to low-velocity shocks from cloud-cloud collisions, this can be used to pinpoint the youngest, thus, likely prestellar massive structures. Based on the line ratio of the (5-4) to the (2-1) line, our study reveals a trend of changing excitation conditions that lead to brighter emission in the (5-4) line towards more evolved sources. Our analysis delivers a more robust estimate of SiO column density and abundance than previous studies and questions the decrease of jet activity in massive clumps as a function of age.

SATELLITE QUENCHING AND GALACTIC CONFORMITY AT 0.3 &lt; z &lt; 2.5*

ABSTRACT We measure the evolution of the quiescent fraction and quenching efficiency of satellites around star-forming and quiescent central galaxies with stellar mass log ( M cen / M &CircleDot; ) &gt; 10.5 ?> at 0.3 &lt; z &lt; 2.5 ?> . We combine imaging from three deep near-infrared-selected surveys (ZFOURGE/CANDELS, Ultra Deep Survey, and UltraVISTA), which allows us to select a stellar-mass complete sample of satellites with log ( M sat / M &CircleDot; ) &gt; 9.3 ?> . Satellites for both star-forming and quiescent central galaxies (“centrals”) have higher quiescent fractions compared to field galaxies matched in stellar mass at all redshifts. We also observe “galactic conformity”: satellites around quiescent centrals are more likely to be quenched compared to the satellites around star-forming centrals. In our sample, this conformity signal is significant at &gsim; 3 &sgr; ?> for 0.6 &lt; z &lt; 1.6 ?> , whereas it is only weakly significant at 0.3 &lt; z &lt; 0.6 ?> and 1.6 &lt; z &lt; 2.5 ?> . Therefore, conformity (and thus satellite quenching) has been present for a significant fraction of the age of the universe. The satellite quenching efficiency increases with increasing stellar mass of the central, but does not appear to depend on the stellar mass of the satellite to the mass limit of our sample. When we compare the satellite quenching efficiency of star-forming centrals with stellar masses 0.2 dex higher than quiescent centrals (which should account for any difference in halo mass), the conformity signal decreases, but remains statistically significant at 0.6 &lt; z &lt; 0.9 ?> . This is evidence that satellite quenching is connected to the star formation properties of the central galaxy as well as to the mass of the halo. We discuss physical effects that may contribute to galactic conformity, and emphasize that they must allow for continued star formation in the central galaxy even as the satellites are quenched.

STAR FORMATION HISTORY AND DUST PRODUCTION: NGC147 AND NGC185

NGC147 and NGC185, paired satellites of the Andromeda galaxy, possess the same order of mass and analogous structures, but they show different star formation and different amounts of interstellar gas and dust. Therefore, we present the first reconstruction of the star formation history of NGC147 and NGC185. Asymptotic Giant Branch stars are highly evolved stars that are brightest in K-band. This maximum K-band magnitude is related to the birth mass of stars. As a result, we have found a 9.9 Gyr old single star formation epoch for NGC185 followed by relatively continuous star formation. NGC147, however, has passed through two star formation episodes; one is as old as ~6 Gyr and the other is as recent as ~850 Myr. Asymptotic Giant Branch stars are also important dust factories; by fitting Spectral Energy Distributions to observed near and mid infrared data for each star, we were able to measure the dust production rates of individual stars; on order of <TEX>$10^{-5}M_{\odot}yr^{-1}$</TEX>. Hence, we estimate the total mass entering the interstellar medium to be <TEX>$1.06{\times}10^{-4}M_{\odot}yr^{-1}$</TEX> and <TEX>$2.89{\times}10^{-4}M_{\odot}yr^{-1}$</TEX> for NGC147 and NGC185.

Growing galaxies via superbubble-driven accretion flows

We use a suite a cooling halo simulations to study a new mechanism for rapid accretion of hot halo gas onto star-forming galaxies. Correlated supernovae events create converging 'superbubbles' in the halo gas. Where these collide, the density increases, driving cooling filaments of low metallicity gas that feed the disc. At our current numerical resolution (20 pc) we are only able to resolve the most dramatic events; these could be responsible for the build-up of galaxy discs after the most massive gas-rich mergers have completed (z < 1). As we increase the numerical resolution, we find that the filaments persist for longer, driving continued late-time star formation. This suggests that SNe-driven accretion could act as an efficient mechanism for extracting cold gas from the hot halo, driving late-time star formation in disc galaxies. We show that such filament feeding leads to a peak star formation rate (SFR) of $\sim 3$ M$_{\rm sun}$ yr$^{-1}$, consistent with estimates for the Milky Way. By contrast, direct cooling from the hot halo ('hot-mode' accretion, not present in the simulations that show filament feeding) falls short of the SNe-driven SFR by a factor of 3-4, and is sustained over a shorter time period. The filaments we resolve extend to $\sim$ 50 kpc, reaching column densities of $\sim 10^{18}$ cm$^{-2}$. We show that such structures can plausibly explain the broad dispersion in Mg II absorption seen along sight lines to quasars. Our results suggest a dual role for stellar feedback in galaxy formation, suppressing hot-mode accretion while promoting cold-mode accretion along filaments. This ultimately leads to more star formation, suggesting that the positive feedback effect outweighs the negative. Finally, since the filamentary gas has higher angular momentum than that coming from hot-mode accretion, we show that this leads to the formation of substantially larger gas discs.

THE GALAXY POPULATION IN VOIDS: ARE ALL VOIDS THE SAME?

The influence of under-dense environments on the formation and evolution of galaxies is studied by analysing the photometric properties of ~200 galaxies residing in voids, taken from our SDSS DR10 void catalogue up to z ~0.055. We split void galaxies into two subsamples based on the luminosity density contrast of their host voids: 'sparse void' {\delta}_s = {\delta} < -0.95 and 'populous void' {\delta}_p = {\delta} > -0.87. We find that galaxies in sparse voids are less massive than galaxies in populous voids. The luminosity distribution of galaxies in populous voids follows the same distribution observed across the SDSS survey in the same redshift range. Galaxies in the sparse voids are also bluer suggesting that they may be going through a relatively slow and continuous star formation. Additionally, we find that the luminosity function of galaxies in populous voids is represented with the Schechter function whereas the same does not hold for sparse voids. Our analysis suggests that the properties of a host void plays a significant role in the formation and evolution of the void galaxies and determining the large scale evolution of voids is an important step to understand what processes regulate the evolution of galaxies.

FORMATION AND EVOLUTION OF NUCLEAR STAR CLUSTERS WITH IN SITU STAR FORMATION: NUCLEAR CORES AND AGE SEGREGATION

Nuclear stellar cluster (NSCs) are known to exist around massive black holes (MBHs) in galactic nuclei. Two formation scenarios were suggested for their origin: (1) Build-up of NSCs from consecutive infall of stellar cluster and (2) Continuous in-situ star-formation. Though the cluster-infall scenario has been extensively studied in recent years, the in-situ formation scenario have been hardly explored. Here we use Fokker-Planck (FP) calculations to study the effects of star formation on the build-up of NSCs and its implications for their long term evolution and their resulting structure. We use the FP equation to describe the evolution of several stellar populations, and add appropriate source terms to account for the effects of newly formed stars. We show that continuous star-formation even 1-2 pc away from the MBH can lead to the build-up of an NSC with properties similar to those of the Milky-way NSC. We also find that the general structure of the old stellar population in the NSC with in-situ star-formation could be very similar to the steady-state Bahcall-Wolf cuspy structure. However, its younger stellar population do not yet achieve a steady state. In particular, formed/evolved NSCs with in-situ star-formation contain differential age-segregated stellar populations which are not yet fully mixed. Younger stellar populations formed in the outer regions of the NSC have a cuspy structure towards the NSC outskirts, while showing a core-like distribution inwards; with younger populations having larger core sizes. In principal, such a structure can give rise to an apparent core-like radial distribution of younger (up to 2-3 Gyrs) stars, as observed in the Galactic center. Such an NSC still preserves an underlying stellar cusp of older stars, that can be potentially be missed by current observations of red-giants.

Revised rate coefficients for H2 and H− destruction by realistic stellar spectra

Understanding the processes that can destroy H$_2$ and H$^-$ species is quintessential in governing the formation of the first stars, black holes and galaxies. In this study we compute the reaction rate coefficients for H$_2$ photo--dissociation by Lyman--Werner photons ($11.2 - 13.6$ eV), and H$^-$ photo--detachment by 0.76 eV photons emanating from self-consistent stellar populations that we model using publicly available stellar synthesis codes. So far studies that include chemical networks for the formation of molecular hydrogen take these processes into account by assuming that the source spectra can be approximated by a power-law dependency or a black-body spectrum at 10$^4$ or $10^5$ K. We show that using spectra generated from realistic stellar population models can alter the reaction rates for photo-dissociation, $\rm k_{\rm{di}}$, and photo-detachment, $\rm k_{\rm{de}}$, significantly. In particular, $\rm k_{\rm{de}}$ can be up to $\sim 2-4$ orders of magnitude lower in the case of realistic stellar spectra suggesting that previous calculations have over-estimated the impact that radiation has on lowering H$_2$ abundances. In contrast to burst modes of star formation, we find that models with continuous star formation predict increasing $\rm k_{\rm{de}}$ and $\rm k_{\rm{di}}$, which makes it necessary to include the star formation history of sources to derive self-consistent reaction rates, and that it is not enough to just calculate J$_{21}$ for the background. For models with constant star formation rate the change in shape of the spectral energy distribution leads to a non-negligible late-time contribution to $\rm k_{\rm{de}}$ and $\rm k_{\rm{di}}$, and we present self-consistently derived cosmological reaction rates based on star formation rates consistent with observations of the high redshift Universe.

Heating and ionization of the primordial intergalactic medium by high-mass X-ray binaries

We investigate the influence of High Mass X-ray Binaries on their high redshift environments. Using a one-dimensional radiative transfer code, we predict the ionization and temperature profiles surrounding a coeval stellar population, composed of main sequence stars and HMXBs, at various times after its formation. We consider both uniform density surroundings, and a cluster embedded in a 10^8 solar mass NFW halo. HMXBs in a constant density environment produce negligible enhanced ionization because of their high-energy SEDs and short lifetimes. In this case, HMXBs only marginally contribute to the local heating rate. For NFW profiles, radiation from main sequence stars cannot prevent the initially ionized volume from recombining since it is unable to penetrate the high density galactic core. However, HMXB photons stall recombinations behind the front, keeping it partially ionized for longer. The increased electron density in these partially ionized regions promotes further cooling, resulting in lower IGM temperatures. In the context of this starburst model, we have shown that HMXBs do not make a major contribution to reionization or IGM heating. However, X-ray escape fractions are high in both density profile cases. Continuous star formation may result in the build up of X-rays over time, reducing the ionization timescale and potentially leading to low level ionization of the distant IGM.

Binary white dwarfs in the halo of the Milky Way

Aims: We study single and binary white dwarfs in the inner halo of the Milky Way in order to learn more about the conditions under which the population of halo stars was born, such as the initial mass function (IMF), the star formation history, or the binary fraction. Methods: We simulate the evolution of low-metallicity halo stars at distances up to ~ 3 kpc using the binary population synthesis code SeBa. We use two different white dwarf cooling models to predict the present-day luminosities of halo white dwarfs. We determine the white dwarf luminosity functions (WDLFs) for eight different halo models and compare these with the observed halo WDLF of white dwarfs in the SuperCOSMOS Sky Survey. Furthermore, we predict the properties of binary white dwarfs in the halo and determine the number of halo white dwarfs that is expected to be observed with the Gaia satellite. Results: By comparing the WDLFs, we find that a standard IMF matches the observations more accurately than a top-heavy one, but the difference with a bottom-heavy IMF is small. A burst of star formation 13 Gyr ago fits slightly better than a star formation burst 10 Gyr ago and also slightly better than continuous star formation $10-13$ Gyrs ago. Gaia will be the first instrument to constrain the bright end of the field halo WDLF, where contributions from binary WDs are considerable. Many of these will have He cores, of which a handful have atypical surface gravities ($\log g < 6$) and reach luminosities $\log(L/L_\odot) > 0$ in our standard model for WD cooling. These so called pre-WDs, if observed, can help us to constrain white dwarf cooling models and might teach us something about the fraction of halo stars that reside in binaries.

Star-formation in nuclear clusters and the origin of the Galactic center apparent core distribution

AbstractNuclear stellar clusters (NSCs) are known to exist around massive black holes (MBHs) in galactic nuclei. Two formation scenarios were suggested for their origin: build-up of NSCs and Continuous in-situ star-formation. Here we study the effects of star formation on the build-up of NSCs and its implications for their long term evolution and their resulting structure. We show that continuous star-formation can lead to the build-up of an NSC with properties similar to those of the Milky-way NSC. We also find that the general structure of the old stellar population in the NSC with in-situ star-formation could be very similar to the steady-state Bahcall-Wolf cuspy structure. However, its younger stellar population does not yet achieve a steady state. In particular, formed/evolved NSCs with in-situ star-formation contain differential age-segregated stellar populations which are not yet fully mixed. Younger stellar populations formed in the outer regions of the NSC have a cuspy structure towards the NSC outskirts, while showing a core-like distribution inwards; with younger populations having larger core sizes.

Continuous Mid-Infrared Star Formation Rate Indicators

AbstractWe present continuous, monochromatic star formation rate (SFR) indicators over the mid-infrared wavelength range of 6–70 μm. We use a sample of 58 star forming galaxies (SFGs) in the Spitzer-SDSS-GALEX Spectroscopic Survey (SSGSS) at z&lt;0.2, for which there is a rich suite of multi-wavelength photometry and spectroscopy from the ultraviolet through to the infrared. The data from the Spitzer infrared spectrograph (IRS) of these galaxies, which spans 5–40 μm, is anchored to their photometric counterparts. The spectral region between 40-70 μm is interpolated using dust model fits to the IRS spectrum anchored by Spitzer 70 and 160 μm photometry. Since there are no sharp spectral features in this region, we expect these interpolations to be robust. This spectral range is calibrated as a SFR diagnostic using several reference SFR indicators to mitigate potential bias. Our band-specific continuous SFR indicators are found to be consistent with monochromatic calibrations in the local universe, as derived from Spitzer, WISE, and Herschel photometry. Additionally, in the era of the James Webb Space Telescope this will become a flexible tool, applicable to any SFG up to z∼3.