Search

We present a comprehensive analysis of the photometric and spectroscopic evolution of SN~2021foa, unique among the class of transitional supernovae for repeatedly changing its spectroscopic appearance from hydrogen-to-helium-to-hydrogen-dominated (IIn-to-Ibn-to-IIn) within 50 days past peak brightness. The spectra exhibit multiple narrow ($\approx$ 300--600~km~s$^{-1}$) absorption lines of hydrogen, helium, calcium and iron together with broad helium emission lines with a full-width-at-half-maximum (FWHM) of $\sim 6000$~km~s$^{-1}$. For a steady, wind-mass loss regime, light curve modeling results in an ejecta mass of $\sim 8$ M$_{\odot}$ and CSM mass below 1 M$_{\odot}$, and an ejecta velocity consistent with the FWHM of the broad helium lines. We obtain a mass-loss rate of $\approx 2$ M$_{\odot} {\rm yr}^{-1}$. This mass-loss rate is three orders of magnitude larger than derived for normal Type II SNe. We estimate that the bulk of the CSM of SN~2021foa must have been expelled within half a year, about 15 years ago. Our analysis suggests that SN~2021foa had a helium rich ejecta which swept up a dense shell of hydrogen rich CSM shortly after explosion. At about 60 days past peak brightness, the photosphere recedes through the dense ejecta-CSM region, occulting much of the red-shifted emission of the hydrogen and helium lines, which results in observed blue-shift ($\sim -3000$~km~s$^{-1}$). Strong mass loss activity prior to explosion, such as those seen in SN~2009ip-like objects and SN~2021foa as precursor emission, are the likely origin of a complex, multiple-shell CSM close to the progenitor star., Comment: Submitted to ApJ, 21 figures, 7 tables

We present LAISS (Lightcurve Anomaly Identification and Similarity Search), an automated pipeline to detect anomalous astrophysical transients in real-time data streams. We deploy our anomaly detection model on the nightly ZTF Alert Stream via the ANTARES broker, identifying a manageable $\sim$1-5 candidates per night for expert vetting and coordinating follow-up observations. Our method leverages statistical light-curve and contextual host-galaxy features within a random forest classifier, tagging transients of rare classes (spectroscopic anomalies), of uncommon host-galaxy environments (contextual anomalies), and of peculiar or interaction-powered phenomena (behavioral anomalies). Moreover, we demonstrate the power of a low-latency ($\sim$ms) approximate similarity search method to find transient analogs with similar light-curve evolution and host-galaxy environments. We use analogs for data-driven discovery, characterization, (re-)classification, and imputation in retrospective and real-time searches. To date we have identified $\sim$50 previously known and previously missed rare transients from real-time and retrospective searches, including but not limited to: SLSNe, TDEs, SNe IIn, SNe IIb, SNe Ia-CSM, SNe Ia-91bg-like, SNe Ib, SNe Ic, SNe Ic-BL, and M31 novae. Lastly, we report the discovery of 325 total transients, all observed between 2018-2021 and absent from public catalogs ($\sim$1% of all ZTF Astronomical Transient reports to the Transient Name Server through 2021). These methods enable a systematic approach to finding the "needle in the haystack" in large-volume data streams. Because of its integration with the ANTARES broker, LAISS is built to detect exciting transients in Rubin data., Comment: 44 pages (68 pages with Appendix), 15 figures, accepted to ApJ

We present the Young Supernova Experiment Data Release 1 (YSE DR1), comprised of processed multi-color Pan-STARRS1 (PS1) griz and Zwicky Transient Facility (ZTF) gr photometry of 1975 transients with host-galaxy associations, redshifts, spectroscopic/photometric classifications, and additional data products from 2019 November 24 to 2021 December 20. YSE DR1 spans discoveries and observations from young and fast-rising supernovae (SNe) to transients that persist for over a year, with a redshift distribution reaching z~0.5. We present relative SN rates from YSE's magnitude- and volume-limited surveys, which are consistent with previously published values within estimated uncertainties for untargeted surveys. We combine YSE and ZTF data, and create multi-survey SN simulations to train the ParSNIP and SuperRAENN photometric classification algorithms; when validating our ParSNIP classifier on 472 spectroscopically classified YSE DR1 SNe, we achieve 82% accuracy across three SN classes (SNe Ia, II, Ib/Ic) and 90% accuracy across two SN classes (SNe Ia, core-collapse SNe). Our classifier performs particularly well on SNe Ia, with high (>90%) individual completeness and purity, which will help build an anchor photometric SNe Ia sample for cosmology. We then use our photometric classifier to characterize our photometric sample of 1483 SNe, labeling 1048 (~71%) SNe Ia, 339 (~23%) SNe II, and 96 (~6%) SNe Ib/Ic. YSE DR1 provides a training ground for building discovery, anomaly detection, and classification algorithms, performing cosmological analyses, understanding the nature of red and rare transients, exploring tidal disruption events and nuclear variability, and preparing for the forthcoming Vera C. Rubin Observatory Legacy Survey of Space and Time., Comment: Accepted to ApJS; 64 pages; 35 figures; 10 tables

We present optical and near-infrared (NIR) observations of the Type Icn supernova (SN Icn) 2022ann, the fifth member of its newly identified class of SNe. Its early optical spectra are dominated by narrow carbon and oxygen P-Cygni features with absorption velocities of 800 km/s; slower than other SNe Icn and indicative of interaction with a dense, H/He-poor circumstellar medium (CSM) that is outflowing slower than a typical Wolf-Rayet wind velocity of $>$1000 km/s. We identify helium in NIR spectra obtained two weeks after maximum and in optical spectra at three weeks, demonstrating that the CSM is not fully devoid of helium. We never detect broad spectral features from SN ejecta, including in spectra extending to the nebular phase, a unique characteristic among SNe~Icn. Compared to other SNe Icn, SN 2022ann has a low luminosity, with a peak o-band absolute magnitude of -17.7, and evolves slowly. We model the bolometric light curve and find it is well-described by 1.7 M_Sun of SN ejecta interacting with 0.2 M_sun of CSM. We place an upper limit of 0.04 M_Sun of Ni56 synthesized in the explosion. The host galaxy is a dwarf galaxy with a stellar mass of 10^7.34 M_Sun (implied metallicity of log(Z/Z_Sun) $\approx$ 0.10) and integrated star-formation rate of log(SFR) = -2.20 M_sun/yr; both lower than 97\% of the galaxies observed to produce core-collapse supernovae, although consistent with star-forming galaxies on the galaxy Main Sequence. The low CSM velocity, nickel and ejecta masses, and likely low-metallicity environment disfavour a single Wolf-Rayet progenitor star. Instead, a binary companion star is likely required to adequately strip the progenitor before explosion and produce a low-velocity outflow. The low CSM velocity may be indicative of the outer Lagrangian points in the stellar binary progenitor, rather than from the escape velocity of a single Wolf-Rayet-like massive star., Comment: 21 pages, 13 figures, to be submitted to MNRAS

Massive black holes (BHs) at the centres of massive galaxies are ubiquitous. The population of BHs within dwarf galaxies, on the other hand, is evasive. Dwarf galaxies are thought to harbour BHs with proportionally small masses, including intermediate mass BHs, with masses $10^{2} < M_{BH} < 10^{6} M_{\odot}$. Identification of these systems has historically relied upon the detection of light emitted from accreting gaseous discs close to the BHs. Without this light, they are difficult to detect. Tidal disruption events (TDEs), the luminous flares produced when a star strays close to a BH and is shredded, are a direct way to probe massive BHs. The rise times of these flares theoretically correlate with the BH mass. Here we present AT2020neh, a fast rising TDE candidate, hosted by a dwarf galaxy. AT2020neh can be described by the tidal disruption of a main sequence star by a 10$^{4.7} - 10^{5.9} M_{\odot}$ BH. We find the observable rate of fast rising nuclear transients like AT2020neh to be rare, at $\lesssim 2 \times 10^{-8}$ events Mpc$^{-3}$ yr$^{-1}$. Finding non-accreting BHs in dwarf galaxies is important to determine how prevalent BHs are within these galaxies, and constrain models of BH formation. AT2020neh-like events may provide a galaxy-independent method of measuring IMBH masses., Comment: Accepted for publication in Nature Astronomy

We present an observational study of the luminous red nova (LRN) AT\,2021biy in the nearby galaxy NGC\,4631. The field of the object was routinely imaged during the pre-eruptive stage by synoptic surveys, but the transient was detected only at a few epochs from $\sim 231$\,days before maximum brightness. The LRN outburst was monitored with unprecedented cadence both photometrically and spectroscopically. AT\,2021biy shows a short-duration blue peak, with a bolometric luminosity of $\sim 1.6 \times 10^{41}$\,erg\,s$^{-1}$, followed by the longest plateau among LRNe to date, with a duration of 210\,days. A late-time hump in the light curve was also observed, possibly produced by a shell-shell collision. AT\,2021biy exhibits the typical spectral evolution of LRNe. Early-time spectra are characterised by a blue continuum and prominent H emission lines. Then, the continuum becomes redder, resembling that of a K-type star with a forest of metal absorption lines during the plateau phase. Finally, late-time spectra show a very red continuum ($T_{\mathrm{BB}} \approx 2050$ K) with molecular features (e.g., TiO) resembling those of M-type stars. Spectropolarimetric analysis indicates that AT\,2021biy has local dust properties similar to those of V838\,Mon in the Milky Way Galaxy. Inspection of archival {\it Hubble Space Telescope} data taken on 2003 August 3 reveals a $\sim 20$\,\msun\ progenitor candidate with log\,$(L/{\rm L}_{\odot}) = 5.0$\,dex and $T_{\rm{eff}} = 5900$\,K at solar metallicity. The above luminosity and colour match those of a luminous yellow supergiant. Most likely, this source is a close binary, with a 17--24\,\msun\ primary component., Comment: 21 pages, 14 figures. Accepted by Astronomy and Astrophysics

We present the results from a dense multi-wavelength (optical/UV, near-infrared (IR), and X-ray) follow-up campaign of the nuclear transient AT2017gge, covering a total of 1698 days from the transient's discovery. The bolometric lightcurve, the black body temperature and radius, the broad H and He I $\lambda$5876 emission lines and their evolution with time, are all consistent with a tidal disruption event (TDE) nature. A soft X-ray flare is detected with a delay of $\sim$200 days with respect to the optical/UV peak and it is rapidly followed by the emergence of a broad He II $\lambda$4686 and by a number of long-lasting high ionization coronal emission lines. This indicate a clear connection between a TDE flare and the appearance of extreme coronal line emission (ECLEs). An IR echo, resulting from dust re-radiation of the optical/UV TDE light is observed after the X-ray flare and the associated near-IR spectra show a transient broad feature in correspondence of the He I $\lambda$10830 and, for the first time in a TDE, a transient high-ionization coronal NIR line (the [Fe XIII] $\lambda$10798) is also detected. The data are well explained by a scenario in which a TDE occurs in a gas and dust rich environment and its optical/UV, soft X-ray, and IR emission have different origins and locations. The optical emission may be produced by stellar debris stream collisions prior to the accretion disk formation, which is instead responsible for the soft X-ray flare, emitted after the end of the circularization process., Comment: Accepted for publication in MNRAS

We use the SMASH survey to obtain unprecedented deep photometry reaching down to the oldest main sequence turn-offs in the colour-magnitude diagrams (CMDs) of the Small Magellanic Cloud (SMC) and quantitatively derive its star formation history (SFH) using CMD fitting techniques. We identify five distinctive peaks of star formation in the last 3.5 Gyr, at $\sim $3, $\sim$2, $\sim$1.1, $\sim $0.45 Gyr ago, and one presently. We compare these to the SFH of the Large Magellanic Cloud (LMC) finding unequivocal synchronicity, with both galaxies displaying similar periods of enhanced star formation over the past $\sim$3.5 Gyr. The parallelism between their SFHs indicates that tidal interactions between the MCs have recurrently played an important role in their evolution for at least the last $\sim$3.5 Gyr, tidally truncating the SMC and shaping the LMC's spiral arm. We show, for the first time, an SMC-LMC correlated SFH at recent times in which enhancements of star formation are localised in the northern spiral arm of the LMC, and globally across the SMC. These novel findings should be used to constrain not only the orbital history of the MCs but also how star formation should be treated in simulations., Comment: 6 pages, 4 figures. Accepted for publication to MNRAS Letters

Using data from a year-long dedicated campaign to observe bright stars, we study the crosstalk channels present in the GPC1 camera. By analyzing these data, we construct a dataset that checks source stars on almost every CCD of every chip within the camera against all possible crosstalk destinations. We use a clustering algorithm to find potential crosstalk occurrences, and then also check all possible combinations (driven by the hardware layout) by eye. This results in a total of 640 rules, with a flux attenuation factor ranging from 2.5x10$^{2}$ for the bright end to 2.5$\times$10$^{4}$ at the faint end. The average value of m$_{cross}$-m$_{src}\approx$-10.25 corresponds to an attenuating factor of 1.25x10$^{4}$, which produces crosstalk ghosts with an average signal-to-noise ratio of 0.64$\pm$0.1 on the bright images. We find no evidence of crosstalk signals between CCDs not connected in the hardware setup. The distribution of attenuation factors is also found to be dependent on crosstalk movement. A clear dependence on cell column offsets is found, consistent with the idea that the source star charge is progressively attenuated during the traversal of cell readout lines. While we can see the trends, the uncertainties on aperture magnitude measurements are large at this stage., Comment: Accepted by PASP

We present multi-wavelength follow-up observations of the ATLAS discovered faint Iax supernova SN 2020kyg that peaked at an absolute magnitude of $M_g \approx -14.9 \pm 0.2$, making it another member of the faint Iax supernova population. The bolometric light curve requires only $\approx 7 \times 10^{-3}$ M$_{\odot}$ of radioactive $^{56}$Ni, with an ejected mass of $M_{\rm ej} \sim 0.4$ M$_{\odot}$ and a low kinetic energy of $E \approx 0.05 \pm 0.02 \times 10^{51}$ erg. We construct a homogeneous volume-limited sample of 902 transients observed by ATLAS within 100 Mpc during a 3.5 year span. Using this sample, we constrain the rates of faint Iax ($M_r \gtrsim -16$) events within 60 Mpc at $12^{+14}_{-8}\%$ of the SN Ia rate. The overall Iax rate, at $15^{+17}_{-9}\%$ of the Ia rate, is dominated by the low-luminosity events, with luminous SNe Iax ($M_r \lesssim -17.5$) like 2002cx and 2005hk accounting for only $0.9^{+1.1}_{-0.5}\%$ of the Ia rate (a 2$\sigma$ upper limit of approximately 3\%). We favour the hybrid CONe WD + He star progenitor channel involving a failed deflagration of a near Chandrasekhar mass white dwarf, expected to leave a bound remnant and a surviving secondary companion, as a candidate explanation for faint Iax explosions. This scenario requires short delay times, consistent with the observed environments of SNe Iax. Furthermore, binary population synthesis calculations have suggested rates of $1-18\%$ of the SN Ia rate for this channel, consistent with our rate estimates., Comment: Accepted to MNRAS after minor revision

The DECam Local Volume Exploration survey (DELVE) is a 126-night survey program on the 4-m Blanco Telescope at the Cerro Tololo Inter-American Observatory in Chile. DELVE seeks to understand the characteristics of faint satellite galaxies and other resolved stellar substructures over a range of environments in the Local Volume. DELVE will combine new DECam observations with archival DECam data to cover ~15000 deg$^2$ of high-Galactic-latitude (|b| > 10 deg) southern sky to a 5$\sigma$ depth of g,r,i,z ~ 23.5 mag. In addition, DELVE will cover a region of ~2200 deg$^2$ around the Magellanic Clouds to a depth of g,r,i ~ 24.5 mag and an area of ~135 deg$^2$ around four Magellanic analogs to a depth of g,i ~ 25.5 mag. Here, we present an overview of the DELVE program and progress to date. We also summarize the first DELVE public data release (DELVE DR1), which provides point-source and automatic aperture photometry for ~520 million astronomical sources covering ~5000 deg$^2$ of the southern sky to a 5$\sigma$ point-source depth of g=24.3, r=23.9, i=23.3, and z=22.8 mag. DELVE DR1 is publicly available via the NOIRLab Astro Data Lab science platform., Comment: 30 pages, 14 figures, 6 tables; updated to match published version; public data release at https://datalab.noirlab.edu/delve

We present the results of our monitoring campaigns of the luminous red novae (LRNe) AT 2020hat in NGC 5068 and AT 2020kog in NGC 6106. The two objects were imaged (and detected) before their discovery by routine survey operations. They show a general trend of slow luminosity rise, lasting at least a few months. The subsequent major LRN outbursts were extensively followed in photometry and spectroscopy. The light curves present an initial short-duration peak, followed by a redder plateau phase. AT 2020kog is a moderately luminous event peaking at ~7 x 10^40 erg/s, while AT 2020hat is almost one order of magnitude fainter than AT 2020kog, although it is still more luminous than V838 Mon. In analogy with other LRNe, the spectra of AT 2020kog change significantly with time. They resemble those of type IIn supernovae at early phases, then they become similar to those of K-type stars during the plateau, and to M-type stars at very late phases. In contrast, AT 2020hat already shows a redder continuum at early epochs, and its spectrum shows the late appearance of molecular bands. A moderate-resolution spectrum of AT 2020hat taken at +37 d after maximum shows a forest of narrow P Cygni lines of metals with velocities of 180 km/s, along with an Halpha emission with a full-width at half-maximum velocity of 250 km/s. For AT 2020hat, a robust constraint on its quiescent progenitor is provided by archival images of the Hubble Space Telescope. The progenitor is clearly detected as a mid-K type star, with an absolute magnitude of MF606W = -3.33+-0.09 mag and a colour of F606W-F814W = 1.14+-0.05 mag, which are inconsistent with the expectations from a massive star that could later produce a core-collapse supernova. Although quite peculiar, the two objects nicely match the progenitor versus light curve absolute magnitude correlations discussed in the literature., Comment: 14 pages, 9 figures, 2 tables. Accepted for publication in A&A

The GD-1 stream is one of the longest and coldest stellar streams discovered to date, and one of the best objects for constraining the dark matter properties of the Milky Way. Using data from {\it Gaia} DR2 we study the proper motions, distance, morphology and density of the stream to uncover small scale perturbations. The proper motion cleaned data shows a clear distance gradient across the stream, ranging from 7 to 12 kpc. However, unlike earlier studies that found a continuous gradient, we uncover a distance minimum at $\varphi_{1}\approx$-50 deg, after which the distance increases again. We can reliably trace the stream between -85$<\varphi_{1}<$15 deg, showing an even further extent to GD-1 beyond the earlier extension of \citet{Price-Whelan18a}. We constrain the stream track and density using a Boolean matched filter approach and find three large under densities and find significant residuals in the stream track lining up with these gaps. In particular, a gap is visible at $\varphi_{1}$=-3 deg, surrounded by a clear sinusoidal wiggle. We argue that this wiggle is due to a perturbation since it has the wrong orientation to come from a progenitor. We compute a total initial stellar mass of the stream segment of 1.58$\pm$0.07$\times$10$^{4}$ M$_{\odot}$. With the extended view of the spur in this work, we argue that the spur may be unrelated to the adjacent gap in the stream. Finally, we show that an interaction with the Sagittarius dwarf can create features similar to the spur., Comment: Submitted to MNRAS. comments are very welcome

Tidal streams are highly sensitive to perturbations from passing dark matter (DM) subhalos and thus provide a means of measuring their abundance. In a recent paper, we analyzed the distribution of stars along the GD-1 stream with a combination of data from the Gaia satellite and the Pan-STARRS survey, and we demonstrated that the population of DM subhalos predicted by the cold dark matter (CDM) paradigm are necessary and sufficient to explain the perturbations observed in the linear density of stars. In this paper, we use the measurements of the subhalo mass function (SHMF) from the GD-1 data combined with a similar analysis of the Pal 5 stream to provide novel constraints on alternative DM scenarios that predict a suppression of the SHMF on scales smaller than the mass of dwarf galaxies, marginalizing over uncertainties in the slope and normalization of the unsuppressed SHMF and the susceptibility of DM subhalos in the inner Milky Way to tidal disruption. In particular, we derive a 95% lower limit on the mass of warm dark matter (WDM) thermal relics $m_{\rm WDM}>3.6\,\mathrm{keV}$ from streams alone that strengthens to $m_{\rm WDM}>6.2\,\mathrm{keV}$ when adding dwarf satellite counts. Similarly, we constrain the axion mass in ultra-light ("fuzzy") dark matter (FDM) models to be $m_{\rm FDM}>1.4\times10^{-21}\,\mathrm{eV}$ from streams alone or $m_{\rm FDM}>2.2\times10^{-21}\,\mathrm{eV}$ when adding dwarf satellite counts. Because we make use of simple approximate forms of the streams' SHMF measurement, our analysis is easy to replicate with other alternative DM models that lead to a suppression of the SHMF., Comment: JCAP, in press. Code available to fully replicate the analysis at https://github.com/jobovy/dark-matter-constraints-from-stellar-streams

New data from the $\textit{Gaia}$ satellite, when combined with accurate photometry from the Pan-STARRS survey, allow us to accurately estimate the properties of the GD-1 stream. Here, we analyze the stellar density perturbations in the GD-1 stream and show that they cannot be due to known baryonic structures like giant molecular clouds, globular clusters, or the Milky Way's bar or spiral arms. A joint analysis of the GD-1 and Pal 5 streams instead requires a population of dark substructures with masses $\approx 10^{7}$ to $10^9 \ M_{\rm{\odot}}$. We infer a total abundance of dark subhalos normalised to standard cold dark matter $n_{\rm sub}/n_{\rm sub, CDM} = 0.4 ^{+0.3}_{-0.2}$ ($68 \%$), which corresponds to a mass fraction contained in the subhalos $f_{\rm{sub}} = 0.14 ^{+0.11}_{-0.07} \%$, compatible with the predictions of hydrodynamical simulation of cold dark matter with baryons., Comment: 20 pages, 14 figures. Also see companion paper : "Novel constraints on the particle nature of dark matter from stellar streams". Accepted in MNRAS. Modified main text slightly. Updated Appendix B and added a plot. Main results and conclusions remain unchanged

We report a new star formation history for the Tucana dwarf spheroidal galaxy, obtained from a new look at a deep HST/ACS colour-magnitude diagram. We combined information from the main sequence turn-off and the horizontal branch to resolve the ancient star formation rates on a finer temporal scale than previously possible. We show that Tucana experienced three major phases of star formation, two very close together at ancient times and the last one ending between 6 and 8 Gyr ago. We show that the three discrete clumps of stars on the horizontal branch are linked to the distinct episodes of star formation in Tucana. The spatial distribution of the clumps reveals that each generation of stars presents a higher concentration than the previous one. The simultaneous modelling of the horizontal branch and the main sequence turn-off also allows us to measure the amount of mass lost by red giant branch stars in Tucana with unprecedented precision, confirming dwarf spheroidals to be excellent laboratories to study the advanced evolution of low-mass stars., Comment: Accepted for publication in A&A. 10 pages, 11 figures

Using data from Gaia DR2, we study the radial number density profiles of the Galactic globular cluster sample. Proper motions are used for accurate membership selection, especially crucial in the cluster outskirts. Due to the severe crowding in the centres, the Gaia data is supplemented by literature data from HST and surface brightness measurements, where available. This results in 81 clusters with a complete density profile covering the full tidal radius (and beyond) for each cluster. We model the density profiles using a set of single-mass models ranging from King and Wilson models to generalised lowered isothermal limepy models and the recently introduced spes models, which allow for the inclusion of potential escapers. We find that both King and Wilson models are too simple to fully reproduce the density profiles, with King (Wilson) models on average underestimating(overestimating) the radial extent of the clusters. The truncation radii derived from the limepy models are similar to estimates for the Jacobi radii based on the cluster masses and their orbits. We show clear correlations between structural and environmental parameters, as a function of Galactocentric radius and integrated luminosity. Notably, the recovered fraction of potential escapers correlates with cluster pericentre radius, luminosity and cluster concentration. The ratio of half mass over Jacobi radius also correlates with both truncation parameter and PE fraction, showing the effect of Roche lobe filling., Comment: 23 pages, 23 figures. Accepted in MNRAS

We present a new method that incorporates the horizontal branch morphology into synthetic colour-magnitude diagram based star formation history determinations. This method, we call MORGOTH, self-consistently takes into account all the stellar evolution phases up to the early asymptothic giant branch, flexibly modelling red giant branch mass loss. We test MORGOTH on a range of synthetic populations, and find that the inclusion of the horizontal branch significantly increases the precision of the resulting star formation histories. When the main sequence turn-off is detected, MORGOTH can fit the star formation history and the red giant branch mass loss at the same time, efficiently breaking this degeneracy. As part of testing MORGOTH, we also model the observed colour-magnitude diagram of the well studied Sculptor dwarf spheroidal galaxy. We recover a new more detailed star formation history for this galaxy. Both the new star formation history and the red giant branch mass loss we determined for Sculptor with MORGOTH are in good agreement with previous analyses, thus demonstrating the power of this new approach., Comment: 12 pages, 11 figures, accepted for publication in MNRAS

Using deep photometric data from CFHT/Megacam, we study the morphology and density of the GD-1 stream, one of the longest and coldest stellar streams in the Milky Way. Our deep data recovers the lower main sequence of the stream with unprecedented quality, clearly separating it from Milky Way foreground and background stars. An analysis of the distance to different parts of the stream shows that GD-1 lies at a heliocentric distance between 8 and 10 kpc, with only a shallow gradient across 45 deg on the sky. Matched filter maps of the stream density show clear density variations, such as deviations from a single orbital track and tentative evidence for stream fanning. We also detect a clear under-density in the middle of the stream track at $\varphi_{1}$=-45 deg surrounded by overdense stream segments on either side. This location is a promising candidate for the elusive missing progenitor of the GD-1 stream. We conclude that the GD-1 stream has clearly been disturbed by interactions with the Milky Way disk or other sub-halos., Comment: submitted to MNRAS, comments welcome

We analyze age-velocity dispersion relations (AVRs) from kinematics of individual stars in eight Local Group galaxies ranging in mass from Carina ($M_{*} \sim 10^{6}$) to M31 ($M_{*} \sim 10^{11}$). Observationally the $\sigma$ vs. stellar age trends can be interpreted as dynamical heating of the stars by GMCs, bars/spiral arms, or merging subhalos; alternatively the stars could have simply been born out of a more turbulent ISM at high redshift and retain that larger velocity dispersion till present day - consistent with recent IFU studies. To ascertain the dominant mechanism and better understand the impact of instabilities and feedback, we develop models based on observed SFHs of these Local Group galaxies in order to create an evolutionary formalism which describes the ISM velocity dispersion due to a galaxy's evolving gas fraction. These empirical models relax the common assumption that the stars are born from gas which has constant velocity dispersion at all redshifts. Using only the observed SFHs as input, the ISM velocity dispersion and a mid-plane scattering model fits the observed AVRs of low mass galaxies without fine tuning. Higher mass galaxies above $M_{vir} > 10^{11}$ need a larger contribution from latent dynamical heating processes (for example minor mergers), in excess of the ISM model. Using the SFHs we also find that supernovae feedback does not appear to be a dominant driver of the gas velocity dispersion compared to gravitational instabilities - at least for dispersions $\sigma \gtrsim 25$ km/s. Together our results point to stars being born with a velocity dispersion close to that of the gas at the time of their formation, with latent dynamical heating operating with a galaxy mass-dependent efficiency. These semi-empirical relations may help constrain the efficiency of feedback and its impact on the physics of disk settling in galaxy formation simulations., Comment: 21 pages, 10 figures. accepted for publication in MNRAS

We present results from deep and very spatially extended CTIO/DECam $g$ and $r$ photometry (reaching out to $\sim$ 2 mag below the oldest MSTO and covering $\sim$ 20 deg$^2$) around the Sextans dSph. We use this data-set to study the structural properties of Sextans overall stellar population and its different stellar evolutionary phases, as well as to search for signs of tidal disturbance from the MW, which would indicate departure from dynamical equilibrium. We perform the most accurate structural analysis to-date of Sextans' stellar components by applying Bayesian MCMC methods to the individual stars' positions. Surface density maps are built by decontaminating the sample through a matched filter analysis of the CMD, and then analysed for departures from axisymmetry. Sextans is found to be considerably less spatially extended than early studies suggested. No significant distortions or tidal disturbances are found down to a surface brightness of $\sim$ 31.8 mag/arcsec$^{-2}$ in V-band. We identify an overdensity in the central regions that may correspond to previously reported kinematic substructure(s). In agreement with previous findings, old $\&$ metal-poor stars such as BHB stars cover a much larger area than stars in other evolutionary phases, and bright BSs are less spatially extended than faint ones. However, the different spatial distribution of bright and faint BSs appears consistent with the general age/metallicity gradients found in Sextans' stellar component. This is compatible with BSs having formed by evolution of binaries and not necessarily due to the presence of a central disrupted globular cluster, as suggested in the literature. We provide structural parameters for the various populations analyzed and make publicly available the photometric catalogue of point-sources as well as a catalogue of literature spectroscopic measurements with updated membership probabilities., Comment: 21 pages, 14 figures, 9 tables, accepted for publication in Astronomy & Astrophysics (A&A), the associated photometric and spectroscopic catalogues will be available at CDS, abstract abridged for arXiv

We present the first detailed study of the behaviour of the stellar proper motion across the entire Galactic Anti-centre area visible in the Sloan Digital Sky Survey data. We use recalibrated SDSS astrometry in combination with positions from {\it Gaia} DR1 to provide tangential motion measurements with a systematic uncertainty $<$5 kms$^{-1}$ for the Main Sequence stars at the distance of the Monoceros Ring. We demonstrate that Monoceros members rotate around the Galaxy with azimuthal speeds of ~230 kms$^{-1}$, only slightly lower than that of the Sun. Additionally, both vertical and azimuthal components of their motion are shown to vary considerably but gradually as a function of Galactic longitude and latitude. The stellar over-density in the Anti-centre region can be split into two components, the narrow, stream-like ACS and the smooth Ring. According to our analysis, these two structures show very similar but clearly distinct kinematic trends, which can be summarised as follows: the amplitude of the velocity variation in $v_{\phi}$ and $v_z$ in the ACS is higher compared to the Ring, whose velocity gradients appear to be flatter. Currently, no model available can explain the entirety of the data in this area of the sky. However, the new accurate kinematic map introduced here should provide strong constraints on the genesis of the Monoceros Ring and the associated sub-structure., Comment: Submitted to MNRAS. comments welcome

We present the spatially resolved star formation history (SFH) of the Carina dwarf spheroidal galaxy, obtained from deep, wide-field g,r imaging and a metallicity distribution from the literature. Our photometry covers $\sim2$ deg$^2$, reaching up to $\sim10$ times the half-light radius of Carina with a completeness higher than $50\%$ at $g\sim24.5$, more than one magnitude fainter than the oldest turnoff. This is the first time a combination of depth and coverage of this quality has been used to derive the SFH of Carina, enabling us to trace its different populations with unprecedented accuracy. We find that Carina's SFH consists of two episodes well separated by a star formation temporal gap. These episodes occurred at old ($>10$ Gyr) and intermediate ($2$-$8$ Gyr) ages. Our measurements show that the old episode comprises the majority of the population, accounting for $54\pm5\%$ of the stellar mass within $1.3$ times the King tidal radius, while the total stellar mass derived for Carina is $1.60\pm0.09\times 10^{6} M_{\rm{\odot}}$, and the stellar mass-to-light ratio $1.8\pm0.2$. The SFH derived is consistent with no recent star formation which hints that the observed blue plume is due to blue stragglers. We conclude that the SFH of Carina evolved independently of the tidal field of the Milky Way, since the frequency and duration of its star formation events do not correlate with its orbital parameters. This result is supported by the age/metallicity relation observed in Carina, and the gradients calculated indicating that outer regions are older and more metal poor., Comment: Accepted in ApJ (22 pages, 13 figures)

We present [C/Fe] and [N/Fe] abundance ratios and CH({\lambda}4300) and S({\lambda}3883) index measurements for 94 red giant branch (RGB) stars in the Sculptor dwarf spheroidal galaxy from VLT/VIMOS MOS observations at a resolving power R= 1150 at 4020 {\AA}. This is the first time that [N/Fe] abundances are derived for a large number of stars in a dwarf spheroidal. We found a trend for the [C/Fe] abundance to decrease with increasing luminosity on the RGB across the whole metallicity range, a phenomenon observed in both field and globular cluster giants, which can be interpreted in the framework of evolutionary mixing of partially processed CNO material. Both our measurements of [C/Fe] and [N/Fe] are in good agreement with the theoretical predictions for stars at similar luminosity and metallicity. We detected a dispersion in the carbon abundance at a given [Fe/H], which cannot be ascribed to measurement uncertainties alone. We interpret this observational evidence as the result of the contribution of different nucleosynthesis sources over time to a not well-mixed interstellar medium. We report the discovery of two new carbon-enhanced, metal-poor stars. These are likely the result of pollution from material enriched by asymptotic giant branch stars, as indicated by our estimates of [Ba/Fe]> +1. We also attempted a search for dissolved globular clusters in the field of the galaxy by looking for the distinctive C-N pattern of second population globular clusters stars in a previously detected, very metal-poor, chemodynamical substructure. We do not detect chemical anomalies among this group of stars. However, small number statistics and limited spatial coverage do not allow us to exclude the hypotheses that this substructure forms part of a tidally shredded globular cluster., Comment: 18 pages, 14 figures, 3 tables. Accepted to A&A

We have determined the detailed star formation history and total mass of the globular clusters in the Fornax dwarf spheroidal using archival HST WFPC2 data. Colour magnitude diagrams are constructed in the F555W and F814W bands and corrected for the effect of Fornax field star contamination, after which we use the routine Talos to derive the quantitative star formation history as a function of age and metallicity. The star formation history of the Fornax globular clusters shows that Fornax 1, 2, 3 and 5 are all dominated by ancient (>10 Gyr) populations. Cluster Fornax 1,2 and 3 display metallicities as low as [Fe/H]=-2.5 while Fornax 5 is slightly more metal-rich at [Fe/H]=-1.8, consistent with resolved and unresolved metallicity tracers. Conversely, Fornax 4 is dominated by a more metal-rich~([Fe/H]=-1.2) and younger population at 10 Gyr, inconsistent with the other clusters. A lack of stellar populations overlapping with the main body of Fornax argues against the nucleus cluster scenario for Fornax 4. The combined stellar mass in globular clusters as derived from the SFH is (9.57$\pm$0.93)$\times$10$^{5}$ M$_{\odot}$ which corresponds to 2.5$\pm$0.2 percent of the total stellar mass in Fornax. The mass of the four most metal-poor clusters can be further compared to the metal-poor Fornax field to yield a mass fraction of 19.6$\pm$3.1 percent. Therefore, the SFH results provide separate supporting evidence for the unusually high mass fraction of the GCs compared to the Fornax field population., Comment: 11 pages, 11 figures. Accepted in A&A

We present the first detailed quantitative study of the stellar populations of the Sagittarius (Sgr) streams within the Stripe 82 region, using photometric and spectroscopic observations from the Sloan Digital Sky Survey (SDSS). The star formation history (SFH) is determined separately for the bright and faint Sgr streams, to establish whether both components consist of a similar stellar population mix or have a distinct origin. Best fit SFH solutions are characterised by a well-defined, tight sequence in age-metallicity space, indicating that star formation occurred within a well-mixed, homogeneously enriched medium. Star formation rates dropped sharply at an age of ~5-7 Gyr, possibly related to the accretion of Sgr by the MW. Finally, the Sgr sequence displays a change of slope in age-metallicity space at an age between 11-13 Gyr consistent with the Sgr alpha-element knee, indicating that supernovae type Ia started contributing to the abundance pattern ~1-3 Gyr after the start of star formation. Results for both streams are consistent with being drawn from the parent Sgr population mix, but at different epochs. The SFH of the bright stream starts from old, metal-poor populations and extends to a metallicity of [Fe/H]~-0.7, with peaks at ~7 and 11 Gyr. The faint SFH samples the older, more metal-poor part of the Sgr sequence, with a peak at ancient ages and stars mostly with [Fe/H]<-1.3 and age>9 Gyr. Therefore, we argue in favour of a scenario where the faint stream consists of material stripped i) earlier, and ii) from the outskirts of the Sgr dwarf., Comment: 15 pages, 12 figures. ccepted for publication in MNRAS

We present deep photometry of the Carina dwarf Spheroidal galaxy in the B,V filters from CTIO/MOSAIC, out to and beyond the tidal radius. The accurately calibrated photometry is combined with spectroscopic metallicity distributions of Red Giant Branch stars to determine the detailed star formation and chemical evolution history. The star formation history confirms the episodic formation history of Carina and quantifies the duration and strength of each episode in great detail, as a function radius from the centre. Two main episodes of star formation occurred at old (>8 Gyr) and intermediate (2-8 Gyr) ages, both enriching stars starting from low metallicities ([Fe/H]<-2 dex). By dividing the SFH into two components, we determine that 60pm9 percent of the total number of stars formed within the intermediate age episode. Furthermore, within the tidal radius (0.48 degrees or 888 pc) a total mass in stars of 1.07pm0.08 x10^6 M_sun was formed, giving Carina a stellar mass-to-light ratio of 1.8pm0.8. Combining the detailed star formation history with spectroscopic observations of RGB stars, we are able to determine the detailed age-metallicity relation of each episode and the timescale of alpha-element evolution of Carina from individual stars. The oldest episode displays a tight age-metallicity relation over 6 Gyr with steadily declining alpha-element abundances and a possible alpha-element knee at [Fe/H]~ -2.5 dex. The intermediate age sequence displays a more complex age-metallicity relation starting from low metallicity and a sequence in alpha-element abundances with a slope much steeper than observed in the old episode, starting from [Fe/H]=-1.8 dex and [Mg/Fe]~0.4 dex and declining to Mg-poor values ([Mg/Fe]<-0.5 dex). This indicates clearly that both episodes of star formation formed from gas with different abundance patterns, inconsistent with simple evolution in an isolated system., Comment: accepted for publication in A&A. 14 pages, 17 figures

We employ abundances from the Sloan Digital Sky Survey (SDSS) and the Sloan Extension for Galactic Understanding and Exploration (SEGUE) to study the alpha-element distribution of the stellar members of the Sagittarius stream. To test the reliability of SDSS/SEGUE abundances for the study of Sagittarius, we select high-likelihood samples tracing the different components of the Milky Way, and recover known literature alpha-element distributions. Using selection criteria based on the spatial position, radial velocity, distance and colours of individual stars, we obtain a robust sample of Sagittarius-stream stars. The alpha-element distribution of the Sagittarius stream forms a narrow sequence at intermediate metallicities with a clear turn-down, consistent with the presence of an alpha-element "knee". This is the first time that the alpha-element knee of the Sagittarius dwarf galaxy has been detected. Fitting a toy model to our data, we determine that the alpha-knee in Sagittarius takes place at [Fe/H]=-1.27pm0.05, only slightly less metal-poor than the knee in the Milky Way. This indicates that a small number of Sagittarius-like galaxies could have contributed significantly to the build-up of the Milky Way's stellar halo system at ancient times., Comment: 6 pages, 3 figures

Using deep V, B-V wide-field photometry, we have conducted a new study of stellar over-densities in the Fornax dwarf spheroidal galaxy by determining detailed Star Formation Histories from Colour-Magnitude Diagram analysis. We have concentrated on the relatively young stellar component (<4 Gyr old), and compared this to the underlying Fornax field population. We have studied in more detail the previously known inner shell-like structure and shown that it has a well-defined age-metallicity relation with a peak at ~1.5 Gyr, [Fe/H]=-0.6 dex. Comparison to the Fornax centre shows that the over-dense feature is consistent with the age-metallicity relation of young field stars, and likely formed from Fornax gas. This is consistent with a scenario in which the over-density was formed by the re-accretion of previously expelled gas. We have also discovered a new stellar over-density, located 0.3 degrees (0.7 kpc) from the centre, which is only 100 Myr old, with solar metallicity. This feature constitutes some of the youngest, most metal-rich stars stars observed in Fornax to date. It is unclear how the young over-density was formed, although the age and metallicity of stars suggest this feature may represent the last star formation activity of the Fornax dSph., Comment: Accepted for publication in Astronomy & Astrophysics

We present deep photometry in the B,V and I filters from CTIO/MOSAIC for about 270.000 stars in the Fornax dwarf Spheroidal galaxy, out to a radius of r_ell\sim0.8 degrees. By combining the accurately calibrated photometry with the spectroscopic metallicity distributions of individual Red Giant Branch stars we obtain the detailed star formation and chemical evolution history of Fornax. Fornax is dominated by intermediate age (1-10 Gyr) stellar populations, but also includes ancient (10-14 Gyr), and young (<1 Gyr) stars. We show that Fornax displays a radial age gradient, with younger, more metal-rich populations dominating the central region. This confirms results from previous works. Within an elliptical radius of 0.8 degrees, or 1.9 kpc from the centre, a total mass in stars of 4.3x10^7 Msun was formed, from the earliest times until 250 Myr ago. Using the detailed star formation history, age estimates are determined for individual stars on the upper RGB, for which spectroscopic abundances are available, giving an age-metallicity relation of the Fornax dSph from individual stars. This shows that the average metallicity of Fornax went up rapidly from [Fe/H]<-2.5 dex to [Fe/H]=-1.5 dex between 8-12 Gyr ago, after which a more gradual enrichment resulted in a narrow, well-defined sequence which reaches [Fe/H]\sim-0.8 dex, \sim3 Gyr ago. These ages also allow us to measure the build-up of chemical elements as a function of time, and thus determine detailed timescales for the evolution of individual chemical elements. A rapid decrease in [Mg/Fe] is seen for the stars with [Fe/H]>-1.5 dex, with a clear trend in age., Comment: 18 pages, 20 figures

We have combined deep photometry in the B,V and I bands from CTIO/MOSAIC of the Sculptor dwarf spheroidal galaxy, going down to the oldest Main Sequence Turn-Offs, with spectroscopic metallicity distributions of Red Giant Branch stars. This allows us to obtain the most detailed and complete Star Formation History to date, as well as an accurate timescale for chemical enrichment. The Star Formation History shows that Sculptor is dominated by old ($>$10 Gyr), metal-poor stars, but that younger, more metal-rich populations are also present. Using Star Formation Histories determined at different radii from the centre we show that Sculptor formed stars with an increasing central concentration with time. The old, metal-poor populations are present at all radii, while more metal-rich, younger stars are more centrally concentrated. We find that within an elliptical radius of 1 degree, or 1.5 kpc from the centre, a total mass in stars of 7.8$\times10^{6}$ M$_{\odot}$ was formed, between 14 and 7 Gyr ago, with a peak at 13$-$14 Gyr ago. We use the detailed Star Formation History to determine age estimates for individual Red Giant Branch stars with high resolution spectroscopic abundances. Thus, for the first time, we can directly determine detailed timescales for the evolution of individual chemical elements. We find that the trends in alpha-elements match what is expected from an extended, relatively uninterrupted period of star formation continuing for 6$-$7 Gyr. The knee in the alpha-element distribution occurs at an age of 10.9$\pm$1Gyr, suggesting that SNe Ia enrichment began $\approx2\pm$1Gyr after the start of star formation in Sculptor., Comment: 23 pages, 23 figures

The ages of individual Red Giant Branch stars (RGB) can range from 1 Gyr old to the age of the Universe, and it is believed that the abundances of most chemical elements in their photospheres remain unchanged with time (those that are not affected by the 1st dredge-up). This means that they trace the ISM in the galaxy at the time the star formed, and hence the chemical enrichment history of the galaxy. CMD analysis has shown the Carina dwarf spheroidal (dSph) to have had an unusually episodic star formation history (SFH) which is expected to be reflected in the abundances of different chemical elements. We use the VLT-FLAMES spectrograph in HR mode (R~20000) to measure the abundances of several chemical elements in a sample of 35 RGB stars in Carina. We also combine these abundances with photometry to derive age estimates for these stars. This allows us to determine which of two distinct star formation (SF) episodes the stars in our sample belong to, and thus to define the relationship between SF and chemical enrichment during these two episodes. As is expected from the SFH, Carina contains two distinct populations of RGB stars: one old (>10 Gyr), which we have found to be metal-poor ([Fe/H]<-1.5), and alpha-rich ([Mg/Fe]>0.0); the other intermediate age (~2-6 Gyr), which we have found to have a metallicity range (-1.8<[Fe/H]<-1.2) with a large spread in [alpha/Fe] abundance, going from extremely low values ([Mg/Fe]<-0.3) to the same mean values as the older population (<[Mg/Fe]>~0.3). We show that the chemical enrichment history of the Carina dSph was different for each SF episode. The earliest was short (~2-3 Gyr) and resulted in the rapid chemical enrichment of the whole galaxy to [Fe/H] ~ -1.5 with both SNe II and SNe Ia contributions. The subsequent episode occured after a gap of ~3-4 Gyr and appears to have resulted in relatively little evolution in either [Fe/H] or [alpha/Fe]., Comment: Accepted in A&A

We present wide-field photometry of resolved stars in the nearby Sculptor dwarf spheroidal galaxy using CTIO/MOSAIC, going down to the oldest Main Sequence Turn-Off. The accurately flux calibrated wide field Colour-Magnitude Diagrams can be used to constrain the ages of different stellar populations, and also their spatial distribution. The Sculptor dSph contains a predominantly ancient stellar population (>10 Gyr old) which can be easily resolved into individual stars. A galaxy dominated by an old population provides a clear view of ancient processes of galaxy formation unimpeded by overlying younger populations. By using spectroscopic metallicities of RGB stars in combination with our deep Main Sequence Turn-Off photometry we can constrain the ages of different stellar populations with particular accuracy. We find that the known metallicity gradient in Sculptor is well matched to an age gradient. This is the first time that this link with age has been directly quantified. This gradient has been previously observed as a variation in Horizontal Branch properties and is now confirmed to exist for Main Sequence Turn-Offs as well. It is likely the Sculptor dSph first formed an extended metal-poor population at the oldest times, and subsequent more metal-rich, younger stars were formed more towards the centre until the gas was depleted or lost roughly 7 Gyr ago. The fact that these clear radial gradients have been preserved up to the present day is consistent with the apparent lack of signs of recent tidal interactions., Comment: 28 pages, 22 figures