The recent star formation history of NGC 628 on resolved scales

Star formation histories (SFHs) are integral to our understanding of galaxy evolution. We can study recent SFHs by comparing the star formation rate (SFR) calculated using different tracers, as each probes a different timescale. We aim to calibrate a proxy for the present-day rate of change in SFR, dSFR/dt, which does not require full spectral energy distribution (SED) modeling and depends on as few observables as possible, to guarantee its broad applicability. To achieve this, we create a set of models in CIGALE and define a SFR change diagnostic as the ratio of the SFR averaged over the past 5 and 200 Myr, ⟨SFR5⟩/⟨SFR200⟩⁠, probed by the Hα −FUV colour. We apply ⟨SFR5⟩/⟨SFR200⟩ to the nearby spiral NGC 628 and find that its star formation activity has overall been declining in the recent past, with the spiral arms, however, maintaining a higher level of activity. The impact of the spiral arm structure is observed to be stronger on ⟨SFR5⟩/⟨SFR200⟩ than on the star formation efficiency (SFEH2⁠). In addition, increasing disk pressure tends to increase recent star formation, and consequently ⟨SFR5⟩/⟨SFR200⟩⁠. We conclude that ⟨SFR5⟩/⟨SFR200⟩ is sensitive to the molecular gas content, spiral arm structure, and disk pressure. The ⟨SFR5⟩/⟨SFR200⟩ indicator is general and can be used to reconstruct the recent SFH of any star-forming galaxy for which Hα, FUV, and either mid- or far-IR photometry is available, without the need of detailed modeling

High-resolution X-ray spectroscopy of the stellar wind in Vela X-1 during a flare

Context. We present a ~130 ks observation of the prototypical wind-accreting, high-mass X-ray binary Vela X-1 collected with XMM-Newton at orbital phases between 0.12 and 0.28. A strong flare took place during the observation that allows us to investigate the reaction of the clumpy stellar wind to the increased X-ray irradiation. Aims. To examine the wind’s reaction to the flare, we performed both time-averaged and time-resolved analyses of the RGS spectrum and examined potential spectral changes. Methods. We focused on the high-resolution XMM-Newton RGS spectra and divided the observation into pre-flare, flare, and post-flare phases. We modeled the time-averaged and time-resolved spectra with phenomenological components and with the self-consistent photoionization models calculated via CLOUDY and XSTAR in the pre-flare phase, where strong emission lines due to resonant transitions of highly ionized ions are seen. Results. In the spectra, we find emission lines corresponding to K-shell transitions in highly charged ions of oxygen, neon, magnesium, and silicon as well as radiative recombination continua (RRC) of oxygen. Additionally, we observe potential absorption lines of magnesium at a lower ionization stage and features identified as iron L lines. The CLOUDY and XSTAR photoionization models provide contradictory results, either pointing towards uncertainties in theory or possibly a more complex multi-phase plasma, or both. Conclusions. We are able to demonstrate the existence of a plethora of variable narrow features, including the firm detection of oxygen lines and RRC that RGS enables to observe in this source for the first time. We show that Vela X-1 is an ideal source for future high-resolution missions, such as XRISM and Athena.V.G. is supported through the Margarete von Wrangell fellowship by the ESF and the Ministry of Science, Research and the Arts Baden-Württemberg. We acknowledge support from the ESTEC Faculty Visiting Scientist Programme to V.G. S.B. acknowledges financial support from the Italian Space Agency under grant ASI-INAF 2017-14-H.O. Work at LLNL was performed under the auspieces of the U.S. Department of Energy under contract No. DE-AC52-07NA27344. S.M.N. acknowledges funding by the Spanish Ministry MCIU under project RTI2018-096686-B-C21 (MCIU/AEI/FEDER, UE), co-funded by FEDER funds and by the Unidad de Excelencia María de Maeztu, ref. MDM-2017-0765

Abundances of iron-peak elements in disk and bulge giant stars from high-resolution optical spectra

Context. The formation and evolution of the Galactic bulge and the Milky Way is still a debated subject. Observations of, e.g., the X-shaped bulge, cylindrical stellar motions and young stars in the bulge have suggested that the bulge formed through secular evolution of the disk and not through gas dissipation and/or mergers, as thought previously.\\ Data. We use high-resolution optical spectra of 291 K giants in the local disk obtained by the FIES spectrograph on the Nordic Optical Telescope (S/N = 80-100 for most of the disk spectra) and 46 K giants in the bulge obtained by the UVES/FLAMES spectrograph on the ESO Very Large Telescope (S/N = 10-80). \\ Aims. The goal of this work is to examine stellar samples from the thin and thick disk as well as the bulge and measure abundances of six iron-peak elements (Sc, V, Cr, Mn, Co and Ni). These can provide additional observational constraints for Galaxy formation and chemical evolution models and help to understand whether the bulge has emerged from the (thick) disk or not. \\ Methods. We determine the abundances in the Solar neighbourhood and bulge by synthesising line profiles using the programme Spectroscopy Made Easy (SME) and 1-D, spherically-symmetric MARCS model atmospheres under the assumption of local thermodynamic equilibrium (LTE). We also separate the thin and thick disk stellar populations according to their [Ti/Fe]-ratios and radial velocities using a Gaussian Mixture Model (GMM). Moreover, we apply NLTE corrections for Co as well.\\ Results. The [(Sc,V,Co)/Fe] vs. [Fe/H] trends show a clear separation between the disk components, being more enhanced in the thick disk. The same is observed for Ni but to a lesser extent. The trends of Sc, V and Co are also more enhanced in the bulge compared to the thick disk, whereas the [Ni/Fe] ratio is similar in the thick disk and the bulge. Our [Mn/Fe] ratio steadily increases with increasing metallicity at about the same rate in all the three components. For Cr, we find a flat trend around [Cr/Fe] $\sim$0 for the whole metallicity range in the disk and the bulge. We also observe a higher average metallicity in the bulge compared to the disk.\\ Conclusions. The significantly enhanced [(Sc,V,Co)/Fe] ratios that we determine from our data of stars in the bulge, suggest that the local thick disk and the bulge have experienced different chemical enrichment and evolutionary paths. %This could depend on a faster chemical enrichment in the bulge than in the thick disk due to, e.g., a faster star formation rate. However, we are unable to predict the exact evolutionary path of the bulge solely based on these observations. Galactic chemical evolution models could, on the other hand, provide that using these results.Solen och alla stjärnor som vi ser på natthimlen tillhör vår Galax - Vintergatan. Vintergatan är en spiralgalax med en utbuktning i dess centrum - bulben. För att förstå hur Vintergatan blev till måste man ha en bra uppfattning om bulbens ursprung. Under många år trodde vi att vi visste hur bulben och hela Galaxen hade formats, men ny teknologi, förbättrade teleskop och instrument samt simuleringar av galaxutvecklingen har visat oss att det inte var fallet. Förut trodde man att bulben formades våldsamt och snabbare än galaxskivan genom att gas föll in och formade ett sfäriskt klot. I detta fall skulle bulben sakna unga stjärnor, och stjärnorna skulle röra sig med slumpmässiga banor runt Galaxcentrum. Nya observationer har dock visat att bulben snarare är formad som ett ``X’’, att det finns en del mycket unga stjärnor, och att stjärnor rör sig som om de befinner sig i en osynlig cylinder. Allt detta tyder snarare på att bulben förmodligen uppstod från galaxskivan vid en dynamisk instabilitet som fick den att bukta ut i mitten. Förutom stjärnornas åldrar, rörelser och formen på bulben, kan man även studera deras kemiska sammansättning för att få fler ledtrådar. Om bulben uppstod snabbt och oberoende av galaxskivan så kommer även stjärnornas sammansättning att skilja sig avsevärt i dessa delar av Vintergatan. En av svårigheterna med observationer av Vintergatans mitt är att den ligger långt bort från oss på ett avstånd som motsvarar ca 26 000 ljusår. Därför måste stjärnor som man observerar vara tillräckligt ljusstarka. Av denna anledning, är röda jättestjärnor (K-jättar) ett passande alternativ eftersom de är väldigt luminösa tack vare deras storlek. Dessutom lever de relativt länge och därför bevarar de informationen om den unga Vintergatan i sina grundämneshalter. I vårt arbete, mäter vi halter av sex grundämnen som tillhör den så kallade ``järntoppen’’ i periodiska systemet: skandium (Sc), vanadium (V), krom (Cr), mangan (Mn), kobolt (Co) och nickel (Ni). Dessa grundämnen bildas i exploderande massiva stjärnor och vita dvärgar. Vi använder oss av högupplösta observerade spektra av 291 K-jättar i solens närhet samt 46 K-jättar i bulben. För att få fram halter, måste man jämföra observerade spektra med syntetiskt framtagna spektra som skapas utifrån modelatmosfärer med hjälp av atomdata. Eftersom diskstjärnorna har visat sig tillhöra två olika grupper, tunn och tjock disk, separerar vi dem beroende på deras kinematik och kemisk sammansättning av titan för mer homogen analys. Genom att studera halttrender av ``järntopp’’-grundämnen i galaxskivan och bulben, ser vi att trender som tillhör Sc, V och Co samt även Ni, till en mindre grad, är mer förhöjda i tjocka disken jämfört med tunna. Halterna av Sc, V och Co är ännu högre i bulben än i tjocka disken, medan Ni ligger på ungefär samma nivå i dessa två Galaktiska komponenter. Disk- och bulbtrenderna för Mn överlappar varandra helt och har liknande stigande lutning. Cr-trenderna är platta både i bulben och disken. Skillnaderna i Sc, V och Co i bulben och tjocka disken leder oss till slutsatsen att utvecklingen i dessa områden kan ha varit (något) annorlunda. Förmodligen har stjärnbildningen varit snabbare i bulben. Nu kan, alltså, vår data användas för att säga något om bulbens utveckling. Genom att modellera våra halttrender kan s.k. galaktiska kemiska utvecklingsmodeller sätta starkare villkor på händelseförloppen som ledde till bildandet och utvecklingen av bulben

The recent star formation history of NGC 628 on resolved scales

Star formation histories (SFHs) are integral to our understanding of galaxy evolution. We can study recent SFHs by comparing the star formation rate (SFR) calculated using different tracers, as each probes a different time-scale. We aim to calibrate a proxy for the present-day rate of change in SFR, dSFR/dt, which does not require full spectral energy distribution (SED) modelling and depends on as few observables as possible, to guarantee its broad applicability. To achieve this, we create a set of models in CIGALE and define an SFR change diagnostic as the ratio of the SFR averaged over the past 5 and 200 Myr, (SFR5)/(SFR200), probed by the II alpha-FUV colour. We apply (SFR5)/(SF R-200) to the nearby spiral NGC 628 and find that its star formation activity has overall been declining in the recent past, with the spiral arms, however, maintaining a higher level of activity. The impact of the spiral arm structure is observed to be stronger on (SF R-5)/(SF R-200) than on the star formation efficiency. In addition, increasing disc pressure tends to increase recent star formation, and consequently (SF R-5)/(SF R-200). We conclude that (S F R-5)/ (S F R-200) is sensitive to the molecular gas content, spiral arm structure, and disc pressure. The (S F R-5)/ (S F R2(00)) indicator is general and can be used to reconstruct the recent SFH of any star-forming galaxy for which H-alpha, FUV, and either mid- or far-IR photometry is available, without the need of detailed modelling

Evidence against Anomalous Compositions for Giants in the Galactic Nuclear Star Cluster

Very strong Sc I lines have recently been found in cool M giants in the Nuclear Star Cluster (NSC) in the Galactic center. Interpreting these as anomalously high scandium abundances in the Galactic center would imply a unique enhancement signature and chemical evolution history for NSCs, and a potential test for models of chemical enrichment in these objects. We present high resolution K-band spectra (NIRSPEC/Keck II) of cool M giants situated in the solar neighborhood and compare them with spectra of M giants in the NSC. We clearly identify strong Sc I lines in our solar neighborhood sample as well as in the NSC sample. The strong Sc I lines in M giants are therefore not unique to stars in the NSC and we argue that the strong lines are a property of the line formation process that currently escapes accurate theoretical modeling. We further conclude that for giant stars with effective temperatures below approximately 3800 K these Sc I lines should not be used for deriving the scandium abundances in any astrophysical environment until we better understand how these lines are formed. We also discuss the lines of vanadium, titanium, and yttrium identified in the spectra, which demonstrate a similar striking increase in strength below 3500 K effective temperature