Constant Star Formation Research Articles

Long-period variable stars in NGC 147 and NGC 185 – I. Their star formation histories

NGC 147 and NGC 185 are two of the most massive satellites of the Andromeda galaxy (M 31). Close together in the sky, of similar mass and morphological type dE, they possess different amounts of interstellar gas and tidal distortion. The question therefore is, how do their histories compare? Here we present the first reconstruction of the star formation histories of NGC 147 and NGC 185 using long-period variable stars. These represent the final phase of evolution of low- and intermediate-mass stars at the asymptotic giant branch, when their luminosity is related to their birth mass. Combining near-infrared photometry with stellar evolution models, we construct the mass function and hence the star formation history. For NGC 185 we found that the main epoch of star formation occurred 8.3 Gyr ago, followed by a much lower, but relatively constant star formation rate. In the case of NGC 147, the star formation rate peaked only 7 Gyr ago, staying intense until ~ 3 Gyr ago, but no star formation has occurred for at least 300 Myr. Despite their similar masses, NGC 147 has evolved more slowly than NGC 185 initially, but more dramatically in more recent times. This is corroborated by the strong tidal distortions of NGC 147 and the presence of gas in the centre of NGC 185.

Cosmological simulations of dwarf galaxies with cosmic ray feedback

We perform zoom-in cosmological simulations of a suite of dwarf galaxies, examining the impact of cosmic rays (CRs) generated by supernovae, including the effect of diffusion. We first look at the effect of varying the uncertain CR parameters by repeatedly simulating a single galaxy. Then we fix the comic ray model and simulate five dwarf systems with virial masses range from 8 to 30 × 1010 M⊙. We find that including CR feedback (with diffusion) consistently leads to disc-dominated systems with relatively flat rotation curves and constant star formation rates. In contrast, our purely thermal feedback case results in a hot stellar system and bursty star formation. The CR simulations very well match the observed baryonic Tully–Fisher relation, but have a lower gas fraction than in real systems. We also find that the dark matter cores of the CR feedback galaxies are cuspy, while the purely thermal feedback case results in a substantial core.

Modelling nova populations in galaxies

Theoretical modelling of the evolution of classical and recurrent novae plays an important role in studies of binary evolution, nucleosynthesis and accretion physics. However, from a theoretical perspective the observed statistical properties of novae remain poorly understood. In this paper, we have produced model populations of novae using a hybrid binary population synthesis approach for differing star formation histories (SFHs): a starburst case (elliptical-like galaxies), a constant star formation rate case (spiral-like galaxies) and a composite case (in line with the inferred SFH for M31). We found that the nova rate at 10\;Gyr in an elliptical-like galaxy is $\sim 10-20$ times smaller than a spiral-like galaxy with the same mass. The majority of novae in elliptical-like galaxies at the present epoch are characterized by low mass white dwarfs (WDs), long decay times, relatively faint absolute magnitudes and long recurrence periods. In contrast, the majority of novae in spiral-like galaxies at 10\;Gyr have massive WDs, short decay times, are relatively bright and have short recurrence periods. The mass loss time distribution for novae in our M31-like galaxy is in agreement with observational data for Andromeda. However, it is possible that we underestimate the number of bright novae in our model. This may arise in part due to the present uncertainties in the appropriate bolometric correction for novae.

LEO P: AN UNQUENCHED VERY LOW-MASS GALAXY*

ABSTRACT Leo P is a low-luminosity dwarf galaxy discovered through the blind H i Arecibo Legacy Fast ALFA survey. The H i and follow-up optical observations have shown that Leo P is a gas-rich dwarf galaxy with active star formation, an underlying older population, and an extremely low oxygen abundance. We have obtained optical imaging with the Hubble Space Telescope to two magnitudes below the red clump in order to study the evolution of Leo P. We refine the distance measurement to Leo P to be 1.62 ± 0.15 Mpc, based on the luminosity of the horizontal branch stars and 10 newly identified RR Lyrae candidates. This places the galaxy at the edge of the Local Group, ∼0.4 Mpc from Sextans B, the nearest galaxy in the NGC 3109 association of dwarf galaxies of which Leo P is clearly a member. The star responsible for ionizing the H ii region is most likely an O7V or O8V spectral type, with a stellar mass ≳25 M ⊙. The presence of this star provides observational evidence that massive stars at the upper end of the initial mass function are capable of being formed at star formation rates as low as ∼10−5 M ⊙ yr−1. The best-fitting star formation history (SFH) derived from the resolved stellar populations of Leo P using the latest PARSEC models shows a relatively constant star formation rate over the lifetime of the galaxy. The modeled luminosity characteristics of Leo P at early times are consistent with low-luminosity dSph Milky Way satellites, suggesting that Leo P is what a low-mass dSph would look like if it evolved in isolation and retained its gas. Despite the very low mass of Leo P, the imprint of reionization on its SFH is subtle at best, and consistent with being totally negligible. The isolation of Leo P, and the total quenching of star formation of Milky Way satellites of similar mass, implies that the local environment dominates the quenching of the Milky Way satellites.

STELLAR POPULATIONS AND THE STAR FORMATION HISTORIES OF LSB GALAXIES. V. WFC3 COLOR–MAGNITUDE DIAGRAMS

We present WFC3 observations of three low surface brightness (LSB) galaxies from the Schombert et. al LSB catalog that are within 11 Mpc of the Milky Way. Deep imaging at F336W, F555W and F814W allow the construction of the V-I color-magnitude diagrams (CMD) to M_I = -2. Overall 1869, 465 and 501 stellar sources are identified in the three LSB galaxies F415-3, F608-1 and F750-V1. The spatial distribution of young blue stars matches the H-alpha maps from ground-based imaging, indicating that star formation in LSB galaxies follows the same style as in other irregular galaxies. Several star complexes are identified, matching regions of higher surface brightness as seen from ground-based imaging. The color-magnitude diagrams for each LSB galaxy has the similar morphology to Local Volume (LV) dwarf galaxies, i.e. a blue main sequence, blue and red He burning branches and asymptotic giant branch (AGB) stars. The LSB CMD's distinguish themselves from nearby dwarf CMD's by having a higher proportional of blue main sequence stars and fewer AGB stars than expected from their mean metallicities. Current [Fe/H] values below -0.6 are deduced from the position of the red helium-burning branch (rHeB) stars in the V-I diagram. The distribution of stars on the blue helium-burning branch (bHeB) and rHeB from the U-V and V-I CMD indicate a history of constant star formation for the last 100 Myrs.

Trident: A three-pronged galaxy survey

Context. Lyman alpha emitting galaxies (LAEs) are used to probe the distant universe and are therefore important for galaxy evolution studies and for providing clues to the nature of the epoch of reionization, but the exact circumstances under which Lyman alpha escapes a galaxy are still not fully understood. Aims. The Trident project is designed to simultaneously examine Lyman alpha, H-alpha and Lyman Continuum emission from galaxies at redshift z~2, thus linking together these three aspects of ionising radiation in galaxies. In this paper, we outline the strategy of this project and examine the properties of LAEs in the GOODS North field. Methods. We performed a narrowband LAE survey in GOODS North using existing and two custom made filters at the Nordic Optical Telescope with MOSCA. We use complementary broad band archival data in the field to make a careful candidate selection and perform optical to near-IR SED fitting. We also estimate far-infrared luminosities by matching our candidates to detections in Spitzer/MIPS 24{\mu}m and Herschel/PACS catalogs. Results. We find a total of 25 LAE candidates, probing mainly the bright end of the LAE luminosity function with L_Ly {\alpha} ~ 1-15e42 erg/s. They display a range of masses of ~0.5-50e9 M_solar, and average ages from a few tens of Myr to 1 Gyr when assuming a constant star formation history. The majority of our candidates also show signs of recent elevated star formation. Three candidates have counterparts in the GOODS-Herschel far-IR catalogue, with luminosities consistent with ultra-luminous infrared galaxies (ULIRGs). Conclusions. The wide range of parameters derived from our SED fitting, as well as part of our sample being detected as ULIRGs, seems to indicate that at these Lyman alpha luminosities, LAEs do not necessarily have to be young dwarfs, and that a lack of dust is not required for Lyman alpha to escape.

Core-collapse supernova rate synthesis within 11 Mpc

The 11 Mpc H-alpha and Ultraviolet Galaxy (11HUGS) Survey traces the star formation activity of nearby galaxies. In addition within this volume the detection completeness of core-collapse supernovae (CCSNe) is high therefore by comparing these observed stellar births and deaths we can make a sensitive test of our understanding of how stars live and die. In this paper, we use the results of the Binary Population and Spectral Synthesis (BPASS) code to simulate the 11HUGS galaxies H-alpha and far-ultraviolet (FUV) star formation rate indicators (SFRIs) and simultaneously match the core-collapse supernova (CCSN) rate. We find that stellar population including interacting binary stars makes little difference to the total CCSN rate but increases the H-alpha and FUV fluxes for a constant number of stars being formed. In addition they significantly increase the predicted rate of type Ibc supernovae (SNe) relative to type II SNe to the level observed in the 11HUGS galaxies. We also find that instead of assuming a constant star formation history (SFH) for the galaxies our best fitting models have a star formation rate (SFR) that peaked more than 3 Myrs ago.

Reconstructing the star formation history of the Milky Way disc(s) from chemical abundances

We develop a chemical evolution model in order to study the star formation history of the Milky Way. Our model assumes that the Milky Way is formed from a closed box-like system in the inner regions, while the outer parts of the disc experience some accretion. Unlike the usual procedure, we do not fix the star formation prescription (e.g. Kennicutt law) in order to reproduce the chemical abundance trends. Instead, we fit the abundance trends with age in order to recover the star formation history of the Galaxy. Our method enables one to recover with unprecedented accuracy the star formation history of the Milky Way in the first Gyrs, in both the inner (R<7-8kpc) and outer (R>9-10kpc) discs as sampled in the solar vicinity. We show that, in the inner disc, half of the stellar mass formed during the thick disc phase, in the first 4-5 Gyr. This phase was followed by a significant dip in the star formation activity (at 8-9 Gyr) and a period of roughly constant lower level star formation for the remaining 8 Gyr. The thick disc phase has produced as many metals in 4 Gyr as the thin disc in the remaining 8 Gyr. Our results suggest that a closed box model is able to fit all the available constraints in the inner disc. A closed box system is qualitatively equivalent to a regime where the accretion rate, at high redshift, maintains a high gas fraction in the inner disc. In such conditions, the SFR is mainly governed by the high turbulence of the ISM. By z~1 it is possible that most of the accretion takes place in the outer disc, while the star formation activity in the inner disc is mostly sustained by the gas not consumed during the thick disc phase, and the continuous ejecta from earlier generations of stars. The outer disc follows a star formation history very similar to that of the inner disc, although initiated at z~2, about 2 Gyr before the onset of the thin disc formation in the inner disc.

Star formation and stellar mass assembly in dark matter haloes: from giants to dwarfs

The empirical model of Lu et al. 2014 is updated with recent data and used to study galaxy star formation and assembly histories. At $z > 2$, the predicted galaxy stellar mass functions are steep, and a significant amount of star formation is hosted by low-mass haloes that may be missed in current observations. Most of the stars in cluster centrals formed earlier than $z\approx 2$ but have been assembled much later. Milky Way mass galaxies have had on-going star formation without significant mergers since $z\approx 2$, and are thus free of significant (classic) bulges produced by major mergers. In massive clusters, stars bound in galaxies and scattered in the halo form a homogeneous population that is old and with solar metallicity. In contrast, in Milky Way mass systems the two components form two distinct populations, with halo stars being older and poorer in metals by a factor of $\approx 3$. Dwarf galaxies in haloes with $M_{\rm h} < 10^{11}h^{-1}M_{\odot}$ have experienced a star formation burst accompanied by major mergers at $z > 2$, followed by a nearly constant star formation rate after $z = 1$. The early burst leaves a significant old stellar population that is distributed in spheroids.

The initial mass function and star formation law in the outer disc of NGC 2915

Using Hubble Space Telescope (HST) Advanced Camera for Surveys/Wide Field Camera data we present the photometry and spatial distribution of resolved stellar populations in the outskirts of NGC 2915, a blue compact dwarf with an extended H i disc. These observations reveal an elliptical distribution of red giant branch stars, and a clumpy distribution of main-sequence stars that correlate with the H i gas distribution. We constrain the upper-end initial mass function (IMF) and determine the star formation law (SFL) in this field, using the observed main-sequence stars and an assumed constant star formation rate. Previously published Hα observations of the field, which show one faint H ii region, are used to provide further constraints on the IMF. We find that the main-sequence luminosity function analysis alone results in a best-fitting IMF with a power-law slope α = −2.85 and upper-mass limit |$\rm M_u = 60\,\mathrm{M}_{\odot }$|⁠. However, if we assume that all Hα emission is confined to H ii regions then the upper-mass limit is restricted to |$\rm M_u \lesssim 20\, \mathrm{M}_{\odot }$|⁠. For the luminosity function fit to be correct, we have to discount the Hα observations implying significant diffuse ionized gas or escaping ionizing photons. Combining the HST photometry with H i imaging, we find the SFL has a power-law index N = 1.53 ± 0.21. Applying these results to the entire outer H i disc indicates that it contributes 11–28 per cent of the total recent star formation in NGC 2915, depending on whether the IMF is constant within the disc or varies from the centre to the outer region.

ALMA OBSERVATIONS OF THE HOST GALAXY OF GRB 090423 ATz= 8.23: DEEP LIMITS ON OBSCURED STAR FORMATION 630 MILLION YEARS AFTER THE BIG BANG

We present rest-frame far-infrared (FIR) and optical observations of the host galaxy of GRB090423 at z=8.23 from the Atacama Large Millimeter Array (ALMA) and the Spitzer Space Telescope, respectively. The host remains undetected to 3-sigma limits of Fnu(222 GHz)<33 microJy and Fnu(3.6 micron)<81 nJy. The FIR limit is about 20 times fainter than the luminosity of the local ULIRG Arp220, and comparable to the local starburst M82. Comparing to model spectral energy distributions we place a limit on the IR luminosity of L_IR(8-1000 micron)<3e10 Lsun, corresponding to a limit on the obscured star formation rate of SFR_IR<5 Msun/yr; for comparison, the limit on the unobscured star formation rate from Hubble Space Telescope rest-frame UV observations is SFR_UV<1 Msun/yr. We also place a limit on the host galaxy stellar mass of <5e7 Msun (for a stellar population age of 100 Myr and constant star formation rate). Finally, we compare our millimeter observations to those of field galaxies at z>4 (Lyman break galaxies, Ly-alpha emitters, and submillimeter galaxies), and find that our limit on the FIR luminosity is the most constraining to date, although the field galaxies have much larger rest-frame UV/optical luminosities than the host of GRB090423 by virtue of their selection techniques. We conclude that GRB host galaxies at z>4, especially those with measured interstellar medium metallicities from afterglow spectroscopy, are an attractive sample for future ALMA studies of high redshift obscured star formation.

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.

The critical radiation intensity for direct collapse black hole formation: dependence on the radiation spectral shape

It has been proposed that supermassive black holes (SMBHs) are originated from direct-collapse black holes (DCBHs) that are formed at z ≳ 10 in the primordial gas in the case where H2 cooling is suppressed by strong external radiation. In this work, we study the critical specific intensity Jcrit required for DCBH formation for various radiation spectral shapes by a series of one-zone calculations of a collapsing primordial-gas cloud. We calculate the critical specific intensity at the Lyman–Werner (LW) bands |$J_{\rm LW,21}^{\rm crit}$| (in units of 10−21 erg s−1 Hz−1 sr−1 cm−2) for realistic spectra of metal-poor galaxies. We find that Jcrit is not sensitive to the age or metallicity for the constant star formation galaxies with |$J_{\rm LW,21}^{\rm crit}=1300-1400$|⁠, while Jcrit decreases as galaxies become older or more metal-enriched for the instantaneous starburst galaxies. However, for the young (the age < 100 Myr) and/or extremely metal poor (Z < 5 × 10−4 Z⊙) instantaneous starburst galaxies, such dependence is not strong and |$J_{\rm LW,21}^{\rm crit}= 1000-1400$|⁠. We also find that Jcrit is solely determined by the ratio between the H− and H2 photodissociation rate coefficients, |$k_{\rm H^-,pd}/k_{\rm H_2,pd}$|⁠, with which we develop a formula to estimate Jcrit for a given spectrum. The higher value of Jcrit for the realistic spectra than those expected in the literature significantly reduces the estimated DCBH number density nDCBH. By extrapolating the result of Dijkstra, Ferrara & Mesinger, we obtain nDCBH ∼ 10−10 cMpc−3 at z = 10, which is roughly consistent with the observed number density of high-redshift SMBHs nSMBH ∼ 10−9 cMpc−3 at z ∼ 6, considering large uncertainties in the estimation.

ON THE ONSET OF SECONDARY STELLAR GENERATIONS IN GIANT STAR-FORMING REGIONS AND MASSIVE STAR CLUSTERS

Here we consider the strong evolution experienced by the matter reinserted by massive stars, both in giant star forming regions driven by a constant star formation rate, and in massive and coeval superstar clusters. In both cases we take into consideration the changes induced by stellar evolution on the number of massive stars, the number of ionizing photons and the integrated mechanical luminosity of the star forming regions. The latter is at all times compared with the critical luminosity that defines, for a given size, the lower mechanical luminosity limit above which the matter reinserted via strong winds and supernova explosions suffers frequent and recurrent thermal instabilities that reduce its temperature and pressure and inhibit its exit as part of a global wind. Instead, the unstable reinserted matter is compressed by the pervasive hot gas, and photoionization maintains its temperature at T $\sim$ 10$^4$ K. As the evolution proceeds, more unstable matter accumulates and the unstable clumps grow in size. Here we evaluate the possible self-shielding of thermally unstable clumps against the UV radiation field. Self shielding allows for a further compression of the reinserted matter which rapidly develops a high density neutral core able to absorb in its outer skin the incoming UV radiation. Under such conditions the cold (T $\sim$ 10 K) neutral cores soon surpass the Jeans limit and become gravitationally unstable, causing a new stellar generation with the matter reinserted by former massive stars. We present the results of several calculations of this positive star formation feedback scenario promoted by strong radiative cooling and mass loading.

Local‐density driven clustered star formation: Model and (some) implications

AbstractA positive power‐law trend between the local surface densities of molecular gas, Σgas, and young stellar objects, Σstars, in molecular clouds of the solar neighbourhood has recently been identified. How it relates to the properties of embedded clusters has so far not been investigated. To that purpose, we model the development of the stellar component of molecular clumps as a function of time and local volume density. Specifically, we associate the observed volume density gradient of molecular clumps to the density‐dependent free‐fall time and we obtain the molecular clump star formation history by applying a constant star formation efficiency per free‐fall time, ϵff. The model reproduces naturally the observed (Σgas, Σstars) relation quoted above. The consequences of our model in terms of cluster survivability after residual star‐forming gas expulsion and in terms of star age distribution in young gas‐free clusters are discussed. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

THE INCIDENCE OF STELLAR MERGERS AND MASS GAINERS AMONG MASSIVE STARS

Because the majority of massive stars are born as members of close binary systems, populations of massive main-sequence stars contain stellar mergers and products of binary mass transfer. We simulate populations of massive stars accounting for all major binary evolution effects based on the most recent binary parameter statistics and extensively evaluate the effect of model uncertainties. Assuming constant star formation, we find that of a sample of early-type stars are the products of a merger resulting from a close binary system. In total we find that of massive main-sequence stars are the products of binary interaction. We show that the commonly adopted approach to minimize the effects of binaries on an observed sample by excluding systems detected as binaries through radial velocity campaigns can be counterproductive. Systems with significant radial velocity variations are mostly pre-interaction systems. Excluding them substantially enhances the relative incidence of mergers and binary products in the non-radial velocity variable sample. This poses a challenge for testing single stellar evolutionary models. It also raises the question of whether certain peculiar classes of stars, such as magnetic O stars, are the result of binary interaction and it emphasizes the need to further study the effect of binarity on the diagnostics that are used to derive the fundamental properties (star-formation history, initial mass function, mass-to-light ratio) of stellar populations nearby and at high redshift.

Stellar Populations and the Star Formation Histories of LSB Galaxies: III. Stellar Population Models

AbstractA series of population models are designed to explore the star formation history of gas-rich, low surface brightness (LSB) galaxies. LSB galaxies are unique in having properties of very blue colors, low Hα emission and high gas fractions that indicated a history of constant star formation (versus the declining star formation models used for most spirals and irregulars). The model simulations use an evolving multi-metallicity composite population that follows a chemical enrichment scheme based on Milky Way observations. Color and time sensitive stellar evolution components (i.e., BHB, TP-AGB and blue straggler stars) are included, and model colors are extended into theSpitzerwavelength regions for comparison to new observations. In general, LSB galaxies are well matched to the constant star formation scenario with the variation in color explained by a fourfold increase/decrease in star formation over the last 0.5 Gyrs (i.e., weak bursts). Early-type spirals, from the S4G sample, are better fit by a declining star formation model where star formation has decreased by 40% in the last 12 Gyrs.

Stellar Populations and the Star Formation Histories of LSB Galaxies: IV Spitzer Surface Photometry of LSB Galaxies

AbstractSurface photometry at 3.6 μm is presented for 61 low surface brightness (LSB) galaxies (μo&lt;19 3.6 μm mag arcsecs−2). The sample covers a range of luminosity from −11 to −22 in M3.6 and size from 1 to 25 kpc. The morphologies in the mid-IR are comparable to those in the optical with 3.6 μm imaging reaches similar surface brightness depth as ground-based optical imaging. A majority of the resulting surface brightness profiles are single exponential in shape with very few displaying upward or downward breaks. The mean V − 3.6 colour of LSB is 2.3 with a standard deviation of 0.5. Colour-magnitude and two-colour diagrams are well matched to models of constant star formation, where the spread in colour is due to small changes in the star formation rate (SFR) over the last 0.5 Gyrs as also suggested by the specific SFR measured by Hα.

Abundance gradients in spiral discs: is the gradient inversion at high redshift real?

We compute the abundance gradients along the disc of the Milky Way by means of the two-infall model: in particular, the gradients of oxygen and iron and their temporal evolution. First, we explore the effects of several physical processes which influence the formation and evolution of abundance gradients. They are (i) the inside-out formation of the disc, (ii) a threshold in the gas density for star formation, (iii) a variable star formation efficiency along the disc, (iv) radial flows and their speed and (v) different total surface mass density (gas plus stars) distributions for the halo. We are able to reproduce at best the present day gradients of oxygen and iron if we assume an inside-out formation, no threshold gas density, a constant efficiency of star formation along the disc and radial gas flows. It is particularly important the choice of the velocity pattern for radial flows and the combination of this velocity pattern with the surface mass density distribution in the halo. Having selected the best model, we then explore the evolution of abundance gradients in time and find that the gradients in general steepen in time and that at redshift z ∼ 3 there is a gradient inversion in the inner regions of the disc, in the sense that at early epochs the oxygen abundance decreases towards the Galactic Centre. This effect, which has been observed, is naturally produced by our models if an inside-out formation of the disc and a constant star formation efficiency are assumed. The inversion is due to the fact that in the inside-out formation a strong infall of primordial gas, contrasting chemical enrichment, is present in the innermost disc regions at early times. The gradient inversion remains also in the presence of radial flows, either with constant or variable speed in time, and this is a new result.

The chemical case for no winds in dwarf irregular galaxies

We argue that isolated gas-rich dwarf galaxies -- in particular, dwarf irregular (dIrr) galaxies -- do not necessarily undergo significant gas loss. Our aim is to investigate whether the observed properties of isolated, gas-rich dwarf galaxies, not affected by external environmental processes, can be reproduced by self-consistent chemo-photometric infall models with continuous star formation histories and no mass or metals loss. The model is characterized by the total mass of primordial gas available to the object, its characteristic collapse timescale, and a constant star formation efficiency. A grid of 144 such models has been computed by varying these parameters, and their predictions (elemental abundances, stellar and gas masses, photometric colors) have been compared with a set of observations of dIrr galaxies obtained from the literature. It is found that the models with moderate to low efficiency are able to reproduce most of the observational data, including the relative abundances of nitrogen and oxygen.