The Star Formation History of Leo I

1998

We present deep HST F555W (V) and F814W (I) observations of a central field in the Local Group dwarf spheroidal (dSph) galaxy Leo I. The resulting color-magnitude diagram (CMD) reaches I \simeq 26 and reveals the oldest ~10-15 Gyr old turnoffs. Nevertheless, a horizontal branch is not obvious in the CMD. Given the low metallicity of the galaxy, this likely indicates that the first substantial star formation in the galaxy may have been somehow delayed in Leo I in comparison with the other dSph satellites of the Milky Way. The subgiant region is well and uniformly populated from the oldest turnoffs up to the 1 Gyr old turnoff, indicating that star formation has proceeded in a continuous way, with possible variations in intensity but no big gaps between successive bursts, over the galaxy's lifetime. The structure of the red-clump of core He-burning stars is consistent with the large amount of intermediate-age population inferred from the main sequence and the subgiant region. In spite of the lack of gas in Leo I, the CMD clearly shows star formation continuing until 1 Gyr ago and possibly until a few hundred Myrs ago in the central part of the galaxy.

Monthly Notices of the Royal Astronomical Society, 2007

We have obtained calcium abundances and radial velocities for 102 red giant branch (RGB) stars in the Leo I dwarf spheroidal galaxy (dSph) and 74 RGB stars in the Leo II dSph using the Low Resolution Spectrograph (LRIS) on the Keck I 10-meter Telescope. We report on the calcium abundances [Ca/H] derived from the strengths of the Ca II triplet absorption lines at 8498, 8542, 8662Å in the stellar spectra using a new empirical Ca II triplet calibration to [Ca/H]. The two galaxies have different average [Ca/H] values of −1.34 ± 0.02 for Leo I and −1.65 ± 0.02 for Leo II with intrinsic abundance dispersions of 1.2 and 1.0 dex, respectively. The typical random and total errors in derived abundances are 0.10 and 0.17 dex per star. For comparison to existing literature, we also converted our Ca II measurements to [Fe/H] on the scale of Carretta and Gratton (1997) though we discuss why this may not be the best determinant of metallicity; Leo I has a mean [Fe/H] = −1.34 and Leo II has a mean [Fe/H] = −1.59. The metallicity distribution function of Leo I is approximately Gaussian in shape with an excess at the metal-rich end, while that of Leo II shows an abrupt cutoff at the metalrich end. The lower mean metallicity of Leo II is consistent with the fact that it has a lower luminosity, hence lower total mass, than Leo I; thus the evolution of Leo II may have been affected more by mass lost in galactic winds. Our direct and independent measurement of the metallicity distributions in these dSph will allow a more accurate star-formation histories to be derived from future analysis of their CMDs.

Proceedings of the International Astronomical Union, 2005

We present VLT-FORS1 color-magnitude diagrams (CMDs) reaching the oldest main-sequence turnoffs for three fields at two galactocentric distances in the Fornax dSph galaxy. We derive the star formation history (SFH) for each field through comparison with synthetic CMDs and show that the main epoch of star formation has extended to more recent epochs toward the center of the galaxy. A small burst of star formation about 2 Gyr ago is a robust feature in the SFH of all fields. It might be related to the shell structure found in Fornax by Coleman et al. (2004), which in turn has been interpreted as the relic of a merger event that occurred in Fornax about 2 Gyr ago.

The Astrophysical Journal, 1999

We present deep HST F555W (V ) and F814W (I) observations of a central Ðeld in the Local Group dwarf spheroidal (dSph) galaxy Leo I. The resulting color-magnitude diagram (CMD) reaches I^26 and reveals the oldest^10È15 Gyr old turno †s. Nevertheless, a horizontal branch is not obvious in the CMD. Given the low metallicity of the galaxy, this likely indicates that the Ðrst substantial star formation in the galaxy may have been somehow delayed in Leo I in comparison with the other dSph satellites of the Milky Way. The subgiant region is well and uniformly populated from the oldest turno †s up to the 1 Gyr old turno †, indicating that star formation has proceeded in a continuous way, with possible variations in intensity but no big gaps between successive bursts, over the galaxyÏs lifetime. The structure of the red clump of core He-burning stars is consistent with the large amount of intermediate-age population inferred from the main sequence and the subgiant region. In spite of the lack of gas in Leo I, the CMD clearly shows star formation continuing until 1 Gyr ago and possibly until a few hundred Myr ago in the central part of the galaxy.

Monthly Notices of the Royal Astronomical Society, 2008

We present a study of the evolved stellar populations in the dwarf spheroidal galaxy Leo II, based on JHK s observations obtained with the near-infrared array WFCAM at the UKIRT telescope. Combining the new data with optical data, we derived photometric estimates of the distribution of global metallicity [M/H] of individual red giant stars from their V −K s colours. Our results are consistent with the metallicities of RGB stars obtained from Ca ii triplet spectroscopy, once the age effects are considered. The photometric metallicity distribution function has a peak at [M/H] = −1.74 (uncorrected) or [M/H] = −1.64 ± 0.06 (random) ±0.17 (systematic) after correction for the mean age of Leo II stars (9 Gyr). The distribution is similar to a Gaussian with σ [M/H] = 0.19 dex, corrected for instrumental errors. We used the new data to derive the properties of a nearly complete sample of asymptotic giant branch (AGB) stars in Leo II. Using a near-infrared two-colour diagram, we were able to obtain a clean separation from Milky Way foreground stars and discriminate between carbonand oxygen-rich AGB stars, which allowed to study their distribution in K s -band luminosity and colour. We simulate the JHK s data with the trilegal population synthesis code together with the most updated thermally pulsing AGB models, and using the star formation histories derived from independent work based on deep HST photometry. After scaling the mass of Leo II models to the observed number of upper RGB stars, we find that present models predict too many O-rich TP-AGB stars of higher luminosity due to a likely under-estimation of either their mass-loss rates at low metallicity, and/or their degree of obscuration by circumstellar dust. On the other hand, the TP-AGB models are able to reproduce the observed number and luminosities of carbon stars satisfactorily well, indicating that in this galaxy the least massive stars that became carbon stars should have masses as low as ∼ 1 M ⊙ .

Proceedings of the International Astronomical Union, 2018

We have studied young stellar populations and star clusters in the dwarf irregular galaxy Leo A using multicolor (B, V, R, I, Hα) photometry data obtained with the Subaru Suprime-Cam and two-color photometry results measured on archival HST/ACS F475W & F814W frames. The analysis of the main sequence (MS) and blue supergiant (BSG – “blue loop”) stars enabled us to study the star formation history in the Leo A galaxy during the last ~200 Myr. Also, we have discovered 5 low-mass (≲ 400 M⊙) star clusters within the ACS field. This finding, taking into account a low metallicity environment and a yet-undetected molecular gas in Leo A, constrains star formation efficiency estimates and scenarios. Inside the well known “hole” in the H i column density map (Hunter et al. 2012) we found a shock front (prominent in Hα), implying an unseen progenitor and reminding the “hole” problems widely discussed by Warren et al. (2011).

2008

We have taken deep images of an outlying field in the Local Group dwarf irregular galaxy IC 1613 with the WFPC2 aboard the Hubble Space Telescope in the standard broad-band F555W (V, 8 orbits) and F814W (I, 16 orbits) filters. The photometry reaches to V = 27.7 (M V = +3.4) and I = 27.1 (M I = +2.8) at the 50% completeness level, the deepest to date for an isolated dwarf irregular galaxy. We analyze the resulting colormagnitude diagram (CMD) and compare it with CMDs created from theoretical stellar models using three different methods to derive a star formation history (SFH) as well as constrain the chemical evolution for IC 1613. All three methods find an enhanced star formation rate (SFR), at roughly the same magnitude (factor of 3), over roughly the same period (from 3 to 6 Gyr ago). Additionally, all three methods were driven to similar age-metallicity relationships (AMR) which show an increase from [Fe/H] ≈ −1.3 at earliest times to [Fe/H] ≈ −0.7 at present. Good agreement is found between the AMR which is derived from the CMD analysis and that which can be inferred from the derived SFH at all but the earliest ages. The agreement between the three models and the selfconsistency of the derived chemical enrichment history support the reality of the derived SFH of IC 1613 and, more generally, are supportive of the practice of constructing galaxy SFHs from CMDs. A comparison of the newly observed outer field with an earlier studied central field of IC 1613 shows that the SFR in the outer field has been significantly depressed during the last Gyr. This implies that the optical scale length of the galaxy has been decreasing with time and that comparison of galaxies at intermediate redshift with present day galaxies should take this effect into account. Comparing the CMD of the outer field of IC 1613 with CMDs of Milky Way dSph companions, we find strong similarities between IC 1613 and the more distant dSph companions (Carina, Fornax, Leo I, and Leo II) in that all are dominated by star formation at intermediate ages. In particular, the SFH and AMR for IC 1613 and Leo I are indistinguishable. This implies that dIrr galaxies cannot be distinguished from dSphs by their intermediate age stellar populations. This type of a SFH may also be evidence for slower or suppressed early star formation in dwarf galaxies due to photoionization after the reionization of the universe by background radiation. Assuming that IC 1613 is typical of a dIrr evolving in isolation, since most of the star formation occurs at intermediate ages, these dwarf systems cannot be responsible for the fast chemical enrichment of the IGM which is seen at high redshift. There is no evidence for any large amplitude bursts of star formation in IC 1613, and we find it highly unlikely that analogs of IC 1613 have contributed to the excess of faint blue galaxies in existing galaxy redshift surveys.

2009

Retrieving the Star Formation History (SFH) of a galaxy out of its integrated spectrum is the central goal of stellar population synthesis. Recent advances in evolutionary synthesis models have given new breath to this old field of research. Modern spectral synthesis techniques incorporating these advances now allow the fitting of galaxy spectra on an Å-by-Å. These detailed fits are useful for a number of studies, like emission line, stellar kinematics, and specially galaxy evolution. Applications of this semi-empirical approach to mega data sets are teaching us a lot about the lives of galaxies. The STARLIGHT spectral synthesis code is one of the tools which allows one to harness this favorable combination of plentifulness of data and models. To illustrate this, we show how SFHs vary across classical emission line diagnostic diagrams. Systematic trends are present along both the star-forming and active-galaxy sequences. We also briefly describe experiments with new versions of evol...

The Astrophysical Journal, 2009

The central kpc of the Milky Way might be expected to differ significantly from the rest of the Galaxy with regard to gas dynamics and the formation of young stellar objects (YSOs). We probe this possibility with mid-infrared observations obtained with IRAC and MIPS on Spitzer and with MSX. We use color-color diagrams and spectral energy distribution (SED) fits to explore the nature of YSO candidates (including objects with 4.5µm excesses possibly due to molecular emission). There is an asymmetry in the distribution of the candidate YSOs, which tend to be found at negative Galactic longitudes; this behavior contrasts with that of the molecular gas, approximately 2/3 of which is at positive longitudes. The small scale height of these objects suggests that they are within the Galactic center region and are dynamically young. They lie between two layers of infrared dark clouds (IRDCs) and may have originated from these clouds. We identify new sites for this recent star formation by comparing the mid-IR, radio, submillimeter, and methanol maser data. The methanol masers appear to be associated with young, embedded YSOs characterized by 4.5µm excesses. We use the SEDs of these sources to estimate their physical characteristics; their masses appear to range from ∼ 10 to ∼ 20 M. Within the central 400 × 50 pc (|l| < 1.3 • and |b| < 10) the star formation rate based on the identification of Stage I evolutionary phase of YSO candidates is about 0.14 M yr −1. Given that the majority of the sources in the population of YSOs are classified as Stage I objects, we suggest that a recent burst of star formation took place within the last 10 5 years. This suggestion is also consistent with estimates of star formation rates within the last ∼ 10 7 years showing a peak around 10 5 years ago. Lastly, we find that the Schmidt-Kennicutt Law applies well in the central 400 pc of the Galaxy. This implies that star formation does not appear to be dramatically affected by the extreme physical conditions in the Galactic center region.

A sample of 69 galaxies with radial velocities of less than 2500 km s À1 was selected from the H i Parkes All-Sky Survey (HIPASS) and imaged in broadband B and R and narrowband H, to deduce details about star formation in nearby disk galaxies while avoiding surface brightness selection effects. The sample is dominated by late-type, dwarf disks (mostly Sc and Sm galaxies) with exponential disk scale lengths of $1–5 kpc. The HIPASS galaxies, on average, have lower star formation rates (SFRs), are bluer, and have lower surface brightness than an optically selected sample. H ii regions were detected in all but one of the galaxies. Many galaxies had as few as two to five H ii regions. The galaxies' H equivalent widths, colors, and SFRs per unit of H i mass are best explained by young mean ages ($3–5 Gyr, according to Schmidt-law models) with star formation histories in which the SFRs were higher in the past. Comparison of the surface brightness coverage of the HIPASS galaxies with that of an optically selected sample shows that such a sample may miss $10% of the local galaxy number density and could possibly miss as much as 3%–4% of the SFR density. The amount lower surface brightness galaxies contribute to the total luminosity density may be insignificant, but this conclusion is somewhat dependent on how the fluxes of these objects are determined.