Red Giant Branch Population Research Articles

The Metal‐poor Halo of the Andromeda Spiral Galaxy (M31)

We present spectroscopic observations of red giant branch (RGB) stars over a large expanse in the halo of the Andromeda spiral galaxy (M31), acquired with the DEIMOS instrument on the Keck II 10 m telescope. Using a combination of five photometric/spectroscopic diagnostics?(1) radial velocity, (2) intermediate-width DDO51 photometry, (3) Na I equivalent width (surface gravity-sensitive), (4) position in the color-magnitude diagram, and (5) comparison between photometric and spectroscopic [Fe/H] estimates?we isolate over 250 bona fide M31 bulge and halo RGB stars located in 12 fields ranging from R = 12 to 165 kpc from the center of M31 (47 of these stars are halo members with R > 60 kpc). We derive the M31 spheroid (bulge and halo) metallicity distribution function and find it to be systematically more metal-poor with increasing radius, shifting from [Fe/H] = -0.47 ? 0.03 (? = 0.39) at R 60 kpc, assuming [?/Fe] = 0.0. These results indicate the presence of a metal-poor RGB population at large radial distances out to at least R = 160 kpc, thereby supporting our recent discovery of a stellar halo in M31 (structural component with an R-2 power-law surface brightness profile). This component has a distinct metallicity distribution from M31's bulge. If we assume an ?-enhancement of [?/Fe] = +0.3 for M31's halo, we derive [Fe/H] = -1.5 ? 0.1 (? = 0.7). Therefore, the mean metallicity and metallicity spread of this newly found remote M31 RGB population are similar to those of the Milky Way halo.

Near-IR photometry of asymptotic giant branch stars in the dwarf elliptical galaxy NGC 147

Near-infrared J, H and K' images were used to investigate the stellar contents of the asymptotic giant branch (AGB) population in the nearby dwarf elliptical galaxy NGC 147. The obtained (K, J - K) and (K, H - K) color-magnitude diagrams contain stars of AGB and red giant branch populations, where the former consists of a group of bright blue stars, a dominant population of M giant and a red C star population. We identified 91 AGB C stars in NGC 147 with the mean absolute magnitude and colors of = -7.56, = 1.81 and = 0.74. The estimated number ratio of C stars to M giant stars (C/M) is 0.16 ± 0.02. The estimated local C/M ratios of 0.14 ± 0.02 for the inner region (r 70) indicate a weak radial gradient. The mean bolometric magnitude of 91 C stars in NGC 147 is = -4.32 ± 0.49. The bolometric luminosity function of M giant stars in NGC 147 extends up to M bol = -5.8 mag, and that of only C stars spans -5.6 < M bol < -3.5. The color histograms of AGB stars in (J - K) and (H - K) show a main peak containing M giant stars, a red tail containing C stars with a weak excess, and a blue excess possibly due to bright blue foreground stars and AGB stars younger than M giants. The comparison of the theoretical isochrone models with the color distribution and the brightness of AGB stars indicates that most of the bright M giants in NGC 147 were formed at log(t yr ) ∼ 8.4 with a range of 8.2 ∼ 8.6. Ages of the majority of M giant stars in NGC 147 are similar to those of younger M giant stars in its companion NGC 185, while their formation occurred more recently than the older M giant stars in NGC 185 by ∼2 Gyr.

Spectrophotometry of Sextans A and B: Chemical Abundances of HiiRegions and Planetary Nebulae

We present the results of high-quality long-slit spectroscopy of planetary nebulae (PNe) and H II regions in the two dwarf irregular (dIrr) galaxies Sextans A and B, which belong to a small group of galaxies just outside the Local Group. The observations were obtained with the New Technology Telescope ESO Multi-Mode Instrument. In Sextans A we obtained the element abundances in its only known PN and in three H II regions with the classical Te method. The oxygen abundances in these three H II regions of Sextans A are all consistent within the individual rms uncertainties, with an average 12 + log(O/H) = 7.54 ± 0.06. The oxygen abundance of the PN in Sextans A is, however, significantly higher: 12 + log(O/H) = 8.02 ± 0.05. This PN is even more enriched in nitrogen and helium, suggesting a classification as a PN of type I. The PN abundances of S and Ar, which are presumably unaffected by nucleosynthesis in the progenitor star, are well below those in the H II regions, indicating lower metallicity at the epoch of the PN progenitor formation (~1.5 Gyr ago, according to our estimates based on the PN parameters). In Sextans B we obtained spectra of one PN and six H II regions. Element abundances with the Te method could be derived for the PN and three of the H II regions. For two of these H II regions, which have a separation of only ~70 pc in projection, the oxygen abundances do not differ within the rms uncertainties, with a mean of 12 + log(O/H) = 7.53 ± 0.05. The third H II region, which is about 0.6 kpc northeast of the first two, is twice as metal-rich, with 12 + log(O/H) = 7.84 ± 0.05. This suggests considerable inhomogeneity in the present-day metallicity distribution in Sextans B. Whether this implies a general chemical inhomogeneity among populations of comparable age in Sextans B, and thus a metallicity spread at a given age, or whether we happen to see the short-lived effects of freshly ejected nucleosynthesis products prior to their dispersal and mixing with the ambient interstellar medium will require further study. For the PN we measured an O/H ratio of 12 + log(O/H) = 7.47 ± 0.16, consistent with that of the low-metallicity H II regions. We discuss the new metallicity data for the H II regions and PNe in the context of the published star formation histories and published abundances of the two dIrr galaxies. Both dIrrs show generally similar star formation histories in the sense of continuous star formation with amplitude variations but differ in their detailed enrichment timescales and star formation rates as a function of time. If we combine the photometrically derived estimates for the mean metallicity of the old red giant branch population in both dIrrs with the present-day metallicity of the H II regions, both dIrrs have experienced chemical enrichment by at least 0.8 dex (lower limit) throughout their history.

Probing the Red Giant Branch Phase Transition: Near‐Infrared Photometry of Six Intermediate‐Age Large Magellanic Cloud Clusters

This is the first of a series of papers devoted to a global study of the photometric properties of the red stellar sequences in a complete sample of the Large Magellanic Cloud clusters, by means of near-infrared array photometry. Deep J, H, Ks photometry and accurate color-magnitude diagrams down to K ≈ 18.5, i.e., ≈1.5 mag below the red He clump, for six intermediate-age clusters (namely, NGC 1987, NGC 2108, NGC 2190, NGC 2209, NGC 2231, NGC 2249) are presented. A quantitative estimate of the population ratios (by number and luminosity) between red giant branch (RGB) and He-clump stars for each target cluster is provided and discussed in the framework of probing the so-called RGB phase transition (Ph-T). By using the Elson & Fall s-parameter as an age indicator, the observed RGB population shows a sharp enhancement (in both number and luminosity) at s = 36. Obviously, the corresponding absolute age strictly depends on the details of theoretical models adopted to calibrate the s-parameter. Curiously, the currently available calibrations of the s-parameter in terms of age based on canonical (by Elson & Fall) and overshooting (Girardi and coworkers) models provide ages that well agree within 10%, suggesting that the full development of the RGB occurs at t ≈ 700 Myr and is a relatively fast event (δt ≈ 300 Myr). However, the RGB Ph-T epoch derived from the overshooting calibration of the s-parameter turns out to be significantly earlier than the epoch provided by the recent evolutionary tracks by Girardi and coworkers. A new calibration of the s-parameter based on high-quality color-magnitude diagrams and updated models is urged to address the origin of this discrepancy and finally establish the epoch of the RGB Ph-T.

Stellar content of NGC 404 – The nearest S0 Galaxy

We report V−band and I−band CCD stellar and surface photometry of the galaxy NGC 404 , taken with HST WFPC2 and the 2.5 m Nordic telescope. The colour-magnitude diagram for the stars in this galaxy is typical of that of spheroidal systems (i.e. it lacks luminous, young stars but contains a large number of Asymptotic Giant Branch (AGB) and Red Giant Branch (RGB) stars). The disk of the galaxy is mostly dominated by red giant stars while its bulge consists of both the AGB and RGB population. Using the distance indicator technique, based on the tip of the red giant branch (TRGB), we find a distance of 3.42 ± 0.25 Mpc for this galaxy. Global photometric characteristic of this galaxy were then measured out to a radius of 3 � , giving V = 10. m 04 and I = 8. m 95. We find ( V − I) = 0.3 for the nucleus of NGC 404. The integral colour of the galaxy changes slightly along the radius, and its mean value is (V − I) = 1.1. On the HST images situated at 9 � from the galaxy center there are many red giants. This means that the size of the disk of NGC 404 exceeds 20 kpc. The value of the mean metallicity of the red giants in the disk is (Fe/H) = −1.11.

The Star Formation History of the Local Group Dwarf Elliptical Galaxy NGC 185. I. Stellar Content

?????We present VI CCD photometry of ~16,000 stars in a 72 ? 72 field of the Local Group dwarf elliptical galaxy NGC 185. The resulting VI color-magnitude diagram reveals a dominant red giant branch population, an important number of luminous red stars located above the tip of the red giant branch, and a number of blue and yellow stars. Besides the nucleus, our field also covers a large, less crowded area of the galaxy. We show color-magnitude diagrams at six different distances from the nucleus. The red giant branch becomes substantially narrower at larger distances from the nucleus, while the photometry gets deeper. In this paper, we concentrate on investigating the contribution of the observational effects (mainly crowding) to this observed gradient. Although we cannot rule out here the possibility that this trend partially originates in a gradient of the characteristics of the stellar populations of the galaxy with radius, we show that a strong radial gradient exists in the observational effects that can mimic a gradient in the real properties (e.g., age, metallicity) of the stellar population. A distance modulus of m - M = 23.95 ? 0.10 has been obtained from the tip of the red giant branch, in good agreement with previous estimates. The average stellar metallicity is estimated to be [Fe/H] = -1.43 ? 0.15, and decreases for increasing galactocentric distance.