Asymptotic giant branch stars in the Phoenix dwarf galaxy

Aims. We present a study aimed at deriving constraints on star formation at intermediate ages from the evolved stellar populations in the dwarf irregular galaxy UKS 2323-326. These observations were also intended to demonstrate the scientific capabilities of the multi-conjugated adaptive optics demonstrator (MAD) implemented at the ESO Very Large Telescope as a test-bench of adaptive optics (AO) techniques. Methods. We perform accurate, deep photometry of the field using J and K s band AO images of the central region of the galaxy. Results. The near-infrared (IR) colour-magnitude diagrams clearly show the sequences of asymptotic giant branch (AGB) stars, red supergiants, and red giant branch (RGB) stars down to ∼1 mag below the RGB tip. Optical-near-IR diagrams, obtained by combining our data with Hubble Space Telescope observations, provide the best separation of stars in the various evolutionary stages. The counts of AGB stars brighter than the RGB tip allow us to estimate the star formation at intermediate ages. Assuming a Salpeter initial mass function, we find that the star formation episode at intermediate ages produced ∼6 × 10 5 M of stars in the observed region.

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“…other recent near-IR studies, e.g., Gullieuszik et al 2007a,b;Menzies et al 2008). It is tempting Fig.…”

Section: The Evolved Stellar Populations Of Uks 2323-326mentioning

confidence: 67%

Resolving stellar populations outside the Local Group: MAD observations of UKS 2323-326

Gullieuszik

Greggio

Held

et al. 2008

A&A

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“…These additional targets were selected among the brightest sources in the [3.6] band, but quite often they were fainter than the magnitude cutoff criteria (M 3.6 μm ≤ −9 mag), and they did not always satisfy our selection criteria in terms of the [3.6]-[4.5] color. Moreover, 10 of these additional targets have been reported as candidate carbon stars based on their photometry (Battinelli & Demers 2000, 2004Menzies et al 2008). In total, 79 stars were observed in four dIrr galaxies: 19 in Pegasus, 14 in Phoenix, 15 in Sextans A, and 31 in WLM.…”

Section: Target Selectionmentioning

confidence: 99%

“…Although some candidates were listed in the literature as candidates for carbon stars (Battinelli & Demers 2000, 2004Menzies et al 2008), we still observed them to fill the free space on MXU masks. In total we observed ten photometric candidates, eight of which are spectroscopically confirmed as carbon stars in this work and are presented in Fig.…”

Section: Carbon Starsmentioning

confidence: 99%

Identification of dusty massive stars in star-forming dwarf irregular galaxies in the Local Group with mid-IR photometry

Britavskiy

Bonanos

Mehner

et al. 2015

A&A

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Context. Increasing the statistics of spectroscopically confirmed evolved massive stars in the Local Group enables the investigation of the mass loss phenomena that occur in these stars in the late stages of their evolution. Aims. We aim to complete the census of luminous mid-IR sources in star-forming dwarf irregular (dIrr) galaxies of the Local Group. To achieve this we employed mid-IR photometric selection criteria to identify evolved massive stars, such as red supergiants (RSGs) and luminous blue variables (LBVs), by using the fact that these types of stars have infrared excess due to dust. Methods. The method is based on 3.6 μm and 4.5 μm photometry from archival Spitzer Space Telescope images of nearby galaxies. We applied our criteria to four dIrr galaxies: Pegasus, Phoenix, Sextans A, and WLM, selecting 79 point sources that we observed with the VLT/FORS2 spectrograph in multi-object spectroscopy mode. Results. We identified 13 RSGs, of which 6 are new discoveries, as well as two new emission line stars, and one candidate yellow supergiant. Among the other observed objects we identified carbon stars, foreground giants, and background objects, such as a quasar and an early-type galaxy that contaminate our survey. We use the results of our spectroscopic survey to revise the mid-IR and optical selection criteria for identifying RSGs from photometric measurements. The optical selection criteria are more efficient in separating extragalactic RSGs from foreground giants than mid-IR selection criteria, but the mid-IR selection criteria are useful for identifying dusty stars in the Local Group. This work serves as a basis for further investigation of the newly discovered dusty massive stars and their host galaxies.

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“…There is, however, a very clear tail of longer period stars representing a small younger population. These are missing from the dwarf spheroidals (Menzies et al 2008;2010;Whitelock et al 2009), as might be expected given their commonly accepted star formation histories (although note that the total number of stars in the dwarf spheroidals is much smaller than in NGC 6822). Fig.…”

Section: Agb Variables In Ngc 6822mentioning

confidence: 89%

“…The observations discussed here form part of a study of AGB stars in Local Group galaxies and detailed analysis have been published for the Phoenix, Leo I and Fornax dwarf spheroidals by Menzies et al (2008Menzies et al ( , 2010 and Whitelock et al (2009). A preliminary study of the variables in NGC 6822 was made by Nsengiyumva (2010) for his MSc thesis, but the detailed analysis is still underway, and this account highlights only a few preliminary findings.…”

Section: Introductionmentioning

confidence: 99%