Very gas-rich extremely metal-poor blue void dwarfs

TL;DR

This study identifies and characterizes extremely gas-rich, low-metallicity blue dwarf galaxies in the Lynx-Cancer void, revealing their properties and discussing their formation and evolution in the context of low-metallicity galaxy populations.

Contribution

It reports the discovery and detailed analysis of a rare class of very metal-poor, gas-rich dwarf galaxies in voids, expanding the known sample and providing insights into their evolution.

Findings

Found a small group of extremely metal-poor, gas-rich dwarf galaxies in the Lynx-Cancer void.

These galaxies are faint, low surface brightness, with young stellar populations of a few Gyr.

Recent searches doubled the known number of such objects in nearby voids.

Abstract

Half-dozen of extreme representatives of void dwarf galaxy population were found in our study of evolutionary status of a hundred galaxies in the nearby Lynx-Cancer void. They are very gas-rich, extremely low-metallicity [7.0 < 12+log(O/H)< ~7.3] objects, with blue colours of outer parts. The colours indicate the ages of the oldest visible stellar population of one to a few Gyr. They all are intrinsically faint, mostly Low Surface Brightness dwarfs, with M_B range of -9.5 to -14 mag. Thus, their finding is a subject of the severe observational selection. The recent advancement in search for such objects in other nearby voids resulted in doubled their total number. We summarize all available data on this group of unusual void dwarf galaxies and discuss them in the general context of very low metallicity galaxies and their possible formation and evolutionary scenarios.

Full text

††volume: 344††journal: Dwarf Galaxies: From the Deep Universe to the Present††editors: K. McQuinn, S. Stierwalt, eds.

1 Introduction

The low-metallicity galaxies with gas metallicity $Z<Z$⊙/10 (known number more than 350) remain very rare objects. Even more so are XMP galaxies with $Z<Z$⊙/20 (or $12+\log$(O/H) $\lesssim 7.38$, about 50 known galaxies, [Guseva et al. (2017), Guseva et al. (2017)]). XMP galaxies are important as the best local proxies for forming galaxies in the early Universe. Most of known low-metallicity galaxies are found via optical spectroscopy of star-forming galaxies. There is a problem of dearth of low-metallicity dwarfs ([Sanchez2017, Sanchez Almeida et al. (2017)]. One of the exits is the existence of many quiscent, LSB dwarfs with low or subtle SFR, missed by optical redshift surveys. The alternative means, such as A) blind wide-angle HI surveys (ALFALFA, [Haynes et al. (2018), Haynes et al. (2018)]), B) colour-morphological selection of blue dIr (e.g., [James et al. (2017), James et al. (2017)], [Hsyu et al., Hsyu et al. (2018)]) and C) the unbiased study of all galaxies in the nearby voids, open additional channels to identify ’quiscent’ XMP dwarfs. We describe and discuss the most unusual XMP dwarfs found in the nearby voids.

2 XMP dwarfs in the Lynx-Cancer, Eridanus and other nearby voids

Here we briefly summarize our recently published results on the unbiased study of Nearby Void galaxies ([Pustilnik, et al. (2010), Pustilnik, et al. (2010)], [Pustilnik2011, Pustilnik & Tepliakova (2011)], [Chengalur & Pustilnik (2013), Chengalur & Pustilnik (2013)], [Perepeletal14, Perepelitsyna, Pustilnik & Kniazev (2014)], [PustilnikMartin16, Pustilnik & Martin (2016)], [Pustilnik, Perepelitsyna, & Kniazev (2016), Pustilnik, Perepelitsuna & Kniazev (2016)], [Chengalur, Pustilnik & Egorova (2017), Chengalur, Pustilnik & Egorova(2017)], [Kniazev, Egorova & Pustilnik (2018), Kniazev, Egorova & Pustilnik (2018)]) or results from the papers in preparation.

1. Late-type void galaxies have in general reduced gas metallicity and elevated gas content, both in average by $\sim$40% with respect of reference samples of similar galaxies in the denser environment of the Local Volume.

2. In voids all dwarfs with $M_{\rm B}>-13$ and with measured O/H, appear to be in the low metallicity regime (O/H $<$ (O/H)⊙/10).

3. A dozen void dwarfs have $12+\log$(O/H) $<\sim$7.20, that is $Z_{\rm gas}<\sim Z$⊙/30. Of them, 4-5 dwarfs have O/H at or near the so-called ’floor’ level ($Z=Z$⊙/50).

4. The most gas-rich void dwarfs have M(HI)/$L_{\rm B}=6-26$, or $M_{*}$/$M_{\rm gas}\lesssim 0.01-0.02$.

5. Blue colours in the outer parts outside the Star-Forming (SF) regions indicate relatively young stellar populations. Its time since the onset of SF $t_{\rm SF}<\sim 1-3$ Gyr for several most extreme XMP dwarfs. Their baryon masses (consisting practically of gas) fall in the range of $\sim 10^{7}$ M*⊙*  to $3\times 10^{8}$ M*⊙*.

3 Diversity of XMP dwarfs and prospects of deeper insights

There exist several scenarios explaining the existence of the most metal-poor dwarfs. They include:

A) probably the most common scenario of the substantial metal loss via the enriched gas outflow (wind) caused by SNe explosions during the episodes of elevated SF. One of the best examples is the nearest XMP dIr Leo P with $12+\log$(O/H) = 7.17. Its carefully studied SF history based on the deep HST photometry, along with all other available data, indicates the loss of $\sim$95% metals produced during its cosmological evolution ([McQuinn et al. (2015), McQuinn et al. (2015)]);

B) temporary strong local dilution of metals in the regions of current SF by blobs of the ambient intergalactic medium due to so called ’Cold Accretion’ along cosmological filaments of low-Z gas ($Z\sim Z$⊙/50) (many examples by Sanchez Almeida & co-authors). However, [Filho et al. (2015), Filho et al. (2015)], and [Sanchez Almeida et al. (2016), Sanchez Almeida et al. (2016)]) also find that the low-metallicity dwarfs favor voids.

C) Inflow of very metal-poor unprocessed ($Z\gtrsim Z$⊙/50) gas from the distant periphery of gas-rich dwarfs to the central region of straburst due to the induced loss of gas stability by an external disturber ([Ekta & Chengalur (2010), Ekta & Chengalur (2010)]).

D) Additional effect of void environment: slower evolution due to the significantly reduced rate of galaxy interactions and probable delayed dwarf formation in shallow gravitational potential of negative density contrast. Such objects are expected to be true Very Young Galaxies (VYG, as defined by Tweed et al. 2018). The SF galaxy J0811+4730 at $D=180$ Mpc with $Z_{\rm gas}=Z$⊙/50 (Izotov et al. 2018) with only young stellar population is a probable VYG. The most interesting candidates to such nearby VYGs with lower SFR are presented here. A couple dozen similar candidates in our Nearby Void Galaxy sample (Pustilnik et al. 2018, MNRAS, submitted) await for the additional checks.

The work is supported by grant of Russian Foundation for Basic Research No. 18-52-45008.