scholarly journals The Distance and Dynamical History of the Virgo Cluster Ultradiffuse Galaxy VCC 615

2022 ◽  
Vol 924 (2) ◽  
pp. 87
Author(s):  
J. Christopher Mihos ◽  
Patrick R. Durrell ◽  
Elisa Toloba ◽  
Patrick Côté ◽  
Laura Ferrarese ◽  
...  
Keyword(s):  
Dwarf Galaxy ◽  
Hubble Space Telescope ◽  
Virgo Cluster ◽  
Cluster Center ◽  
Main Body ◽  
The Galaxy ◽  
Orbit Modeling ◽  
History Of ◽  
Red Giant ◽  
Distance Estimator

Abstract We use deep Hubble Space Telescope imaging to derive a distance to the Virgo Cluster ultradiffuse galaxy (UDG) VCC 615 using the tip of the red giant branch (TRGB) distance estimator. We detect 5023 stars within the galaxy, down to a 50% completeness limit of F814W ≈ 28.0, using counts in the surrounding field to correct for contamination due to background sources and Virgo intracluster stars. We derive an extinction-corrected F814W tip magnitude of m tip , 0 = 27.19 − 0.05 + 0.07 , yielding a distance of d = 17.7 − 0.4 + 0.6 Mpc. This places VCC 615 on the far side of the Virgo Cluster (d Virgo = 16.5 Mpc), at a Virgocentric distance of 1.3 Mpc and near the virial radius of the main body of Virgo. Coupling this distance with the galaxy’s observed radial velocity, we find that VCC 615 is on an outbound trajectory, having survived a recent passage through the inner parts of the cluster. Indeed, our orbit modeling gives a 50% chance the galaxy passed inside the Virgo core (r < 620 kpc) within the past gigayear, although very close passages directly through the cluster center (r < 200 kpc) are unlikely. Given VCC 615's undisturbed morphology, we argue that the galaxy has experienced no recent and sudden transformation into a UDG due to the cluster potential, but rather is a long-lived UDG whose relatively wide orbit and large dynamical mass protect it from stripping and destruction by the Virgo cluster tides. Finally, we also describe the serendipitous discovery of a nearby Virgo dwarf galaxy projected 90″ (7.2 kpc) away from VCC 615.

scholarly journals A New Age and Distance for I Zw 18, the Most Metal-Poor Galaxy in the Nearby Universe

2008 ◽  
Vol 4 (S255) ◽  
pp. 387-391
Author(s):  
A. Aloisi ◽  
G. Clementini ◽  
M. Tosi ◽  
F. Annibali ◽  
R. Contreras ◽  
...  
Keyword(s):  
High Resolution ◽  
Early Universe ◽  
Dwarf Galaxy ◽  
Hubble Space Telescope ◽  
Local Universe ◽  
Photometric Observations ◽  
The Galaxy ◽  
Cepheid Variables ◽  
History Of ◽  
Blue Compact Dwarf Galaxy

AbstractThe blue compact dwarf galaxy I Zw 18 holds the record of the lowest metallicity ever observed in the local universe. As such, it represents the closest analog to primordial galaxies in the early universe. More interestingly, it has recurrently been regarded as a genuinely young galaxy caught in the process of forming in the nearby universe. However, stars of increasingly older ages are found within I Zw 18 every time deeper high-resolution photometric observations are performed with the Hubble Space Telescope (HST): from the original few tens of Myrs to, possibly, several Gyrs. Here we summarize the history of I Zw 18 age and present an ongoing HST/ACS project which allowed us to precisely derive the galaxy distance by studying its Cepheid variables, and to firmly establish the age of its faintest resolved populations.

scholarly journals The Galaxy's Eating Habits

2004 ◽  
Vol 217 ◽  
pp. 406-411 ◽  
Author(s):  
M. E. Putman ◽  
C. Thom ◽  
B. K. Gibson ◽  
L. Staveley-Smith
Keyword(s):  
Eating Habits ◽  
Dwarf Galaxy ◽  
Galactic Disk ◽  
Neutral Hydrogen ◽  
Main Body ◽  
Gaseous Component ◽  
The Galaxy ◽  
Gaseous Stream ◽  
Gaseous Halo ◽  
Current Orbit

The possibility of a gaseous halo stream which was stripped from the Sagittarius dwarf galaxy is presented. The total mass of the neutral hydrogen along the orbit of the Sgr dwarf in the direction of the Galactic Anti-Center is 4 − 10 × 106 M⊙ (at 36 kpc, the distance to the stellar debris in this region). Both the stellar and gaseous components have negative velocities in this part of the sky, but the gaseous component extends to higher negative velocities. We suggest this gaseous stream was stripped from the main body of the dwarf 0.2 – 0.3 Gyr ago during its current orbit after a passage through a diffuse edge of the Galactic disk with a density > 10−4 cm−3. The gas would then represent the dwarf's last source of star formation fuel and explains how the galaxy was forming stars 0.5-2 Gyr ago.

scholarly journals Dissecting star formation in the “Atoms-for-Peace” galaxy

2018 ◽  
Vol 614 ◽  
pp. A130 ◽  
Author(s):  
K. George ◽  
P Joseph ◽  
P. Côté ◽  
S. K. Ghosh ◽  
J. B. Hutchings ◽  
...  
Keyword(s):  
Star Formation ◽  
Dwarf Galaxies ◽  
Initial Conditions ◽  
Dwarf Galaxy ◽  
Main Body ◽  
Star Forming ◽  
Uv Imaging ◽  
Star Forming Regions ◽  
The Galaxy ◽  
Tidal Tails

Context. The tidal tails of post-merger galaxies exhibit ongoing star formation far from their disks. The study of such systems can be useful for our understanding of gas condensation in diverse environments. Aims. The ongoing star formation in the tidal tails of post-merger galaxies can be directly studied from ultraviolet (UV) imaging observations. Methods. The post merger galaxy NGC7252 (“Atoms-for-Peace” galaxy) is observed with the Astrosat UV imaging telescope (UVIT) in broadband NUV and FUV filters to isolate the star-forming regions in the tidal tails and study the spatial variation in star formation rates. Results. Based on ultraviolet imaging observations, we discuss star-forming regions of ages <200 Myr in the tidal tails. We measure star formation rates in these regions and in the main body of the galaxy. The integrated star formation rate (SFR) of NGC7252 (i.e., that in the galaxy and tidal tails combined) without correcting for extinction is found to be 0.81 ± 0.01 M⊙ yr−1. We show that the integrated SFR can change by an order of magnitude if the extinction correction used in SFR derived from other proxies are taken into consideration. The star formation rates in the associated tidal dwarf galaxies (NGC7252E, SFR = 0.02 M⊙ yr−1 and NGC7252NW, SFR = 0.03 M⊙ yr−1) are typical of dwarf galaxies in the local Universe. The spatial resolution of the UV images reveals a gradient in star formation within the tidal dwarf galaxy. The star formation rates show a dependence on the distance from the centre of the galaxy. This can be due to the different initial conditions responsible for the triggering of star formation in the gas reservoir that was expelled during the recent merger in NGC7252.

scholarly journals The Evolutionary History of the Milky Way

1996 ◽  
Vol 171 ◽  
pp. 3-10
Author(s):  
K.C. Freeman
Keyword(s):  
Evolutionary History ◽  
Milky Way ◽  
Dwarf Galaxy ◽  
Galactic Disk ◽  
Small Satellite ◽  
Galactic Bulge ◽  
The Galaxy ◽  
History Of ◽  
Satellite Galaxies

The accretion of small satellite galaxies appears to have been important in the formation of the metal-poor halo of the Galaxy. The disrupting Sgr dwarf galaxy and the recent discovery of a young, metal-poor component of the halo indicate that this is a continuing process. The evolution of the galactic disk, and some consequences of the bar-like nature of the galactic bulge are briefly discussed.

scholarly journals The curious case of the companion: evidence for cold accretion onto a dwarf satellite near the isolated elliptical NGC 7796

2018 ◽  
Vol 620 ◽  
pp. A133 ◽  
Author(s):  
T. Richtler ◽  
M. Hilker ◽  
K. Voggel ◽  
T. H. Puzia ◽  
R. Salinas ◽  
...  
Keyword(s):  
Galaxy Evolution ◽  
Dwarf Galaxy ◽  
Broad Band ◽  
Hii Regions ◽  
Diagnostic Tools ◽  
Strong Line ◽  
Main Body ◽  
Surprising Result ◽  
Star Forming ◽  
The Galaxy

Context. The isolated elliptical (IE) NGC 7796 is accompanied by an interesting early-type dwarf galaxy, named NGC 7796-DW1. It exhibits a tidal tail, very boxy isophotes, and multiple nuclei or regions (A, B, and C) that are bluer than the bulk population of the galaxy, indicating a younger age. These properties are suggestive of a dwarf–dwarf merger remnant. Aims. Dwarf–dwarf mergers are poorly understood, but may have a high importance for dwarf galaxy evolution. We want to investigate the properties of the dwarf galaxy and its components to find more evidence for a dwarf–dwarf merger or for alternative formation scenarios. Methods. We use the Multi-Unit Spectroscopic Explorer (MUSE) at the VLT to investigate NGC 7796-DW1. We extract characteristic spectra to which we apply the STARLIGHT population synthesis software to obtain ages and metallicities of the various population components of the galaxy. This permits us to isolate the emission lines for which fluxes and flux ratios can be measured and to which strong-line diagnostic tools can be applied. Results. The galaxy’s main body is old and metal-poor. A surprising result is the extended line emission in the galaxy, forming a ring-like structure with a projected diameter of 2.2 kpc. The line ratios fall into the regime of HII-regions, although OB-stellar populations cannot be identified by spectral signatures. The low Hα surface brightnesses indicate unresolved star-forming substructures, which means that broad-band colours are not reliable age or metallicity indicators. Nucleus A is a relatively old (7 Gyr or older) and metalpoor super star cluster, most probably the nucleus of the dwarf, now displaced. The star-forming regions B and C show younger and distinctly more metal-rich components. The emission line ratios of regions B and C indicate an almost solar oxygen abundance, if compared with radiation models of HII regions. Oxygen abundances from empirical calibrations point to only half-solar. The ring-like Hα-structure does not exhibit signs of rotation or orbital movements. Conclusions. NGC 7796-DW1 occupies a particular role in the group of transition-type galaxies with respect to its origin and current evolutionary state, being the companion of an IE. The dwarf–dwarf merger scenario is excluded because of the missing metal-rich merger component. A viable alternative is gas accretion from a reservoir of cold, metal-rich gas. NGC 7796 has to provide this gas within its X-ray bright halo. As illustrated by NGC 7796-DW1, cold accretion may be a general solution to the problem of extended star formation histories in transition dwarf galaxies.

scholarly journals Nearby Blue Compact Dwarf Galaxy NGC 6789

1999 ◽  
Vol 192 ◽  
pp. 253-258
Author(s):  
I. Drozdovsky ◽  
N. Tikhonov
Keyword(s):  
Surface Brightness ◽  
Dwarf Galaxy ◽  
High Surface ◽  
Low Surface Brightness ◽  
The Galaxy ◽  
Metal Abundance ◽  
The Mean ◽  
Red Giant ◽  
Halo Population ◽  
Blue Compact Dwarf Galaxy

We present the results of a detailed BVRI and Hα study of the isolated nearby blue compact dwarf (BCD) galaxy NGC 6789. Judging from the literature the observed galaxy has not yet been resolved into stars up to now. On CCD frames obtained with 6m BTA telescope and 2.5m Nordic telescope the galaxy is well resolved. Its colour-magnitude diagram confirms the two component (core-halo) galaxy morphology, which consists of two stellar populations distinct in structure and colour: an inner high surface-brightness young population within 150 pc from the center of the galaxy, and a relatively low surface-brightness intermediate-age population extending out to at least 600 pc. The distance to the galaxy, estimated from the tip of the red giant branch (TRGB) is 2.1 Mpc which places NGC 6789 close to the Local Group. From the mean colour of the RGB, the mean metal abundance of the halo population is estimated as [Fe/H] ≃ −1 dex.

scholarly journals Is Fornax 4 the nuclear star cluster of the Fornax dwarf spheroidal galaxy?

2020 ◽  
Vol 495 (4) ◽  
pp. 4518-4528
Author(s):  
S Martocchia ◽  
E Dalessandro ◽  
M Salaris ◽  
S Larsen ◽  
M Rejkuba
Keyword(s):  
Stellar Population ◽  
Hubble Space Telescope ◽  
Detailed Comparison ◽  
Observational Evidence ◽  
Star Cluster ◽  
The Other ◽  
Horizontal Branch ◽  
The Galaxy ◽  
Red Giant

ABSTRACT Fornax 4 is the most distinctive globular cluster in the Fornax dwarf spheroidal. Located close to the centre of the galaxy, more metal-rich and potentially younger than its four companions (namely, Fornax clusters number 1, 2, 3, and 5), it has been suggested to have experienced a different formation than the other clusters in the galaxy. Here, we use Hubble Space Telescope/WFC3 photometry to characterize the stellar population content of this system and shed new light on its nature. By means of a detailed comparison of synthetic horizontal branch and red giant branch with the observed colour–magnitude diagrams, we find that this system likely hosts stellar sub-populations characterized by a significant iron spread up to Δ[Fe/H] ∼ 0.4 dex and possibly by also some degree of He abundance variations ΔY ∼ 0.03. We argue that this purely observational evidence, combined with the other peculiarities characterizing this system, supports the possibility that Fornax 4 is the nuclear star cluster of the Fornax dwarf spheroidal galaxy. A spectroscopic follow-up for a large number of resolved member stars is needed to confirm this interesting result and to study in detail the formation and early evolution of this system and more in general the process of galaxy nucleation.

scholarly journals From the inner to outer Milky Way: a photometric sample of 2.6 million red clump stars

2020 ◽  
Vol 495 (3) ◽  
pp. 3087-3103 ◽  
Author(s):  
Madeline Lucey ◽  
Yuan-Sen Ting ◽  
Nesar S Ramachandra ◽  
Keith Hawkins
Keyword(s):  
Spectral Energy Distribution ◽  
Spectral Energy ◽  
Contamination Rate ◽  
Formation History ◽  
The Galaxy ◽  
Scientific Potential ◽  
History Of ◽  
Nitrogen Ratio ◽  
Red Clump ◽  
Red Giant

ABSTRACT Large pristine samples of red clump stars are highly sought after given that they are standard candles and give precise distances even at large distances. However, it is difficult to cleanly select red clumps stars because they can have the same Teff and log g as red giant branch stars. Recently, it was shown that the asteroseismic parameters, $\rm {\Delta }$P and $\rm {\Delta \nu }$, which are used to accurately select red clump stars, can be derived from spectra using the change in the surface carbon to nitrogen ratio ([C/N]) caused by mixing during the red giant branch. This change in [C/N] can also impact the spectral energy distribution. In this study, we predict the $\rm {\Delta }$P, $\rm {\Delta \nu }$, Teff, and log g using 2MASS, AllWISE, Gaia, and Pan-STARRS data in order to select a clean sample of red clump stars. We achieve a contamination rate of ∼20 per cent, equivalent to what is achieved when selecting from Teff and log g derived from low-resolution spectra. Finally, we present two red clump samples. One sample has a contamination rate of ∼20 per cent and ∼405 000 red clump stars. The other has a contamination of ∼33 per cent and ∼2.6 million red clump stars that includes ∼75 000 stars at distances &gt;10 kpc. For |b| &gt; 30 deg, we find ∼15 000 stars with contamination rate of ∼9 per cent. The scientific potential of this catalogue for studying the structure and formation history of the Galaxy is vast, given that it includes millions of precise distances to stars in the inner bulge and distant halo where astrometric distances are imprecise.

scholarly journals A DECam view of the diffuse dwarf galaxy Crater II: the colour–magnitude diagram

2019 ◽  
Vol 490 (3) ◽  
pp. 4121-4132 ◽  
Author(s):  
A R Walker ◽  
C E Martínez-Vázquez ◽  
M Monelli ◽  
A K Vivas ◽  
G Bono ◽  
...  
Keyword(s):  
Star Formation ◽  
Dwarf Galaxy ◽  
Semimajor Axis ◽  
Multiple Star ◽  
Chemical Enrichment ◽  
Rr Lyrae ◽  
Magnitude Diagram ◽  
History Of ◽  
Red Giant ◽  
Central Concentration

ABSTRACT We present a deep Blanco/DECam colour–magnitude diagram (CMD) for the large but very diffuse Milky Way satellite dwarf galaxy Crater II. The CMD shows only old stars with a clearly bifurcated subgiant branch (SGB) that feeds a narrow red giant branch. The horizontal branch (HB) shows many RR Lyrae and red HB stars. Comparing the CMD with [Fe/H] = −2.0 and [α/Fe] = + 0.3 alpha-enhanced BaSTI isochrones indicates a mean age of 12.5 Gyr for the main event and a mean age of 10.5 Gyr for the brighter SGB. With such multiple star formation events Crater II shows similarity to more massive dwarfs that have intermediate age populations, however for Crater II there was early quenching of the star formation and no intermediate age or younger stars are present. The spatial distribution of Crater II stars overall is elliptical in the plane of the sky, the detailed distribution shows a lack of strong central concentration, and some inhomogeneities. The 10.5 Gyr subgiant and upper main-sequence stars show a slightly higher central concentration when compared to the 12.5 Gyr population. Matching to Gaia DR2 we find the proper motion of Crater II: μαcos δ = −0.14 ± 0.07, μδ = −0.10 ± 0.04 mas yr−1, approximately perpendicular to the semimajor axis of Crater II. Our results provide constraints on the star formation and chemical enrichment history of Crater II, but cannot definitively determine whether or not substantial mass has been lost over its lifetime.

scholarly journals The Multiply Imaged Strongly Lensed Supernova Refsdal

2015 ◽  
Vol 11 (A29B) ◽  
pp. 822-824
Author(s):  
Patrick Kelly
Keyword(s):  
Rest Frame ◽  
Hubble Space Telescope ◽  
Galaxy Cluster ◽  
Supernova Explosion ◽  
Gravitational Lens ◽  
Cluster Center ◽  
Arrival Times ◽  
The Galaxy ◽  
Multiple Paths ◽  
Massive Galaxy

AbstractIn 1964, Sjur Refsdal first considered the possibility that the light from a background supernova could traverse multiple paths around a strong gravitational lens towards us. He showed that the arrival times of the supernova's light would depend on the cosmic expansion rate, as well as the distribution of matter in the lens. I discussed the discovery of the first such multiply imaged supernova, which exploded behind the MACS J1149.6+2223 galaxy cluster. We have obtained Hubble Space Telescope grism and ground-based spectra of the four images of the supernova, which form an Einstein Cross configuration around an elliptical cluster member. These spectra as well as rest-frame optical light curves have allowed us to learn about the properties of the peculiar core-collapse supernova explosion, which occurred 4.3 Gyr after the Bang Bang, and contain information about the lenses matter distribution as well as their stellar populations. A delayed image of the supernova is expected close to the galaxy cluster center as early as this Fall, and will serve as an unprecedented probe of the potential of a massive galaxy cluster.

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