Hα survey of nearby dwarf galaxies

Monthly Notices of the Royal Astronomical Society, Sep 2012

We present the Hα imaging data and flux measurements for 30 dwarf galaxies in the Local Volume. The Hα fluxes are used to derive the galaxy star-formation rate (SFR). The sample of observed galaxies is characterized by the following parameters: a median distance of 7.5 Mpc, median blue absolute magnitude of −14.8 mag and median SFR of −2.0 dex. Two dSph members of the Local Group, Cetus and Leo IV, do not show signs of star formation, with rates of −5.4 and −7.0 dex, respectively. The BCD galaxy ESO 553−46 has one of the highest specific SFRs among the Local Volume galaxies.

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Hα survey of nearby dwarf galaxies

Hα survey of nearby dwarf galaxies S. S. Kaisin 1 I. D. Karachentsev 1 S. Ravindranath 0 0 Inter-University Centre for Astronomy and Astrophysics , Pune , India 1 Special Astrophysical Observatory, Russian Academy of Sciences , N. Arkhyz , Russia A B S T R A C T We present the Hα imaging data and flux measurements for 30 dwarf galaxies in the Local Volume. The Hα fluxes are used to derive the galaxy star-formation rate (SFR). The sample of observed galaxies is characterized by the following parameters: a median distance of 7.5 Mpc, median blue absolute magnitude of −14.8 mag and median SFR of −2.0 dex. Two dSph members of the Local Group, Cetus and Leo IV, do not show signs of star formation, with rates of −5.4 and −7.0 dex, respectively. The BCD galaxy ESO 553−46 has one of the highest specific SFRs among the Local Volume galaxies. - 1 I N T R O D U C T I O N Since 2004, a survey programme to map Hα emission in galaxies belonging to the Local Volume (LV) with distance D < 10 Mpc has been underway using the 6-m telescope at the Special Astrophysical Observatory of the Russian Academy of Sciences (SAO RAS). This project is approaching completeness and the basic results have been published in a series of papers: Karachentsev et al. (2005) , Kaisin & Karachentsev (2006 , 2008), Kaisin et al. (2007) , Kaisin, Karachentsev & Kaisina (2011 ) and Karachentsev & Kaisin (2007 , 2010). In parallel to our survey, similar observations of nearby galaxies in the Hα line were also performed in Cambridge and Arizona universities; a review of these is published by Kennicutt et al. (2008) . At present, the degree of completeness of the Hα survey of LV objects exceeds 90 per cent in the Northern Hemisphere, but the observations in the Southern Hemisphere have not been so successful so far. Essential input to the Hα survey of northern LV galaxies was introduced by observations with the 6-m Big Telescope Altazimuthal (BTA) telescope of the SAO RAS. For a continuation of the survey into the southern sky, we used the facilities of the 2-m telescope of Girawali Observatory (IUCAA) situated at a geographic latitude of +18◦. In this paper we present results from the observing runs during 2009–2010, which have yielded Hα fluxes and SFR for 30 nearby dwarf galaxies. 2 O B S E RVAT I O N S A N D I M AG E P R O C E S S I N G Observations of the nearby dwarf galaxies were performed using the 2-m telescope at the IUCAA Girawali Observatory, located near Pune, India. The average seeing was 1.5 arcsec during most of the observing period. Images of the galaxies were recorded with the IUCAA Faint Object Spectrograph and camera instrument equipped with an EEV 2K × 2K CCD, which provides a 10.5 arcmin field of view with a resolution of 0.31 arcsec per pixel. The images were obtained via an interference Hα filter with an effective wavelength 6563 Å and bandwidth FWHM = 84 Å, and also with the standard wide-band R filter to subtract a continuum. The typical exposure time was 1200 s in the Hα line and 600 s in the continuum. Since the range of radial velocities in our sample is small, we used the same filters for all the observed objects. The European Southern Observatory (ESO) Munich Image Data Analysis System (MIDAS) astronomy data-reduction package was used to process our obtained data. To confirm the quality of the results, comparisons between different nights and standard stars were performed. The observational data were processed in the standard way, including bias-subtracting, flat-fielding by twilight flats, removing cosmic rays and sky-background subtracting. The next operation was to align the emission-line image with continuum ones. Here, PSF (point spread function) matching was done to match the resolutions of the Hα image and continuum images. Then the images in the continuum were normalized to Hα images using 10–20 stars and subtracted. The measured Hα fluxes of the galaxies were calibrated with spectrophotometric standard stars from Hamuy et al. (1992 , 1994) obtained during the same night. The internal errors for the Hα flux measurements were typically about 0.10 dex. The major contribution to the error was from variable atmospheric conditions. The conditions of our observations are given in column 11 of Table 1, where the colon means that the sky was not photometric. We did not correct Hα fluxes for the emission of the [N II] doublet, which is likely to be small, ∼0.05 dex, in the case of dwarf galaxies Lee et al. (2009) . Images of the galaxies we have observed are shown as a mosaic in Fig. 1. The left and right images of each galaxy correspond to the Hα + continuum and the Hα − continuum. The angular scale and north and east directions are indicated by the horizontal bars and arrows. The main characteristics of the observed galaxies are listed in Table 1. The table columns contain the following: (1) galaxy name; (2) equatorial coordinates for epoch J2000.0; (3) morphological type from de Vaucouleurs digital classification; (4) radial velocity relative to the Local Group centroid (in km s−1); (5) distance to the galaxy in Mpc with indication of the method used (‘rgb’ via the tip of red giant branch, ‘tf’ from the Tully–Fisher relation, ‘mem’ from the membership of a galaxy in a group, ‘h’ via the radial velocity for the Hubble parameter H0 = 73 km s−1 Mpc−1); (6, 7) observed integrated and extinction-corrected Hα flux of the galaxy on a logarithmic scale in units of [erg cm−2 s−1]; here the Galactic extinction was accounted for according to Schlegel, Finkbeiner & Davis (1998 ) and the internal extinction via equation (5) from Karachentsev et al. (2004) , assuming the extinction in Hα to be 0.538 of the extinction in the B band; (8) integrated star-formation rate in the galaxy calculated from the canonical relation [SFR] = 0.945 × 109Fc(Hα)D2, where the distance to the galaxy is expressed in Mpc Kennicutt (1998) ; (9) absolute B magnitude of the galaxy corrected for extinction; (10) logarithm of the hydrogen mass of the galaxy taken from HIPASS Meyer et al. (2004) ; Koribalski et al. (2004) ; (11) observation conditions. 3 S O M E I N D I V I D UA L P R O P E R T I E S O F T H E O B S E RV E D G A L A X I E S Cetus dSph. This dwarf spheroidal galaxy is a peripheral companion to Andromeda (M31), being at a projected distance of 620 kpc from it. Like KKR 25 and Apples I, Cetus is a rare example of an isolated dSph system that has no normal galaxy within 0.5 Mpc. Bouchard, Carignan & Staveley-Smith (2006 ) found an H I cloud that is probably associated with the dwarf. Hα emission from the Cetus dSph is not detected by us. The history of star formation in Cetus was studied by Monelli et al. (2010) based on deep Hubble Space Telescope (HST)/Advanced Camera for Surveys (ACS) observations. UGC 1056. A blue compact dwarf (BCD), which is a companion to the bright spiral NGC 628. Hα and UV emission are concentrated in its core. It was imaged in Hα by Kennicutt et al. (2008) . ESO 483−013. An isolated BCD galaxy with Hα emission in the centre. It is mistakenly classified by the NASA/IPAC Extragalactic Database (NED) as of lenticular type. It was imaged in Hα by Kennicutt et al. (2008) . UGC 3174 = DDO 34. A dwarf spiral galaxy with a dozen small H II regions. It was imaged in Hα by Kennicutt et al. (2008) . ESO 553−046. An isolated BCD galaxy with an extremely high star-formation rate per luminosity unit. UGCA 127 sat. A dwarf irregular galaxy without radial velocity. Probably it is a companion to the spiral UGCA 127, the distance of which derived from the Tully–Fisher relation was ascribed to the companion itself. On the western side of the galaxy disc there is an unresolved H II region separate from the total diffuse Hα emission. WHI B0619−07. This dwarf spiral galaxy residing in a zone of strong extinction is another companion to UGCA 127. A bright star is projected into its central part. ESO 490−017. A nearby dIr galaxy, the distance of which was determined via the tip of the red giant branch Karachentsev et al. (2003) . The H I map and velocity field of ESO 490−017 were obtained with the Giant Metrewave Radio Telescope (GMRT) by Begum et al. (2008) . CGMW1−260 and IC 2171. Both these Sdm galaxies have similar radial velocities and distance estimates from the Tully–Fisher relation. NGC 2283. This is a Scd galaxy with a majority of compact H II regions. Judging by its radial velocity and Tully–Fisher distance, NGC 2283 belongs to a scattered group at low galactic latitude, |b| ∼ 10◦, together with two previous galaxies. KKSG 9. A BCD galaxy with a high hydrogen mass-to-luminosity ratio. Probably it has an extended H I envelope, which may be verified by observations in the H I line with aperture synthesis. The distance to KKSG 9 was estimated from its apparent membership of the NGC 2283 group. ESO 558−011. A galaxy of Magellanic (Sm) type with faint H II regions. HIPASS J0801−21. A dwarf irregular galaxy but without signs of Hα emission. Its coordinates in the NED are given with an error. ESO 495−008 and ESO 497−004. Both these galaxies with low radial velocities are situated outside the Local Volume, judging by their Tully–Fisher distance estimates. HIPASS J0916−23b = ESO 497−035. A companion to the spiral galaxy NGC 2835. ESO 565−003 and 6dFJ0939351−250735. Isolated dIr galaxies with small emission knots. 6dFJ0956376−092911. A single BCD galaxy undetected in HIPASS. KKSG 15, MCG-01-26-009, KKSG 17 and UGCA 193. Probable companions to the lenticular galaxy NGC 3115. UGCA 193 was imaged in Hα by Kennicutt et al. (2008) . UGC 6145. This is a dIr companion to the spiral NGC 3521. It shows faint diffuse Hα emission. The H I map and velocity field of ESO 490−017 were obtained with the GMRT by Begum et al. (2008) . Leo IV. A dwarf spheroidal companion to the Milky Way recently discovered by Belokurov et al. (2007) . It does not show any signs of Hα emission. Its structure and star-formation history were studied by Sand et al. (2010) . A probable companion to Leo V (de Jong et al. 2010) . KKSG 33. A dSph companion to the ‘Sombrero’ (NGC 4594) without signs of emission in H I or Hα. IC 3647 = KDG 180 = VCC 1857. A dSph galaxy with unreliably measured radial velocity from Binggeli, Sandage & Tammann (1985 ). It is a probable member of the Virgo cluster, without apparent emission in Hα and H I. NGC 4700. This Sd galaxy has a radial velocity that is much higher than the expected one under a Tully–Fisher distance of 6.5 Mpc. 2MFGC 15085. One of the few galaxies with relatively low radial velocity situated in front of the Local Void centre. A faint diffuse companion is seen ∼2 arcmin north-west of it. 4 D I S C U S S I O N Of the 30 galaxies in Table 1, there are 4 objects (UGC 1056, ESO 483−013, UGC 3174 and UGCA 193) for which the Hα fluxes have been measured by others. All four estimates were made by Kennicutt et al. (2008) . A comparison of our estimates and theirs (K08) shows that the average arithmetic difference log Four − log FK08 = −0.04 ± 0.09 and the mean square difference of the logarithm of the fluxes is 0.16. This indicates a satisfactory agreement of the independent measurements (in spite of the imperfect weather conditions), although the external flux error turns out to be about 1.5 times the internal one. The objects observed by us are characterized by a large diversity of luminosity, morphological types and other parameters. The median distance to them is 7.5 Mpc, the median absolute blue magnitude is MB = −14.8 mag and the median star-formation rate is log[SFR] = −2.0. Two dwarf spheroidal member of the Local Group – Cetus and Leo IV – do not show any signs of current star formation on the level of (10−5–10−7) M yr−1, while the Sc galaxy NGC 2283 transforms its gas into stars at a rate of ∼1 M yr−1. The distribution of the observed galaxies versus their absolute magnitude MB and SFR is presented in Fig. 2 (squares). The galaxies with only an upper limit for the Hα flux are not indicated in the figure. For comparison, we also present another 450 galaxies from the Local Volume (D < 10), shown by circles. The SFR data for them are taken from the literature. The line corresponds to a constant SFR per unit luminosity. Another diagram is presented in Fig. 3, in which the global SFR of galaxies is compared with their mass of neutral hydrogen MH I. The galaxies with only an upper limit for the SFR or MH I are not indicated. In this diagram the dashed line corresponds to a fixed SFR per unit MH I and the solid line traces the known Kennicutt–Schmidt relatioship SFR ∝ MH1.I5. As is seen from the data of columns (8)–(10) in Table 1, the star-formation rate of a galaxy correlates with its luminosity as well as with the amount of neutral hydrogen. To characterize the evolution status of a galaxy, one can use the ‘past–future’ diagram {P∗, F∗}, where the dimensionless and distance-independent parameters P∗ = log ([SFR]T/LB) and F ∗ = log (MH I/[SFR]T ) determine the productivity of star formation in the past over the whole cosmic time T = 13.7 Gyr and how long the star formation can continue in future with the present gas resource. Actually, the P∗ parameter is equivalent to the widely used ‘specific star-formation rate’ normalized to the cosmic time T and the parameter F∗ corresponds to the time-scale to exhaust the H I mass (in units of T), also often used by many authors. The titles of {P∗, F∗} quadrants conditionally indicates the evolution state of galaxies in them. For instance, a galaxy with constant SFR and constant mass-toluminosity ratio is located at the origin of the {P∗, F∗} plane if the galaxy luminosity can be reproduced over the cosmic time T, and the galaxy has a suitable H I amount to keep the observed SFR during the next T term. Most bulgeless spiral galaxies and irregular dwarfs on the {P∗, F∗} diagram are concentrated around the origin {P∗ = 0, F∗=0}. Therefore, the bulk of local late-type galaxies reside at the midpoint of their star-forming evolution. Nevertheless, the diagnostic diagram in Fig. 4 exhibits a significant scatter of galaxies on P∗, F∗ parameters. As one can see, the sample of galaxies in Fig. 4 is slightly elongated along the diagonal F∗ = −P∗. Such a feature appears when the star formation in dwarf galaxies (they constitute a majority in our sample) is characterized by burst activity changing the rate of star formation over an order of magnitude. In particular, we may consider the BCD galaxy ESO 553−046 to be just at the stage of a star-formation burst. Under the present value of SFR, the galaxy is able to reproduce its total luminosity within only 1.5 Gyr and its gas resource would be sufficient to keep the present star-formation rate on a time-scale ∼1/30 of the cosmic age. Many other examples of nearby dwarf galaxies currently undergoing global starbursts are presented by Lee et al. (2009) . Note that Stinson et al. (2007) simulated evolution of dwarf irregular galaxies, taking into account the effects of gas outflows due to the wind from SNs, and found cyclic bursts of star formation on a scale of 0.3 Gyr with an amplitude of about (2–3) mag for dwarf systems of very low mass. On the other hand, as was shown by Lee et al. (2009) , the dwarfs that are currently experiencing massive global bursts consist of only 6 per cent from their total number, and AC K N OW L E D G M E N T S R E F E R E N C E S Makarov & Kaisina ( 2012 ) contains 825 galaxies residing in the Local Volume . Among them, there are about 500 galaxies with measured Hα fluxes and about 700 galaxies having estimates of Explorer (GALEX) Martin et al. ( 2005 ). We shall use this extended ported by RUS - IND RFBR grant 10-02-92650 , RFBR grant 10- 02-00123 and the Ministry of Education and Science of . the Russian Federation under contract 14.740.11.0901 Begum A. , Chengalur J. N. , Karachentsev I. D. , Sharina M. E. , Kaisin S. 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S. S. Kaisin, I. D. Karachentsev, S. Ravindranath. Hα survey of nearby dwarf galaxies, Monthly Notices of the Royal Astronomical Society, 2012, 2083-2098, DOI: 10.1111/j.1365-2966.2012.21501.x